Time  Speaker & Topic of the Talk 

29.07.2020
Zoom Seminar 
Michael Marthaler Lattice Models and Quantum Computing without Quantum Error Correction HQS Quantum Simulations Current quantum computers have relatively high error probabilities per operation. Therefore, any application has to use an extremely small number of operations and ideally be somewhat insensitive to the errors (e.g. decoherence). We discuss the number of operations needed for simulation of the electronic structure problem. We show that the number of operations and the mode of error accumulation seems somewhat favorable for lattice models. Given current limitations of quantum computers it is also important to compare to classical solvers like DMRG to understand at which point a quantum computer can really deliver a speed up of interest. We will also discuss our current work using DMRG and how it connects to our quantum computing goals. 
15.07.2020
Zoom Seminar 
Titus Neupert Solving a Small Hubbard Model on an IBM Quantum Computer Condensed Matter Theory, Department of Physics University of Zurich Fully programmable quantum computing and simulation devices have hit several milestones over the past few years, with a massively growing involvement of industrial research from companies such as IBM, Google, and Microsoft. The emerging quantum computing technology promises to be useful for applications such as quantum machine learning, but notably also for intrinsically quantummechanical calculations that appear in material science, quantum chemistry, or quantum manybody physics. In this talk, I will show how stateoftheart quantum hardware performs in this domain of quantum problems. Concretely, I focus on solving the ground state structure of a fermionic Hubbard model on a small cluster of sites and demonstrate that qualitatively and quantitatively accurate results can be obtained. This is enabled by exploiting the symmetries of the problem, employing a hybrid quantumclassical variational algorithm, and a Lanczosbased error mitigation scheme. I will introduce each of these steps in detail and discuss the potential of the general workflow for upscaling. 
08.07.2020
Zoom Seminar 
Jacob Biamonte Introduction to Modern Quantum Algorithms: Optimisation, Simulation and Machine learning Laboratory for Quantum Information Processing Skolkovo Institute of Science and Technology, Moscow, Russia This lecture begins with the promise of idealised, error corrected, quantum algorithms. With this goal defined, we then outline the contemporary capacity of quantum enhanced processors. Increasing the control and capacity of quantum simulators has resulted in a new class of quantum devices, utilising an iterative classicaltoquantum feedback process. This socalled “variational” approach to quantum computation was formally proven (in the noisefree setting) to represent a universal model of quantum computation. An exciting global research effort to understand the variational model is redefining the field of quantum computation. While the ultimate capacity of variational algorithms remains unknown, these methods represent an attractive approach due to the ease at which these algorithms can be realized experimentally. Contrary to analog quantum simulation, the variational approach forgoes the requirement of realising the targeted Hamiltonian directly in the laboratory, thus allowing the study of a wide variety of previously intractable target models experimentally. This comes at the cost of increased measurements and classical pre and postprocessing.

01.07.2020
Zoom Seminar 
Luca de' Medici Electronic Compressibility and HighT_{c} Superconductivity: New Links ESPCI, Paris In multiorbital Hubbard models including strong intraatomic exchange a socalled "Hund's metal" phase is realized that shows, among other hallmarks, typically a strong differentiation in the degree of correlation of the conduction electrons based on their orbital character. This selective physics is also a key player in ironbased superconductors (FeSC), where a wealth of experimental evidences validates this theoretical picture. We will show that at the frontier between this Hund's metal phase and a conventional metal, the electronic compressibility is strongly enhanced or even divergent, and that the FeSC that have a high T_{c} are placed in our simulations on this frontier. This theoretical evidence of enhanced compressibility at the frontier between a phase with selective correlations and a more conventional metal is found in cuprates. We will outline the main indications for a common scenario of highT_{c} superconductivity. 
24.06.2020
Zoom Seminar 
Leonardo Degiorgi Nematicity in the Charge Dynamics of IronBased Superconductors ETH Zürich, Department Physik The divergent nematic susceptibility, obeying a simple CurieWeiss power law over a large temperature interval, is empirically found to be a ubiquitous signature in several ironbased superconductors across their dopingtemperature phase diagram. I will discuss the impact of nematicity in the optical response of selected iron pnictide and chalcogenide materials, over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. First, I will focus my attention on FeSe where we reveal an astonishing anisotropy of the optical response in the midinfraredtovisible spectral range. This bears testimony to an important polarization of the underlying electronic structure, due to an orbitalordering mechanism, supplemented by orbital selective band renormalization. The farinfrared response of the charge dynamics in FeSe moreover allows establishing the link to the dc resistivity, emphasizing scenarios based on scattering by anisotropic spinfluctuations. Second, I will offer a comprehensive optical investigation of the optimally holedoped Ba_{0.6}K_{0.4}Fe_{2}As_{2}, for which we show that the stressinduced optical anisotropy in the infrared spectral range is reversible upon sweeping the applied stress and occurs only below the superconducting transition temperature. These findings demonstrate that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome. 
17.06.2020
Zoom Seminar 
Hermann Suderow Real Space Imaging of Electronic Correlations Department Física de la Materia Condensada, Instituto Nicolás Cabrera, IFIMAC, Universidad Autónoma de Madrid  Spain I will briefly explain the basics of Scanning Tunneling Spectroscopy (STS), focusing on techniques to study strongly correlated electron systems at dilution refrigeration temperatures. I will first report on the discovery of a onedimensional charge density wave (1DCDW) which is a Moiré pattern between the atomic lattice and a hot spot for electronic scattering in the bandstructure of the hidden order (HO) state of URu_{2}Si_{2}. The Moiré is produced by fracturing the crystal in presence of a dynamical spin mode at low temperatures and its presence suggests that charge interactions are among the most relevant features competing with HO in URu_{2}Si_{2}. Then, I will discuss the consequences of band hybridization in the local density of states which lead to new insight into the local density of states on smallsized atomically flat areas in URu_{2}Si_{2}. Finally, I will briefly present suprising results in feedback driven atomic size Josephson junctions, which lead to an AC Josephson signal that will likely improve Josephson Scanning Spectroscopy. 
03.06.2020
Zoom Seminar 
Xavier Waintal Introduction to Quantum Computing by a Skeptic Quantum Photonics, Electronics and Engineering Laboratory (PHELIQS) Commissariat à l'énergie atomique et aux énergies alternatives (CEA) Grenoble, France I will expose the concepts of quantum computing for physicists and from a physicist's point of view. I will go through the construction of quantum error corrections and point out the difficulties and physical limitations for implementing this framework. I will, in particular, address the surface code and the concept of correctable versus noncorrectable errors. This presentation is intended to an audience of physicists that mostly know about what the qubits are but not how we are supposed to make a computer out of them.

27.05.2020
Zoom Seminar 
Jens Müller Learning from Noise  Fluctuation Spectroscopy of Correlated Electrons in Molecular Conductors Goethe University Frankfurt Socalled 1/ftype fluctuations are ubiquitous in nature, and can be found in such diverse contexts as light curves of quasars, the human heartbeat, earthquakes, road traffic or classical music. In this seminar, we aim to give a broad and pedagogical overview of such fluctuation phenomena in condensedmatter systems and discuss how 'noise can be turned into a signal' by the method of fluctuation spectroscopy. We first give an example where understanding the microscopic origin of the fluctuations in semiconductorbased Hall sensors helps to improve the signaltonoise ratio and hence the device performance. In the main part of the talk we then will discuss recent results on the lowfrequency dynamics of strongly correlated electrons in lowdimensional molecular metals, which may be considered model systems for studying the Mott metalinsulator transition, a key phenomenon in manybody physics. Our findings range from glassy structural dynamics, critical slowing down of charge fluctuations at the Mott transition, and recent results on the formation of polar nanoregions in chargedriven ferroelectrics. 
20.02.2020

Rolf Lortz Topological Superconducting Phases in 2D and 3D Materials Hong Kong University of Science and Technology In this talk I give an overview of our recent research on 2D and 3D topological superconducting materials. I will focus on the doped topological insulator Bi_{2}Se_{3} and on heterostructures between a quantum anomalous Hall insulator and a superconductor. A nematic topological superconductor has an order parameter symmetry, which spontaneously breaks the crystalline symmetry in its superconducting state. This state can be observed, for example, by thermodynamic or upper critical field experiments in which a magnetic field is rotated with respect to the crystalline axes, but also directly from the anisotropic gap symmetry in scanning tunneling probe experiments. We present a study on the upper critical field of the Nbdoped Bi_{2}Se_{3} for various magnetic field orientations parallel to the basal plane of the Bi2Se3 layers. The data clearly demonstrate a twofold symmetry that breaks the threefold crystal symmetry. This provides strong experimental evidence that Nbdoped Bi_{2}Se_{3} is a nematic topological superconductor similar to the Cu and Srdoped Bi_{2}Se_{3}, and rules out earlier suggestions that the finite magnetic moment of the intercalated Nb ions could instead induce a chiral superconducting state. We then show that in doped Bi_{2}Se_{3}, the nematic order arises from a multicomponent order parameter where superconductivity is the primary order and the nematic order an intertwined secondary order. Such a state of matter with a multicomponent order parameter can give rise to a vestigial order. In the vestigial phase, the primary order is only partially melted, leaving a remaining symmetry breaking behind, an effect driven by strong classical or quantum fluctuations. We present the observation of a partially melted superconductor in which pairing fluctuations condense at a separate phase transition and form a nematic state with broken Z_{3} symmetry Highresolution thermal expansion, specific heat and magnetization measurements reveal that this symmetry breaking occurs at T_{nem}≈3.8 K above T_{c}≈3.25 K, along with an onset of superconducting fluctuations. Thus, before Cooper pairs establish longrange coherence at T_{c}, they fluctuate in a way that breaks the rotational invariance at Tnem and induces a distortion of the crystalline lattice. With the recent discovery of the quantum anomalous Hall insulator, which exhibits the conductive quantum Hall edge states without external magnetic field, it becomes possible to create a novel topological superconductor by introducing superconductivity into these edge states. In this case, two distinct topological superconducting phases with one or two dispersive chiral Majorana edge modes were theoretically predicted, characterized by Chern numbers (N) of 1 and 2, respectively. We present spectroscopic evidence from Andreev reflection experiments for the presence of chiral Majorana modes in a Nb / (Cr_{0.12}Bi_{0.26}Sb_{0.62})_{2}Te_{3} heterostructure with distinct signatures attributed to two different topological superconducting phases. The results are in qualitatively good agreement with the theoretical predictions. 
18.02.2020

Gediminas Simutis Tuning Quantum Magnets Laboratoire de Physique des Solides, Université Paris  Saclay & CNRS When cooled down, most magnetic materials form longranged structures of the magnetic moments. The situation is markedly different when strong quantum fluctuations are present in low dimensional materials or lattices with highly frustrated networks of spins. Such systems collectively called quantum magnets may exhibit unconventional magnetism and even maintain a fluctuating quantum state down to the lowest temperatures. 
07.01.2020

Max Hirschberger Scalar spin chirality on the nanoscale: Material exploration and emergent electrodynamics RIKEN CEMS, Center for Emergent Matter Science, Japan Scalar spin chirality is a pervasive concept in contemporary research on magnetism in condensed matter, especially as relates to the coupling between magnetic order and the conduction electrons. In metallic magnets, noncoplanar spin structures on the nanoscale can exert a giant emergent magnetic field on charge carriers, with potential applications in spintronics. The emergent magnetic field may also be harnessed in the future design of new types of correlated systems with protected surface modes.
In this talk, I will outline our work on magnets with noncoplanar order on different length scales:
(1) The canted ferromagnetic state in the correlated pyrochlore oxide Nd_{2}Mo_{2}O_{7} is a representative example of noncoplanarity within a single crystallographic unit cell [1,2]. We discuss our experimental efforts of tuning the chemical potential in this material by chemical substitution, and new theoretical insight into the qualitatively different effects of scalar spin chirality and spinorbit coupling on the electronic band structure [3].
(2) The metallic antiferromagnet Dy_{3}Ru_{4}Al_{12} with breathing Kagome network of rareearth sites serves as an example of an intermediate class (λ_{mag} ~ 1.5 nm): Its noncoplanar magnetic order (in zero field) can be described as antiferromagnetic stacking of strongly coupled spintrimers with tilted allin or allout configuration [4]. We reveal high sensitivity of the trimer arrangement to external magnetic field, which leads to a large anomalous Hall effect when net scalar spinchirality emerges above 1 Tesla.
(3) Finally, I will discuss our recent work on the formation of nanometersized skyrmion spinvortices in the centrosymmetric rare earth intermetallics Gd_{2}PdSi_{3} [5] and Gd_{3}Ru_{4}Al_{12} [6] with Heisenberg Gd3+ moments and modulation length λ_{mag} ~ 2.53 nm. In contrast to the more widely studied case of skyrmions in noncentrosymmetric material platforms such as bulk B20 compounds or magnetic interfaces (λ_{mag} ~ 5200 nm), skyrmions emerge here in absence of the DzyaloshinskiiMoriya interaction. We report the phase diagrams of these compounds, reveal their magnetic order using scattering and real space imaging techniques, and discuss their large topological Hall and Nernst responses [7].
