ConQuMat - TRR360

TRR 360 Sonderseminar: Optimization and Application for Cryogenic Magnetic Refrigeration Using Yb-based Intermetallic Compounds Yasuyuki Shimura
2024-06-28 10:00 - 11:00 (Europe/Berlin) | University of Augsburg, Room 242-R
Optimization and Application for Cryogenic Magnetic Refrigeration Using Yb-based Intermetallic Compounds
Prof. Dr. Yasuyuki Shimura
Hiroshima University, Japan

Venue

TRR 360 Seminar: Discerning and mitigating complex quantum correlations in many-body dynamics Jan Budich
2024-07-17 14:00 - 15:00 | Technical University Munich + Zoom
Discerning and mitigating complex quantum correlations in many-body dynamics
Jan BudichTU Dresden
The proliferation of complex quantum correlations in many-body dynamics poses a formidable challenge both to quantum information science at a fundamental level and to classical variational approaches such as tensor network methods at a practical computational level. In this talk, after a brief introduction to the broader field, we discuss two of our recent contributions towards discerning various sources of quantum correlations and mitigating dynamical growth of entanglement. First, noting that entanglement depends on how a system is partitioned into subsystems (entanglement cut), we explore the potential of dynamically adjusting the entanglement cut as a strategy for reducing entanglement growth. Second, regarding the dynamics of mixed states such as locally perturbed thermal states, we demonstrate how tools from differential geometry such as parallel transport can be harnessed to tame the temporal growth of quantum correlations in a purified state corresponding to the physical mixed state.

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TRR 360 Seminar: Strain Tuning of Frustrated Magnets and Mott Insulators: Stress Reduces Frustration Andrej Pustogov
2024-07-23 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Strain Tuning of Frustrated Magnets and Mott Insulators: Stress Reduces Frustration
Andrej Pustogov TU Wien
Tuning materials properties on demand is at the heart of condensed matter science. Charge transport and magnetism are strongly linked to the overlap of electronic wave functions and can be, thus, manipulated by varying the electronic bandwidth through chemical substitution or physical pressure. Yet, a controlled variation of the symmetry, anisotropy and frustration of transfer integrals and exchange interactions remained inaccessible so far.
Here, we explore Mott insulators subject to strong antiferromagnetic interactions, where geometrical frustration suppresses magnetic order entirely [1,2] or down to very low temperatures TN << J [4,7]. Utilizing the recent advancements in strain tuning of unconventional superconductors [8,9], we apply uniaxial stress to fine-tune the Mott transition with unprecedented precision in a triangular-lattice compound [2]. Through the slope of the metal-insulator boundary in the temperature-pressure phase diagram we pinpoint the nonmagnetic ground state of the most intensely studied quantum-spin-liquid candidate [1-3]. By applying in situ uniaxial pressure to a clean, well-studied kagome-lattice compound without disorder [4-6], we obtain direct control of antiferromagnetic order within one single crystal [7]. As the applied stress reduces the frustration strength, TN is enhanced by 10% [7]. Our pioneering endeavors [2,7] demonstrate uniaxial strain as a powerful tool to tune interacting spins on frustrated lattices – towards stabilizing novel, exotic, possibly even quantum entangled spin states.
[1] B. Miksch, A. Pustogow, M. Javaheri Rahim, A. A. Bardin, K. Kanoda, J. A. Schlueter, R. Hübner, M. Scheffler, M. Dressel, Science 372, 276-279 (2021)[2] A. Pustogow, Y. Kawasugi, H. Sakurakoji, N. Tajima, Nat. Commun. 14, 1960 (2023)[3] Y. Kawasugi, S. Yamazaki, A. Pustogow, N. Tajima, J. Phys. Soc. Jpn. 92, 065001 (2023)[4] P. Puphal, M. Bolte, D. Sheptyakov, A. Pustogow, K. Kliemt, M. Dressel, M. Baenitz, C. Krellner, J., Mater. Chem. C 5, 2629 (2017)[5] T. Biesner et al., Adv. Quantum Technol. 2022, 2200023 (2022)[6] D. Chatterjee et al., Phys. Rev. B 107, 125156 (2023)[7] Jierong Wang, Y.-S. Su, M. Spitaler, K.M. Zoch, C. Krellner, P. Puphal, S.E. Brown, and A. Pustogow, Phys. Rev. Lett. 131, 256501 (2023)[8] C.W. Hicks et al., Science 344, 283 LP (2014)[9] A. Chronister et al., npj Quantum Mater. 7, 113 (2022)

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TRR 360 Seminar: Hidden multipolar order and singlet magnetism in f-electrons systems Sergii Khmelevskyi
2024-10-22 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Hidden multipolar order and singlet magnetism in f-electrons systems
Dr. Sergii Khmelevskyi
TU Wien
The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such “hidden orders” (HOs) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f-shells that may harbor high-rank multipole moments. We show how the ground-state order and magnetic excitations of a prototypical HO system NpO2, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. A primary non collinear order of time odd rank 5 (triakontadipolar) moments has been predicted. We show also that exotic non-chiral magnetic order in PrO2 is a results of a strong high order multipolar interactions within whole |JM> ground state multiplet. The multipolar interactions also leads to the peculiar magnetization process in PrO2 resulting in the exotic meta-magnetic transition in the applied magnetic field and fully dominate the ordinary dipolar Heisenberg exchange.
As has been shown by Kotliar and Haule in 2007 the canonical and perhaps most investigated metallic “hidden” order materials URu2Si2 can also develop a “hidden” multipolar order (hexadecapolar) due to localized 5f2 configuration at low temperature. Whereas at higher temperatures a hybridization of the localized 5f-levels leads to the Kondo behavior (“Kondo arrest scenario”) and at very low temperature a hidden order phase coexists with superconductivity. We reveal on basis of correlated ab-intio calculations a close analogy between normal state behavior of URu2Si2 and newly discovered heavy fermion UTe2 superconductor.

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Introduction to the electronic lab book ELabFTW
2024-10-30 09:00 - 10:30
An introduction to the electronic lab book eLabFTW is scheduled for next Wednesday, October 31st at 9:00 a.m. in the seminar room of EPII (room 344, north) at the University of Augsburg.This will be a hybrid seminar, conducted in parallel online.The Zoom link and login details are provided by email to all TRR 360 staff members.

TRR 360 Seminar: Imaging superconductors at high pressure using a nanoscale quantum sensor Christopher Laumann
2024-11-06 14:00 - 15:00 | Technical University Munich + Zoom
Imaging superconductors at high pressure using a nanoscale quantum sensor
Christopher LaumannBoston University, USA

Pressure alters the physical, chemical and electronic properties of matter. By compressing a material between two opposing brilliant cut diamonds, the diamond anvil cell enables tabletop experiments to reach pressures more than a million times that of atmospheric pressure. Since its development over half a century ago, it has enabled experiments to directly access pressure as a thermodynamic tuning parameter and has had a dramatic impact on quantum science, chemistry and materials physics. Among these impacts, a tremendous amount of recent attention has focused on the discovery of superconductivity in a class of hydrogen-based materials. When compressed to megabar pressures, these so-called super-hydrides are believed to exhibit the highest known critical temperatures, and have led to a nascent field that is equal parts exciting and controversial. Part of this controversy stems from the nature of the tool itself: especially at high pressures, it is tremendously challenging to extract local information from within a diamond anvil cell.

We describe a new approach to directly “see” the physics inside the sample chamber of a diamond anvil cell at ultra-high pressures. The basic idea is deceptively simple: we directly integrate a thin layer of quantum sensors, the NV defect, into the surface of the diamond anvil that is actually applying the pressure. We demonstrate the ability to perform optical diffraction-limited imaging of both stress fields and magnetism, with the latter allowing us to image the magnetic field expulsion associated with superconductivity. Applying our techniques to cerium hydride, we observe the dual signatures of superconductivity: diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping both the diamagnetic response and flux trapping, we directly image the geometry of superconducting regions, showing marked inhomogeneities at the micron scale.

Primary ref:[1] P. Bhattacharyya et al., Nature 627, 73–79 (2024).

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TRR 360 Seminar: Multi-Extreme THz ESR Study of the Pressure-Induced Phase Transition in the Shastry-Sutherland Model Substance SrCu2(BO3)2 Hitoshi Ohta
2024-11-12 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Multi-Extreme THz ESR Study of the Pressure-Induced Phase Transition in the Shastry-Sutherland Model Substance SrCu2(BO3)2
Prof. Dr. Hitoshi Ohta
Molecular Photoscience Research Center, Kobe University, JapanResearch Center for Development of Far-Infrared Region, University of Fukui, Japan
We have been developing THz ESR under multi-extreme conditions, such as high magnetic field, high pressure and low temperature in Kobe. It covers the frequency region between 0.03 and 7 THz,1 the temperature region between 1.8 and 300 K,1 the magnetic field region up to 55 T,1 and the pressure region is extended from 1.5 GPa2 to 2.5 GPa using the hybrid-type piston-cylinder pressure cell.3 It also includes mechanically detected ESR4 measurements using a commercially available membrane-type surface stress sensor, which is the extension from our micro-cantilever ESR5. As an example, application of high pressure THz ESR to understand the pressure-induced phase transition in the Shastry-Sutherland model substance SrCu2(BO3)2 will be discussed. Using our technique, we were able to determine the pressure where the pressure-induced phase transition occurs, and obtain the pressure dependence of exchange interactions in SrCu2(BO3)2 [Ref. 6]. Comparison of our results with the theory will be discussed.

H. Ohta et al., J. Low Temp. Phys. 2013, 170, 511.
T. Sakurai et al., Rev. Sci. Inst. 2007, 78, 065107
K. Fujimoto et al., Appl. Mag. Res. 2013, 44, 893; H. Ohta et al., J. Phys. Chem. B 2015, 119, 13755; T. Sakurai et al., J. Mag. Res., 2015, 259,108.
H. Takahashi et al., Rev. Sci. Intrum. 2018, 89, 036108
H. Ohta et al., AIP Conf. Proceedings 2006, 850, 1643; E. Ohmichi et al., Rev. Sci. Instrum. 2008, 79, 103903; H. Takahashi et al., Appl. Phys. Lett. 2015, 107, 182405.
T. Sakurai et al., J. Phys. Soc. Jpn. 87 (2018) 033701

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TRR 360 Seminar: Tba Urban Seifert
2024-12-04 14:00 - 15:00 | Technical University Munich + Zoom
Tba
Dr. Urban Seifert
TU Dresden

Venue

TRR 360 Seminar: Directional amplification, non-Hermitian topology, and optomechanical realisation of the bosonic Kitaev chain Andreas Nunnenkamp
2024-12-18 14:00 - 15:00 | Technical University Munich + Zoom
Directional amplification, non-Hermitian topology, and optomechanical realisation of the bosonic Kitaev chain
Prof. Andreas Nunnenkamp
Universität Wien

Directional amplification, in which signals are selectively amplified depending on their propagation direction, has attracted much attention as a key resource for applications, including quantum information processing. I will present a unifying framework based on non-Hermitian topology to understand non-reciprocity and directional amplification in driven-dissipative cavity arrays. Specifically, I will unveil a one-to-one correspondence between a non-zero topological invariant defined on the spectrum of the dynamic matrix and regimes of directional amplification, in which the end-to-end gain grows exponentially with the number of cavities. I will show how to restore the bulk-boundary correspondence for one paradigmatic class of non-Hermitian Hamiltonians. Finally, I will present experiments on a bosonic analog of the fermionic Kitaev chain in an nano-optomechanical network. Our work opens up new avenues in topological photonics and non-Hermitian phases of matter.

C.C. Wanjura, M. Brunelli, and A. Nunnenkamp,
Topological framework for directional amplification in driven-dissipative cavity arrays,
Nat Commun 11, 3149 (2020).
https://doi.org/10.1038/s41467-020-16863-9

M. Brunelli, C.C. Wanjura, and A. Nunnenkamp,
Restoration of the non-Hermitian bulk-boundary correspondence via topological amplification,
SciPost Phys. 15, 173 (2023).
https://scipost.org/10.21468/SciPostPhys.15.4.173

J.J. Slim, C.C. Wanjura, M. Brunelli et al.,
Optomechanical realization of the bosonic Kitaev chain,
Nature 627, 767 (2024).
https://doi.org/10.1038/s41586-024-07174-w

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TRR 360 Seminar: Tba Mark Goerbig
2025-01-29 14:00 - 15:00 | Technical University Munich + Zoom
Tba
Prof. Mark Goerbig
Université Paris Sud, Orsay

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TRR 360 Seminar: Cavity electrodynamics of correlated materials in Fabry-Pèrot optical resonators Giacomo Jarc
2025-02-11 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Cavity electrodynamics of correlated materials in Fabry-Pèrot optical resonators
Dr. Giacomo Jarc
Friedrich-Alexander-Universität, Erlangen-Nürnberg

The possibility of engineering collective properties of bulk materials through cavity electrodynamics has been so far mostly explored in theoretical works and theory-based experimental proposals [1]. Only recently, the development of new experimental platforms [2, 3] has started to unlock the full potential of cavity-control of solid-state quantum materials and shed light on new pathways for light-matter coupling in a cavity-confined environment.
In this seminar, I will discuss our recent study on the cavity-mediated thermal control of the metal-to-insulator transition in the charge-density-wave (CDW) material 1T-TaS2 [4], which show the possibility of reversible cavity-manipulation of a phase transition in a correlated material. We embed 1T-TaS2 into cryogenic tunable terahertz cavities [3] and show that a switch between conductive and insulating behaviours, triggered by a large change in the sample temperature, is obtained by mechanically tuning the distance between the cavity mirrors and their alignment. The large thermal modification observed is indicative of a Purcell-like scenario in which the spectral profile of the cavity modifies the energy exchange between the material and the external electromagnetic field [5].
When the cavity is tuned resonantly to the multiple IR-active vibrations of the CDW dielectric phase of 1T-TaS2, we reveal the THz signatures of a multimode vibro-polariton mixing, originating from the collective strong coupling of the CDW vibrations with the cavity field [6]. The estimated components of the polaritonic wave-functions show that the observed multimode polaritons have character from all the vibrational resonances as a consequence of the cavity-mediated hybridization. The Rabi splittings between the hybridized phonons are suppressed across the insulator-to-metal transition as a consequence of the screening of the free charges. Intriguingly, we reveal that in proximity of the phase transition, the phonon dissipations trigger the onset of a weak vibrational coupling regime in the cavity-confined material.
When instead the cavity frequency is tuned within the spectral range of the continuum Drude excitation of 1T-TaS2, we reveal that the quality factor of the cavity, which quantifies the dissipative response of the coupled system, is sensitive to the material phase. We measure indeed the lifetime of the cavity photons to decrease passing from the insulating to the metallic state. This evidence points to a scenario in which the free charges responsible for the Drude excitation can effectively couple to the cavity field and subsequently modulate the light-matter coupling within the Fabry-Pèrot resonator.
Our findings show how cavity electrodynamics can play a role in the intricate equilibrium physics of complex materials, possibly providing a new tool to control and engineer their cooperative properties.

[1] F. Schlawin, D. M. Kennes, and M. A. Sentef., “Cavity quantum materials”, Applied Physics Reviews 9, 011312 (2022).
[2] G. Jarc, S. Y. Mathengattil, F. Giusti, M. Barnaba, A. Singh, A. Montanaro, F. Glerean, E. M. Rigoni, S. Dal Zilio, S. Winnerl, and D. Fausti, “Tunable cryogenic terahertz cavity for strong light-matter coupling in complex materials”, Rev. Sci. Instrum. 93, 033102 (2022).
[3] F. Appugliese, J. Enkner, G. Parravicini-Bagliani, M. Beck, C. Reichl, W. Wegscheider, G. Scalari, C. Ciuti, and J. Faist, “Breakdown of topological protection by cavity vacuum fields in the integer quantum Hall effect”, Science 375, 6584, 1030-1034 (2022).
[4] G. Jarc, S. Y. Mathengattil, A. Montanaro, F. Giusti, E. M. Rigoni, R. Sergo, F. Fassioli, S. Winnerl, S. Dal Zilio, D. Mihailovic, P. Prelovsek, M. Eckstein, and D. Fausti, “Cavity-mediated thermal control of metal-to-insulator transition in 1T-TaS2”, Nature 622, 487-492 (2023).
[5] F. Fassioli, J. Faist, M. Eckstein, and D. Fausti, “Controlling radiative heat through cavity electrodynamics”, arXiv:2403.00851v1 [physics.optics] (2024).
[6] G. Jarc, A. Montanaro, S. Y. Mathengattil, E. M. Rigoni, S. Dal Zilio, and D. Fausti, “Multimode vibrational coupling across the insulator-to-metal transition in 1T-TaS2 in THz cavities”, J. Chem. Phys. 161, 154711 (2024).

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TRR 360 Seminar: Energy relaxation and dynamics in strongly correlated materials N. Peter Armitage
2025-02-13 14:00 - 15:00 | University of Augsburg, Room S-288 + Zoom
Energy relaxation and dynamics in strongly correlated materials
Prof. Dr. N. Peter Armitage
Johns Hopkins University, Baltimore, USA

If a physical system is perturbed from equilibrium, the rate that it equilibrates is an important measure of its physics. In condensed matter systems, we are used to measuring such rates in the context of linear response to electromagnetic fields. For instance, the rate that current decays in a metal after an electric field impulse can be related to the width of its low-frequency “Drude” response in the optical conductivity. The rate that polarization decays after polling a liquid with an E field corresponds to the width of the broad peak in the Debye relaxational functional form. In contrast, the rate of energy relaxation is a fundamental rate that governs many processes in solids, but which is unfortunately not measured straightforwardly via conventional electrodynamic linear response. However quite generically, this rate can be measured in non-linear chi3 spectroscopies. I will discuss recent technical developments in the form of THz range 2D coherent spectroscopy (and its relatives) that allow us to get new information about energy relaxation in correlated and topological metals, as well as disordered electron glasses. I will discuss a number of systems and phenomena in which unconventional dynamics and energy relaxation govern their low energy behavior. I will give number of examples of the power of these new techniques to strongly interacting metals, Dirac semimetals, collective modes in superconductors, electron glasses, and 1D spin chains.

[1] Fahad Mahmood et al., “Observation of a marginal Fermi glass”, Nature Physics (2021).
[2] D. Barbalas et al., “Energy Relaxation and Dynamics in the Correlated Metal Sr2⁢RuO4 via Terahertz Two-Dimensional Coherent Spectroscopy”, Phys. Rev. Lett. 134, 036501 (2025).
[3] K. Katsumi et al., “Revealing novel aspects of light-matter coupling in terahertz two-dimensional coherent spectroscopy: the case of the amplitude mode in superconductors”, Phys. Rev. Lett. 132, 256903 (2024).
[4] R. Bhandia et al., “”Anomalous electronic energy relaxation and soft phonons in the Dirac semimetal Cd3As2“, Phys. Rev. B 110, 075131 (2024).
[5] R. Bhandia et al. “Energy and momentum relaxation through the Curie temperature in an itinerant ferromagnet”, https://arxiv.org/abs/2412.08749.

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TRR 360 Seminar: Optically induced quantum spin-disordered state in the Kitaev material α-RuCl3 Paul H. M. van Loosdrecht
2025-02-25 14:00 - 15:00 | University of Augsburg, Room S-288 + Zoom
Optically induced quantum spin-disordered state in the Kitaev material α-RuCl3
Prof. Dr. Paul H. M. van Loosdrecht
Universität zu Köln
One way to characterize quantum spin-liquids is through the fractionalization of spin excitations. A prime example of this is found in the exactly solvable Kitaev model of spin-1/2 moments with anisotropic exchange interactions on a tri-coordinated lattice. To find examples of this kind of physics in nature turns out to be challenging. The currently best-known examples of materials in which Kitaev-like physics plays a central role are the layered spin-orbit entangled J=1/2 systems Na2IrO3, α-Li2IrO3, and α-RuCl3. However, these materials all possess additional interactions, which, among other, lead to a magnetically ordered state at low temperature preventing the formation of a pure Kitaev spin-liquid (KSL) state. Apart from the ongoing quest for materials showing a true KSL ground state, one can also destabilize the magnetic order in the existing materials, which potentially can induce the sought-after KSL state. In this contribution I will discuss two methods to destabilize magnetic order in α-RuCl3. The first one is through the application of an in-plane magnetic field. Though it has been shown by various authors that this indeed leads to suppression of the ordered state in α-RuCl3, the nature of the field-induced state is still not fully clear. The second approach is a pump-probe method which creates holon and doublon excitations. These excitations are found to couple efficiently to magnetic excitations which in turn disorder the magnetically ordered state. For sufficiently high excitation densities the magnetic order is fully suppressed, leading to a quantum disordered magnetic state.

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TRR 360 Seminar: Spin-Orbit Coupling and Emergent Superconductivity in Oxide Heterostructures Dr. Denis Maryenko
2025-03-25 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Spin-Orbit Coupling and Emergent Superconductivity in Oxide Heterostructures
Dr. Denis Maryenko
RIKEN Center for Emergent Matter Science, Wako, Japan
Oxide compounds encompass a diverse class of materials hosting a rich spectrum of quantum phenomena, driven by the interplay of spin-orbit coupling (SOC), electron correlations, and lattice effects. In particular, materials with inherently strong SOC have drawn significant attention due to their potential to stabilize novel electronic phases with implications for future technologies. Controlling these materials and their interfaces provides a powerful route to engineering emergent quantum states.
The perovskite KTaO₃, with its pronounced SOC, has recently emerged as a promising platform for exploring unconventional superconductivity. However, precisely tailoring interfacial electronic states to induce conductivity in KTaO₃ remains a key challenge. In this talk, we present our recent progress on epitaxially grown KTaO₃-based heterostructures, focusing on the LaTiO₃/KTaO₃ interface. Our findings reveal the systematic emergence of superconductivity at this interface, which can be tuned via electric and magnetic fields, pointing to novel superconducting phases. These results provide new insights into the interplay between superconductivity and SOC in low-dimensional systems and establish a robust platform for investigating emergent physics in two-dimensional superconductors.

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TRR 360 Sonderseminar: Localized Magnons in the Generalized Model of the Sawtooth Chain with Dzyaloshinskii-Moriya Interactions Vadim Ohanyan
2025-04-10 14:00 - 15:00 | University of Augsburg, Room S-439
Localized Magnons in the Generalized Model of the Sawtooth Chainwith Dzyaloshinskii-Moriya Interactions

Dr. Vadim Ohanyan

Yerevan State University, Armenia

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TRR 360 Sonderseminar: Probing ultrafast Electron and Spin Dynamics in Momentum, Space, and Time Martin Aeschlimann
2025-05-15 14:00 - 15:00 | University of Augsburg, Room S-288 + Zoom
Probing ultrafast Electron and Spin Dynamics in Momentum, Space, and Time

Prof. Dr. Martin Aeschlimann

Department of Physics and Research Center OPTIMAS, RPTU Kaiserslautern-Landau

Competing interactions of spin with charge and lattice, determined by the spin-orbit interaction, give rise to rich phase diagrams of states in novel correlated electron materials. In such materials, magnetically ordered phases are very often in direct competition with other ordered phases, such as a spin- or charge-ordered phase. Unfortunately, the dominant interaction responsible for the formation of a particular phase is often difficult to determine at thermal equilibrium, so that a fundamental understanding of the underlying competing interactions is out of reach using static measurements. Time-resolved spectroscopy techniques have the potential to overcome these limitations by temporally driving the material system out of equilibrium. The subsequent relaxation pathways are then determined by the spin-charge lattice interactions, which can be studied using different photoemission techniques. In this talk I will show how recent developments in ultrafast light sources and photoemission detector technology have paved the way for a completely new generation of time- and spin-resolved photoemission experiments. With this tool at hand we can directly observe the temporal evolution of excited carriers and spins in energy, momentum space and time, providing an unprecedented insight into the fundamental energy and (angular) momentum dissipation mechanisms even in complex condensed matter.

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TRR 360 Seminar: !! Canceled due to illness of the speaker !!Designing Quantum Materials with Light Michael Sentef
2025-05-20 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
The seminar talk has been canceled due to illness of the speaker!
Designing Quantum Materials with Light
Prof. Dr. Michael Sentef
Universität Bremen
Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany
In recent years, light-driven quantum materials science has undergone a fundamental transformation. What was once a theoretical vision—the ability to control and manipulate emergent properties of materials on ultrafast timescales—has now become a reality [1]. This progress has been enabled by rapid advancements in shaping laser pulses, probing nonequilibrium dynamics with femtosecond resolution, and developing sophisticated theoretical approaches to describe light-driven many-body systems [2]. As a result, we are now entering an era in which quantum materials can be actively “designed” and controlled using tailored light fields.
A cornerstone of this approach is Floquet engineering, which exploits periodic driving to coherently modify electronic states and induce novel phases of matter. I will briefly review key experimental and theoretical developments in realizing Floquet states in quantum materials and discuss their implications for controlling competing orders. However, despite its promise, Floquet engineering also faces intrinsic limitations, particularly due to heating effects and decoherence, which can constrain its applicability as a general tuning mechanism.
Moving beyond conventional Floquet approaches, a new frontier is emerging: cavity quantum materials [3]. By embedding materials in tailored quantum-electrodynamical environments, such as optical cavities, it is possible to enhance light-matter interactions and create hybrid light-matter states with fundamentally new properties. Unlike classical laser-driven schemes, cavity-mediated interactions can modify quantum fluctuations and collective excitations even in thermal equilibrium, offering a novel route to control material properties without direct external driving. I will highlight recent advances in this field, both from theoretical [4] and experimental [5,6] perspectives, and specifically discuss how strong correlations in cavity quantum materials provide new opportunities for engineering competing electronic orders through light-matter hybridization. This emerging paradigm may open pathways toward controlling superconductivity, charge density waves, and other ordered phases in a fundamentally new way.

Figure: Top left – illustration of a quantum material driven by a classical laser drive. Top right – cavity-embedded quantum material, opening the possibility to use vacuum fluctuations of light to control emergent properties. Bottom – setup of the McIver lab for on-chip THz spectroscopy of quantum materials in a plasmonic cavity [5].
[1] A. de la Torre et al., Rev. Mod. Phys. 93, 041002 (2021)
[2] F. Caruso et al., arXiv:2501.06752
[3] F. Schlawin et al., Applied Physics Reviews 9, 011312 (2022)
[4] M. A. Sentef et al., Phys. Rev. Research 2, 033033 (2020)
[5] G. Kipp, H. Bretscher, et al., arXiv:2403.19745
[6] B. Le Dé et al., Journal of Physics: Materials 5, 024006 (2022); I. Keren, T. Webb, D. Basov, et al., forthcoming

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TRR 360 Seminar: RIXS interferometry: cluster Mott insulators and bond-directional excitations in Kitaev materials Markus Grüninger
2025-05-27 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
RIXS interferometry: cluster Mott insulators and bond-directional excitations in Kitaev materials
Prof. Dr. Markus Grüninger
Universität zu Köln

Cluster Mott insulators provide a platform for novel types of quantum magnets with unconventional magnetic moments. Electrons are localized on a cluster such as a dimer and occupy quasimolecular cluster orbitals. In cluster Mott insulators, RIXS interferometry reveals the symmetry and character of excited states, offering a powerful tool to study the electronic structure. We systematically investigate the quasimolecular properties of dimers, trimers, and tetrahedra in the 5d transition-metal compounds Ba3MIr2O9, Ba4NbIr3O12, and GaTa4Se8. Furthermore, we discuss the applicability of the cluster picture for 3d GaV4S8.
RIXS interferometry can also be applied to Kitaev materials such as Na2IrO3 and α-Li2IrO3 and shows distinctive fingerprints of Kitaev physics. Kitaev exchange restricts spin-spin correlations to nearest neighbors, giving rise to a dimer-like dynamical structure factor. Bond-directional exchange implies the existence of bond-directional excitations. Making use of polarization selection rules, RIXS reveals this bond-directional character of magnetic excitations in Na2IrO3.

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TRR 360 Seminar: Randomness, disorder and compromises in magnetic systems Ellen Fogh
2025-06-10 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Randomness, disorder and compromises in magnetic systems
Prof. Dr. Ellen Fogh
Technical University of Munich

The interplay between randomness, disorder and frustration in magnetic systems may bring about interesting ground states and behaviours. Many real materials display intrinsic inhomogeneity, as a result of impurities or (counter)ion substitution, that results in site or bond disorder. This is known as quenched randomness, and the loss of translational symmetry it entails makes the system challenging to study theoretically. However, quenched randomness in magnetic systems can lead to specific ground states with no long-ranged order, including glassy states and the random-singlet state but also various long-range ordered states. Some of these phases of matter are closely related to certain types of quantum spin liquids and thus raise the question of whether randomness in a frustrated system can produce qualitatively different types of quantum coherence, as opposed to only destroying such coherence. Here, I discuss three specific cases of how randomness and disorder have profound consequences for physical behaviour of such magnetic systems.

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TRR 360 Seminar: Some exotic phases for spin-1/2 on the triangular lattice Sylvain Capponi
2025-06-17 14:00 - 15:00 | Technical University Munich + Zoom
Some exotic phases for spin-1/2 on the triangular lattice
Prof. Dr. Sylvain Capponi
Laboratoire de Physique Théorique, Université de Toulouse, CNRS, UPS, France

The triangular lattice is at the origin of magnetic frustration both for Ising or Heisenberg models, as shown by Wannier and Anderson respectively. Hence it is a natural testbed to stabilize unconventional phases of matter. In this talk, I will present two recent numerical studies discussing
(i) the possibility of a Dirac spin liquid in the J1-J2 Heisenberg model [1](ii) the possibility of a supersolid phase in the XXZ model [2] and its relevance to recent scattering experiments on K2Co(SeO3)2

[1] A. Wietek, S. Capponi, and A. Läuchli, Phys. Rev. X 14, 021010 (2024)[2] S. Capponi and A. Läuchli, in preparation

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TRR 360 Seminar: Effect of disorder in a frustrated system Mayukh Majumder
2025-06-24 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Effect of disorder in a frustrated system
Dr. Mayukh Majumder
Department of Physics, Shiv Nadar University, India

Disorder is unavoidable in any real-life quantum many-body system. Though in several cases, the disorder often hinders us from elucidating the actual physics of a system, in some cases, unusual properties can be seen, and exotic phases can be formed due to the presence of disorder. Those interesting phases do not develop in a clean or disorder-free system. Some examples of those exotic phases include the quantum Griffiths phase near quantum criticality, spin glass, random spin singlet state, etc. In the talk, I will discuss the presence of an exotic state, namely the random spin singlet state, stabilized by disorder in a new Copper-based three-dimensional quantum spin liquid candidate.

Venue

TRR 360 Seminar: Signatures of fractional quantum Hall states in few-body systems Cecile Repellin
2025-06-25 14:00 - 15:00 | Technical University Munich + Zoom
Signatures of fractional quantum Hall states in few-body systems
Dr. Cecile Repellin
Université Grenoble Alpes, CNRS, LPMMC, Grenoble, France

Realizing strongly correlated topological phases of ultracold gases is a central goal for quantum gas experiments. Due to the difficulty in preparing these phases, ongoing experiments are focusing on ensembles of few atoms, and the preparation of a fractional quantum Hall state of two bosonic atoms has been achieved [1]. Beyond their preparation, the characterization of these few-body states poses a unique challenge due to their small system size. I will discuss which signatures can be used, and show that hallmark fingerprints of fractional quantum Hall phases, such as a quantized Hall conductivity [2], or chiral edge modes[3], can be extracted in few-particle systems through local density measurements.

[1] Léonard, J., Kim, S., Kwan, J. et al., Realization of a fractional quantum Hall state with ultracold atoms. Nature 619, 495–499 (2023).[2] Repellin, C., Léonard, J. and Goldman, N., Chern insulators of few bosons in a box: Hall plateaus from center-of-mass drifts and density profiles. Phys. Rev. A 102, 063316 (2020).[3] Binanti, F., Goldman, N., Repellin, C, Spectroscopy of edge and bulk collective modes in fractional Chern insulators. Phys. Rev. Research 6, L012054 (2024).

Venue

TRR 360 Sonderseminar: Emergence of novel quantum phases in Yb-based effective spin-1/2 systems with competing exchange interactions Takahiro Onimaru
2025-07-03 11:00 - 12:00 | University of Augsburg, Room S-254 + Zoom
Emergence of novel quantum phases in Yb-based effective spin-1/2 systems with competing exchange interactions

Prof. Dr. Takahiro Onimaru

Hiroshima University, Japan

Venue

TRR 360 Seminar: Frustrated Triangular Lattice Magnetism in Delafossite Type Yb- (and Cr-based) Systems- from spin liquid to field induced order Michael Baenitz
2025-07-08 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Frustrated Triangular Lattice Magnetism in Delafossite Type Yb- (and Cr-based) Systems- from spin liquid to field induced order
Dr. Michael Baenitz
Max-Planck-Institute for Chemical Physics of Solids

Venue

TRR 360 Sonderseminar: Phase diagram of the heavy-fermion superconductor YbRh2Si2 at ultra-low temperatures Manuel Brando
2025-07-08 17:15 - 18:15 | University of Augsburg, Room S-254 + Zoom
Phase diagram of the heavy-fermion superconductor YbRh2Si2 at ultra-low temperatures

Dr. Manuel Brando

Max-Planck-Institute for Chemical Physics of Solids, Dresden

Venue

TRR 360 Seminar: Exploration of Hybrid Skyrmion Systems using Resonant Elastic X-ray Scattering Thorsten Hesjedal
2025-07-22 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Exploration of Hybrid Skyrmion Systems using Resonant Elastic X-ray Scattering
Prof. Dr. Thorsten Hesjedal
Clarendon Laboratory, University of Oxford, UK

A major challenge in topological magnetism lies in the three-dimensional (3D) exploration of the magnetic textures. Recent efforts have focused on understanding how 2D skyrmion sheets vertically stack to form distinct types of 3D topological strings, how these strings behave when clamped at an interface, how they couple across interfaces of dissimilar materials, and how these ordered states can be manipulated. Our team has advanced the capabilities of polarization- and energy-dependent resonant elastic x-ray scattering (REXS) to probe the 3D structure of magnetic skyrmions and related topological objects, as well as their dynamics. In this talk, I will present the latest examples of hybrid skyrmion systems, with a particular focus on 3D objects at or near surfaces and their dynamical properties.

Venue

TRR 360 Seminar: Randomness, disorder and compromises in magnetic systems Ellen Fogh
2025-07-30 14:00 - 15:00 | Technical University Munich + Zoom

TRR 360 Seminar: Imaging Magnetic Dynamics with In-situ and Ultrafast Lorentz Microscopy Sascha Schäfer
2025-11-04 16:00 - 17:00 | University of Augsburg, Room S-288 + Zoom
Imaging Magnetic Dynamics with In-situ and Ultrafast Lorentz Microscopy
Prof. Dr. Sascha Schäfer
Department of Physics, University of Regensburg, Regensburg, GermanyRegensburg Center for Ultrafast Nanoscopy (RUN), Regensburg, Germany
The interaction of magnetic materials with ultrashort light pulses provides intriguing glimpses into the inner workings of the magnetic couplings within a material but also offers an important experimental handle to locally manipulate magnetic textures and phases. Whereas many studies in the field of ultrafast magnetism have utilized various optical pump-probe methodologies, these approaches often lack the required spatial resolution for mapping the nanoscale details of magnetic textures. Here, I will present two methodologies to address ultrafast nanoscale magnetic dynamics in Lorentz microscopy.
In the first part of the talk, I will discuss how in-situ Lorentz microscopy with femtosecond optical excitation [1] can unravel nonlinearities in the magnetic response upon light excitation, focusing on the example of light-induced switching processes in the magnetic texture of Fe3Sn2 thin films [2]. This material hosts dipolar skyrmions [3] which are composed of chiral Bloch-like domain walls in the interior of the film and chiral Néel-like caps at each film surface. Femtosecond light pulses allow for the stochastic switching of the Néel cap chirality. The analysis of switching probabilities yields insights into the importance of interactions within and in-between skyrmions.
In the second part of the talk, I will discuss the current state of ultrafast transmission electron microscopy [4], specifically ultrafast Lorentz microscopy [6], in which femtosecond or picosecond electron pulses enable the stroboscopic imaging of optically or current-driven magnetic dynamics. I will highlight the impact of novel femtosecond electron sources, including radiofrequency beam choppers [7] and laser-driven cold-field emitter sources [8], for different magnetic imaging techniques in electron microscopy, as well as emerging possibilities due to fast electron detectors [9].
[1] T. Eggebrecht; M. Möller et al., Phys. Rev. Lett. 118, 097203 (2017). [2] A Kovács, J. T. Weber et al., Comm. Mat. 6, 223, (2025).[3] L. Kong et al., Phys. Rev. B 107, 174425 (2023).[4] Feist et al., Ultramicroscopy 176, 63 (2017).[5] Feist et al., Nature 521, 200 (2015). [6] N. Rubiano da Silva et al., Phys. Rev. X 8, 031052 (2018).[7] C. Liu et al., Nat. Mat. 24, 406 (2025).[8] A. Schröder et al., Ultramicroscopy 275, 114158 (2025).[9] A. Schröder et al., Ultramicroscopy 256, 113881 (2024).

Venue

TRR 360 Seminar:!! Canceled due to illness of the speaker !! Sebastian Diehl
2025-11-12 14:00 - 15:00 | Technical University Munich + Zoom
Tba
Prof. Dr. Sebastian Diehl
University of Cologne

Venue

TRR 360 Sonderseminar: Computational design of quantum materials using density functional theory and beyond Manish Verma
2025-11-18 16:00 - 17:00 | University of Augsburg, Room S-288
Computational design of quantum materials using density functional theory and beyond

Dr. Manish Verma

Institute for Theoretical Physics and AstrophysicsComputational Quantum MaterialsJulius-Maximilians-Universität Würzburg

Computational design of quantum materials using density functional theory (DFT) and many-body techniques-both perturbative and non-perturbative, has become a powerful tool for exploring key physical phenomena such as strong correlations, metal-to-insulator transitions (MIT), magnetism, and thermoelectricity etc. In this direction, designing novel electronic properties in artificial transition metal oxide (TMO) heterostructures, distinct from their bulk counterparts, has emerged as a new paradigm enabled by modern layer-by-layer growth techniques and the unique nature of d-electrons. I will start my talk by discussing the mechanisms driving the MIT and magnetism in ultrashort-period superlattices (SL), namely (SrVO3)1/(SrTiO3)1(001) [1, 2] and (LaNiO3)1/(LaAlO3)1(001) [3], where confinement and epitaxial strain play fundamental roles. Subsequently, I will present the MIT in Ca-doped LaMnO3, which arises from the interplay between strain and chemical doping [4]. TMO-based SL are also known to exhibit high thermoelectric response, in addition to their environmental friendliness and stability. In this context, I will discuss the doping-induced robust p-type thermoelectric response in ultrashort-period (SrMnO3)1/(SrTiO3)1(001) SL, obtained by employing Boltzmann transport theory within constant-relaxation time approach. Next, I will then present my results on strategies for reducing lattice thermal conductivity in artificial oxide superlattices, obtained using many-body perturbation theory calculations of phonon-phonon interactions. Transition-metal oxides are further known to display strong correlation effects due to their d-electrons. In this regard, LiV2O4 stands out as an enigmatic heavy fermion compound lacking localized f-orbital states. I will present DFT combined with dynamical mean-field theory (DFT+DMFT) results that elucidate the origin of heavy-fermion behavior, supported by angle-resolved photoemission spectroscopy (ARPES) measurements [5].

[1] M. Verma, B. Geisler, and R. Pentcheva, Phys. Rev. B 100, 165126 (2019).[2] M. Verma and R. Pentcheva, Phys. Rev. Research 4, 033013 (2022).[3] M. Verma and R. Pentcheva, Phys. Rev. Research 6 (1), 013189 (2024).[4] S. S. Hong et al., Science 368, 71 (2020).[5] D. Oh et al., Proceedings of the National Academy of Sciences 122 (45), e2518213122 (2025).

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