IQMT Seminar
Time Speaker & Topic of the Talk
Wed 15:00
Hybrid Zoom / CN B.425 R.206
Christos Panagopoulos
Nanyang Technological University, Singapore
Chirality Engineering in Quantum Matter

Quantum geometry manifests itself in a plethora of materials and architectures including topological insulators, Archimedean magnets, Weyl/Dirac semimetals, transition metal oxides and dichalcogenides, and can also be engineered via interaction with electromagnetic fields. The emergent internal structure can radically alter our understanding of quantum mechanical wavefunctions of charge carriers and quasiparticles, yielding modular transport, optical, magnetic responses that are critically interlinked. I will introduce our efforts to geometrically tune the microscopic interactions responsible for macroscopic quantum coherences, through two recent examples. Namely, the first architecture in which superconductivity and magnetism interact via distinct topological solitons and a novel physical qubit platform based on the energy-level quantization of the helicity degree of freedom.

Wed 11:00
Hybrid Zoom / CN B.425 R.206
The role of synthesis experts in quantum materials and technology research

High-quality crystals are required for experiments in solid state physics and also industrial research fields. Such crystals are often synthesized by experts (as known as sample growers). They also study physical properties of materials and conduct joint research with other researchers for further study and even for mass production. However, the work of synthesis experts is in some ways unknown to physicists. Based on my experience studying superconductors [1, 2], magnetic memory materials [3], and battery materials in academia and industry, I will introduce and explain this aspect to researchers and students.
[1] N. H. Sung, C. J. Roh, K. S. Kim, and B. K. Cho, Possible multigap superconductivity and magnetism in single crystals of superconducting La2Pt3Ge5 and Pr2Pt3Ge5, Phys. Rev. B 86, 224507 (2012)
[2] Y. K. Kim, N. H. Sung, J. D. Denlinger, B. J. Kim, Observation of a d-wave gap in electron-doped Sr2IrO4, Nature Physics 12, 37 (2016)
[3] N. H. Sung, F. Ronning, J. D. Thompson, and E. D. Bauer, Magnetic phase dependence of the anomalous Hall effect in Mn3Sn single crystals, Appl. Phys. Lett. 112, 132406 (2018)

Wed 11:00
Hybrid Zoom / CN B.425 R.206
Synthesis and characterization of Ba1-xCaxNi2As2 with thermodynamic, transport and spectroscopic methods

In many unconventional superconductors, new types of exotic electronic orders are often found in proximity to the superconducting phase. These can consist in charge density waves, various types of magnetic orders or electronic nematicity. In many cases, the nature of the interplay between these orders and superconductivity remains elusive. They can be simply coexisting, competing or even intertwined with superconductivity, and their impact on the superconducting transition temperature is often unclear. Understanding the normal state of unconventional superconductors is generally the first important step towards unveiling the mechanism of superconductivity in these materials.
In this thesis, we study the superconductor BaNi2As2 which is structurally identical to the parent compounds of high-temperature Fe-based superconductors, BaFe2As2, but exhibits a completely different set of electronic phases, none of them being in particular magnetic in nature. Recently, investigations of Sr substituted Ba1-xSrxNi2As2 attracted attention with a sixfold enhancement of Tc and the discovery of an electronic nematic phase by elasto-resistance measurements [1]. Here, we have explored the effect of an alternative substitution. In this talk, I present the first study of novel Ca-substituted Ba1-xCaxNi2As2. First, we report the growth procedure for the synthesis of high-quality single crystals with a substitution level of up to 10 %. The main goal is to create a first phase diagram for Ba1-xCaxNi2As2 based on specific heat and electrical transport measurements. In this context, we determined the temperatures of a triclinic transition TS, a minimum in dR/dT Tmin, and the superconducting transition Tc for several samples. We observed a decrease of TS from 137 K to below 100 K (on cooling) and a slow increase of Tc by about 0.2 K, an observation which strongly contrasts to that made upon Sr substitution in the absence of a significant contraction of the unit cell. In addition, we have carried out a first study of the lattice dynamics of this system, focusing in particular on the behavior of the Eg,1 Raman active vibration. Earlier studies had revealed a new type of lattice-driven nematicity [2] and its occurrence in the Ca-substituted system was studied.
[1] Eckberg et al., Nature Physics 16, 346-350 (2020)
[2] Yao et al., (2022)

Wed 14:00
Online - Zoom Meeting
Santanu Pakhira
Ames Laboratory
Novel magnetism in 122-type itinerant pnictide system and Eu-based magnetic topological materials

The 122-type iron pnictides have opened up a wide research field after the discovery of superconductivity (SC) with chemical doping by suppressing the long-range antiferromagnetic (AFM) order of parent compounds. In the absence of SC, isotructural cobalt pnictides are itinerant quantum magnets and are described to consist of competing magnetic interactions. In the first part of my talk, I will present the novel magnetic properties of some doped ACo2As2 (A = Ca, Sr) compounds. CaCo2-yAs2 is a unique itinerant magnetic system with strong frustration and exhibits A-type antiferromagnetic (AFM) ordering below TN ≈ 52 K. Although frustration in local moment systems are well studied, itinerant frustrated magnetic systems are largely unexplored. Such magnetically unstable states are fragile and can be easily chemically tuned to different exotic quantum states. Both electron- and hole-doping onto the Co-site through Fe and Ni substitutions strongly suppress the AFM ordering in the system followed by a carrier-tuned Stoner transition [1, 2]. In the absence of long-range magnetic ordering, strong quasi-1D ferromagnetic quantum spin-fluctuations develop in these doped-CaCo2-yAs2 along with a signature of non-Fermi-liquid behavior. In SrCo2As2, no long-range magnetic order is detected in down to at least 50 mK, but stripe AF spin fluctuations exist. We found that Pd substitution trigger long-range AF order in the system. In the second part of my talk I will discuss some of the important results on Eu-based magnetic topological materials. Understanding the complex interplay of magnetism and band topology is quite important to discover exotic quantum phenomena in magnetic topological materials. Recently, topological Dirac surface states are reported in trigonal A-type AFM compounds EuMg2Bi2 and EuSn2As2. We have shown that in the AFM state, an unusual low-field induced spin reorientation in the three-fold AFM domains is observed for both the compounds associated with a weak in-plane anisotropy. H-T magnetic phase diagrams are constructed from the combined results of magnetic, heat capacity, resistivity, and neutron diffraction data [3, 4]. The phase boundary between the AFM and PM states is consistent with the molecular-field-theory prediction for spin S = 7/2.
[1] B. G. Ueland et. al., Phys. Rev. B 104 (2021) L220410
[2] S. Pakhira et. al., Phys. Rev. B 104 (2021) 094420
[3] S. Pakhira et. al., Phys. Rev. B 101 (2020) 214407
[4] S. Pakhira et. al., Phys. Rev. B 104 (2021) 174427

Wed 11:00
Hybrid Zoom / CN B.425 R.206
Mads Hansen
Institut Néel, CNRS
LaFeSiO1-δ: A Novel Superconducting Member of the Iron-Silicide Family

Since their discovery in 2008, iron-based superconducting pnictides (As, P...) and chalcogenides (Te, Se...) have become a well-established class of unconventional superconductors, spanning multiple structural families, with Tc up to 55 K in bulk materials [1]. This category of superconductors has recently been extended to a new subgroup of materials where pnictogen/chalcogen atoms are replaced by Si in LaFeSiH (Tc ~ 10 K) and LaFeSiFx (Tc ~ 9 K) [2,3].
By using a topotactical approach, we were able to intercalate oxygen in intermetallic LaFeSi, resulting in the novel crystallogenide, LaFeSiO1-δ. Our study, based on complementary experimental probes, reveals that this crystallogenide is superconducting with Tc ~ 10 K [4].
Considering its strongly squeezed Fe-Si anion height, below 1 Å, this Tc is surprisingly high, challenging the quasi-universal relationship between structure and superconductivity in Fe-based compounds [5]. The unique crystal structure of this new compound has a strong impact on its electronic properties where the doping is absorbed in a non-rigid-band fashion, resulting in a distinct fermiology. Specifically, the electron pockets, usually observed around the M point in the prototypical Fermi surface of iron-based superconductors, are strongly suppressed, while the hole-like pockets at the Γ point are retained. This, in turn, is detrimental to the usual (π,π)-nesting. The lack of such nesting suggests a departure from s± pairing mechanism.
Furthermore, above Tc, resistivity shows a non-Fermi liquid behaviour, implying significant electronic correlations. This correlated behaviour is also visible in NMR where anti-ferromagnetic fluctuations are observed at low temperatures.
[1] H. Hosono, K. Kuroki, Physica C 514, 399 (2015).
[2] F. Bernardini et al., Phys. Rev. B 97, 1462 (2018).
[3] J.-B. Vaney et al., Nature Communications. 13, 100504 (2022).
[4] M. F. Hansen et al., npj Quantum Materials - in review (2021).
[5] C. H. Lee et al, Solid State Communications, 152, 8 (2012)

Tue 16:00
Hybrid Zoom / CS Sem.R. 3.01
Philippe Goldner
Chimie Paristech, ENS Paris
Emerging Rare Earth-Based Platforms for Optical Quantum Technology

Wed 11:00
Hybrid Zoom / CN B.425 R.206
Jazmín Aragón Sanchez
Centro Atómico Bariloche, CNEA, CONICET Universidad Nacional de Cuyo, Argentina
Impact of atomic defects in the bulk electronic states of FeSe1-xSx superconducting crystals

Understanding the impact of atomic-scale defects in the electronic structure of Fe-based superconductors is of key importance for assessing how critical are the occurrence of these features for the establishment of superconductivity in these compounds. With this aim, here we study single crystals of FeSe1-xSx, the Fe-based compound with the simplest crystal structure, consisting of a pile up of superconducting layers. We apply scanning tunnelling microscopy (STM) to reveal that this compound presents S-doping induced defects as well as diluted dumbbell defects associated to an Fe vacancy. We measure the electronic structure of the samples by means of X-ray photoemission spectroscopy (XPS) and reveal that the spectral shape of the peaks of some of the Se and Fe core levels can only be adequately described by considering a dominant plus a smaller second component of the electronic states. We find this result for both, pure as well as S-doped samples, irrespective that they present extra crystal defects associated to the substitution of Se by S atoms. Structural and resistivity characterization, as well as the spectral shape and energy location of the peaks in XPS spectra, indicate our samples do not present intergrowths of the hexagonal phase. Even though in our STM topographies only about 4% of the imaged Se atoms are involved in dumbbell defects, according to DFT calculations these defects entail a significant modification of the electronic clouds of the eight Se and Fe atoms surrounding the Fe vacancy. Furthermore, in the case of FeSe films, if the density of dumbbells increases, superconductivity is suppressed. Thus, we suggest the second component in our XPS spectra is associated to the ubiquitous dumbbell defects in FeSe. These impact of atomic defects in the binding energy energy and spectral shape of the core levels in FeSe1-xSx highlights the subtle interplay between the crystal structure and the bulk electronic states in Fe-based superconductors.

Tue 16:00
Hybrid Zoom / CS Sem.R. 3.01
Yanina Fasano
Centro Atómico Bariloche, CNEA, CONICET, Universidad Nacional de Cuyo, Argentina
Leibniz Institute for Solid State and Materials Research, Dresden
Hyperuniformity in Vortex Matter in type-II Superconductors

Many biological, material and mathematical systems share the special property of being hyperuniform, namely of presenting vanishing infinite-wavelength density fluctuations. This means that the density of the constituent objects is homogeneous in the large scale, as in a perfect crystal, although they can be isotropic and disordered like a liquid. Hyperuniform structures present a structure factor that algebraically decays to zero when decreasing the reciprocal-space wavenumber q due to the suppression of particle density fluctuations at large direct-space length-scales. This "hidden order" present in disordered systems can be affected by the type of disorder of the host medium where the objects are nucleated. Vortex matter nucleated at the surface of superconducting samples with weak point-like disorder are ordered and disordered hyperuniform systems.[1,2] In this work we study experimentally and theoretically how planar correlated disorder in the medium affects this property. We show that correlated strong disorder generated by planes of defects traversing the whole sample thickness suppress the hyperuniformity of the structure. This particular type of disorder produces a structure factor that decays at small wavenumbers but saturates in the limit q->0 due to persisting vortex density fluctuations at large lengthscales. We also show that this suppression is concomitant to anisotropic vortex density fluctuations.
[1] G. Rumi, J. Aragón Sánchez, F. Elías, R. Cortés Maldonado, J. R. Puig, N. R. Cejas Bolecek, G. Nieva, M. Konczykowski, Y. Fasano, A.B. Kolton, Phys. Rev. Res. 1 (2019) 033057
[2] J. B. Llorens, I. Guillamón, I. García-Serrano, R. Córdoba, J. Sesé, J.M. De Teresa, M. R. Ibarra, S. Vieira, M. Ortuño, H. Suderow, Phys. Rev. Res. 2 (2020) 033133

Wed 16:00
Join Zoom Meeting
Susmita Roy
University of Colorado, Boulder
Conventional, Time-Resolved Raman, and Inelastic Neutron Scattering of Strongly Correlated Materials

The interplay between charge, spin and lattice degrees of freedom creates exotic phases in strongly correlated materials. Raman and Neutron scatterings are valuable tools to study these phases because of their sensitivity towards these elementary excitations. I will first present the time-resolved Raman scattering results on optimally doped YBa2Cu3O6.9 superconductor which focuses on relaxation timescales and interactions between electrons and phonons. Experimental advances we have made allow for future time-resolved experiments in which the pump frequency can be tuned from the near-to mid-infrared, opening the possibility to pump particular infrared-active modes and observe their effect on Raman-active modes. Further, I will discuss the Raman scattering results which will illuminate the impact of flash sintering on the structure of layered oxide structure of cuprate Pr2CuO4. The combined comprehensive inelastic neutron scattering measurements of Ba8Ga16Ge30 with lattice dynamical calculations based on the density functional theory will be presented to elucidate the effect of the occupational disorder on heat-carrying phonons. Also, I will show the Raman scattering results of spin- 1/2 quantum spin liquid Ba4Ir3O10 which will provide the signatures of spinons and damped phonons.

Tue 16:00
IQMT & PHI Seminar / Hybrid Zoom & CS-P R. 3-1
Yolita Eggeler
Laboratorium für Elektronenmikroskopie des Karlsruher Instituts für Technologie
Exploring Magnetic Phenomena on the Nanoscale of Functional Materials Using Analytical In Situ Transmission Electron Microscopy

Wed 11:00
Hybrid Zoom / CN B.425 R.206
Huanhuan Shi
TU Dresden
Exfoliation and Functionalization of 2D Semiconductors via Wet Chemistry

2D semiconductors have remarkable physical, optical and electrical properties and high surface areas. These intrinsic properties make 2D semiconductors promising candidates for versatile applications such as (opto)electronics, energy storage and conversion. However, scalable synthesis of high quality 2D semiconductors remains a significant barrier for their device integrations and practical applications. Therefore, the development of a novel chemical exfoliation process which aims at high yield synthesis of high quality 2D materials while maintaining good solution processability is of great concern.
Here, we focus on the solution production of emerging 2D materials (In2Se3 , MXene and black phosphorus) with high-quality by wet-chemical exfoliation methods and address their applications for high performance (opto)electronics and supercapacitors.
[1] H. Shi, M. Li, A. Shaygan Nia, M. Wang, S. Park, Z. Zhang, M. R. Lohe, S. Yang, X. Feng, Adv. Mater. 32 (2020) 1907244
[2] H. Shi, P. Zhang, Z. Liu, S. Park, M. R. Lohe, Y. Wu, A. S. Nia, S. Yang, X. Feng, Angew. Chem., Int. Ed. 60 (2021) 8689
[3] H. Shi, S. Fu, Y. Liu, C. Neumann, M. Wang, H. Dong, P. Kot, M. Bonn, H. Wang, A. Turchanin, Oliver G. Schmidt, A. Shaygan Nia, S. Yang, X. Feng, Adv. Mater. 33 (2021) 2105694

Fri 11:00
Hybrid Zoom / CN B.425 R.206
Alexander Grimm
Quantum Information Processing with Schrödinger-Cat Qubits

Quantum two-level systems are routinely used to encode qubits but tend to be inherently fragile, leading to errors in the encoded information. Quantum error correction (QEC) addresses this challenge by encoding effective qubits into more complex quantum systems. Unfortunately, the hardware overhead associated with QEC can quickly become very large.
In contrast, a qubit that is intrinsically protected against a subset of quantum errors can be encoded into superpositions of two opposite-phase oscillations in a resonator, so-called Schrödinger-cat states. This "Schrödinger-cat qubit" has the potential to significantly reduce the complexity of QEC. In a recent experiment, we have demonstrated the stabilization and operation of such a qubit through the interplay between Kerr nonlinearity and single-mode squeezing in a superconducting microwave resonator.
In this talk, I will review some key concepts of QEC and situate our approach within the field. I will give an overview of the cat qubit, followed by an outlook on different applied and fundamental research directions it enables.

Fri 14:00
Zoom Seminar
Wahib Aggoune
Institut für Physik & IRIS Adlershof Humboldt-Universität zu Berlin
Tuning two-dimensional electron and hole gases at LaInO3/BaSnO3 interfaces by polar distortions, termination, and thickness

Two-dimensional electron gases (2DEG), arising due to quantum confinement at interfaces between transparent conducting oxides, have received tremendous attention in view of electronic applications. The challenge is to find a material system that exhibits both a high charge-carrier density and mobility at room temperature. In this talk, we will explore the potential of interfaces formed by two lattice-matched wide-gap oxides of emerging interest, i.e., the polar, orthorhombic perovskite LaInO3 and the non-polar, cubic perovskite BaSnO3, employing density-functional theory and many-body perturbation theory. We first present the properties of the pristine components. For periodic heterostructures, we show that the polar discontinuity at the interface is mainly compensated by electronic relaxation through charge transfer from the LaInO3 to the BaSnO3 side. This leads to the formation of a 2DEG hosted by the highly-dispersive Sn-s-derived conduction band and a 2D hole gas of O-p character, strongly localized inside LaInO3. Remarkably, structural distortions through octahedra tilts induce a depolarization field counteracting the polar discontinuity, and thus increasing the critical (minimal) LaInO3 thickness, required for the formation of a 2DEG. These polar distortions decrease with increasing LaInO3 thickness, enhancing the polar discontinuity and leading to a 2DEG density f 0.5 electron per unit-cell surface. Interestingly, in non-periodic heterostructures, these distortions lead to a decrease of the critical thickness, thereby enhancing and delocalizing the 2DEG. We rationalize how polar distortions, termination, and thickness can be exploited in view of tailoring the 2DEG characteristics, and why this material is superior to the most studied prototype LaAlO3/SrTiO3.

Tue 16:00
Zoom Seminar
Xing He
Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina, USA
Understanding Phonon Anharmonicity Using Neutron / X-ray Scattering and First-Principles Simulation

Phonon anharmonicity is crucial to understand thermal transport properties of materials, phase transition mechanisms, as well as atomic disorder/diffusions. Time-of-flight inelastic neutron scattering instrument covering large four-dimensional momentum and energy space, is suitable to probe phonon frequencies and lifetimes as a function of temperature. Moreover, first-principles simulation including temperature effects, such as ab initio molecular dynamics, and recent machine-learning accelerated molecular dynamics provide tools for large atomic scale and time scale analysis of anharmonic systems. In this talk, we combine inelastic neutron scattering, neutron/x-ray diffuse scattering, and first-principles calculation to probe and decipher phonon anharmonicities in multiple perovskite materials for both oxide and halide. Oxide perovskites, exhibiting multiple phase transitions and lattice instability and complex ferroelectric polarization behaviors, have drawn interest for decades, with recent emerging quantum critical behaviors, anomalous thermal transport properties, and thermal hall effects at low temperature. While halide perovskite, softer than oxide, has attracted attention due to high performance in photovoltaic, radiation detectors, potential thermoelectric materials, and optoelectronic properties coupling with large atomic fluctuation. Here, we probe and rationalize the phonon eigenvector anharmonicity in oxide perovskite as approaching quantum critical point, large octahedron instability and strong diffuse scattering in halide perovskite, and selective coupling of atomic disorder with phonons in oxide perovskite.
[1] Xing He et al., Phys. Rev. Lett. 124 (2020) 145901
[2] T. Lanigan-Atkins, Xing He et al., Nat. Mater. 20 (2021) 977
[3] Xing He et al., arXiv preprint arXiv:2112.04717 (2021)

Tue 16:00
Zoom Seminar
Senthil Kumar Kuppusamy
Coherent light-matter interactions in rare-earth ion containing molecules

Tue 16:00
Zoom Seminar
Evgeni Ilichev
Leibniz Institute of Photonic Technology, Jena
Dual Shapiro Steps in Superconducting Nanowire

We demonstrated current quantization in a superconducting NbN nanowire subject to microwave radiation. The observed current steps, dual to Shapiro steps in Josephson junctions, result from photon assisted tunneling of coherent quantum phase slips. To suppress fluctuations the nanowires are incorporated into a compact high impedance environment. Without radiation the nanowires exhibit supercurrent-like behavior at high bias and current blockade at low bias. The demonstrated effect is promising for the development of the quantum current standard; a missing element in the Quantum Metrology Triangle. We observe the effect with radiation up to 31 GHz, and sharp steps up to 26 GHz, corresponding to a current step of 8.3 nA.

Tue 16:00
Zoom Seminar
Patrick Winkel
Superconducting Quantum Circuits for Hybrid Architectures

Superconducting quantum circuits (SQCs) constitute a versatile hardware platform for the realization of novel quantum technologies. While the quantum coherence of SQCs has been improved significantly over the last two decades, there are other quantum degrees of freedom, which outperform SQCs in some figures of merit. For that reason, it might proof beneficial to combine these different technologies in a quantum hybrid architecture. Within the scope of my PhD thesis, I developed a non-linear inductive circuit element based on granular aluminum (grAl) - a disordered superconductor with high kinetic inductance - which can be operated in strong external magnetic fields, an essential requirement for the applicability in a hybrid system. As a proof of concept, the quantum coherence and non-linearity of the element is demonstrated by the implementation of a transmon qubit, in which the conventional Josephson junction is substituted by the grAl element. In order to improve the signal-to-noise ratio of the measurement, I developed a non-degenerate parametric amplifier based on long arrays of Josephson junctions, with up to 1800 elements. The novelty of the amplifier concept is to use multiple eigenmodes to cover the desired frequency range between 1 and 10 GHz.

Tue 16:00
Zoom Seminar
Loic Doussoulin
IQMT-KIT & Institut Néel (Grenoble)
Iron-based Superconductors in High Magnetic Fields

Iron-based superconductors are a major discovery in the history of superconductivity. The electronic properties of FeSe, the simplest iron-based superconductor, suggest the existence of fluctuations that could lead to the observation of vortex lattice melting associated with Pauli pair breaking. I will therefore present my thesis work on this subject. Specific heat measurements have been performed up to 35T and thus offer the possibility to study the complete diagram. Using a scaling analysis, the existence of Gaussian thermal fluctuations is shown and at the same time allows us to find the upper critical field. The latter shows signs of a Pauli-limited behaviour. The melting of the vortex lattice is studied from the perspective of thermal as well as quantum fluctuations. The presence of a FFLO phase will also discussed.

Tue 16:00
PHI-IQMT Zoom + SR 3/1
Kristin Willa
Thermodynamic signatures of short-range magnetic correlations in UTe2

The recent discovery of superconductivity in the heavy fermion UTe2 [1] has led to an enormous interest in this material. It hosts various unusual phenomena like metamagnetism, field-reentrant superconductivity and pressure-induced magnetic and superconducting phases [2]. The extremely high upper critical fields suggest a spin-triplet p-wave state [3] while there have also been experimental indications for chiral as well as multi- component superconductivity. As intriguing as the superconducting state is, there are also still many open questions concerning the normal state. While no ordered magnetic moment has been observed, a mixture of ferromagnetic, antiferromagnetic and valence fluctuations has been proposed.
In this talk, I will present our thermodynamic (thermal expansion, specific heat, magnetostriction and susceptibility) measurements on the normal state of UTe2 [4]. In all of these quantities, clear evidence for a broad Schottky-like anomaly around T*=12K is observed with a calculated entropy of roughly R*ln(2). Combining our thermodynamic measurements with previously published high magnetic-field transport data [5] allows us to construct an H-T phase diagram which resembles that of the ferromagnetic superconductor URhGe. From our thermodynamic data, we calculate the electronic Grüneisen parameter related to T*. It is found to be comparable in magnitude to that of the metamagnetic field indicating a common origin. Finally, enhanced Wilson and Korringa ratios suggest that the existence of short-range ferromagnetic fluctuations cannot be ruled out, although no direct evidence for their existence has been found to date.

[1] S. Ran et al., Science 365, 684 (2019)
[2] D. Braithwaite et al., Communications Physics 2, 147 (2019)
[3] S. Ran et al., Nat. Phys. 15, 1250 (2019)
[4] K. Willa et al., PRB 104, 205107 (2021)
[5] W. Knafo et al., JPSJ 88, 063705 (2019)

Tue 16:00
Zoom + SR 3/1
David Hunger
Physikalisches Institut & IQMT, KIT

Tue 16:00
Zoom + SR 3/1
Adrian Merritt
Forschungszentrum Jülich, Juelich Center for Neutron Scattering
Phonon Anomalies Observed by Inelastic Scattering in the Iron-based Superconductors

Inelastic neutron and x-ray scattering are powerful techniques to probe lattice and magnetic dynamics of materials. I will discuss these techniques and their application to the iron-based superconductors. In particular, I will discuss investigations of nematicity in superconducting iron pnictides such as FeSe and Co-doped BaFe2As2. Nematicity is common in electronic phases of high-Tc superconductors, particularly in the Fe-based systems. We extracted temperature-dependent nematic correlation length from the anomalous softening of acoustic phonons in FeSe, underdoped Ba0.97Co0.03Fe2As2 and optimally doped Ba0.94Co0.06Fe2As2. In all cases, we find that the nematic correlation length is well described by a power law (T-T0)-1/2 extending over a wide temperature range. We attributed this mean-field behavior and the extended fluctuation regime to a sizable nemato-elastic coupling, which may be detrimental to superconductivity. I will also show recent, similar measurements of FeSe under pressure, where a rich phase diagram can be revealed by high-pressure. This includes not only a modification of the structural transition temperature, but the development of a concurrent magnetic-structural transition not seen at ambient pressure and a significant increase in the superconducting transition temperature. Access to this phase reveals interesting phenomena at both the structural and superconducting transitions which we were able to observe.

Tue 16:00
Zoom + SR 3/1
Martin Gärttner
Physikalisches Institut, Kirchhoff-Institut für Physik und Institut für Theoretische Physik der Universität Heidelberg
Representing Many-Body Quantum States with Neural Networks

Wed 11:00
Zoom Seminar
Yulia Tymoshenko
Technical University of Dresden
Magnetic excitations in bond-frustrated helimagnet ZnCr2Se4

Low-energy spin excitations in any long-range ordered magnetic system in the absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating from the ordering wave vectors. In helimagnets, these modes hybridize into the so-called helimagnon excitations. We employ neutron spectroscopy supported by theoretical calculations to investigate the magnetic excitation spectrum of the isotropic Heisenberg helimagnet ZnCr2Se4 with a cubic spinel structure, in which spin-3/2 magnetic Cr3+ ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the (0,0,qh) ordering vector, low-energy magnetic excitations in the single-domain spiral phase show soft helimagnon modes with a small energy gap of ∼ 0.17 meV, emerging from two orthogonal wave vectors (qh,0,0) and (0,qh,0) where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spin-wave theory and only acquire a finite gap due to higher-order quantum-fluctuation corrections. To further investigate anisotropic low-temperature properties of the cubic spinel helimagnet ZnCr2Se4 we combined neutron scattering, thermal conductivity, ultrasound velocity, and dilatometry measurements. In an applied magnetic field, neutron spectroscopy shows a complex and non-monotonic evolution of the spin-wave spectrum across the quantum-critical point that separates the spin-spiral phase from the field-polarized ferromagnetic phase at high fields. A pseudo-Goldstone spin gap vanishes at this quantum critical point, restoring the cubic symmetry in the magnetic subsystem. The anisotropy imposed by the spin helix has only a minor influence on the lattice structure and sound velocity but has a much stronger effect on the heat conductivities measured parallel and perpendicular to the magnetic propagation vector. The thermal transport is anisotropic at T = 2 K, highly sensitive to an external magnetic field, and likely results directly from magnonic heat conduction. We also report long-time thermal relaxation phenomena, revealed by capacitive dilatometry, which are due to magnetic domain motion related to the destruction of the single-domain magnetic state, initially stabilized in the sample by the application and removal of magnetic field. Our results are likely universal for a broad class of helimagnetic materials in which a discrete lattice symmetry is spontaneously broken by the magnetic order.

Tue 16:00
Zoom Seminar
Vibhuti Rai
Karlsruher Institut für Technologie, Physikalisches Institut
Light Emission from Single Self-decoupled Molecules in a Scanning Tunnelling Microscope

Tue 16:00
Zoom Seminar
Francesco Valenti
Karlsruher Institut für Technologie, Physikalisches Institut
Diagnostics and abatement of quasiparticle poisoning in superconducting quantum circuits

Tue 16:00
Zoom Seminar
Larissa Kohler
Karlsruher Institut für Technologie, Physikalisches Institut
Tracking Brownian motion in three dimensions and characterization of individual nanoparticles using a fiber-based high-finesse microcavity

Tue 16:00
Zoom Seminar
Daria Gusenkova
Karlsruher Instituts für Technologie, Physikalisches Institut
Quantum non-demolition dispersive readout of a superconducting artificial atom using large photon numbers

Wed 11:00
Zoom Seminar
Giovanna Morigi
Saarland University
Dissipative control of quantum systems

In the first part of this talk a protocol is discussed for preparing a spin chain in a generic many-body state in the asymptotic limit of tailored nonunitary dynamics. The dynamics require the spectral resolution of the target state, optimized coherent pulses, engineered dissipation, and feedback. As an example, we discuss the preparation of an entangled antiferromagnetic state, and argue that the procedure can be applied to chains of trapped ions or Rydberg atoms. In the second part we propose a protocol which achieves fast adiabatic dynamics in a Landau-Zener problem by implementing a quantum-non-demolition (QND) measurement of the spin. In the limit where the effective dynamics can be described by a Born-Markov master equation, the QND measurement induces an effective dephasing and dissipative dynamics which enforce adiabaticity. We show that the resulting fidelity of the adiabatic transfer significantly increases with the strength of the QND coupling. We then discuss perspectives of applying these dynamics for quantum annealers.

Tue 16:00
Zoom Seminar
Nabeel Aslam
Harvard University, Department of Physics
Quantum Sensors for Nanoscale Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) is successfully applied in many fields such as biology, chemistry, and condensed matter physics. The technique is non-invasive and reveals rich information about the underlying structure of the sample. However, due to NMR's limited sensitivity, the samples typically must be macrosco-pic in size, preventing the detection of single molecules or of two-dimensional materials. This limitation calls for a new approach.
Nitrogen-vacancy (NV) centers in diamond can be applied as quantum sensors and present an alternative path to magnetic resonance on the nanoscale. They are optically addressable and possess exceptionally long coherence times, even at ambient conditions. With the creation of shallow NV centers, it is now possible to detect the NMR signal on the nanoscale [1,2]. The measurement of a single proton spin [3] and of a monolayer of a two-dimensional material [4] shows the impressive sensitivity of the method. Nevertheless, the technique is still in its infancy and several challenges, such as low spectral resolution and the lack of colocalization between sample and sensor, need to be addressed.
In my presentation I will discuss how these limitations can be overcome by designing the quantum sensor system. Particularly I will describe the use of long-lived quantum memories based on single nuclear spins in diamond. Combining these quantum memories with the quantum sensors enables us to improve the spectral resolution by four orders of magnitude. This leads to the chemical shift detection and thereby identification of molecules on the nanoscale [5]. Another crucial aspect is the colocalization of sample and sensor spins, which we are addressing by nanofabricating the diamond surface.
In the talk I will also present our efforts to couple the quantum sensors to magnetic molecules and to two-dimensional metal-organic frameworks (MOF). These chemically tunable systems can be employed as sen-sor spins. Due to the reduced sample to sensor distance a significantly enhanced sensitivity can be expected.
[1] T. Staudacher et al. Science 339 (2013) 561
[2] H. Mamin et al. Science 339 (2013) 557
[3] A.Sushkov et al. PRL 113 (2014) 197601
[4] I. Lovchinsky et al. Science 355 (2017) 6324
[5] N. Aslam et al. Science 357 (2017) 67

Wed 15:00
Zoom Seminar
Mai Ye
Rutgers University, Department of Physics and Astronomy
Critical dynamics and quantum coherence in excitonic insulator Ta2NiSe5

Excitonic insulator is a quantum coherent phase resulting from the formation of a macroscopic population of electron-hole pairs. Because crystal structural symmetries are broken at the transition temperature Tc, it is difficult to determine whether a particular transition is of excitonic or structural origin. The nature of transition for the candidate material Ta2NiSe5 (Tc = 328K) is currently under debate, with both excitonic [1] and structural [2] explanations proposed. We report Raman-scattering results on Ta2NiSe5 to explore its critical excitonic fluctuations above Tc and emergent coherence below Tc [3]. The overdamped excitonic mode in the quadrupolar symmetry channel softens above Tc, while the optical phonon modes show no softening behavior. Moreover, the softening of the acoustic phonon mode [4] can be accounted for by its coupling to the excitonic mode, i.e. intrinsic ferroelastic instability is absent. On cooling, coherent superposition of band states at the gap edge gradually emerges. From these results, we demonstrate that the phase transition of Ta2NiSe5 is of excitonic nature. We further show that sulfur doping suppresses the excitonic contribution to ordering [5].
For Ta2NiS5, we identify a phase transition induced by ferroelastic instability at 120K, and a sharp in-gap exciton mode.
[1] Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009); Y. F. Lu et al., Nat. Commun. 8, 14408 (2017)
[2] E. Baldini et al., arXiv: 2007.02909; A. Subedi, Phys. Rev. Materials 4, 083601 (2020)
[3] M. Ye et al., arXiv: 2102.07912 (2021); P. A. Volkov et al., arXiv: 2007.07344 (2020)
[4] A. Nakano et al., Phys. Rev. B 98, 045139 (2018)
[5] P. A. Volkov et al., arXiv: 2104.07032 (2021)

Wed 11:00
Zoom Seminar
Alexander Petrovic
Nanyang Technological University, Singapore
Coupling Topological Solitons in Hybrid Quantum Materials

Topological solitons in condensed matter are stable excitations characterised by a non-zero “winding number” or topological charge. Their particle-like dynamics render them ideal for information-handling applications, including cryogenic memory and synaptic logic devices. Recently, combining topological solitons in chiral magnets (skyrmions) and superconductors (vortices) has attracted considerable theoretical attention, largely due to the possibility of creating a topological superconductor by imprinting the non-collinear exchange field from skyrmions onto s-wave Cooper pairs. Vortex cores in the resultant px + i py superconducting phase would harbour the Majorana fermions which have been so keenly sought for building quantum computers with increased resilience to environmental decoherence. However, it has so far proven extremely challenging to develop materials which simultaneously exhibit superconductivity and topological spin textures at low temperature.
I will outline the design principles for a new class of “hybrid” multilayer materials, which superpose a thin superconducting film onto a chiral magnetic layer. In such heterostructures, the characteristic length scales of both superconducting and magnetic orders may be tuned via the multilayer geometry and an applied magnetic field. Following the hierarchy ξ < rsk < λ (where ξ, rsk, and λ are the coherence length, skyrmion radius, and penetration depth, respectively), we stabilise skyrmions with rsk ≈ 50nm in a IrFeCoPt multilayer and demonstrate that their stray field nucleates antivortices in a neighbouring 25nm Nb film. The antivortices principally reveal themselves via an emergent magnetic moment antiparallel to the external field, but also influence the bulk flux dynamics in an applied current or field gradient. Although the stray field dominates the interaction between chiral magnet and superconductor in our films, we additionally detect a weak Rashba-Edelstein coupling in exchange-coupled heterostructures which enhances antivortex formation.
I will discuss methods to further optimise the skyrmion-(anti)vortex coupling in such multilayers, with a view towards eventual quantum computational applications. Beyond the skyrmion-vortex example described here, the general concept of linking topological solitons from distinct quantum phases promises to facilitate spatially heterogeneous magnetoelectric coupling at nanometer length scales, with fascinating consequences for the hybrid soliton dynamics and functionality.

Tue 16:00
Zoom Seminar
Tim Wolz
Karlsruher Instituts für Technologie, Physikalisches Institut
Controlling Cavity Magnon Polariton Properties

Wed 11:00
Zoom Seminar
Alsu Gazizulina
Institute for Quantum Phenomena in Novel Materials, HZ Berlin
Structural and Magnetic Properties of the Spin-Dimer Compound Ba3-xSrxCr2O8

The compounds Sr3Cr2O8 and Ba3Cr2O8 are insulating dimerized antiferromagnets with Cr5+ magnetic ions. These spin-1/2 ions form hexagonal bilayers with a strong intradimer antiferromagnetic interaction that leads to a singlet ground state and gapped triplet states. The Cr5+ ions surrounded by oxygen ions in a tetrahedral coordination are Jahn-Teller active. I will discuss the effect on the structural and magnetic properties of Sr3Cr2O8 by introducing chemical disorder upon replacing Sr by Ba. Mixed compounds Ba3-xSrxCr2O8 with x = 2.9 and x = 2.8 were grown in a four-mirror-type optical floating-zone furnace. There is a distinct suppression of the orbital-lattice fluctuation regime with increasing Ba content. This stands in contrast to the linear behavior exhibited by unit cell volumes, atomic positions, and intradimer spin-spin exchange interactions. The magnetic properties of these compounds were studied by magnetization measurements. Inelastic neutron-scattering measurements on Ba0.1Sr2.9Cr2O8 were performed in order to determine the interaction constants and the spin gap for x = 2.9. The intradimer interaction constant is found to be about 4% smaller than that of pure Sr3Cr2O8, whereas the interdimer exchange interaction is smaller by 7%. These results indicate a noticeable change in the magnetic properties by a random substitution effect.

Wed 11:00
Zoom Seminar
Senthil Kumar Kuppusamy
Karlsruhe Institute of Technology
Spin-Switchable and Luminescent Molecular Systems for Advanced Applications

Molecule-based switching and light-addressable systems have been in focus to harness spintronics and quantum technology applications. Spin-crossover (SCO) complexes featuring spin-state-dependent physical property variations are one of the pillars of molecular magnetism and a prominent example of spin-switchable material. Such complexes exhibiting bistable spin-state switching characteristics in the bulk and spin-state dependent conductance switching at the single-molecule level are desirable for developing molecule-based memory and spintronics, respectively, applications [1,2]. In the first part, a concise and systematic overview of - from the bulk-state to single-molecule level - functional SCO complexes studied for device applications will be presented [3,4]. In the second part, proposals to develop luminescent lanthanoid (Ln(III)) complexes as light-addressable systems for quantum technology applications will be presented. Lanthanoid complexes feature sharp inter-configurational f-f electronic transitions, which are well isolated from the environment by the outer 5d and 6s orbitals, and nuclear spin-levels with long-lived spin lifetimes. These features combined with the possible physical property tuning via molecular engineering approaches render lanthanoid complexes suitable for implementing quantum information processing (QIP) schemes. While lanthanoid molecule-based qubits and qdits have been reported in the literature [5,6] direct optical addressing of nuclear spin-based hyperfine qbits of lanthanoid complexes are yet to be reported. To begin with, we have prepared a series of Eu(III) complexes and shown optical polarisation of nuclear spin-levels in them and obtained optical coherence lifetime (T2,opt) in the μs range. To progress further, we propose to synthesize isotopically enriched Ln(III) complexes following isotopologues chemistry [7]. This will enable us to precisely engineer the hyperfine splitting of nuclear spin-levels (151Eu vs. 153Eu), tune the optical properties - for example, emission wavelength and lifetime - and improve the coherence lifetime by reducing the environmental fluctuations arising from phonon and spin baths. We also propose to study the optically addressable Ln(III) molecular systems on the surface and in resonant cavities to harness the utility of the systems as qbits and quantum memories [8,9] and use two-color laser pulses as the excitation source of Ln(III)-based f-f transitions to eliminate detection issues arising from light scattering [10]. Overall, we propose to develop a new and unexplored field of light-addressable Ln(III) molecular systems [11] as hyperfine qbits and qdits for quantum technology applications.
[1] K. S. Kumar and M. Ruben, Coord. Chem. Rev. 2017, 346(1), 176-205.
[2] K. S. Kumar and M. Ruben, Angew. Chem. Int. Ed. 2020, DOI: 10.1002/anie.201911256.
[3] K. S. Kumar, M. Studniarek, B. Heinrich, J. Arabski, G. Schmerber, M. Bowen, S. Boukari, E. Beaurepaire, J. Dreiser, and M. Ruben, Adv. Mater. 2018, 30(11), 1705416.
[4]. E. Burzuri, A. Garcia-Fuente, V. Garcia-Suarez, K. S. Kumar, M. Ruben, J. Ferrer, and HSJ Van Der Zant, Nanoscale, 2018, 10, 7905-7911.
[5] C. Godfrin, A. Ferhat, R. Ballou, S. Klayatskaya, M. Ruben, W. Wernsdorfer, and F. Balestro, Phy. Rev. Lett. 2017, 119, 187702.
[6] A. Gaita-Arino, F. Luis, S. Hill, and E. Coronado, Nat. Chem. 2019, 11, 301-309.
[7] W. Wernsdorfer and M. Ruben, Adv. Mater. 2019, 31, 1806687.
[8] D. D. Awschalom, R. Hanson, J. Wrachtrup, and B. B. Zhou, Nat. Phot. 2018, 12, 516-527.
[9] A. I. Lvovsky, B. C. Sanders, and W. Tittel, Nat. Phot. 2009, 3, 706-714.
[10] Y-M. He et al. Nat. Phy. 2019, 15, 941-946.
[11] K. S. Kumar, D. Serrano, A. Nonat, B. Heinrich, L. Karmazin, L. Charbonniere, P. Goldner and M. Ruben, Nat. Commun. (Revised final version submitted) arXiv:2006.09831.

Wed 11:00
Zoom Seminar
Shreenanda Ghosh
Institute for Solid State and Materials Physics, TU Dresden
Manipulation of Time Reversal Symmetry Breaking Superconductivity in Sr2RuO4 by Uniaxial Stress

Although the normal-state electronic structure of Sr2RuO4 is known with exceptional precision, even after two decades of research, the symmetry of its certainly unconventional superconducting state is under strong debate, e.g., the long-time favoured spin-triplet px ± i py state is ruled out by recent NMR experiments [1]. However, in general time-reversal-symmetry breaking (TRSB) superconductivity indicates complex two-component order parameters. Probing Sr2RuO4 under uniaxial stress offers the possibility to lift the degeneracy between such components [2]. One key prediction for Sr2RuO4, a splitting of the superconducting and TRSB transitions under uniaxial stress has not been observed so far. I will show results of muon spin relaxation (μSR) measurements on Sr2RuO4 placed under uniaxial stress, wherein a large stress-induced splitting between the onset temperatures of superconductivity and TRSB was observed [3]. Moreover, at high stress beyond the Van Hove singularity, a new spin density wave ordered phase was detected for the first time. In order to perform μSR measurements under uniaxial stress, a custom piezoelectric based pressure cell was developed [4]. This cell is going to be useful for a range of other materials, in which the Fermi surface or magnetic interaction strengths can be tuned leading to strong modifications of the electronic state.
[1] A. Pustogow, et al., Nature 574, 72 (2019); K. Ishida et al., J. Phys. Soc. Jpn. 89, 034712 (2020)
[2] C. Hicks, et al., Science 344, 283 (2014)
[3] V. Grinenko, S. Ghosh et al., arXiv:2001.08152, accepted in Nature Physics
[4] S. Ghosh et al., Review of Scientific Instruments 91, 103902 (2020)

Wed 11:00
Zoom Seminar
Michael Buchhold
Institute for Theoretical Physics, University of Cologne
Measurement-Induced Phase Transitions in Monitored Fermion Systems

A wave function exposed to measurements undergoes pure state dynamics, with deterministic unitary and probabilistic measurement induced state updates, defining a quantum trajectory. For many-particle systems, the competition of these different elements of dynamics can give rise to a scenario similar to quantum phase transitions. However, due to the stochastic nature of the wave function this type of phase transition does not manifest itself in common observable averages, obtained from the statistically averaged density matrix, and have instead mainly been observed in the dynamics entanglement. Hence, they are often termed entanglement transitions.
Here we establish a novel type of entanglement transition between a regime of logarithmic entanglement growth, and a quantum Zeno regime obeying an area law, in continuously monitored fermion dynamics. We analyze the phase transition from to different perspectives:
(i) numerical simulations of monitored lattice fermions and
(ii) an analytical field theory for monitored Dirac fermions, interpolating between the microscopic measurement dynamics and the macroscopic observables.
We identify the relevant degrees of freedom for describing the phase transition and show that their dynamics is governed by a non-Hermitian quantum Sine-Gordon model. This yields a physical picture for the phase transition in terms of a depinning from the measurement operator eigenstates induced by unitary dynamics, and places it into the BKT universality class.

Wed 11:00
Zoom Seminar
Chunqing Deng
Experimental Group, Alibaba Quantum Laboratory (AQL)
Fluxonium Qubits for Ultra-High-Fidelity and Scalable Quantum Processors

The success of superconducting quantum computing (SQC) has so far been largely built upon the transmon qubit. Finding an alternative qubit that drastically outperforms transmon represents one of the most fundamental and exciting frontiers of SQC. The fluxonium qubit stands out as a promising candidate, due to its long coherence times and large anharmonicity. Furthermore, fluxonium can be directly integrated into the existing circuit-QED schemes for scaling. For these reasons, fluxonium is our qubit platform of choice at Alibaba Quantum Laboratory.
In this talk, I will present our design and experiments of a 2D fluxonium circuit, which exhibits high coherence, high tunability and strong coupling. With T1 > 200 μs at around the sweet spot, we achieve a dielectric loss tangent as low as 1.3 10-6, and an estimated 1/f flux noise amplitude at best 1.4 μΦ0/√Hz, both numbers approaching the state of the art. Using a high-bandwidth flux bias, we demonstrate robust qubit reset and readout, together with fast one- and two-qubit gates, thus paving the path toward a fluxonium-based, ultra-high-fidelity multi-qubit processor.

Wed 11:00
Zoom Seminar
Lukas Gerhard
IQMT - Karlsruhe Institute of Technology
Controlling Individual Nano-Objects with a Scanning Tunneling Microscope

The scanning tunneling microscope (STM), relying on the overlap of wavefunctions of a sharp tip and a conductive sample, has proven to be an ideal tool for both measurement and manipulation of objects on the nanoscale. I will present an overview of our experimental techniques to control and read out the state of nano-objects with a low-temperature STM and their application to single molecule junctions [1]. We have extended the possibilities of conventional STM and built up an STM with a highly efficient light collection setup, which enables us to control and detect electron photon interaction on the nanoscale [2]. Recently, we have provided first evidence of sharp emission lines from individual self-decoupled molecules, avoiding non-radiative processes and quenching by the underlying metal substrate. I will present possibilities of how to further boost the quantum efficiency of electroluminescence of individual quantum objects. In the near future, our already record high external quantum yields will allow us to study statistics of photons emitted from single molecules, i.e. single-photon or entangled photon nature of the emitted light, with the help of a coincidence setup that is currently being finished. Medium-term plans include reversing the path of light, which opens up access to time-dependent processes in individual quantum objects.
[1] Gerhard, L. et al. An electrically actuated molecular toggle switch. Nature Communications 8, 14672 (2017).
[2] Rai, V. et al. Boosting Light Emission from Single Hydrogen Phthalocyanine Molecules by Charging. Nano Lett. 20, 7600-7605 (2020).

Tue 16:00
Zoom Seminar
Slava Dobrovitski
QuTech and Kavli Institute of Nanoscience - Delft University of Technology
Spins in Diamond for Nanoscale Sensing and Quantum Information Processing

Understanding and controlling quantum spins in solids is an exciting scientific endeavor. Besides fundamental interest in non-equilibrium many-spin dynamics, this research is needed for applications in nanomagnetism, spintronics, quantum information, and advanced sensing at nanoscale. The nitrogen-vacancy (NV) centers in diamond constitute a particularly promising platform for many solid-state quantum technologies. I will overview the effort, and present our work on quantum spin dynamics and control of individual electronic and nuclear spins associated with NV centers. I will discuss how controlling and protecting the coherent dynamics of coupled spins enables accurate quantum gates on spin qubits in diamond, and how such gates allow development of the quantum registers with solid-state spin qubits [1]. I will talk about extending this approach into the area of nanoscience, which results in very sensitive nanoscale tomography with single-spin resolution [2], and about using these advances for implementing small/medium-scale quantum registers in diamond [3,4].
[1] T. van der Sar et al., Nature 484, 82 (2012).
[2] T. H. Taminiau et al., Phys. Rev. Lett. 109, 137602 (2012).
[3] T. H. Taminiau et al., Nature Nano. 9, 171 (2014).
[4] M. H. Abobeih et al., Nature 576, 411 (2019).

Wed 11:00
Zoom Seminar
Tom Lacmann
Single crystal growth, chemical and hydrostatic pressure tuning of BaNi2As2

Many superconducting materials show an interplay of superconductivity, charge density waves and structural phases which can be tuned by chemical doping and hydrostatic pressure. Understanding this interplay is crucial in revealing the microscopic origin of these different phases. In the superconductor BaNi2As2 tetragonal, orthorhombic and triclinic structural phases as well as an incommensurate (IC-CDW) and a commensurate charge density wave (C-CDW) are reported. In this study, high quality phosphorus doped BaNi2(As1-xPx)2 crystals were synthesized with a flux method to apply chemical pressure to the crystals. The crystals were characterized by different experimental methods. In addition, x-ray diffraction (XRD) experiments using a four circle diffractometer were performed to investigate the different electronic and structural phases of BaNi2(As1-xPx)2. High pressure XRD studies were performed in order to compare the effect of chemical pressure due to phosphorus doping with the effect of hydrostatic pressure. The x-ray investigations confirm the existence of a Immm orthorhombic phase as proposed by Merz et al. [1] by means of the orthorhombic splitting of the (800) Bragg peak. The observed splitting is small (δ ∼ 10-4) and increases linearly on cooling. The XRD measurements show strengthening of the IC-CDW and orthorhombic phase under phosphorus doping due to the larger temperature window of the phases. In addition, the IC-CDW was found at temperatures higher than the orthorhombic phase with relevant diffuse scattering signal observable up to room temperature. We verified the suppression of the triclinic/commensurable CDW (C-CDW) phase under phosphorus doping as reported by Kudo et al. [2]. However, under hydrostatic pressure BaNi2As2 showed a different behavior: The triclinic phase was only slightly suppressed and the initial C-CDW is totally suppressed. In addition, above 7.5 GPa the beginning IC-CDW in the tetragonal/orthorhombic phase is suppressed and a cascade of new superstructures appeared. Under a pressure of 10 GPa, new and much more complicated superstructures emerge.
[1] M. Merz et al., arXiv:2012.05024
[2] K. Kudo et al., PRL 109 (2012) 097002

Mon 14:00
Zoom Seminar
Laerte Patera
Department of Chemistry, Technical University of Munich, Garching
Imaging the Effect of Electron Transfer at the Atomic Scale: From Single Molecules to Light-Harvesting Quantum Materials

Electron transfer plays a crucial role in many chemical processes, from photosynthesis to combustion and corrosion. For instance, light-driven charge transfer can be exploited to drive carbon dioxide conversion into chemical fuels, being of enormous relevance to steer a carbon-free energy cycle. However, the effect of electron transfer on the electronic structure of organic molecules and light-harvesting interfaces remains largely unclear, limiting our current understanding of the mechanisms lying at the heart of sunlight-driven chemical transformations. Unveiling these fundamental aspects requires the development of experimental tools allowing the observation of (light-induced) electron transfer down to the single molecule level.
In this talk, I will present a novel imaging approach, based on atomic force microscopy (AFM), allowing mapping the orbital structure of single molecules upon electron transfer. In this way, we unveiled the effects of electron transfer and polaron formation on the single-orbital scale [1]. To understand the light-induced charge transfer process, we introduced a second microscopy technique, which combines an AFM setup with a visible-light source, allowing for the imaging of excitonic wavefunctions with angstrom resolution. Finally, I will show how this novel experimental approach opens for the study of photoexcited states in two-dimensional quantum materials.
[1] L. L. Patera, F. Queck, P. Scheuerer and J. Repp, Nature 566, 245-248 (2019)

Wed 11:00
Zoom Seminar
Jochen Kalt
Laboratory for Applications of Synchrotron Radiation Department of Physics, KIT
Lattice Dynamics of α-FeSi2 Nanostructures

The reduction of the dimensions of crystals to the nanometer length scale induces significant deviations in the phonon dispersions and the phonon density of states of nanostructures compared to their bulk counterparts and novel vibrational phenomena emerge. Due to the inherently small scattering volume of nanostructures, however, the determination of their lattice dynamics remains a challenge.
In this talk, I will present recent results on the lattice dynamics of α-phase FeSi2 nanoislands [1] and nanowires [2], epitaxially grown on Si(111) and Si(110) surfaces, respectively. Iron silicides are a particularly interesting member of the technologically important class of transition metal silicides, since it is the only representative that forms metallic and semiconducting phases. The phonon density of states (PDOS) of the nanostructures was obtained by nuclear inelastic scattering, a technique based on the Mössbauer effect, which is uniquely suitable for the investigation of nanostructures. Additionally, the phonon dispersions and Phonon DOS of α-phase FeSi2were determined by ab initio calculations. The experimental results reveal a distinct anisotropy of the atomic vibrations along and across the nanowires, which is not present in the nano islands. In both cases, the results can completely be understood by taking into account the specific orientation of the α-FeSi2 unit cell on the Si surface. Furthermore, both kinds of nanostructures show a distinct damping of the PDOS features upon reduction of the characteristic sizes, which can be understood by modeling of the experimental results with the ab initio calculations.
[1] J. Kalt et al., Phys. Rev. B 101 (2020) 165406
[2] J. Kalt et al., Phys. Rev. B 102 (2020) 195414

Wed 11:00
Zoom Seminar
Shangxiong Huangfu
University of Zurich, Department of Physics
Complex magnetic properties in Pr4Ni3O8

The nickelate Pr4Ni3O8 features quasi-two-dimensional layers consisting of three stacked square-planar NiO2 planes, in a similar way to the well-known cuprate superconductors. Motivated by the discovery of possible superconductivity, herein we have synthesized Pr4Ni3O8 by topotactic reduction of Pr4Ni3O10, and report on measurements of powder-neutron diffraction, magnetization and muon-spin rotation (μSR). Although without detectable long-range magnetic order, we observe complex magnetic behaviours which should be intrinsic in Pr4Ni3O8. Pr4Ni3O8 exhibits typical spin-glass behaviour with a distinct magnetic memory effect in the temperature range from 2 to 300 K and a freezing temperature ≈ 68 K. Moreover, the analysis of μSR spectra indicates two magnetic processes characterized by remarkably different relaxation rates: a slowly relaxing signal, resulting from paramagnetic fluctuations of Pr/Ni ions, and a fast-relaxing signal, indicating the presence of short-range correlated interaction. This short-range interaction strengthens substantially below ≈ 70 K. We conclude a complicated spin-freezing process in Pr4Ni3O8 governed by these multiple magnetic interactions. It is possible that the complex magnetism in Pr4Ni3O8 is detrimental to the occurrence of superconductivity.

Tue 16:00
Zoom Seminar
Riccardo Comin
Massachusetts Institute of Technology, Department of Physics
Visualizing the Birth (or at least Infancy) of Charge-Density Waves

Strongly correlated electron systems are a natural host for spontaneous electronic symmetry breaking phenomena, which lead to emergent electronic phases often characterized by long-range order and collective behavior. Superconductivity and charge-density-waves are two prime examples of ordered electronic phases seeded by strong interactions, despite their very different macroscopic traits – the former exhibiting dissipationless conduction while in the latter charge motion is often hindered as a result of electrons freezing into a ‘supercrystal’. As in many other fields, our understanding of the mechanisms that promote these collective phenomena requires capturing their formation in the act, as the new phase emerges from a high-symmetry state (in this case, a correlated electron fluid).
In this talk, I will show how resonant soft X-ray scattering can be used to gain unprecedented sensitivity to the formation of charge-density-waves at an early stage. I will discuss two studies, one focused on charge order in the family of copper oxide high-temperature superconductors (cuprates), the other on charge-density-waves in quasi-1D material ZrTe3.
In the cuprates, using X-ray scattering at the Cu L3-resonance (~930 eV) we found an electronic ‘Wigner glass’ phase where electrons seemingly organize in a uncommon, ordered phase with broken-translational symmetry but full rotational symmetry. This state underscores the quantum nature of the electronic fluid in these materials and provides one of the missing links to understand the birth of an unconventional superconductor from a Mott insulating state [1].
In ZrTe3, we used coherent X-ray scattering at the Te M-edge (~630 eV) to visualize the formation of the electronic superlattice and the precursor ‘seeds’ from which charge-density-waves develop into a long range ordered phase in quasi-1D systems. In the same study, we examined the dynamics of density-wave formation using photon correlation spectroscopy and proposed a connection between the mechanisms of CDW formation and the instabilities of the Fermi surface [2].
[1] M. Kang et al., Evolution of charge order topology across a magnetic phase transition in cuprate superconductors, Nature Physics 15 (2019) 335
[2] L. Yue et al., Distinction between pristine and disorder-perturbed charge density waves in ZrTe3, Nature Communications 11 (2020) 98

Wed 11:00
Zoom Seminar
Eli Zeldov
Weizmann Institute of Science, Rehovot, Israel
Mapping the Twist-Angle Disorder and Unconventional Landau Levels in Magic Angle Graphene

The emergence of flat bands and of strongly correlated and superconducting phases in twisted bilayer graphene crucially depends on the interlayer twist angle upon approaching the magic angle. Utilizing a scanning nanoSQUID-on-tip, we attain tomographic imaging of the Landau levels and derive nanoscale high precision maps of the twist-angle disorder in high quality hBN encapsulated devices, which reveal substantial twist-angle gradients and a network of jumps [1]. We show that the twist-angle gradients generate large gate tunable in-plane electric fields, unscreened even in the metallic regions, which drastically alter the quantum Hall state by forming edge channels in the bulk of the samples. The correlated states are found to be particularly fragile with respect to twist-angle disorder. We establish the twist-angle disorder as a fundamentally new kind of disorder, which alters the local band structure and may significantly affect the correlated and superconducting states. The talk will also describe direct imaging of the QH edge states in monolayer graphene revealing their internal structure of counterpropagating equilibrium topological and nontopological currents [2].
[1] A. Uri, S. Grover, Y. Cao, J. A. Crosse, K. Bagani, D. Rodan-Legrain, Y. Myasoedov, K. Watanabe, T. Taniguchi, P. Moon, M. Koshino, P. Jarillo-Herrero, and E. Zeldov, Nature 581, 47 (2020).
[2] A. Uri, Y. Kim, K. Bagani, C. K. Lewandowski, S. Grover, N. Auerbach, E. O. Lachman, Y. Myasoedov, T. Taniguchi, K. Watanabe, J. Smet, and E. Zeldov, Nat. Physics 16, 164 (2020).

Wed 11:00
Zoom Seminar
Ana Akrap
University of Fribourg, Department of Physics
Understanding the Ground States of Topological Systems

Nowadays we know of many gapless electronic phases with conical bands, such as graphene, Dirac semimetals, and Weyl semimetals. Their low-energy excitations resemble truly relativistic particles, creating an interesting analogy between the two branches of modern physics which were until now only loosely connected. This is how condensed matter physicists can now explore low-energy phenomena which were previously thought to occur only in high-energy physics. The key is understanding the physics at a milli-electron-volt scale. To access electronic structures at these low energies, we combine Landau level spectroscopy, infrared spectroscopy at zero magnetic field, and effective Hamiltonian models. Because of their inherent low energy scales, our materials are often easy to tune by external parameters such as temperature, high pressure and magnetic field, and have rich phase diagrams.
I will illustrate our approach for several Dirac and Weyl semimetals focusing on ZrTe5.

Wed 11:00
Zoom Seminar
Michael Marthaler
HQS Quantum Simulations
Lattice Models and Quantum Computing without Quantum Error Correction

Current quantum computers have relatively high error probabilities per operation. Therefore, any application has to use an extremely small number of operations and ideally be somewhat insensitive to the errors (e.g. decoherence). We discuss the number of operations needed for simulation of the electronic structure problem. We show that the number of operations and the mode of error accumulation seems somewhat favorable for lattice models. Given current limitations of quantum computers it is also important to compare to classical solvers like DMRG to understand at which point a quantum computer can really deliver a speed up of interest. We will also discuss our current work using DMRG and how it connects to our quantum computing goals.

Wed 11:00
Zoom Seminar
Titus Neupert
Condensed Matter Theory, Department of Physics University of Zurich
Solving a Small Hubbard Model on an IBM Quantum Computer

Fully programmable quantum computing and simulation devices have hit several milestones over the past few years, with a massively growing involvement of industrial research from companies such as IBM, Google, and Microsoft. The emerging quantum computing technology promises to be useful for applications such as quantum machine learning, but notably also for intrinsically quantum-mechanical calculations that appear in material science, quantum chemistry, or quantum many-body physics. In this talk, I will show how state-of-the-art quantum hardware performs in this domain of quantum problems. Concretely, I focus on solving the ground state structure of a fermionic Hubbard model on a small cluster of sites and demonstrate that qualitatively and quantitatively accurate results can be obtained. This is enabled by exploiting the symmetries of the problem, employing a hybrid quantum-classical variational algorithm, and a Lanczos-based error mitigation scheme. I will introduce each of these steps in detail and discuss the potential of the general workflow for upscaling.

Wed 11:00
Zoom Seminar
Jacob Biamonte
Laboratory for Quantum Information Processing
Skolkovo Institute of Science and Technology, Moscow, Russia
Introduction to Modern Quantum Algorithms: Optimisation, Simulation and Machine learning

This lecture begins with the promise of idealised, error corrected, quantum algorithms. With this goal defined, we then outline the contemporary capacity of quantum enhanced processors. Increasing the control and capacity of quantum simulators has resulted in a new class of quantum devices, utilising an iterative classical-to-quantum feedback process. This so-called “variational” approach to quantum computation was formally proven (in the noise-free setting) to represent a universal model of quantum computation. An exciting global research effort to understand the variational model is redefining the field of quantum computation. While the ultimate capacity of variational algorithms remains unknown, these methods represent an attractive approach due to the ease at which these algorithms can be realized experimentally. Contrary to analog quantum simulation, the variational approach forgoes the requirement of realising the targeted Hamiltonian directly in the laboratory, thus allowing the study of a wide variety of previously intractable target models experimentally. This comes at the cost of increased measurements and classical pre- and postprocessing.
The talk should be accessible to graduate students in physics.
- J. Biamonte et al. , Quantum Machine Learning, Nature 549, 195 (2017)
- J. Biamonte, Universal Variational Quantum Computation, arXiv:1903.04500 (2019)
- J. V. Akshay et al. , Reachability Deficits in Quantum Approximate Optimization, Phys. Rev. Lett. 104, 090504 (2020)

Wed 11:00
Zoom Seminar
Luca de' Medici
ESPCI, Paris
Electronic Compressibility and High-Tc Superconductivity: New Links

In multi-orbital Hubbard models including strong intra-atomic exchange a so-called "Hund's metal" phase is realized that shows, among other hallmarks, typically a strong differentiation in the degree of correlation of the conduction electrons based on their orbital character. This selective physics is also a key player in iron-based superconductors (FeSC), where a wealth of experimental evidences validates this theoretical picture. We will show that at the frontier between this Hund's metal phase and a conventional metal, the electronic compressibility is strongly enhanced or even divergent, and that the FeSC that have a high Tc are placed in our simulations on this frontier. This theoretical evidence of enhanced compressibility at the frontier between a phase with selective correlations and a more conventional metal is found in cuprates. We will outline the main indications for a common scenario of high-Tc superconductivity.

Wed 11:00
Zoom Seminar
Leonardo Degiorgi
ETH Zürich, Department Physik
Nematicity in the Charge Dynamics of Iron-Based Superconductors

The divergent nematic susceptibility, obeying a simple Curie-Weiss power law over a large temperature interval, is empirically found to be a ubiquitous signature in several iron-based superconductors across their doping-temperature phase diagram. I will discuss the impact of nematicity in the optical response of selected iron pnictide and chalcogenide materials, over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. First, I will focus my attention on FeSe where we reveal an astonishing anisotropy of the optical response in the mid-infrared-to-visible spectral range. This bears testimony to an important polarization of the underlying electronic structure, due to an orbital-ordering mechanism, supplemented by orbital selective band renormalization. The far-infrared response of the charge dynamics in FeSe moreover allows establishing the link to the dc resistivity, emphasizing scenarios based on scattering by anisotropic spin-fluctuations. Second, I will offer a comprehensive optical investigation of the optimally hole-doped Ba0.6K0.4Fe2As2, for which we show that the stress-induced optical anisotropy in the infrared spectral range is reversible upon sweeping the applied stress and occurs only below the superconducting transition temperature. These findings demonstrate that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome.

Wed 11:00
Zoom Seminar
Hermann Suderow
Department Física de la Materia Condensada, Instituto Nicolás Cabrera, IFIMAC, Universidad Autónoma de Madrid - Spain
Real Space Imaging of Electronic Correlations

I will briefly explain the basics of Scanning Tunneling Spectroscopy (STS), focusing on techniques to study strongly correlated electron systems at dilution refrigeration temperatures. I will first report on the discovery of a one-dimensional charge density wave (1D-CDW) which is a Moiré pattern between the atomic lattice and a hot spot for electronic scattering in the bandstructure of the hidden order (HO) state of URu2Si2. The Moiré is produced by fracturing the crystal in presence of a dynamical spin mode at low temperatures and its presence suggests that charge interactions are among the most relevant features competing with HO in URu2Si2. Then, I will discuss the consequences of band hybridization in the local density of states which lead to new insight into the local density of states on small-sized atomically flat areas in URu2Si2. Finally, I will briefly present suprising results in feedback driven atomic size Josephson junctions, which lead to an AC Josephson signal that will likely improve Josephson Scanning Spectroscopy.

Wed 14:00
Zoom Seminar
Xavier Waintal
Quantum Photonics, Electronics and Engineering Laboratory (PHELIQS)
Commissariat à l'énergie atomique et aux énergies alternatives (CEA)
Grenoble, France
Introduction to Quantum Computing by a Skeptic

I will expose the concepts of quantum computing for physicists and from a physicist's point of view. I will go through the construction of quantum error corrections and point out the difficulties and physical limitations for implementing this framework. I will, in particular, address the surface code and the concept of correctable versus non-correctable errors. This presentation is intended to an audience of physicists that mostly know about what the qubits are but not how we are supposed to make a computer out of them.
After the main talk, there will be a more specialized discussion session "Is quantum supremacy real?" for those who are interested in details of the recent Google experiment on quantum supremacy.

Wed 11:00
Zoom Seminar
Jens Müller
Goethe University Frankfurt
Learning from Noise - Fluctuation Spectroscopy of Correlated Electrons in Molecular Conductors

So-called 1/f-type fluctuations are ubiquitous in nature, and can be found in such diverse contexts as light curves of quasars, the human heartbeat, earthquakes, road traffic or classical music. In this seminar, we aim to give a broad and pedagogical overview of such fluctuation phenomena in condensed-matter systems and discuss how 'noise can be turned into a signal' by the method of fluctuation spectroscopy. We first give an example where understanding the microscopic origin of the fluctuations in semiconductor-based Hall sensors helps to improve the signal-to-noise ratio and hence the device performance. In the main part of the talk we then will discuss recent results on the low-frequency dynamics of strongly correlated electrons in low-dimensional molecular metals, which may be considered model systems for studying the Mott metal-insulator transition, a key phenomenon in many-body physics. Our findings range from glassy structural dynamics, critical slowing down of charge fluctuations at the Mott transition, and recent results on the formation of polar nanoregions in charge-driven ferroelectrics.

Thu 11:00
Rolf Lortz
Hong Kong University of Science and Technology
Topological Superconducting Phases in 2D and 3D Materials

In this talk I give an overview of our recent research on 2D and 3D topological superconducting materials. I will focus on the doped topological insulator Bi2Se3 and on heterostructures between a quantum anomalous Hall insulator and a superconductor. A nematic topological superconductor has an order parameter symmetry, which spontaneously breaks the crystalline symmetry in its superconducting state. This state can be observed, for example, by thermodynamic or upper critical field experiments in which a magnetic field is rotated with respect to the crystalline axes, but also directly from the anisotropic gap symmetry in scanning tunneling probe experiments. We present a study on the upper critical field of the Nb-doped Bi2Se3 for various magnetic field orientations parallel to the basal plane of the Bi2Se3 layers. The data clearly demonstrate a two-fold symmetry that breaks the three-fold crystal symmetry. This provides strong experimental evidence that Nb-doped Bi2Se3 is a nematic topological superconductor similar to the Cu- and Sr-doped Bi2Se3, and rules out earlier suggestions that the finite magnetic moment of the intercalated Nb ions could instead induce a chiral superconducting state. We then show that in doped Bi2Se3, the nematic order arises from a multicomponent order parameter where superconductivity is the primary order and the nematic order an intertwined secondary order. Such a state of matter with a multi-component order parameter can give rise to a vestigial order. In the vestigial phase, the primary order is only partially melted, leaving a remaining symmetry breaking behind, an effect driven by strong classical or quantum fluctuations. We present the observation of a partially melted superconductor in which pairing fluctuations condense at a separate phase transition and form a nematic state with broken Z3 symmetry High-resolution thermal expansion, specific heat and magnetization measurements reveal that this symmetry breaking occurs at Tnem≈3.8 K above Tc≈3.25 K, along with an onset of superconducting fluctuations. Thus, before Cooper pairs establish long-range coherence at Tc, they fluctuate in a way that breaks the rotational invariance at Tnem and induces a distortion of the crystalline lattice. With the recent discovery of the quantum anomalous Hall insulator, which exhibits the conductive quantum Hall edge states without external magnetic field, it becomes possible to create a novel topological superconductor by introducing superconductivity into these edge states. In this case, two distinct topological superconducting phases with one or two dispersive chiral Majorana edge modes were theoretically predicted, characterized by Chern numbers (N) of 1 and 2, respectively. We present spectroscopic evidence from Andreev reflection experiments for the presence of chiral Majorana modes in a Nb / (Cr0.12Bi0.26Sb0.62)2Te3 heterostructure with distinct signatures attributed to two different topological superconducting phases. The results are in qualitatively good agreement with the theoretical predictions.

Tue 11:00
Gediminas Simutis
Laboratoire de Physique des Solides, Université Paris - Saclay & CNRS
Tuning Quantum Magnets

When cooled down, most magnetic materials form long-ranged structures of the magnetic moments. The situation is markedly different when strong quantum fluctuations are present in low dimensional materials or lattices with highly frustrated networks of spins. Such systems collectively called quantum magnets may exhibit unconventional magnetism and even maintain a fluctuating quantum state down to the lowest temperatures.
Sometimes one of the best ways to reveal these new ground states and their excitations is by perturbing such systems. In this talk, I will present several ways to modify the quantum magnets using chemical substitution as well as application of high pressure. First, I will show how spectral modifications can be achieved by diluting low dimensional magnets and what effects the impurities have on magnetic ordering as well as the universal scaling properties [1]. I will then talk about a different way to perturb such systems: high pressure. In particular, I will discuss the search and discovery of new phases in bond-frustrated iridate and ruthenate magnets. The links between the exact modification of the lattice and the resulting magnetic phases will be demonstrated in some of Kitaev materials [2].
Finally, I will discuss arguably the most interesting perturbation - carrier doping. After an overview of previous efforts to dope a spin liquid, I will present our ongoing study of the interplay between the charge and magnetic degrees of freedom in an iridium-based geometrically frustrated hyperkagome lattice [3].
[1] Phys. Rev. Lett. 111, 067204 (2013); Phys. Rev. 95, 054409 (2017)
[2] Phys. Rev. B 98, 104421 (2018); Phys. Rev. Lett. 120, 237202 (2018)
[3] Phys. Rev. Lett. 99, 137207 (2007); Scientific Reports 4, 6818 (2014)

Tue 14:30
Max Hirschberger
RIKEN CEMS, Center for Emergent Matter Science, Japan
Scalar spin chirality on the nanoscale: Material exploration and emergent electrodynamics

Scalar spin chirality is a pervasive concept in contemporary research on magnetism in condensed matter, especially as relates to the coupling between magnetic order and the conduction electrons. In metallic magnets, non-coplanar spin structures on the nanoscale can exert a giant emergent magnetic field on charge carriers, with potential applications in spintronics. The emergent magnetic field may also be harnessed in the future design of new types of correlated systems with protected surface modes. In this talk, I will outline our work on magnets with non-coplanar order on different length scales: (1) The canted ferromagnetic state in the correlated pyrochlore oxide Nd2Mo2O7 is a representative example of non-coplanarity within a single crystallographic unit cell [1,2]. We discuss our experimental efforts of tuning the chemical potential in this material by chemical substitution, and new theoretical insight into the qualitatively different effects of scalar spin chirality and spin-orbit coupling on the electronic band structure [3]. (2) The metallic antiferromagnet Dy3Ru4Al12 with breathing Kagome network of rare-earth sites serves as an example of an intermediate class (λmag ~ 1.5 nm): Its non-coplanar magnetic order (in zero field) can be described as antiferromagnetic stacking of strongly coupled spin-trimers with tilted all-in or all-out configuration [4]. We reveal high sensitivity of the trimer arrangement to external magnetic field, which leads to a large anomalous Hall effect when net scalar spin-chirality emerges above 1 Tesla. (3) Finally, I will discuss our recent work on the formation of nanometer-sized skyrmion spin-vortices in the centrosymmetric rare earth intermetallics Gd2PdSi3 [5] and Gd3Ru4Al12 [6] with Heisenberg Gd3+ moments and modulation length λmag ~ 2.5-3 nm. In contrast to the more widely studied case of skyrmions in non-centrosymmetric material platforms such as bulk B20 compounds or magnetic interfaces (λmag ~ 5-200 nm), skyrmions emerge here in absence of the Dzyaloshinskii-Moriya interaction. We report the phase diagrams of these compounds, reveal their magnetic order using scattering and real space imaging techniques, and discuss their large topological Hall and Nernst responses [7].
[1] Y. Taguchi et al., Science 291, 2573
[2] Y. Taguchi et al., Physical Review Letters 90, 257202
[3] M. Hirschberger et al., manuscript in preparation
[4] S. Gao, M. Hirschberger et al., arXiv:1908.07728
[5] T. Kurumaji, T. Nakajima, M. Hirschberger, et al., Science 365, 914
[6] M. Hirschberger, T. Nakajima, et al., arXiv:1812.02553, accepted to Nat. Comms.
[7] M. Hirschberger, L. Spitz, et al., arXiv:1910.06027