Zeit | Sprecher & Vortragsthema |
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06.07.2022
Online - Zoom Meeting |
Santanu Pakhira Novel magnetism in 122-type itinerant pnictide system and Eu-based magnetic topological materials Ames Laboratory 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.
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22.06.2022
Hybrid Zoom / CN B.425 R.206 |
Mads Hansen LaFeSiO1-δ: A Novel Superconducting Member of the Iron-Silicide Family Institut Néel, CNRS 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]. |
21.06.2022
Hybrid Zoom / CS Sem.R. 3.01 |
Philippe Goldner Emerging Rare Earth-Based Platforms for Optical Quantum Technology Chimie Paristech, ENS Paris |
01.06.2022
Hybrid Zoom / CN B.425 R.206 |
Jazmín Aragón Sanchez Impact of atomic defects in the bulk electronic states of FeSe1-xSx superconducting crystals Centro Atómico Bariloche, CNEA, CONICET Universidad Nacional de Cuyo, Argentina 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. |
31.05.2022
Hybrid Zoom / CS Sem.R. 3.01 |
Yanina Fasano Hyperuniformity in Vortex Matter in type-II Superconductors Centro Atómico Bariloche, CNEA, CONICET, Universidad Nacional de Cuyo, Argentina Leibniz Institute for Solid State and Materials Research, Dresden 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.
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18.05.2022
Join Zoom Meeting |
Susmita Roy Conventional, Time-Resolved Raman, and Inelastic Neutron Scattering of Strongly Correlated Materials University of Colorado, Boulder 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. |
17.05.2022
IQMT & PHI Seminar / Hybrid Zoom & CS-P R. 3-1 |
Yolita Eggeler Exploring Magnetic Phenomena on the Nanoscale of Functional Materials Using Analytical In Situ Transmission Electron Microscopy Laboratorium für Elektronenmikroskopie des Karlsruher Instituts für Technologie |
11.05.2022
Hybrid Zoom / CN B.425 R.206 |
Huanhuan Shi Exfoliation and Functionalization of 2D Semiconductors via Wet Chemistry TU Dresden 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. |
29.04.2022
Hybrid Zoom / CN B.425 R.206 |
Alexander Grimm Quantum Information Processing with Schrödinger-Cat Qubits Paul-Scherrer-Institut 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. |
18.03.2022
Zoom Seminar |
Wahib Aggoune Tuning two-dimensional electron and hole gases at LaInO3/BaSnO3 interfaces by polar distortions, termination, and thickness Institut für Physik & IRIS Adlershof Humboldt-Universität zu Berlin 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. |
15.02.2022
Zoom Seminar |
Xing He Understanding Phonon Anharmonicity Using Neutron / X-ray Scattering and First-Principles Simulation Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina, USA 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.
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01.02.2022
Zoom Seminar |
Senthil Kumar Kuppusamy Coherent light-matter interactions in rare-earth ion containing
molecules INT & IQMT, KIT |
18.01.2022
Zoom Seminar |
Evgeni Ilichev Dual Shapiro Steps in Superconducting Nanowire Leibniz Institute of Photonic Technology, Jena 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. |
11.01.2022
Zoom Seminar |
Patrick Winkel Superconducting Quantum Circuits for Hybrid Architectures IQMT, KIT 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. |
07.12.2021
Zoom Seminar |
Loic Doussoulin Iron-based Superconductors in High Magnetic Fields IQMT-KIT & Institut Néel (Grenoble) 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. |
23.11.2021
PHI-IQMT Zoom + SR 3/1 |
Kristin Willa Thermodynamic signatures of short-range magnetic correlations in UTe2 IQMT, KIT 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. |
16.11.2021
PHI-IQMT Zoom + SR 3/1 |
David Hunger Laserschutzbelehrung
Physikalisches Institut & IQMT, KIT |
09.11.2021
PHI-IQMT Zoom + SR 3/1 |
Adrian Merritt Phonon Anomalies Observed by Inelastic Scattering in the Iron-based Superconductors Forschungszentrum Jülich, Juelich Center for Neutron Scattering 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. |
26.10.2021
PHI-IQMT Zoom + SR 3/1 |
Martin Gärttner Representing Many-Body Quantum States with Neural Networks Physikalisches Institut, Kirchhoff-Institut für Physik und Institut für Theoretische Physik der Universität Heidelberg |
13.10.2021
PHI-IQMT Zoom Seminar |
Yulia Tymoshenko Magnetic excitations in bond-frustrated helimagnet ZnCr2Se4 Technical University of Dresden 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. |
20.07.2021
PHI-IQMT Zoom Seminar |
Vibhuti Rai Light Emission from Single Self-decoupled Molecules in a Scanning Tunnelling Microscope Karlsruher Institut für Technologie, Physikalisches Institut |
13.07.2021
PHI-IQMT Zoom Seminar |
Francesco Valenti Diagnostics and abatement of quasiparticle poisoning in superconducting quantum circuits Karlsruher Institut für Technologie, Physikalisches Institut |
06.07.2021
PHI-IQMT Zoom Seminar |
Larissa Kohler Tracking Brownian motion in three dimensions and characterization of individual nanoparticles using a fiber-based high-finesse microcavity Karlsruher Institut für Technologie, Physikalisches Institut |
29.06.2021
PHI-IQMT Zoom Seminar |
Daria Gusenkova Quantum non-demolition dispersive readout of a superconducting artificial atom using large photon numbers Karlsruher Instituts für Technologie, Physikalisches Institut |
12.05.2021
Zoom Seminar |
Giovanna Morigi Dissipative control of quantum systems Saarland University 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. |
11.05.2021
Zoom Seminar |
Nabeel Aslam Quantum Sensors for Nanoscale Magnetic Resonance Spectroscopy Harvard University, Department of Physics 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. |
28.04.2021
Zoom Seminar |
Mai Ye Critical dynamics and quantum coherence in excitonic insulator Ta2NiSe5 Rutgers University, Department of Physics and Astronomy 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]. |
21.04.2021
Zoom Seminar |
Alexander Petrovic Coupling Topological Solitons in Hybrid Quantum Materials Nanyang Technological University, Singapore 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.
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20.04.2021
Zoom Seminar |
Tim Wolz Controlling Cavity Magnon Polariton Properties Karlsruher Instituts für Technologie, Physikalisches Institut |
31.03.2021
Zoom Seminar |
Alsu Gazizulina Structural and Magnetic Properties of the Spin-Dimer Compound Ba3-xSrxCr2O8 Institute for Quantum Phenomena in Novel Materials, HZ Berlin 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. |
17.03.2021
Zoom Seminar |
Senthil Kumar Kuppusamy Spin-Switchable and Luminescent Molecular Systems for Advanced Applications Karlsruhe Institute of Technology 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.
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10.03.2021
Zoom Seminar |
Shreenanda Ghosh Manipulation of Time Reversal Symmetry Breaking Superconductivity in Sr2RuO4 by Uniaxial Stress
Institute for Solid State and Materials Physics, TU Dresden 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.
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03.03.2021
Zoom Seminar |
Michael Buchhold Measurement-Induced Phase Transitions in Monitored Fermion Systems Institute for Theoretical Physics, University of Cologne 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. |
24.02.2021
Zoom Seminar |
Chunqing Deng Fluxonium Qubits for Ultra-High-Fidelity and Scalable Quantum Processors Experimental Group, Alibaba Quantum Laboratory (AQL) 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. |
03.02.2021
Zoom Seminar |
Lukas Gerhard Controlling Individual Nano-Objects with a Scanning Tunneling Microscope IQMT - Karlsruhe Institute of Technology 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.
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02.02.2021
PHI-IQMT Zoom Seminar |
Slava Dobrovitski Spins in Diamond for Nanoscale Sensing and Quantum Information Processing QuTech and Kavli Institute of Nanoscience - Delft University of Technology 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].
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20.01.2021
Zoom Seminar |
Tom Lacmann Single crystal growth, chemical and hydrostatic pressure tuning of BaNi2As2 IQMT - KIT 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.
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18.01.2021
Zoom Seminar |
Laerte Patera Imaging the Effect of Electron Transfer at the Atomic Scale:
From Single Molecules to Light-Harvesting Quantum Materials Department of Chemistry, Technical University of Munich, Garching 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. |
13.01.2021
Zoom Seminar |
Jochen Kalt Lattice Dynamics of α-FeSi2 Nanostructures Laboratory for Applications of Synchrotron Radiation Department of Physics, KIT 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.
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16.12.2020
Zoom Seminar |
Shangxiong Huangfu Complex magnetic properties in Pr4Ni3O8 University of Zurich, Department of Physics 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. |
17.11.2020
Zoom Seminar |
Riccardo Comin Visualizing the Birth (or at least Infancy) of Charge-Density Waves Massachusetts Institute of Technology, Department of Physics 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). |
23.09.2020
Zoom Seminar |
Eli Zeldov Mapping the Twist-Angle Disorder and Unconventional Landau Levels in Magic Angle Graphene Weizmann Institute of Science, Rehovot, Israel 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].
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16.09.2020
Zoom Seminar |
Ana Akrap Understanding the Ground States of Topological Systems University of Fribourg, Department of Physics 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. |
29.07.2020
Zoom Seminar |
Michael Marthaler Lattice Models and Quantum Computing without Quantum Error Correction HQS Quantum Simulations 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. |
15.07.2020
Zoom Seminar |
Titus Neupert Solving a Small Hubbard Model on an IBM Quantum Computer Condensed Matter Theory, Department of Physics University of Zurich 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. |
08.07.2020
Zoom Seminar |
Jacob Biamonte Introduction to Modern Quantum Algorithms: Optimisation, Simulation and Machine learning Laboratory for Quantum Information Processing Skolkovo Institute of Science and Technology, Moscow, Russia 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.
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01.07.2020
Zoom Seminar |
Luca de' Medici Electronic Compressibility and High-Tc Superconductivity: New Links ESPCI, Paris 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. |
24.06.2020
Zoom Seminar |
Leonardo Degiorgi Nematicity in the Charge Dynamics of Iron-Based Superconductors ETH Zürich, Department Physik 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. |
17.06.2020
Zoom Seminar |
Hermann Suderow Real Space Imaging of Electronic Correlations Department Física de la Materia Condensada, Instituto Nicolás Cabrera, IFIMAC, Universidad Autónoma de Madrid - Spain 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. |
03.06.2020
Zoom Seminar |
Xavier Waintal Introduction to Quantum Computing by a Skeptic Quantum Photonics, Electronics and Engineering Laboratory (PHELIQS) Commissariat à l'énergie atomique et aux énergies alternatives (CEA) Grenoble, France 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.
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27.05.2020
Zoom Seminar |
Jens Müller Learning from Noise - Fluctuation Spectroscopy of Correlated Electrons in Molecular Conductors Goethe University Frankfurt 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. |
20.02.2020
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Rolf Lortz Topological Superconducting Phases in 2D and 3D Materials Hong Kong University of Science and Technology 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. |
18.02.2020
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Gediminas Simutis Tuning Quantum Magnets Laboratoire de Physique des Solides, Université Paris - Saclay & CNRS 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. |
07.01.2020
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Max Hirschberger Scalar spin chirality on the nanoscale: Material exploration and emergent electrodynamics RIKEN CEMS, Center for Emergent Matter Science, Japan 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].
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