IQMT Seminar
Time Speaker & Topic of the Talk
13.02.2024
Tue 16:00
Hybrid Zoom / CS B. 30.23 R. 3-1
Georgios Katsaros
Institute of Science and Technology, Austria
Understanding the physics of Majorana Nanowire devices

07.02.2024
Wed 11:00
Zoom
Sudip Chakraborty
Condensed Matter Physics Division, Saha Institute of Nuclear Physics, India
Novel magnetic ground state of new ternary intermetallic R2IrSi3-series and Heusler alloys

In the modern technological landscape, the seamless functioning of everyday technologies hinges on the advancements in magnetic materials. This seminar highlights the pivotal role played by magnetic materials, with the primary focus on R2IrSi3-type intermetallic compounds (R = Gd - Ho) and some novel Heusler alloys. The research investigates previously unexplored R2IrSi3-type compounds, unveiling their distinct crystallographic structures as well as the influence of minute atomic vacancies on their magnetic properties. The magnetic behaviors of these compounds, including complex magnetic transitions and ground state degeneracy, are thoroughly analyzed. Experimental studies on Gd2Ir0.97Si2.97 and Tb2Ir0.95Si2.95 reveal ground state degeneracy along with multiple magnetic transitions in both the materials. The role of atomic vacancies on the magnetic properties of these two compounds could be understood by studying the physical properties of stoichiometric Dy2IrSi3, revealing the vacancy-induced short range ferromagnetic ordering at high temperatures in these compounds. Dy2IrSi3 also exhibits a rather large value of adiabatic temperature change close to its Néel temperature (~6.6 K). The other member of the series, Ho2IrSi3, has been synthesized in single-crystalline form and an enigmatic broad hump in the heat capacity data signifies a considerable crystalline electric field splitting of 4f-level. Our investigation on some selected members of different itinerant moment Heusler systems, viz. full-, half- and inverse Heusler alloys also divulge some very interesting features. For example, the inverse Heusler alloy, Fe2RuGe, defies theoretical predictions with a high-temperature magnetic ordering, that we have attributed to rather large Bader charge transfer. RuMnGa, a supposedly non-magnetic compound, had earlier been reported to exhibit a ferromagnetic ground state, contrary to theoretical expectations. Through our detailed study, we have shown that the magnetic ordering in this system is induced by minute presence of off-stoichiometry in composition. Rh2FeAl demonstrates high-temperature magnetic ordering and Griffith's phase-like behavior, a first among Heusler compounds. The seminar presents a short glimpse in the very interesting aspects of correlations of crystal structure as well as atomic disorders and vacancies in relation to the magnetic ground states of the concerned materials and calls for further exploration. It also sheds light on the rich tapestry of magnetic materials, offering new insights and avenues for future research and innovation.

06.02.2024
Tue 16:00
Hybrid Zoom / CS B. 30.23 R.3-1
Martin Spiecker
Physikalisches Institut, KIT
Two-level system hyperpolarization using a quantum Szilard engine

30.01.2024
Tue 16:00
Hybrid Zoom / CS B.30.23 R.3-1
Namrata Bansal
Physikalisches Institut, KIT
Magnetism, Skyrmions and Magnon-Phonon Coupling in two-dimensional van der Waals Material Fe3GeTe2

23.01.2024
Tue 16:00
Hybrid Zoom / CS B. 30.23 R.3-1
Julian Ferrero
Physikalisches Institut, KIT
Optimising semiconductor qubit circuit performance using electric field cooling

16.01.2024
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Martin Baaske
Huygens-Kamerlingh Onnes Laboratory, Leiden und Max-Planck-Institut für Biophysik, Frankfurt
DNA-loops and Label-free plasmonic single-molecule detection

13.12.2023
Wed 10:00
CS B.30.23, R.6-1
Benjamin Lienhard
Princeton University, USA
Interfacing with Quantum Information Processors - From Readout to Control

To realize the vision of useful quantum computing, balancing the effort required for quantum system control, particularly in measurements during system characterization and calibration, is essential. The effort should be low enough to compensate for system parameter changes and fast enough for periodic recalibration. While theoretical models offer insights into the general structures of quantum control landscapes, they may fall short in fully representing real quantum systems. On the contrary, complete system characterization can provide accurate numerical models, but this process is often cumbersome. In contrast, model-free learning control, although resource-intensive, presents a data-driven calibration technique. As quantum systems increase in size, both avenues become more measurement-intensive.
During this presentation, I will delve into the protocols we have devised to enhance superconducting qubit readout. Additionally, I will introduce a novel technique that combines residual modeling with neural networks to extract quantum processor dynamics. This approach provides a comprehensive representation of experimentally observed dynamics with minimal effort. The significance of this low-resource methodology lies in its ability to facilitate the calibration of closed-system models, allowing for the initiation of reinforcement-learning gate-calibration agents. This approach holds the potential to scale up and address the challenges associated with quantum control, offering a promising trajectory for the future of quantum computation.

12.12.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Dr. Taner Esat
Forschungszentraum Jülich
Standing molecules for quantum sensing

06.12.2023
Wed 11:00
Hybrid Zoom / CN B.425 R.206
Rudi Hackl
Walther-Meißner-Institut, Garching
Gap anisotropy, enhanced phonon-phonon coupling, and unusual Fano shape of the amplitude mode in CsV3Sb5

Our polarization-resolved Raman spectra of the kagome metal CsV3Sb5 reveal significant anisotropies of a large and a small gap in the electronic excitation spectra below the charge density wave (CDW) transition at TCDW = 94 K. The projections observed in A1g and E2g symmetry suggest that a gap may open up also on the Fermi surface encircling the Γ-point if the lowest-order A1g vertex is relevant. The simulations using density-functional theory (DFT) reproduce the observed spectral changes and show that both the stability of phonons and spectroscopy support the tri-hexagonal (inverse Star of David, iSoD) rather than the Star-of-David (SoD) distortion which cannot be distinguished on the basis of the phonon selection rules. The temperature dependent line width of the low-energy optical A1g mode indicates enhanced phonon-phonon coupling below TCDW. The A1g amplitude mode (AM) develops an unexpected Fano-type line shape which describes the coupling of an isolated oscillator and a continuum. The coincidence of a large electronic gap, an enhanced anharmonic phonon-phonon coupling, and the Fano shape of the AM suggests a strong-coupling phonon-driven CDW transition rather than of a Fermi surface instability or an exotic mechanism.

05.12.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Dennis Rieger
Physikalisches Institut, KIT
Gralmonium: Granular Aluminum Nano-Junction Fluxonium Qubit

15.11.2023
Wed 11:00
Zoom
Dr. Sitaram Ramakrishnan
Department of Quantum Matter, AdSE Hiroshima University, Higashi-Hiroshima, Japan
Atypical first-order charge density wave transition in 3D R2Ir3Si5 (R = Lu, Er, Ho) and its interplay with magnetism

R2Ir3Si5 (R = Lu, Er, Ho) at ambient conditions crystallizes in the orthorhombic Ibam, U2Co3Si5 structure type [1]. Previously, physical property measurements were performed for single-crystal Lu2Ir3Si5 where both charge density wave (CDW) and superconductivity (SC) was found at 200 K and 2 K [1]. However, the symmetry of the crystal in the CDW phase was left unexplored. We have studied both Lu2Ir3Si5 and Er2Ir3Si5 via single crystal x-ray diffraction (SXRD) down to 20 K where we found the structure is actually is triclinic I-1 despite the strong monoclinic distortion of the lattice accompanied satellite reflections at q = δ(121), δ = 0.23 ∼ 0.25 and that the Ir-Ir band is responsible for the CDW [2, 3]. For Er2Ir3Si5 we also found from magnetic susceptibility that the CDW is strongly coupled with the magnetism [3].

Currently, upon investigation of another isostructural compound Ho2Ir3Si5 we find that it follows a similar pattern [4]. All three compounds undergo a first-order charge density wave transition (CDW) at 200 K for Lu, 150 K for Er and 110 K for Ho. Although, the CDW does not reside on the R atoms, the TCDW is influenced by it. Here I will present the atomic mechanisms of the CDW and its behavior with magnetism in the R2Ir3Si5 family elucidating the similarities and subtle differences between them.

[1] N. Sangeetha et al., Phys. Rev. B 91, 205131, 2015.
[2] S. Ramakrishnan et al., Phys. Rev. B 101, 060101(R), 2020.
[3] S. Ramakrishnan et al., Phys. Rev. B 104, 054116, 2021.
[4] S. Ramakrishnan et al., Chem. Mater, 35, 1980, 2023.

14.11.2023
Tue 16:00
Zoom
Lior Ella
Quantum Machines, Israel
Demonstration of long-range quantum state teleportation

Adaptive quantum circuits, in which gates are conditioned on mid-circuit measurement results, are emerging as enables for expanding the capabilities of NISQ processors beyond what is possible today. In particular, they allow for efficient preparation of topologically-ordered and non-abelian states of matter [1] and for observing measurement induced phase transitions [2]. The canonical example of an adaptive circuit is state teleportation, which serves both as a fundamental building block in measurement-based quantum computation schemes as well as allows for quantum state routing in constant-depth between distant qubits [3]. In this work, we demonstrate for the first time teleportation-based routing across a chain of 7 superconducting qubits. This requires both ultra-low latency real time feedback with logical classical operations within the coherence time, as well as the generation of a highly entangled state between all qubits. This work paves the way for the efficient simulation of exotic states of matter on NISQ processors by greatly expanding the capabilities of present-day superconducting quantum processors.
[1] https://arxiv.org/pdf/2302.01917
[2] https://arxiv.org/abs/2303.04792
[3] https://arxiv.org/abs/2204.0418

16.10.2023
Mon 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Alexandre Blais
Sherbrooke University, Canada
The mysterious case of qubit readout in circuit QED

Circuit quantum electrodynamics (cQED) has emerged as a powerful platform for quantum computation and for the investigation of quantum optics at microwave frequencies. A key step in all cQED experiments is qubit readout. Based on microwave drives, it is expected that by increasing the drive amplitude, we can get faster and better qubit measurements. However, experiments show that as the drive amplitude is increased, the quality of the readout drops quickly, something that severely limits qubit readout in the laboratory. In this talk, we aim at understanding this phenomenon which has puzzled the field for over 15 years. We begin by reviewing the qubit measurement process in circuit QED, and present numerical results of the measurement dynamics. Our findings reveal signatures of ‘qubit ionization’ where the qubit is promoted to highly excited states by the readout drive. We further explore the connections between these observations and the emerge of chaos, and compare our results to recent experiments.

Remark: Prof. Blais is one of the inventors of circuit quantum electrodynamics (cQED), which this year celebrates two decades since the publication of its seminal papers
Blais et al. PRA 69, 062320 (2004)
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.69.062320
and
Wallraff et al. Nature 431, 162 (2004)
https://www.nature.com/articles/nature02851

25.07.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Qigang Zhong
Universität Gießen
On-surface molecular reactions studied by atomically precise imaging and manipulation

18.07.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Christian Ast
MPI-FKF Stuttgart
Superconducting Quantum Interference at the Atomic Scale

23.06.2023
Fri 14:00
Hybrid Zoom / CN B.425 R.206
Pintu Das
Indian Institute of Technology Delhi, New Delhi, India
Topological Hall effect due to nanoscale magnetic skyrmions in ultra-thin film multilayers

In the last decade, the Hall effect family has been enriched by a new type, called Topological Hall effect (THE) which describes a contribution to transverse current that is not usually covered in the ordinary or anomalous Hall effect (AHE). Whereas AHE result from spin orbit interaction, the THE is a manifestation of exchange interaction of the spin of the carriers with the underlying magnetic textures. Recent experimental and theoretical investigations suggest that a magnetic skyrmion or a skyrmion lattice possessing a nonzero topological charge (topologically nontrivial spin texture) may result in THE. In this talk, I will discuss the results of THE observed in the skyrmion phase for Pt|Al|Co|Pt based ultra- thin film multilayer stacks. Other than observing a correlation with the evolution of skyrmion in the system, our analysis suggests undetected skyrmions resulting in a large THE comparable with the value observed for the skyrmions which are detected by magnetic force microscopy. Our electrical fluctuation experiments indicate an increase in fluctuations as a skyrmion phase evolves in the system. I will discuss the possible scenarios to understand our experimental data.

20.06.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Maximilian Kristen
Physikalisches Institut des KIT
Noise and dissipation in superconducting granular aluminum circuits

13.06.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Felix Lüpke
Peter-Grünberg-Institut, Forschungszentrum Jülich
Quantum states in van der Waals heterostructures studied by Scanning Tunneling Microscopy

06.06.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Maximilian Pallmann
Physikalisches Institut, KIT
Purcell-enhanced emission and collective effects of NV centers in diamond coupled to a microcavity

25.05.2023
Thu 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Katharina Kaiser
CNRS – IPCMS, Straßburg, Frankreich
Charging dynamics of single molecules in STM

09.05.2023
Tue 16:00
Hybrid Zoom / CN B.425 R.206 / CS B.30.23, R.3-1
Wolfgang Pfaff
University of Illinois at Urbana-Champaign
Plug & Play Quantum Circuits

Superconducting quantum circuits are a leading platform for building large-scale quantum systems, such as quantum processors or simulators, from the bottom up. Circuit quantum electrody- namics provides us with excellent control over single artificial atoms, microwave photons, and their interactions. Scaling circuits to larger and larger numbers of qubits, however, is an ongoing challenge: Owing to inevitable imperfections in designing and fabricating devices, building large systems with precisely defined interactions is very difficult.
In this talk, I will present our efforts to build circuits in a ‘plug-and-play‘ fashion from isolated and separated components. By utilizing drive-controlled qubit-photon interactions we aim to realize modular and reconfigurable quantum networks in which qubits exchange quantum information through microwave photons. We are exploring how fast and high-fidelity two-qubit gates can be performed, quantum information may be distributed, and how remote entanglement may be stabilized through a common bath, for instance through engineered nonreciprocal qubit-photon interactions.
Our work may provide insight on how quantum devices can be scaled more robustly, and how distributed quantum states can be stabilized in open systems

25.04.2023
Tue 16:00
Zoom
Nils Marquardt
CEA Grenoble
Enlightening spin triplet superconductivity in UTe2

08.03.2023
Wed 15:00
Hybrid Zoom / CN B.425 R.206
Simulation of superconductivity on an IBM quantum computer using error mitigation protocols

We perform digital simulations of nonequilibrium dynamics of a small superconductor on an IBM quantum computer. With the toy model realized on three qubits, we discuss various error-mitigation protocols that allow us to improve the simulation results. In particular, we have developed and benchmarked a new approach that turns the cross-talk effects into depolarizing noise channel using randomized compiling.

03.03.2023
Fri 11:00
Hybrid Zoom / CN B.425 R.206
Dmitry Reznik
Department of Physics, University of Colorado - Boulder
Observing quantum materials in real time by pump-probe Raman scattering

Pump-probe experiments open a completely new way to study and control quantum materials. Their behavior away from thermal equilibrium, generation of metastable phases, photoexcited states, quantum computing-related phenomena, chemical reactions, energy dissipation, etc. can now be measured in real time with unprecedented precision due to rapid advances in ultrafast lasers and x-ray sources. In this talk I describe the time-resolved Raman scattering lab at the University of Colorado. It shares an ultrafast laser system with an angle-resolved photoemission (ARPES) setup, which allows investigation of bosonic and fermionic excitations under the same driven nonequilibrium conditions. I will discuss new physics that was uncovered using time-resolved Raman in three projects completed at the University of Colorado. The first one combined the two techniques to directly follow electron-hole excitations as well as the increase in the population of the Raman-active G-phonon in graphite after an excitation by an intense laser pulse. This work uncovered a new relaxation pathway later confirmed by others: The energy is first transferred to optical phonons near the zone boundary K-points, which then decay into G-phonons via phonon-phonon scattering. This work showed that phonon-phonon interactions must be included in any calculations of hot carrier relaxation in optical absorbers even for short timescales. I will then switch gears to a prototypical copper oxide superconductor YBa2Cu3O6+x. First, I will present our results on ultrafast magnetic dynamics where we followed time-evolution of the two-magnon Raman scattering in an insulating antiferromagnetic sample. Then I will discuss nonequilibrium dynamics of the apical oxygen phonon as a new window into electronic states and electron-phonon coupling.

22.02.2023
Wed 11:00
Hybrid Zoom / CN B.425 R.206
Jeremy Sourd
Laboratoire Ondes et Matière d'Aquitaine, Institut de Chimie de la Matière Condensée de Bordeaux, University of Bordeaux, France
Superconductivity and Kondo lattice effects in La1-xCexFeSiH: An interdisciplinary approach to a strongly correlated electron system

Strongly correlated electron systems refer to a broad class of materials in which exotic phases of matter such as unconventional superconductivity are observed. Very often, the strong electronic correlation is associated with atomic Coulomb repulsion in localized d or f orbitals. My PhD thesis [1] was the occasion for an interdisciplinary work between solid state chemistry and theoretical physics. Starting from the iron based superconductor LaFeSiH, we studied the family of intermetallic hydrides La1-xCexFeSiH (0 ≤ x ≤ 1), in which rich physical properties might emerge from the interaction between cerium 4f and iron 3d electrons. On one side, we realized the synthesis of those compounds, together with structural analysis. The physical properties measurements permits to identify superconductivity, incoherent Kondo effect and coherent Kondo lattice regime. Furthermore, we used a systematic methodology to extract phenomenologically some temperature scales and plot a phase diagram temperature vs cerium concentration x [2]. On the other side, we studied some effective model Hamiltonians adapted to the correlated electrons present in those compounds. In particular, we considered a non-local Kondo hybridization between the cerium 4f orbital and the iron 3d orbital. We showed that this non local hybridization leads to a pocket selective doping effect of iron by cerium, related to the symmetry of the cerium low energy crystal field Kramers doublet. We also proposed some experimental signatures of this effect through light-matter experiments, namely ARPES, optical conductivity and Raman spectra [3].
Key words: strong correlations, heavy fermions, iron based superconductors, non-local Kondo coupling, ZrCuSiAs compounds
[1] Phd thesis: Synthèses, mesures physiques et modélisation de composés intermétalliques RTSiH (R=La, Ce, T=Fe, Ru) à électrons fortement corrélés : supraconductivité, cohérence Kondo et magnétisme quantique (available online in french)
[2] J. Sourd, B. Vignolle, E. Gaudin, S. Burdin, S. Tencé (in preparation)
[3] J. Sourd, E. Gaudin, S. Tencé, S. Burdin (in preparation)

15.02.2023
Wed 11:00
Hybrid Zoom / CN B.425 R.206
Ina Schäfer
KIT, Institut für Informationssicherheit und Verlässlichkeit (KASTEL)
Quantum Informatics - From Quantum Gates to Quantum Software Engineering

While quantum computing hardware is becoming more and more available, the demand for quantum software is also increasing. As in classical computing, the expectation is that after quantum computing hardware has reached a certain maturity, the main value creation in quantum computing will be obtained from quantum software. In order to facilitate the use of quantum computing to solve industrial-scale applications, advances in quantum informatics, and especially in quantum software engineering are needed. In this presentation, I will explore the relationship between quantum computing and classical software engineering by focussing on three main aspects. First, I will show what can be learnt from classical programming language and compiler technology for the implementation of quantum programming languages. Second, for scaling the development of quantum programs, I will present first results on design patterns for quantum programming. Third, in order to ensure correctness of quantum programs, I will focus on verification techniques and correctness-by-construction development for quantum programs.

14.02.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Oliver Gröning
Eidgenössische Materialprüfungs- und Forschungsanstalt (Empa), Materials Science and Technology
Exploiting the chirality of the intermetallic compound PdGa on its surfaces

The chirality of biomolecules is a key structural property for determining their physiological activity. A prominent example is the difference in taste between R-limonene (rectus, right) and S-limonene (sinister, left) where the former smells of oranges and the latter of lemons. Accordingly, the synthesis of enantio-pure products is of the utmost relevance in biochemistry and the pharmaceutical industry. Despite this importance, the potential of chiral metal surfaces for asymmetric catalysis is practically unexplored, not least because the crystal structure of elemental metals is achiral. The intermetallic compound Pd1Ga1 belongs to the space group space group mo. 198, P213 and its bulk structure as well as all its terminating surfaces are chiral. In my presentation, I will introduce the surface structure and properties of the low Miller-index surfaces of Pd1Ga1 investigated by Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (IV-LEED). Then I will review the transfer of the surface chirality to chiral and prochiral molecular systems, where very high enantiomeric excess of up to 99 % can be observed. The chirality of the surface can also manifest itself in directional molecular motion, which we observe in the highly directed rotation of acetylene molecules. The rotation of the molecule can be temperature activated or triggered by the tunneling current of the STM tip, in which case the rotation frequency is highly dependent on the bias voltage. In this activated rotation regime, I will discuss the role of friction as a necessary component to yield the directional rotation.

13.02.2023
Mon 11:00
Hybrid Zoom / CN B.425 R.206
Pascal Reiss
MPI for Solid State Research, Stuttgart
High-pressure studies of electronic structures - past, present and future

Hydrostatic pressure is one of the most powerful and successful tuning parameters for the experimental investigation of quantum materials. Ranging from the realisation of unique crystal structures to the observation of record-high transition temperatures in conventional and unconventional superconductors, hydrostatic pressure has led to the discovery of many mysterious phases. However, our understanding of such high-pressure phases is often very limited, in particular regarding their electronic structure. The main reason is the necessity for a pressure cell, its various components as well as the pressure transmitting media. These render many high- pressure experimental techniques very challenging or flat out impossible, including many thermodynamic or photoemission experiments. In this talk, I will give an overview of various pressure techniques and how we used them recently to reveal the electronic structure of FeSe-based superconductors. Our work has not only provided groundbreaking insights into nematic quantum criticality [1], but also revealed the presence of a quantum Griffiths phase [2], as well as a fragile nature of the high-Tc phase under large pressures [3]. Moreover, I will present our recent developments towards low-temperature pressure quenches which overcome the technical limitations arising from the presence of the various pressure components in order to carry out previously impossible thermodynamic measurements on metastable high-pressure phases [4].
[1] P. Reiss et al., Nat. Phys. 16, 89 (2020)
[2] P. Reiss et al., Phys. Rev. Lett. 127, 246402 (2021)
[3] P. Reiss et al., arXiv:2212.06824 (2022)
[4] N. Menyuk et al., Phys. Rev. 177, 942 (1969); L. Deng et al., Proc. Natl. Acad. Sci. 118, 6 (2021);
B. Wolf et al., Phys. Rev. B 106, 134432 (2022); C. Chu et al., J. Supercond. Nov. Magn. 35, 987 (2022)

07.02.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Magnetic ordering in the Kondo lattice

Heavy fermion materials are a rich research topic due to their anomalous low-temperature properties. While it is generally accepted that these phenomena are caused by the interplay between itinerant and localized electrons (resulting in the eponymous "heavy" particles), the details of this strongly correlated state are still unclear. Some topics of interest are the quasiparticle dynamics and possible magnetic ordering.In this talk I will present some theoretical calculations for the Kondo lattice model, the simplest model of a heavy fermion material. The problem is attacked using bond fermion theory, a concep- tually simple approach that nonetheless retains much of the strong correlation expected from the system. Special focus is put on antiferromagnetic order, as well as the influence of external magnetic fields and geometric frustration.

03.02.2023
Fri 11:00
Hybrid Zoom / CN B.425 R.206
Elena Hassinger
Institut für Festkörper- und Materialphysik, Technische Universität Dresden
Interplay of ordered states in CeRh2As2

Quantum materials are interesting because they host exotic states such as unconventional superconductivity that are valuable for applications. For example, finding a topological odd-parity superconductor would be a breakthrough for quantum computing. The recent discovery of two-phase superconductivity in CeRh2As2, is a big step in this direction, since the observed transition is from even to odd-parity superconductivity. It is rooted in the presence of two Ce sites with locally broken inversion symmetry. The phase-diagram presents clear experimental evidence that local inversion-symmetry breaking is a crucial parameter in the study of quantum materials that can induce odd-parity superconductivity. However, with one material realization, we are far from controlling this parameter. My research currently intends studying exotic quantum states that emerge in materials with locally broken inversion symmetry. By measuring bulk transport and magnetic properties in extreme conditions of very low temperature, high magnetic field and high hydrostatic and uniaxial pressure, we investigate and tune the delicate interplay of local inversion-symmetry breaking with correlated electrons, magnetic, and orbital degrees of freedom and superconductivity. I will give an overview of recent experimental investigations of CeRh2As2 from our group.

01.02.2023
Wed 10:00
Zoom
Rolf W. Lortz
Department of Physics, The Hong Kong University of Science & Technology
Uncovering the secrets of unusual superconductors

In this talk I will provide a relatively broad overview of my work in the field of unusual superconducting states, from thermodynamic characterization of bulk single crystals to quantum spectroscopy in nanoscale devices. I will give several short research examples of how sophisticated home-made calorimetric probes can provide a wealth of information on unusual superconducting states, ranging from phonon spectroscopy and electron-phonon coupling (which is usually only accessible by neutron scattering and tunneling spectroscopy), to the detection of the tiny signature of thermal fluctuations and vortex melting in classical superconductors (where it was previously widely believed to be experimentally inaccessible), to the detection of the superconducting properties in arrays of elementary sub-nanowires, where the specific heat reveals an extreme enhancement of the superconducting critical temperature caused solely by the nanostructuring. I will then discuss three selected topics in more detail: the Fulde-Ferrell-Larkin-Ovchinnikov state (which we have discovered in three different superconductors: the organic superconductor k-(ET)2Cu(NCS)2, the Fe-based superconductor KFe2As3, and most recently in the transition-metal dichalcogenide superconductor NbS2), nematic and topological superconductivity in doped Bi2Se3 and monolayers of NbSe2, and a new platform for the detection of localized Majorana modes in mesoscopic quasi one-dimensional quantum anomalous Hall insulator/superconductor heterostructures.

31.01.2023
Tue 11:00
Hybrid Zoom / CN B.425 R.206
Diego A. Zocco
Institute of Solid State Physics, Vienna University of Technology (TU Wien)
Materials at the intersection of strong electronic correlations and nontrivial topology

The ability to control topological states at will holds great promise for its application in topological quantum devices. Heavy fermion compounds are well known for their excellent tunability, and provide an exceptional platform to explore the intersection of strong electronic correlations and electronic topology. In this talk, I will focus on tuning experiments performed on heavy fermion semimetals in which Weyl nodes form in a setting of broken inversion and preserved time-reversal symmetries. In Ce3Bi4Pd3, which exemplifies the notion of Weyl-Kondo semimetal (WKSM) [1,2], and in which a giant spontaneous Hall effect has been observed [3], we found that the application of relatively modest magnetic fields suppresses the topological signatures; we attribute this effect to the annihilation of Weyl nodes at a topological quantum phase transition [4]. CeRu4Sn6 is another WKSM candidate material [5]. Interestingly, inelastic neutron scattering experiments revealed that the material is quantum critical without tuning [6]. I will present pressure-tuning investigations of CeRu4Sn6, and discuss whether the WKSM state may nucleate out of quantum critical fluctuations [7].
Financial support has been provided by the Austrian Science Fund (I2535, I4047, P29279, and I5868-FOR5249- QUAST), EU’s Horizon 2020 Research and Innovation Programme (824109-EMP), and the European Research Council (Advanced Grant 101055088).
[1] S. Dzsaber et al., Phys. Rev. Lett. 118, 246601 (2017)
[2] H.-H. Lai et al., Proc. Natl. Acad. Sci. U.S.A. 115, 93 (2018)
[3] S. Dzsaber et al., Proc. Natl. Acad. Sci. U.S.A. 118, e2013386118 (2021)
[4] S. Dzsaber et al., Nat. Commun. 13, 5729 (2022)
[5] Y. Xu et al., Phys. Rev. X 7, 011027 (2017)
[6] W. T. Fuhrman et al., Sci. Adv. 7/21, eabf9134 (2021)
[7] D. M. Kirschbaum et al., in preparation.

31.01.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Mikhail Feigel’man
Floralis & LPMMC, University Grenoble -Alpes
Gapful electrons in a vortex core of granular superconductor

24.01.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Aparajita Singha
Max-Planck-Institute for Solid State Research, Stuttgart
Non-invasive imaging and coherent control of surface-supported single spins

17.01.2023
Tue 16:00
Hybrid Zoom / CS B.30.23, R.3-1
Markus Ternes
RWTH Aachen
From excitations of individual atomic and molecular spins to topological quasiparticle excitations in a Kondo lattice