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.

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

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.

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 YBa₂Cu₃O_6+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.

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 La_1-xCe_xFeSiH (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)

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.

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.

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-T_c 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)

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.

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.

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.

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.