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.

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 large-scale problems. I conclude by discussing generalizations to mixed quantum states.

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 feedback-based quantum algorithms (FQAs) as an alternative paradigm that is optimization-free and applicable to a broad range of applications. Within this paradigm, quantum circuit parameter values are assigned in a layer-wise manner using a deterministic, measurement-based 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.

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 anti-localization 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 Hikami-Larkin-Nagaoka 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 cm²V^-1s^-1 at room temperature. This means that good thermoelectric properties of nanoparticulate Bi₂Te₃ near room temperature are also determined in part by the existence of surface charge carriers.

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 (SrTiO₃). In the second part, I will present measurements of the stress-strain relation of bulk strontium ruthenate (Sr₂RuO₄) 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

At low temperatures, practically all amorphous materials show a variety of anomalous properties. These are believed to arise from structural material defects forming two-level 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.
With the advent of superconducting quantum circuits, new tools became available for the study of TLS. By exploiting the fragility of the circuit's coherent quantum state, it is now possible to resolve and characterize even single TLS defects.
After a brief review of the physics of TLS defects and the limitations of the standard tunneling model, I will present our experiments where we use superconducting qubits as interfaces to single defects. A focus lies on techniques to tune TLS defects by applied mechanical strain or electric fields, which have proven useful to reveal the TLS' positions and their interactions with other TLS, phonons, and quasiparticles. Such new insights may become especially valuable since TLS defects have a widespread importance as a source of noise that affects various devices.

Magnetic topological materials, such as magnetic Dirac and Weyl semimetals, and intrinsic magnetic topological insulators, in which topologically non-trivial 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 EuMnBi₂, the evidence for the possible impact of magnetism on Dirac fermions is obtained via a detailed neutron diffraction study of the spin-flop transition [1]. In the two-dimensional van der Waals honeycomb ferromagnets CrSiTe₃ and CrGeTe₃, 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 spin-wave excitations [2]. Furthermore, in the magnetic Weyl semimetal Mn₃Sn, an unusual magnetic phase transition that is driven by emergent many-body effects can be revealed via our combined polarised neutron scattering study and band-structure calculations [3].
[1] F. Zhu et al., Phys. Rev. Research 2, 043100 (2020)
[2] F. Zhu et al., Sci. Adv. 7, eabi7532 (2021)
[3] X. Wang et al., (submitted).

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

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

In many unconventional superconductors, new types of exotic electronic orders are often found in proximity to the superconducting phase. These can consist in charge density waves, various types of magnetic orders or electronic nematicity. In many cases, the nature of the interplay between these orders and superconductivity remains elusive. They can be simply coexisting, competing or even intertwined with superconductivity, and their impact on the superconducting transition temperature is often unclear. Understanding the normal state of unconventional superconductors is generally the first important step towards unveiling the mechanism of superconductivity in these materials.
In this thesis, we study the superconductor BaNi₂As₂ which is structurally identical to the parent compounds of high-temperature Fe-based superconductors, BaFe₂As₂, but exhibits a completely different set of electronic phases, none of them being in particular magnetic in nature. Recently, investigations of Sr substituted Ba_1-xSr_xNi₂As₂ attracted attention with a sixfold enhancement of T_c and the discovery of an electronic nematic phase by elasto-resistance measurements [1]. Here, we have explored the effect of an alternative substitution. In this talk, I present the first study of novel Ca-substituted Ba_1-xCa_xNi₂As₂. First, we report the growth procedure for the synthesis of high-quality single crystals with a substitution level of up to 10 %. The main goal is to create a first phase diagram for Ba_1-xCa_xNi₂As₂ based on specific heat and electrical transport measurements. In this context, we determined the temperatures of a triclinic transition T_S, a minimum in dR/dT T_min, and the superconducting transition T_c for several samples. We observed a decrease of T_S from 137 K to below 100 K (on cooling) and a slow increase of T_c by about 0.2 K, an observation which strongly contrasts to that made upon Sr substitution in the absence of a significant contraction of the unit cell. In addition, we have carried out a first study of the lattice dynamics of this system, focusing in particular on the behavior of the E_g,1 Raman active vibration. Earlier studies had revealed a new type of lattice-driven nematicity [2] and its occurrence in the Ca-substituted system was studied.
[1] Eckberg et al., Nature Physics 16, 346-350 (2020)
[2] Yao et al., (2022)

The 122-type iron pnictides have opened up a wide research field after the discovery of superconductivity (SC) with chemical doping by suppressing the long-range antiferromagnetic (AFM) order of parent compounds. In the absence of SC, isotructural cobalt pnictides are itinerant quantum magnets and are described to consist of competing magnetic interactions. In the first part of my talk, I will present the novel magnetic properties of some doped ACo₂As₂ (A = Ca, Sr) compounds. CaCo_2-yAs₂ is a unique itinerant magnetic system with strong frustration and exhibits A-type 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 hole-doping onto the Co-site through Fe and Ni substitutions strongly suppress the AFM ordering in the system followed by a carrier-tuned Stoner transition [1, 2]. In the absence of long-range magnetic ordering, strong quasi-1D ferromagnetic quantum spin-fluctuations develop in these doped-CaCo_2-yAs₂ along with a signature of non-Fermi-liquid behavior. In SrCo₂As₂, no long-range magnetic order is detected in down to at least 50 mK, but stripe AF spin fluctuations exist. We found that Pd substitution trigger long-range AF order in the system. In the second part of my talk I will discuss some of the important results on Eu-based magnetic topological materials. Understanding the complex interplay of magnetism and band topology is quite important to discover exotic quantum phenomena in magnetic topological materials. Recently, topological Dirac surface states are reported in trigonal A-type AFM compounds EuMg₂Bi₂ and EuSn₂As₂. We have shown that in the AFM state, an unusual low-field induced spin reorientation in the three-fold AFM domains is observed for both the compounds associated with a weak in-plane anisotropy. H-T magnetic phase diagrams are constructed from the combined results of magnetic, heat capacity, resistivity, and neutron diffraction data [3, 4]. The phase boundary between the AFM and PM states is consistent with the molecular-field-theory prediction for spin S = 7/2.
[1] B. G. Ueland et. al., Phys. Rev. B 104 (2021) L220410
[2] S. Pakhira et. al., Phys. Rev. B 104 (2021) 094420
[3] S. Pakhira et. al., Phys. Rev. B 101 (2020) 214407
[4] S. Pakhira et. al., Phys. Rev. B 104 (2021) 174427

Since their discovery in 2008, iron-based superconducting pnictides (As, P...) and chalcogenides (Te, Se...) have become a well-established class of unconventional superconductors, spanning multiple structural families, with 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].
By using a topotactical approach, we were able to intercalate oxygen in intermetallic LaFeSi, resulting in the novel crystallogenide, LaFeSiO_1-δ. Our study, based on complementary experimental probes, reveals that this crystallogenide is superconducting with T_c ~ 10 K [4].
Considering its strongly squeezed Fe-Si anion height, below 1 Å, this T_c is surprisingly high, challenging the quasi-universal relationship between structure and superconductivity in Fe-based compounds [5]. The unique crystal structure of this new compound has a strong impact on its electronic properties where the doping is absorbed in a non-rigid-band fashion, resulting in a distinct fermiology. Specifically, the electron pockets, usually observed around the M point in the prototypical Fermi surface of iron-based superconductors, are strongly suppressed, while the hole-like pockets at the Γ point are retained. This, in turn, is detrimental to the usual (π,π)-nesting. The lack of such nesting suggests a departure from s^± pairing mechanism.
Furthermore, above T_c, resistivity shows a non-Fermi liquid behaviour, implying significant electronic correlations. This correlated behaviour is also visible in NMR where anti-ferromagnetic fluctuations are observed at low temperatures.
[1] H. Hosono, K. Kuroki, Physica C 514, 399 (2015).
[2] F. Bernardini et al., Phys. Rev. B 97, 1462 (2018).
[3] J.-B. Vaney et al., Nature Communications. 13, 100504 (2022).
[4] M. F. Hansen et al., npj Quantum Materials - in review (2021).
[5] C. H. Lee et al, Solid State Communications, 152, 8 (2012)

Understanding the impact of atomic-scale defects in the electronic structure of Fe-based superconductors is of key importance for assessing how critical are the occurrence of these features for the establishment of superconductivity in these compounds. With this aim, here we study single crystals of FeSe_1-xS_x, the Fe-based compound with the simplest crystal structure, consisting of a pile up of superconducting layers. We apply scanning tunnelling microscopy (STM) to reveal that this compound presents S-doping induced defects as well as diluted dumbbell defects associated to an Fe vacancy. We measure the electronic structure of the samples by means of X-ray photoemission spectroscopy (XPS) and reveal that the spectral shape of the peaks of some of the Se and Fe core levels can only be adequately described by considering a dominant plus a smaller second component of the electronic states. We find this result for both, pure as well as S-doped samples, irrespective that they present extra crystal defects associated to the substitution of Se by S atoms. Structural and resistivity characterization, as well as the spectral shape and energy location of the peaks in XPS spectra, indicate our samples do not present intergrowths of the hexagonal phase. Even though in our STM topographies only about 4% of the imaged Se atoms are involved in dumbbell defects, according to DFT calculations these defects entail a significant modification of the electronic clouds of the eight Se and Fe atoms surrounding the Fe vacancy. Furthermore, in the case of FeSe films, if the density of dumbbells increases, superconductivity is suppressed. Thus, we suggest the second component in our XPS spectra is associated to the ubiquitous dumbbell defects in FeSe. These impact of atomic defects in the binding energy energy and spectral shape of the core levels in FeSe_1-xS_x highlights the subtle interplay between the crystal structure and the bulk electronic states in Fe-based superconductors.

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

The interplay between charge, spin and lattice degrees of freedom creates exotic phases in strongly correlated materials. Raman and Neutron scatterings are valuable tools to study these phases because of their sensitivity towards these elementary excitations. I will first present the time-resolved Raman scattering results on optimally doped YBa₂Cu₃O_6.9 superconductor which focuses on relaxation timescales and interactions between electrons and phonons. Experimental advances we have made allow for future time-resolved experiments in which the pump frequency can be tuned from the near-to mid-infrared, opening the possibility to pump particular infrared-active modes and observe their effect on Raman-active modes. Further, I will discuss the Raman scattering results which will illuminate the impact of flash sintering on the structure of layered oxide structure of cuprate Pr₂CuO₄. The combined comprehensive inelastic neutron scattering measurements of Ba₈Ga₁₆Ge₃₀ with lattice dynamical calculations based on the density functional theory will be presented to elucidate the effect of the occupational disorder on heat-carrying phonons. Also, I will show the Raman scattering results of spin- 1/2 quantum spin liquid Ba₄Ir₃O₁₀ which will provide the signatures of spinons and damped phonons.

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

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

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

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

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

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