Zeit  Sprecher & Vortragsthema 

08.03.2023
Hybrid Zoom / CN B.425 R.206 
Hugo Perrin, Thibault Scoquart Simulation of superconductivity on an IBM quantum computer using error mitigation protocols TKM KIT 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 errormitigation protocols that allow us to improve the simulation results. In particular, we have developed and benchmarked a new approach that turns the crosstalk effects into depolarizing noise channel using randomized compiling. 
03.03.2023
Hybrid Zoom / CN B.425 R.206 
Dmitry Reznik Observing quantum materials in real time by pumpprobe Raman scattering Department of Physics, University of Colorado  Boulder Pumpprobe 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 computingrelated phenomena, chemical reactions, energy dissipation, etc. can now be measured in real time with unprecedented precision due to rapid advances in ultrafast lasers and xray sources. In this talk I describe the timeresolved Raman scattering lab at the University of Colorado. It shares an ultrafast laser system with an angleresolved 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 timeresolved Raman in three projects completed at the University of Colorado. The first one combined the two techniques to directly follow electronhole excitations as well as the increase in the population of the Ramanactive Gphonon 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 Kpoints, which then decay into Gphonons via phononphonon scattering. This work showed that phononphonon 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 YBa_{2}Cu_{3}O_{6+x}. First, I will present our results on ultrafast magnetic dynamics where we followed timeevolution of the twomagnon 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 electronphonon coupling. 
22.02.2023
Hybrid Zoom / CN B.425 R.206 
Jeremy Sourd Superconductivity and Kondo lattice effects in La_{1x}Ce_{x}FeSiH:
An interdisciplinary approach to a strongly correlated electron system Laboratoire Ondes et Matière d'Aquitaine, Institut de Chimie de la Matière Condensée de Bordeaux, University of Bordeaux, France 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_{1x}Ce_{x}FeSiH (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 nonlocal 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 lightmatter experiments, namely ARPES, optical conductivity and Raman spectra [3]. 
15.02.2023
Hybrid Zoom / CN B.425 R.206 
Ina Schäfer Quantum Informatics  From Quantum Gates to Quantum Software Engineering KIT, Institut für Informationssicherheit und Verlässlichkeit (KASTEL) 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 industrialscale 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 correctnessbyconstruction development for quantum programs. 
14.02.2023
Hybrid Zoom / CS B.30.23, R.31 
Oliver Gröning Exploiting the chirality of the intermetallic compound PdGa on its surfaces Eidgenössische Materialprüfungs und Forschungsanstalt (Empa), Materials Science and Technology The chirality of biomolecules is a key structural property for determining their physiological activity. A prominent example is the difference in taste between Rlimonene (rectus, right) and Slimonene (sinister, left) where the former smells of oranges and the latter of lemons. Accordingly, the synthesis of enantiopure 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 Pd_{1}Ga_{1} 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 Millerindex surfaces of Pd_{1}Ga_{1} investigated by Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (IVLEED). 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
Hybrid Zoom / CN B.425 R.206 
Pascal Reiss Highpressure studies of electronic structures  past, present and future MPI for Solid State Research, Stuttgart 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 recordhigh transition temperatures in conventional and unconventional superconductors, hydrostatic
pressure has led to the discovery of many mysterious phases. However, our understanding of such highpressure
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 FeSebased 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 highT_{c} phase under large pressures [3]. Moreover, I will present our recent developments towards
lowtemperature 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 highpressure phases [4]. 
07.02.2023
Hybrid Zoom / CS B.30.23, R.31 
Matthias Keßler Magnetic ordering in the Kondo lattice IQMT, KIT Heavy fermion materials are a rich research topic due to their anomalous lowtemperature 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
Hybrid Zoom / CN B.425 R.206 
Elena Hassinger Interplay of ordered states in CeRh_{2}As_{2} Institut für Festkörper und Materialphysik, Technische Universität Dresden Quantum materials are interesting because they host exotic states such as unconventional superconductivity that are valuable for applications. For example, finding a topological oddparity superconductor would be a breakthrough for quantum computing. The recent discovery of twophase superconductivity in CeRh_{2}As_{2}, is a big step in this direction, since the observed transition is from even to oddparity superconductivity. It is rooted in the presence of two Ce sites with locally broken inversion symmetry. The phasediagram presents clear experimental evidence that local inversionsymmetry breaking is a crucial parameter in the study of quantum materials that can induce oddparity 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 inversionsymmetry breaking with correlated electrons, magnetic, and orbital degrees of freedom and superconductivity. I will give an overview of recent experimental investigations of CeRh_{2}As_{2} from our group. 
01.02.2023
Zoom 
Rolf W. Lortz Uncovering the secrets of unusual superconductors Department of Physics, The Hong Kong University of Science & Technology 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 homemade calorimetric probes can provide a wealth of information on unusual superconducting states, ranging from phonon spectroscopy and electronphonon 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 subnanowires, 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 FuldeFerrellLarkinOvchinnikov state (which we have discovered in three different superconductors: the organic superconductor k(ET)_{2}Cu(NCS)_{2}, the Febased superconductor KFe_{2}As_{3}, and most recently in the transitionmetal dichalcogenide superconductor NbS_{2}), nematic and topological superconductivity in doped Bi_{2}Se_{3} and monolayers of NbSe_{2}, and a new platform for the detection of localized Majorana modes in mesoscopic quasi onedimensional quantum anomalous Hall insulator/superconductor heterostructures. 
31.01.2023
Hybrid Zoom / CN B.425 R.206 
Diego A. Zocco Materials at the intersection of strong electronic correlations and nontrivial topology Institute of Solid State Physics, Vienna University of Technology (TU Wien) 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 timereversal symmetries. In Ce_{3}Bi_{4}Pd_{3}, which exemplifies the notion of WeylKondo 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]. CeRu_{4}Sn_{6} is another WKSM
candidate material [5]. Interestingly, inelastic neutron scattering experiments revealed that the material is
quantum critical without tuning [6]. I will present pressuretuning investigations of CeRu_{4}Sn_{6}, and discuss whether
the WKSM state may nucleate out of quantum critical fluctuations [7].

31.01.2023
Hybrid Zoom / CS B.30.23, R.31 
Mikhail Feigel’man Gapful electrons in a vortex core of granular superconductor Floralis & LPMMC, University Grenoble Alpes 
24.01.2023
Hybrid Zoom / CS B.30.23, R.31 
Aparajita Singha Noninvasive imaging and coherent control of surfacesupported single spins MaxPlanckInstitute for Solid State Research, Stuttgart 
17.01.2023
Hybrid Zoom / CS B.30.23, R.31 
Markus Ternes From excitations of individual atomic and molecular spins to topological quasiparticle excitations in a Kondo lattice RWTH Aachen 
20.12.2022
Hybrid Zoom / CS B.30.23, R.31 
Christian Arenz Randomized adaptive quantum state preparation Arizona State University Methods for preparing quantum states are an integral component of any quantum technology. In this talk, I present an adaptive method for quantum state preparation that utilizes randomness as an essential component. The method prepares a desired quantum state by adaptively minimizing a cost function without utilizing iterative classical optimization routines. Instead, the target quantum state is prepared through an adaptively constructed quantum circuit, where each adaptive step is informed by feedback from gradient measurements in which the associated tangent space directions are randomized. I provide theoretical arguments and numerical evidence that convergence to the target state can be achieved for almost all initial states. I investigate different randomization procedures and develop lower bounds on the expected cost function change, which allows for drawing connections to barren plateaus and for assessing the applicability of the algorithm to largescale problems. I conclude by discussing generalizations to mixed quantum states. 
19.12.2022
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Alicia Magann Feedbackbased quantum algorithms Sandia National Laboratories Variational quantum algorithms (VQAs) are a significant focus of the quantum computing community. These algorithms operate by wrapping a classical optimization loop around a parameterized quantum circuit, and iteratively searching for the parameter configuration that produces the best solution to the problem under consideration. A critical challenge in VQAs is the difficulty of this classical optimization problem, which can become intractable as the number of quantum circuit parameters increases. I will introduce feedbackbased quantum algorithms (FQAs) as an alternative paradigm that is optimizationfree and applicable to a broad range of applications. Within this paradigm, quantum circuit parameter values are assigned in a layerwise manner using a deterministic, measurementbased feedback law derived from quantum Lyapunov control principles. The use of feedback in this manner guarantees a monotonic improvement in solution quality with respect to the depth of the quantum circuit. I will overview quantum Lyapunov control theory as a motivation for this framework and go on to discuss concrete formulations of FQAs for applications including quantum simulation and combinatorial optimization. I will conclude by presenting results from a hardware implementation and numerical investigations of convergence, scalability, and robustness. 
14.12.2022
Hybrid Zoom / CN B.425 R.206 
Gabi Schierning Interfacedominated topological transport in nanograined Bi_{2}Te_{3} Universität Bielefeld Bismuth telluride is an excellent thermoelectric material and also belongs to the class of three dimensional topological insulators. Therefore, charge carriers with extremely high mobility exist at the crystal surfaces. This is particularly visible in nanoparticulate samples, provided that the nanoparticles used have sufficient surface purity. Compacted nanoparticulate bulk samples exhibit a high density of interfaces. These samples show a pronounced weak antilocalization in the low temperature transport behavior as well as a kink in the electrical resistance at about 5 K. Evaluation of the magnetotransport data using the HikamiLarkinNagaoka model yields coherence lengths of up to 200 nm, which is significantly larger than the average grain size in the studied samples. Using terahertz spectroscopy, the average mobility of the surface charge carriers can be estimated to be about 1000 to 10000 cm^{2}V^{1}s^{1} at room temperature. This means that good thermoelectric properties of nanoparticulate Bi_{2}Te_{3} near room temperature are also determined in part by the existence of surface charge carriers. 
16.11.2022
Hybrid Zoom / CN B.425 R.206 
Hilary Noad Probing the interplay between electronic systems and the lattice in transition metal oxides MaxPlanckInstitut für Chemische Physik fester Stoffe, Dresden A central theme in the modern study of quantum materials is the coupling between magnetic or electronic order and the underlying crystal lattice. Probing this interplay using conventional thermodynamic or transport measurements can be onerous, often requiring the measurement of series of samples that vary, for example, epitaxial strain, isovalent substitution, or isotopic abundance to realize a range of lattice parameters while leaving the chemical nature of the material intact. In this talk, I will describe two novel approaches to studying the coupling between structural and electronic degrees of freedom, first by locally resolving the intrinsic variation of strain within a sample, and then by in situ control of the global strain field. In the first part of my talk, I will discuss how I used a local magnetic probe, scanning superconducting quantum interference device (SQUID) microscopy, to study the influence of the intrinsic twin structure on superconductivity in heterostructures based on strontium titanate (SrTiO_{3}). In the second part, I will present measurements of the stressstrain relation of bulk strontium ruthenate (Sr_{2}RuO_{4}) under uniaxial pressure, revealing a large lattice softening as the material undergoes a Lifshitz transition. These studies demonstrate the utility of novel approaches in revealing the critical role that the lattice plays in the ordered phases of quantum materials 
15.11.2022
Hybrid Zoom / CN B.425 R.206 
David Larbalestier Understanding the real, imperfect nature of REBCO (REBa_{2}Cu_{3}O_{x} with RE=Rare
Earth elements) coated conductors and its implications for ultrahigh magnetic
field used Mechanical Engineering Applied Superconductivity Center and Florida State UniversityTallahassee, Florida, USA 
09.11.2022
Hybrid Zoom / CN B.425 R.206 
Jürgen Lisenfeld Studying atomic tunneling systems with superconducting qubits Karlsruher Institut für Technologie  Physikalisches Institut At low temperatures, practically all amorphous materials show a variety of anomalous properties. These are
believed to arise from structural material defects forming twolevel quantum systems (TLS) which can interact
with photons and phonons. However, despite 50 years of active research, a thorough understanding of the nature
of TLS has remained elusive. 
26.10.2022
Hybrid Zoom / CN B.425 R.206 
Yixi Su Neutron scattering on magnetic topological materials Jülich Centre for Neutron Science JCNS at MLZ, Forschungszentrum Jülich Magnetic topological materials, such as magnetic Dirac and Weyl semimetals, and intrinsic magnetic
topological insulators, in which topologically nontrivial band structures, magnetism and electronic
correlation effects can be intertwined, have recently emerged as an exciting platform to explore exotic
states and novel functionalities. As a unique microscopic probe for magnetism, neutron scattering is
ideally suited for the investigations of magnetic correlations over a wide range of length and time scales
in these emergent quantum materials. In this talk, I will present our recent neutron scattering studies of
magnetic topological materials, with the main aim to demonstrate the fascinating interplay between
topology, magnetism and electronic correlation. In the Dirac semimetal EuMnBi_{2}, the evidence for the
possible impact of magnetism on Dirac fermions is obtained via a detailed neutron diffraction study of
the spinflop transition [1]. In the twodimensional van der Waals honeycomb ferromagnets CrSiTe_{3} and
CrGeTe_{3}, the exotic topological magnon insulators, the bosonic analogue of topological insulators, have
been experimentally realized based on our inelastic neutron scattering study and theoretical analysis of
spinwave excitations [2]. Furthermore, in the magnetic Weyl semimetal Mn_{3}Sn, an unusual magnetic
phase transition that is driven by emergent manybody effects can be revealed via our combined
polarised neutron scattering study and bandstructure calculations [3].

21.09.2022
Hybrid Zoom / CN B.425 R.206 
Christos Panagopoulos Chirality Engineering in Quantum Matter
Nanyang Technological University, Singapore Quantum geometry manifests itself in a plethora of materials and architectures including topological insulators, Archimedean magnets, Weyl/Dirac semimetals, transition metal oxides and dichalcogenides, and can also be engineered via interaction with electromagnetic fields. The emergent internal structure can radically alter our understanding of quantum mechanical wavefunctions of charge carriers and quasiparticles, yielding modular transport, optical, magnetic responses that are critically interlinked. I will introduce our efforts to geometrically tune the microscopic interactions responsible for macroscopic quantum coherences, through two recent examples. Namely, the first architecture in which superconductivity and magnetism interact via distinct topological solitons and a novel physical qubit platform based on the energylevel quantization of the helicity degree of freedom. 
14.09.2022
Hybrid Zoom / CN B.425 R.206 
The role of synthesis experts in quantum materials and technology research Highquality crystals are required for experiments in solid state physics and also industrial research fields. Such crystals are often synthesized by experts (as known as sample growers). They also study physical properties of materials and conduct joint research with other researchers for further study and even for mass production. However, the work of synthesis experts is in some ways unknown to physicists. Based on my experience studying superconductors [1, 2], magnetic memory materials [3], and battery materials in academia and industry,
I will introduce and explain this aspect to researchers and students.

27.07.2022
Hybrid Zoom / CN B.425 R.206 
Fabian Henßler Synthesis and characterization of Ba_{1x}Ca_{x}Ni_{2}As_{2} with thermodynamic, transport and spectroscopic methods
KITIQMT In many unconventional superconductors, new types of exotic electronic orders are often found in proximity to
the superconducting phase. These can consist in charge density waves, various types of magnetic orders or
electronic nematicity. In many cases, the nature of the interplay between these orders and superconductivity
remains elusive. They can be simply coexisting, competing or even intertwined with superconductivity, and their impact on the superconducting transition temperature is often unclear. Understanding the normal state of
unconventional superconductors is generally the first important step towards unveiling the mechanism of
superconductivity in these materials. 
06.07.2022
Online  Zoom Meeting 
Santanu Pakhira Novel magnetism in 122type itinerant pnictide system and Eubased magnetic topological materials Ames Laboratory The 122type iron pnictides have opened up a wide research field after the discovery of superconductivity (SC)
with chemical doping by suppressing the longrange 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 ACo_{2}As_{2} (A = Ca, Sr) compounds. CaCo_{2y}As_{2} is a unique itinerant magnetic system with strong frustration
and exhibits Atype antiferromagnetic (AFM) ordering below T_{N} ≈ 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 holedoping onto the Cosite through Fe and Ni substitutions strongly suppress the AFM ordering in the
system followed by a carriertuned Stoner transition [1, 2]. In the absence of longrange magnetic ordering, strong
quasi1D ferromagnetic quantum spinfluctuations develop in these dopedCaCo_{2y}As_{2} along with a signature of
nonFermiliquid behavior. In SrCo_{2}As_{2}, no longrange magnetic order is detected in down to at least 50 mK, but
stripe AF spin fluctuations exist. We found that Pd substitution trigger longrange AF order in the system.
In the second part of my talk I will discuss some of the important results on Eubased 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 Atype AFM compounds EuMg_{2}Bi_{2} and EuSn_{2}As_{2}. We have shown that in the AFM state, an
unusual lowfield induced spin reorientation in the threefold AFM domains is observed for both the compounds
associated with a weak inplane anisotropy. HT 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 molecularfieldtheory prediction for spin S = 7/2.

22.06.2022
Hybrid Zoom / CN B.425 R.206 
Mads Hansen LaFeSiO_{1δ}: A Novel Superconducting Member of the IronSilicide Family Institut Néel, CNRS Since their discovery in 2008, ironbased superconducting pnictides (As, P...) and chalcogenides (Te, Se...)
have become a wellestablished class of unconventional superconductors, spanning multiple structural families,
with T_{c} 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 (T_{c} ~ 10 K) and LaFeSiF_{x}
(T_{c} ~ 9 K) [2,3]. 
21.06.2022
Hybrid Zoom / CS B.30.23, R.31 
Philippe Goldner Emerging Rare EarthBased 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 FeSe_{1x}S_{x} superconducting crystals Centro Atómico Bariloche, CNEA, CONICET Universidad Nacional de Cuyo, Argentina Understanding the impact of atomicscale defects in the electronic structure of Febased 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 FeSe_{1x}S_{x}, the Febased 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 Sdoping induced defects as well as diluted dumbbell defects associated to an Fe vacancy. We measure the electronic structure of the samples by means of Xray 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 Sdoped 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 FeSe_{1x}S_{x} highlights the subtle interplay between the crystal structure and the bulk electronic states in Febased superconductors. 
31.05.2022
Hybrid Zoom / CS Sem.R. 3.01 
Yanina Fasano Hyperuniformity in Vortex Matter in typeII 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 infinitewavelength 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 reciprocalspace wavenumber q due to the suppression of particle density fluctuations at large directspace lengthscales. 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 pointlike 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.

18.05.2022
Join Zoom Meeting 
Susmita Roy Conventional, TimeResolved 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 timeresolved Raman scattering results on optimally doped YBa_{2}Cu_{3}O_{6.9} superconductor which focuses on relaxation timescales and interactions between electrons and phonons. Experimental advances we have made allow for future timeresolved experiments in which the pump frequency can be tuned from the nearto midinfrared, opening the possibility to pump particular infraredactive modes and observe their effect on Ramanactive 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 Pr_{2}CuO_{4}. The combined comprehensive inelastic neutron scattering measurements of Ba_{8}Ga_{16}Ge_{30} with lattice dynamical calculations based on the density functional theory will be presented to elucidate the effect of the occupational disorder on heatcarrying phonons. Also, I will show the Raman scattering results of spin 1/2 quantum spin liquid Ba_{4}Ir_{3}O_{10} which will provide the signatures of spinons and damped phonons. 
17.05.2022
Hybrid Zoom /CS B.30.23, R.31 
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 SchrödingerCat Qubits PaulScherrerInstitut Quantum twolevel 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 twodimensional electron and hole gases at LaInO_{3}/BaSnO_{3} interfaces by polar distortions, termination, and thickness Institut für Physik & IRIS Adlershof HumboldtUniversität zu Berlin Twodimensional 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 chargecarrier density and mobility at room temperature. In this talk, we will explore the potential of interfaces formed by two latticematched widegap oxides of emerging interest, i.e., the polar, orthorhombic perovskite LaInO_{3} and the nonpolar, cubic perovskite BaSnO_{3}, employing densityfunctional theory and manybody 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 LaInO_{3} to the BaSnO_{3} side. This leads to the formation of a 2DEG hosted by the highlydispersive Snsderived conduction band and a 2D hole gas of Op character, strongly localized inside LaInO_{3}. Remarkably, structural distortions through octahedra tilts induce a depolarization field counteracting the polar discontinuity, and thus increasing the critical (minimal) LaInO_{3} thickness, required for the formation of a 2DEG. These polar distortions decrease with increasing LaInO_{3} thickness, enhancing the polar discontinuity and leading to a 2DEG density f 0.5 electron per unitcell surface. Interestingly, in nonperiodic 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 LaAlO_{3}/SrTiO_{3}. 
15.02.2022
Zoom Seminar 
Xing He Understanding Phonon Anharmonicity Using Neutron / Xray Scattering and FirstPrinciples 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. Timeofflight inelastic neutron
scattering instrument covering large fourdimensional momentum and energy space, is suitable to
probe phonon frequencies and lifetimes as a function of temperature. Moreover, firstprinciples
simulation including temperature effects, such as ab initio molecular dynamics, and recent
machinelearning 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/xray diffuse scattering, and firstprinciples 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.

01.02.2022
Zoom Seminar 
Senthil Kumar Kuppusamy Coherent lightmatter interactions in rareearth 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 supercurrentlike 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 nonlinear 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 nonlinearity 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 signaltonoise ratio of the measurement, I developed a nondegenerate 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. 