Munich Quantum Matter Days

27 Oct 2025, 08:00 → 31 Oct 2025, 16:00 Europe/Berlin

Institute for Advanced Study (IAS)

Recent years have seen significant advancements in our understanding of strongly correlated quantum materials, characterized by extensive many-body entanglement. These developments have been propelled by discoveries of new materials and breakthroughs in experimental techniques.

However, despite simultaneous improvements in the theoretical understanding of ground state properties and their classification, our grasp of experimentally accessible response functions remains incomplete. Advancing our theoretical descriptions of the dynamics of collective excitations is crucial for diagnozing and characterizing new quantum states of matter.

This workshop aims to bridge this gap by fostering theoretical and experimental collaborations to deepen our understanding of quantum materials, with a particular focus on spin-orbit coupled quantum phases of matter.

Image credit to Alexander Grimm

Monday 27 October

Optional FRM II Tour

Day 1: Lunch

Day 1 Talk Session A

1 Interleaved bond and magnetic frustration in quantum materials

Hexagonal lattices with close connectivity can commonly form extended atomic networks of triangular, kagome, and honeycomb patterns, each capable of hosting a broad range of unconventional electronic properties. Key examples are unusual states such as quantum disordered magnetic states or compact localized charge states that are stabilized via geometric magnetic or kinetic frustration. In certain scenarios, hexagonal networks can also realize structurally frustrated charge or bond order instabilities that impart lattice fluctuations capable of renormalizing electronic properties. In this talk, I will discuss our work searching for classes of materials where frustration can emerge in two separate sublattices within the same crystal structure, in particular magnetic and bond frustration. Two examples will be discussed to motivate the possibility of interfacing strong structural fluctuations with neighboring magnetic networks and to provide some general insights into similar lattice instabilities in related compounds.

Speaker: Stephen Wilson (UC Santa Barbara)

2 Compressed kagome metals

Kagome metals are in the focus of current research on quantum materials. Understanding the effect of external stimuli on these materials is a prerequisite for the deliberate optimization and tailoring toward applications. In this talk, I will present the evolution of crystal and electronic structures of the kagome metals of the 135 family under hydrostatic and nonhydrostatic pressures.

Using single-crystal x-ray diffraction, broadband infrared spectroscopy, and ab initio calculations, I will show that CsV3Sb5 undergoes a pressure-induced reconstruction of the Sb 5p bands driven by the 2D-3D crossover and the formation of interlayer Sb-Sb bonds as the c-parameter of the crystal structure decreases upon compression. The ensuing evolution of the Fermi surface gives rise to a nonmonotonic change in the electron-phonon coupling probed experimentally by the displaced Drude peak (localization peak) in the optical conductivity. These results suggest that the reentrant superconductivity observed in CsV3Sb5 is mainly caused by the evolution of the Sb 5p bands and unrelated to the kagome (d-band) part of the electronic structure. The p-bands also determine the stability of the charge-density wave and its suppression at elevated pressures. Therefore, the p-bands and d-bands in the kagome metals should be considered on equal footing.

I will further show how these insights can be extended to CsTi3Bi5 and CsCr3Sb5 where flat d-bands in the vicinity of the Fermi level result in enhanced electronic correlations. I will argue that in CsTi3Bi5 these d-bands are augmented by the 6p bands of Bi, which produce tilted Dirac crossings and show a peculiar evolution under pressure. I will also highlight the importance of probing crystal structures of pressurized kagome metals experimentally.

Speaker: Alexander Tsirlin (University of Augsburg)

Day 1: Coffee Break 1

Day 1 Talk Session B

3 To be announced

Speaker: Bernhard Keimer

4 To be announced

Speaker: Andreas Schnyder

Day 1: Coffee Break 2

Day 1 Talk Session C

5 To be announced

Speaker: Christian Rüegg

6 Spin and pair density waves in 2D altermagnetic metals

Altermagnetism, a recently proposed and experimentally confirmed class of magnetic order, features collinear compensated magnetism with unconventional spin-split bands. We show that in a metallic 2D d-wave altermagnet with [C_2||C_4] symmetry, secondary instabilities can arise. Using an unbiased functional renormalization group approach, we analyze the weak-coupling instabilities of a 2D Hubbard model with a preexisting altermagnetic order inspired by our ab initio electronic structure calculations of realistic material candidates from V2X2O (X = Te, Se) family. We identify two distinct spin density wave (SDW) states that break the underlying altermagnetic [C_2||C_4] symmetry. Additionally, we find spin-fluctuation-induced instabilities leading to a singlet d-wave superconducting state and an unconventional commensurate pair density wave state with extended s-wave and spin-triplet symmetry. We analyze the pairing mechanism and characterize their excitation spectrum, which exhibits Bogoliubov Fermi surfaces or nodal points depending on the gap size.

Speaker: Laura Classen (Technical University Munich)

Tuesday 28 October

Day 2 Talk Session A

7 From continuum excitations to sharp magnons via transverse magnetic field in a triangular lattice antiferromagnet

We report high-resolution inelastic neutron scattering measurements that observe the evolution of the spectrum as a function of transverse magnetic field in a disorder-free material realization of the Ising-like spin-1/2 triangular lattice antiferromagnet. We reveal how the excitation continuum in zero field transforms via an intermediate-field phase with broadened magnons into a spectrum of sharp magnons in the polarized phase at high field. We propose that the origin of the dominant continuum of excitations in zero field is related to the existence of a manifold of mean-field degenerate ground states and compare the experimental data with the expected spectrum of excitations in this case. Leonie Woodland, Ryutaro Okuma, J. Ross Stewart, Christian Balz, and Radu Coldea http://arxiv.org/abs/2505.06398.

Speaker: Radu Coldea (University of Oxford)

8 Quintuplet condensation in the skyrmionic insulator Cu2OSeO3 at ultrahigh magnetic fields

The chiral helimagnet Cu2OSeO3 is the first Mott insulator showing skyrmion lattice phases and a linear magnetoelectric effect, and has been studied extensively at low and intermediate magnetic fields. In this talk, I will discuss the physics of this material at ultrahigh magnetic fields (above 180 T) and, in particular, the existence of a Bose-Einstein condensate (BEC) of magnons, a canted XY ferrimagnet sandwiched between the 1/2 magnetization plateau (one of the widest ever observed) and the fully saturated phase. Unlike previously reported magnon BECs, the total transverse magnetization vanishes identically, and the order parameter is associated with the transverse components of the individual Cu spins. Due to the magnetoelectric coupling, the magnetic order is also accompanied by a characteristic dome-like electric polarization which is crucial for the observation of the condensate via the Faraday rotation effect.

Speaker: Ioannis Rousochatzakis (Loughborough University)

Day 2: Coffee Break 1

Day 2 Talk Session B

9 To be announced

Speaker: Léo Mangeolle

10 To be announced

Speaker: Collin Broholm

Day 2: Lunch

Day 2 Talk Session C

11 To be announced

Speaker: Ellen Fogh

12 Gapless fracton quantum spin liquid and emergent photons in a 2D spin-1 model

Gapless fracton quantum spin liquids are exotic phases of matter described by higher-rank U(1) gauge theories which host gapped and immobile fracton matter excitations as well as gapless photons. Despite well-known field theories, no spin models beyond purely classical systems have been identified to realize these phases. Using error-controlled Green function Monte Carlo, here we investigate a square lattice spin-1 model that shows precise signatures of a fracton quantum spin liquid without indications of conventional ordering. Specifically, the magnetic response exhibits characteristic patterns of suppressed pinch points that accurately match the prediction of a rank-2 U(1) field theory and reveals the existence of emergent photon excitations in 2+1 spacetime dimensions. Remarkably, this type of fracton quantum spin liquid is not only identified in the system's ground state but also in generic low-energy sectors of a strongly fragmented Hilbert space.

Speaker: Johannes Reuther (Helmholtz-Zentrum Berlin und Freie Universität Berlin)

Day 2: Coffee Break 2

Day 2 Talk Session D

13 To be announced

Speaker: Urban Seifert

14 Superconductivity without time-reversal symmetry

Recent experiments on twisted MoTe2 and rhombohedral multilayer graphene have demonstrated that these systems can host superconducting phases that emerge out of normal states where time-reversal symmetry (TRS) is broken in the orbital channel. This is surprising as TRS is the key symmetry leading to the formation of Cooper pairs already at weak coupling. Here, we will theoretically analyze crucial changes in the phenomenology of pairing that result from broken orbital time-reversal symmetry. In the second part of the talk, we will start from a normal state with TRS and then discuss how intentional impurities, e.g., in the form of adatom superlattices on high-temperature superconductors, can be used to engineer non-trivial orbital TRS-breaking textures by spontaneous symmetry breaking inside the superconducting phase.

Speaker: Mathias Scheurer (University of Stuttgart)

Poster Session with "Bavarian Brotzeit" (cold snacks)

15 Frustrated Magnetism Probed by Inelastic Neutron Scattering in SrTm2O4

The non-Kramers $\text{Tm}^{3+}$ ions are usually expected to form a non-magnetic singlet ground state. However, due to mixing between the ground and excited crystal field states, a pseudo-doublet with effective spin ½ can arise as demonstrated for $\text{SrTm}_2\text{O}_4$ by specific heat, magnetic entropy, and electron paramagnetic resonance measurements [1].
Inelastic neutron scattering shows low-lying dispersing crystal field modes along the $\langle 00L\rangle$ direction, which are modelled using random phase approximation and crystal field parameters obtained from an effective charge model [2]. The two inequivalent $\text{Tm}^{3+}$ sites in $\text{SrTm}_2\text{O}_4$ possess different single ion anisotropies (Ising and planar) and $\text{SrTm}_2\text{O}_4$ hosts two magnetic sublattices. The extracted exchange couplings for both sublattices are antiferromagnetic and frustrated with negligible interchain interactions. The frustration and low dimensionality of the system suppress long range magnetic order down to low temperatures despite the pseudo doublet nature of the ground state.
Here, we present magnetic excitation spectra using inelastic neutron scattering of the pseudo-doublet ground state of the non-Kramers compound $\text{SrTm}_2\text{O}_4$ in the low temperature regime ($T \sim J$) and under magnetic field.
When tuning the magnetic field, magnetic Bragg peaks appear at commensurate positions which are not equivalent to the excitation minima, and one observes an increased gap in the excitation spectrum.
The presence of two sublattices with frustrated interactions, together with large single-ion anisotropy, puts $\text{SrTm}_2\text{O}_4$ close to a multitude of nearly degenerate ground states and offers access to very rich new physics.

[1] D. L. Quintero Castro et al., Phys. Rev. B 111, 224409 (2025)
[2] A. Bhat Kademane et al., Journal of Magnetism and Magnetic Materials 551, 169020 (2022)

Speaker: Maximilian Spitaler (ETH Zürich)

16 Unconventional highly fluctuating spin in dirty magnet gamma-Ba3CoNb2O9

The search for the quantum spin liquid state has recently been extended from one-dimensional to three-dimensional systems. Nevertheless, the role of strong quenched disorder in three dimensions remains theoretically less explored due to the complexity of the computational methods. The cubic lattice, being the simplest three-dimensional lattice, provides a useful platform for examining the effects of disorder on quantum states in 3D.

It has been established that gamma-Ba3CoNb2O9 exhibits a disordered cubic lattice [1], characterized by a 1:2 site mixing ratio between magnetic Co2+ and non-magnetic Nb5+. In the low-temperature limit, the effective spin-1/2 state of Co2+ is a consequence of the interplay between spin-orbit coupling and the octahedral crystal fields.

Surprisingly, the frozen magnetic moment, which is typically exhibited in conventional spin glasses, has not been identified in such a disordered magnet. Instead, short-range dynamical magnetic correlations have been observed, which mimic the quantum spin liquid behavior.

In this poster I will present the experimental evidence of short-range dynamical spin correlations [1] in this disordered spin-1/2 cubic lattice, in combination with Muon Spin Rotation, Neutron Diffraction and Neutron Spin Echo. The present study demonstrates that the absence of frozen spin is due to the quantum nature of the spin 1/2, as evidenced by the quantum cluster expansion approximation method.

Reference:
[1] Fanjun Xu, et al. Submitted 2025, under review.

Speaker: Fanjun Xu (Helmholtz-Zentrum Berlin)

17 Identification and Reversible Optical Switching of NV+ Centers in Diamond

Nitrogen-vacancy (NV) centers in diamond consist of a substitutional nitrogen atom adjacent to a carbon vacancy. Due to their remarkable quantum properties, NV centers have emerged as promising candidates for qubits in quantum computing, as well as key components in quantum sensors, quantum circuits, and quantum networks. Their ability to be individually addressed, even at room temperature, makes them highly attractive for various quantum technologies.

NV centers can exist in different charge states, with the negatively charged NV⁻ and the neutral NV⁰ states being the most extensively studied. However, the positively charged NV⁺ center remains largely unexplored due to its optical inactivity, often leading to its classification as a "dark state." Theoretical predictions suggest that NV⁺ should play a crucial role in charge-state transitions and defect dynamics in diamond, yet direct experimental evidence has been challenging to obtain.

In this study, we employed positron annihilation spectroscopy (PAS) in combination with in situ light illumination to investigate the presence and charge-state conversion of NV⁺ centers in nitrogen-implanted and annealed diamond. Our measurements reveal a transition from NV⁺ to NV⁰ upon illumination with photons of energy exceeding a threshold of 1.234(8) eV. Furthermore, in complete darkness, we observe a decay of NV⁰ centers with a characteristic time constant of 1.73(22) hours.

These findings provide direct experimental insight into the charge dynamics of NV centers and the existence of the elusive NV⁺ state. Understanding these transitions is essential for optimizing NV-based quantum technologies, particularly in environments where charge stability is critical.

Speaker: Marcel Dickmann

18 Non-trivial Spin Structures of the DMI-Compound Ba2CuGe2O7

Ba2CuGe2O7 is an incommensurate spiral magnet, characterized by a quasi-2D topology of the magnetic interactions. It is an insulator characterized by a tetragonal, non-centrosymmetric space group. Due to the lack of inversion symmetry the Dzyaloshinskii–Moriya interaction (DMI) leads to the formation of a plethora on non-colinear spin structures [1-5]. The main features of the magnetic structure are due to the Cu2+ ions in a square arrangement in the tetragonal (a,b) plane with dominant nearest-neighbor AF exchange along the diagonal in the (a,b) plane and much weaker FM exchange between planes, leading to a quasi-2D behaviour. The DMI term leads to a long pitch incommensurate, almost AF cycloidal spin spiral in the ground state with its propagation vector confined to the tetragonal plane. The theoretical phase diagram of Ba2CuGe2O7 has recently been updated [6] and now predicts two new phases, positioned between the soliton lattice and the AF cone phase: an AF square and rectangular vortex phase is stabilized in a pocket around 2.2T for fields along the tetragonal c-axis. It is a square lattice of vortices and antivortices, with vortices being topological defects.

The quasi-2D character of Ba2CuGe2O7 also has an impact on the nature of the magnetic phase transitions: At zero field the phase transition was investigated in great detail [4,7]. Instead of a second order phase transition as expected by mean field theory, it was found to be of weak first order, described by the crossover from 2D-incommensurate Brazovskii fluctuations to a 2D AFM Heisenberg model.

Recent neutron diffraction experiments and measurements of the quasi-elastic linewidth have indeed indicated the presence of the vortex-anti-vortex phase in an extended window in magnetic field and temperature. However, neutron data shows both a finite linewidth in momentum space and energy, pointing towards a slowly fluctuating paramagnetic regime with vortex-anti-vortex character at the verge of ordering. We speculate that the absence of long-range order might be caused by the strong 2D character of Ba2CuGe2O7.

[1] A. Zheludev et al. Phys. Rev. B, 54:15163–15170, 1996.
[2] S. Mühlbauer et al. Phys. Rev. B, 84(18):5–8, 2011.
[3] S. Mühlbauer et al. Phys. Rev. B, 86(2):1–12, 2012.
[4] S. Mühlbauer et al. Phys. Rev. B, 96(13):1–9, 2017.
[5] J. Chovan et al. Phys. Rev. B, 65:064433, 3 2002.
[6] B. Wolba. PhD thesis, KIT, 2021.
[7] M. Dembski-Villalta et al. Phys. Rev. B, in review 2024.

Speaker: Sebastian Muehlbauer

19 Possibilities and Challenges for investigating Quantum Materials with the Cold Neutron Spectrometer ThALES

Cold neutron spectroscopy, with its intrinsic sensitivity to magnetism, provides unique insights into quantum materials. The dynamical susceptibility measured with neutrons enables direct validation of microscopic models, such as spin-wave calculations and ab initio theories. Moreover, neutron scattering plays a central role in model-free approaches, for example, the use of quantum witnesses to characterize entanglement in condensed-matter systems [1]. Such studies impose demanding experimental conditions: absolute units for elastic and inelastic scattering, measurements at low temperatures, and, ideally, polarization control for unambiguous verification.

The ThALES spectrometer at the ILL combines high continuous neutron flux with highly tunable resolution, thanks to its flexible primary spectrometer design and well-conditioned incident beam. The dedicated H53 guide system incorporates a virtual source, a velocity selector, and double-focusing PG002, Si111, and H111 monochromators [2]. ThALES can be operated in both unpolarised and fully polarised modes, the latter using either a conventional polarization setup or CryoPad [3].

A recent addition is the innovative multi-analyser system MARMOT [reference], which enhances ThALES’ capabilities by enabling efficient mapping of selected regions in momentum and energy space. MARMOT is based on a novel energy-analysis concept: large arrays of bent silicon blades (≈90 per channel) combined with a position-sensitive detector. Angular multiplexing across 30 channels provides a scattering-angle coverage of ~75°, while its geometry allows truly continuous energy analysis over a wide dynamic range (3.5–7 meV). Although still under construction at the ILL, commissioning of an initial set-up has already begun.

In this contribution, we will present an overview of the ThALES spectrometer and its MARMOT analyser system. We will highlight recent scientific results that showcase their performance, and discuss experimental strategies that leverage their versatile capabilities.

[1] A Scheie et al., https://doi.org/10.1016/j.mtquan.2024.100020
[2] https://www.ill.eu/for-all-users/instruments/instruments-list/thales/description/instrument-layout
[3] Tasset, F., Brown, P.J. et Forsyth J.B. (1988) J. Appl. Phys. 63, 3606-3608.

Speaker: Arno Hiess (Institut Laue Langevin and European Spallation Source)

20 Crystal electric field and phonon excitations in magnetoelectric TbPO4 investigated using inelastic neutron scattering

Magnetoelectric coupling describes the appearance of a macroscopic electric polarization $P_i = \alpha_{ij}H_j$ when the material is exposed to an applied external field $\mathbf{H}$ and vice versa. This effect offers promising opportunities for information technology applications, such as magnetic data storage devices [1].
TbPO$_4$ is a rare-earth material known for its exceptional magnetoelastic and magnetoelectric properties. With a coupling tensor element $\alpha_{xy}$ of 730 ps/m, it exhibits one of the strongest magnetoelectric couplings ever observed in bulk materials [2]. However, its origin remains an unsolved problem, since the underlying magnetic dynamics in these rare-earth systems is often complex and involves interactions between the lattice, the crystal electric field (CEF) and collective excitations. Here, we present the low-energy excitation spectrum of TbPO$_4$ in a transverse magnetic field, investigated using inelastic neutron scattering (INS). The INS data reveal complex dispersive CEF modes (at 0.4 meV, 1.2 meV, and 3.0 meV), which are broad and non-dispersive in the low-field antiferromagnetic phase, but sharpen and disperse in the high-field phase, where the system becomes ferromagnetic accompanied by a strong distortion of the crystal structure [3]. These results show a strong dependence of the low-energy excitations on the transverse magnetic field and, consequently, on the magnetic and structural phase transitions of the system.
Furthermore, we present measurements on the phonon spectrum of TbPO$_4$, allowing for an estimation of the phonon velocities and providing insight into the possible coupling between the CEF modes and collective dynamics.
These results contribute to a deeper understanding of the complex low-energy dynamics in TbPO$_4$ and can lay the basis for further theoretical modeling of its magnetoelastic and magnetoelectric effects.

[1] M. Bibes and A. Barthélémy, Nature Materials 7, 425–426 (2008).
[2] J.-P. Rivera, Eur. Phys. J. B 71, 299-313 (2009).
[3] P. C. Forino, PhD Thesis, DTU (2024).

Speaker: Anni Chen (EPFL)

21 Dynamical response theory of interacting Majorana fermions and its application to generic Kitaev quantum spin liquids in a field

Motivated by the appearance of Majorana fermions in a broad range of correlated and topological electronic systems, we develop a general method to compute the dynamical response of interacting Majorana fermions in the random-phase approximation (RPA). This can be applied self-consistently on top of Majorana mean-field theory (MFT) backgrounds, thereby in particular providing a powerful tool to analyse generic behaviour in the vicinity of (various heavily studied) exactly soluble models. Prime examples are quantum spin liquids (QSL) with emergent Majorana excitations, with the celebrated exact solution of Kitaev. We employ the RPA to study in considerable detail phase structure and dynamics of the extended Kitaev honeycomb KJΓ-model, with and without an applied field. First, we benchmark our method with Kitaev's exactly soluble model, finding a remarkable agreement. The interactions between Majorana fermions even turn out to mimic the effect of local Z2 flux excitations, which we explain analytically. Second, we show how small non-Kitaev couplings J and Γ induce Majorana bound states, resulting in sharp features in the dynamical structure factor in the presence of fractionalisation: such 'spinon excitons' naturally appear, and can coexist and interact with the broad Majorana continuum. Third, for increasing couplings or field, our theory predicts instabilities of the KQSL triggered by the condensation of the sharp modes. From the high symmetry momenta of the condensation we can deduce which magnetically ordered phases surround the KQSL, in good agreement with previous finite-size numerics. We discuss implications for experiments and the broad range of applicability of our method to other QSL and Majorana systems.

Speaker: Peng Rao (Technical University of Munich)

22 High-harmonic generation in multilayer systems

Over the past decades, the study of electronic behavior in novel materials has gained significant attention, driven by both the miniaturization of technology and the discovery of quantum effects at micro- and nanoscale dimensions. These quantum properties can be probed through high-harmonic generation (HHG), a nonlinear process arising from the interaction of a strong laser field with matter. HHG provides access to intrinsic material properties, such as the electronic wave function, enabling the characterization of topological phases and electronic velocities.

HHG has proven to be sensitive to topological phases, as well as to other phenomena including superconductivity, metallic behavior, and interaction-induced insulating phases. Previous studies have shown how topological invariants, such as the Chern number, can be extracted from an experimentally measurable quantity, the circular dichroism in HHG. This quantity is obtained by comparing the response of the system under different laser polarizations. For instance, the Chern number has been computed in the prototype Chern insulators, such as the Haldane model.

In this work, we extend these ideas to two-dimensional correlated van der Waals multilayer systems. Specifically, we exploit the nonlinear coupling between material chirality and laser polarization to test the use of circular dichroism in HHG as a measure of the Chern number in these materials.

Speaker: Jessica Almeida (University of Stuttgart)

23 Magnetism of the distorted kagome material clinoatacamite Cu$_2$Cl(OH)$_3$

Structurally, the mineral clinoatacamite Cu$_2$Cl(OH)$_3$ [1] is closely related to the kagome material herbertsmithite ZnCu$_3$Cl$_2$(OH)$_6$ [2,3]. In clinoatacamite, however, the kagome motif of Cu sites is embedded into a low-symmetry crystal structure [4], which causes a distortion of the kagome plane as well as different site symmetries for the kagome Cu sites. Unlike the quantum spin liquid candidate herbertsmithite, clinoatacamite undergoes a magnetic transition at 18.1 K [1]. Its magnetic ground states have remained inconclusive to date. Here, we will revisit the magnetic properties using single-crystalline material. We will first demonstrate using density-functional theory that the dominant magnetic exchange couplings in this material form non-uniform antiferromagnetic kagome layers of Cu sites with weak ferromagnetic couplings to the interlayer Cu site [5]. Further, we have characterized clinoatacamite by means of thermodynamic measurement techniques, muon spin rotation/relaxation as well as neutron diffraction. We reveal a zero-field magnetic phase diagram which is far more complex than previously anticipated [5]. We discuss our data within a scenario of competing order parameters on the inequivalent Cu sites.

[1] X. G. Zheng et al., Phys. Rev. Lett. 95, 057201 (2005).
[2] P. Mendels et al., Phys. Rev. Lett. 98, 077204 (2007).
[3] P. Khuntia et al., Nat. Phys. 16, 469 (2020).
[4] J. D. Grice et al., Can. Mineral. 34, 73 (1996).
[5] L. Heinze, C. Kastner et al., in preparation.

Speaker: Leonie Heinze (Forschungszentrum Jülich GmbH, JCNS at MLZ)

24 Quadrupole response of spin-3/2 impurities in the gapless Kitaev spin liquid

The Kitaev spin liquid is remarkable not only for its exact solvability via the Majorana representation but also for the presence of local conserved quantities known as plaquette operators. When magnetic impurities are introduced, they can couple to the neighboring spin-1/2 moments while respecting these conserved quantities. Notably, numerical studies have revealed that a spin-3/2 impurity is distinguished by its robust binding of a π-flux in the ground state of the system [1]. The stabilization of π-fluxes in the ground state is essential, as it enables the realization of Ising anyons once time-reversal symmetry is broken.
In this work, we employ the SO(6) Majorana representation of spin-3/2 operators in the Kitaev spin liquid [2] and perform a mean-field analysis of a locally interacting Majorana Hamiltonian that describes the time-reversal-symmetric Kitaev spin liquid with dilute spin-3/2 impurities. We demonstrate that the π-fluxes trapped at impurities, being charge-neutral and spinless, can be detected through their quadrupole response. We further discuss the origin of their remarkable stability in light of Lieb’s flux-phase conjecture [3].
[1] MOT, W.-H. Kao, S. Fujimoto, and N. B. Perkins, npj Quantum Materials 10, 14 (2025)
[2] H.-K. Jin, W. M. H. Natori, F. Pollmann, and J. Knolle, Nature Communications 13, 3813 (2022).
[3] E. H. Lieb, Phys. Rev. Lett. 73, 2158 (1994).

Speaker: Masahiro O. Takahashi (RIKEN Center for Computational Science (R-CCS), Kobe, Hyogo, Japan)

25 RESEDA - a spin - echo spectrometer for quantum matter

The MIEZE (Modulation of Intensity with Zero Effort) technique is a high-resolution spin-echo time-of-flight technique, for which all spin manipulation is carried out upstream of the sample, in contrast to classical neutron spin-echo spectroscopy [1]. This technique is therefore robust against depolarizing conditions at the sample position, therefore permitting us to investigate magnetic or strongly incoherently scattering samples, making it ideal for the study of strongly correlated electron systems.
Perhaps most intriguingly, we have recently demonstrated that the spin and energy degrees of freedom of neutrons traversing RESEDA are entangled, opening up new avenues for probing quantum matter [2].

[1] C. Franz et al., J. Phys. Soc. Jpn. 88, 081002 (2019)
[2] J. C. Leiner et al., Phys. Rev. Applied 22, L031005 (2024)

Speaker: Johanna Jochum

26 Resolving the Magnetic Ground State and Field-Induced Transitions in magnetic Dirac semimetal candidate EuMnSb₂

Magnetic topological semimetals have attracted much research interest due to the possibility to control topological states via their coupling with magnetism and applied magnetic fields. A case of particular interest is EuMnSb₂ due to its rich metamagnetic phases. A precise determination of its intrinsic magnetic structure and interactions is essential to understand the coupling between the electronic topology and magnetism. However, the magnetic structure of EuMnSb₂ is still under debate with inconsistency across different neutron diffraction studies [1-4]. A possible cause of such discrepancies is due to the high neutron absorption cross section of Eu nuclei which severely compromises the accuracy of conventional diffraction measurements. Furthermore, bulk measurements on flux-grown crystals synthesized with Sb flux give different results from Sn-flux crystals [1-6]. In this work, we have performed Spherical Neutron Polarimetry (SNP) to overcome the self-absorption problem and determine the zero-field magnetic structure of a partially untwinned Sn-flux single crystal of EuMnSb₂.. Surprisingly, SNP also reveals a pronounced imbalance between magnetic domains, an effect not reported previously. Furthermore, we track the evolution of several magnetic Bragg peaks under applied magnetic field, providing direct insight into the nature of the metamagnetic transitions. These results demonstrate the capability of SNP to probe strongly absorbing materials and clarify the intrinsic magnetism of EuMnSb₂, offering a robust basis for understanding its coupled magnetic–topological properties. More broadly, they provide key insights into the coupling between magnetism and Dirac-like electronic states in these types of manganese pnictide topological semimetals.

[1] Q. Zhang et al., NPG Asia Materials, 14, 22 (2022)
[2] D. Gong et al., Phys. Rev. B 101, 224422 (2022)
[3] J.M. Wilde et al., Phys. Rev. B 106, 024420 (2022)
[4] J.-R. Soh et al., Phys. Rev. B 100, 174406 (2019)
[5] Sun et al., npj Quantum Materials 6, 94 (2021)
[6] Yi et al., Phys. Rev. B 96, 205103 (2017)

Speaker: Yiu Fung Chiu (University of Oxford)

27 The effect of reductive annealing on spin fluctuations in Nd_(1.85)Ce_(0.15)CuO_(4−δ)

The insulating cuprate Nd2−xCexCuO4−δ (NCCO) becomes superconducting upon reductive post-synthesis annealing. However, the exact nature of the effect of reductive annealing remains a matter of debate. For example, adding additional electrons, by increasing the Ce content, does not produce superconductivity in non-annealed samples. To elucidate the annealing effect, we study the effect of annealing on the magnetic fluctuations. While some other neutron spectroscopy studies have focused on annealed samples, the as-grown samples yet remain unreported.

In this study, we have grown a large single crystal NCCO, split it in two, and annealed one of the pieces. Growing one crystal and then splitting it allows us to study only the effect of annealing and not other factors such as composition. We measured our data on the thermal triple axis spectrometers TAIPAN (ANSTO) and the IN20 (ILL). We studied the spin wave spectrum in the annealed and as-grown samples below and above Tc (26 K). The data shows a large spin pseudogap in the as-grown sample below 26 K, with a smaller spin pseudo-gap in the superconducting sample.

The closing of the spin pseudogap by annealing suggests a connection between the size of the spin pseudogap and the observed superconductivity. Here we hypothesise that the spin pseudogap present in as-grown NCCO can be explained by disorder present in the system. By reductively annealing and therefore providing more pristine CuO2 sheets, we significantly reduce the size of the spin pseudogap, allowing for spin pairing. As a result, we argue that the low-energy spin fluctuations are a linked to superconductivity in electron-doped cuprates, and reductive annealing is necessary to obtain these in the NCCO system. Our study helps to elucidate the interplay between magnetism, superconductivity, and crystal structure in the intriguing NCCO system.

Speaker: Kristine Krighaar (Nanoscience Center, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark)

28 Transient localization by fractionalization

We report disorder-free localization of fractionalized fermions over intermediate timescales, where the quantum fluctuation of visons induced by an external field (i) localizes the Majorana fermions without quench disorder nor extrinsic dynamical disorder and (ii) simultaneously closes the fermion gap. Compelling evidence of its localization is provided by the negligible spreading of energy after a local quench on its ground state; a highly confined lightcone of correlation function; and a vanishing energy current response despite the gapless energy spectrum. The gapless spectrum is confirmed by the numerically obtained gapless dynamical spin spectral function and energy-density fluctuation. We formulate an effective theory with quantum coherent vison disorder for the transient disorder-free localization, giving close agreement to the observed slow transport and gapless spin spectrum. It demonstrates that the disorder-free localization can also occur near equilibrium at low energy, and offer an explanation to the thermal paradox in recent experiments where a linear specific heat coexists with vanishing thermal transport in frustrated Mott insulators with neutral Fermi surfaces.

Speaker: Shi Feng (Technical University of Munich)

29 Two characteristic contributions to the superconducting state of 2H-NbSe2

Multiband superconductivity emerges when multiple electronic bands contribute to the formation of the
superconducting state, allowing for distinct pairing mechanisms and complex gap structures, leading to rich physics
that extends beyond single-band superconductivity. The layered superconductor 2H-NbSe₂, known for its
multiband characteristics, provides a compelling platform to explore these phenomena. This study aims to resolve
the nature of superconductivity in 2H-NbSe₂ by employing small-angle neutron scattering (SANS) to investigate the
field- and temperature-dependent vortex lattice structure in 2H-NbSe₂.
Using SANS, we measured the form factor of the magnetic field in the vortex lattice, gaining microscopic insight
into the bulk superconducting state. Field- and temperature dependent data reveal a significant degree of
interband coupling, with clear evidence of two distinct superconducting bands. At low temperatures and fields, the
two gaps are 13.1 and 6.5 K (Δ0 = 1.88 and 0.94 kBTc); with the larger gap band contributing approximately twothirds
of the superfluid density. Notably, the vortex lattice signal from one band is suppressed at fields well below
Bc2, underscoring the distinct roles of the two bands. The zero-field and zero-temperature penetration depth is
extrapolated to be 160 nm [1].
These findings not only provide critical insights into the multiband nature of 2H-NbSe₂ but also highlight the
intricate role of interband coupling in shaping its vortex lattice properties and superconducting behavior. This study
contributes to the broader debate on multiband superconductivity, offering quantitative evidence to refine
theoretical models.

REFERENCES
[1] A. Alshemi et al., Physical Review Letters, 134(11), 116001 (2025).

Speaker: Ahmed Alshemi (Lund University)

30 Using Positron Annihilation Spectroscopy for Defect Chraracterization in Semiconductor Devices

The rapid progress of artificial intelligence, quantum computing, and other advanced technologies for data processing and sensing has sharply increased the demand for electronic devices that combine high efficiency with superior performance. Achieving these improvements is strongly tied to the scaling down of transistors [1]. This trend in miniaturization has driven the evolution of device architectures beyond conventional silicon-based MOSFETs, leading to the development of alternatives such as thin-film transistors (TFTs) [2], two-dimensional structures [3], and three-dimensional designs like gate-all-around FETs (GAAFETs) [4]. To realize these new device generations, novel material systems are often employed, including transition metal dichalcogenides as well as high-k oxides such as HfO2 and Gd2O5 [5]. Each novel material system inevitably introduces characteristic intrinsic defects as well as interface-specific defect states. An understanding of their origin and properties is critical, since they have a direct impact on device performance and reliability. For the characterization of such material systems, depth-resolved Positron Annihilation Lifetime Spectroscopy (PALS) offers a unique, non-destructive method capable of detecting both the type and size of defects characteristic of a given material. In this study, positron lifetime analysis is combined with the application of external electric fields to manipulate positrons before annihilation, thereby enabling a more detailed understanding of defect structures within Metal-Oxide-Silicon (MOS) capacitors [6].

[1]: K. S. Kim et al., “The future of two-dimensional semiconductors beyond Moore’s law”, Nat. Nanotechnol. 19, 895–906 (2024).
[2]: C. W. A. Yan et al., “Thin-film transistors for integrated circuits: fundamentals and recent process”, Adv. Funct. Mater. 34, 2304409 (2024).
[3]: S. Wang et al., “Two-dimensional devices and integration towards the silicon lines”, Nat. Mat. 21, 1225–1239 (2022).
[4]: S. Yuvaraja et al., “Three-dimensional integrated metal-oxide transistors”, Nat. Electron. 7, 768–776 (2024).
[5]: C. S. Lau et al., “Dielectrics for two-dimensional transition-metal dichalcogenide applications”, ASC Nano 17, 9870–9905 (2023).
[6]: R. Helm et al., “Defect studies in thin-film SiO2 of a metal-oxide-silicon capacitor using drift-assisted positron-annihilation lifetime spectroscopy”, Nanometerials 15, 1142 (2025)

Speaker: Ricardo Helm (Universität der Bundeswehr München)

Wednesday 29 October

Day 3 Talk Session A

31 To be announced

Speaker: Simon Trebst

32 From spin chains to corner-sharing triangles in the compound family ACuTe$_2$O$_6$

Frustration in three-dimensional lattices is most well-studied on the pyrochlore lattice which consists of corner-sharing tetrahedra. Lattices of corner-sharing triangles also have the potential for strong geometrical frustration. An interesting case is the distorted windmill lattice found in materials with the space group P4$_1$32 where the magnetic ions lie on the $12d$ Wyckoff site which is characterized by positional parameter $y$. Here the magnetic ions form a hyperkagome lattice where each magnetic ion participates in two equilateral triangles. In addition each magnetic ion also participates in another inequivalent triangle so that there are in fact three equilateral triangles at each site with one being different from the other two. The relative sizes and interaction strengths of these triangles can be controlled by $y$. This lattice is realized in the compound family ACuTe$_2$O$_6$ A=Sr, Ba, Pb, Ca, where the Cu ions have quantum spin-$\frac{1}{2}$. Here, in addition to the triangular interactions, there are further neighbor interactions that gives rise to spin chains. In this talk the properties of this family will be discussed. They range from spin chains as found in SrCuTe$_2$O$_6$ and BaCuTe$_2$O$_6$ with different types of frustrated interchain interactions due to the weaker triangular interactions, to the spin liquid candidate PbCuTe$_2$O$_6$ where the triangular interactions are dominant and magnetic order is suppressed due to frustration.

Speaker: Prof. Bella Lake (Helmholtz Zentrum Berlin, Technical University Berlin)

Day 3: Coffee Break

Day 3 Talk Session B

33 To be announced

Speaker: Achim Rosch

34 Two-dimensional nonlinear dynamic response of quantum magnets

Two-dimensional nonlinear (2DNL) coherent optical spectroscopy is of great interest in order to deconvolute excitation continua in correlated magnets, potentially allowing to analyze individual quasiparticles, including those of fractionalized magnets. We discuss the relevant response functions for the coupling of spin systems to electric fields and analyze the 2DNL dynamical susceptibilities for several types of quantum magnets, including quantum spin-liquids (QSL) [1,2], incommensurate spiral magnets (ISM) [3], as well as spin-chain systems [4]. For the QSL, we consider the Kitaev magnet, which hosts a quantum spin-liquid, featuring fractionalization in terms of mobile Majorana fermions and static flux-visons. We show that the 2DNL response does not only probe characteristic features of both fractional excitations, but also allows to extract quasiparticle lifetimes from its multi-particle continua. These properties will be discussed over a wide range of temperatures. For the case of 2DNL response from an ISM, we chose the J$_1$-J$_3$ spin-model on the square lattice. Here, some features of the 2DNL spectra are found to be remarkably similar to those of the QSL case. Finally we analyze the case of dimerized spin-chains, where, going beyond a bare quasiparticle approach, we will also comment on the impact of final-state interactions.

[1] Olesia Krupnitska, Wolfram Brenig, Phys. Rev. B 108, 075120 (2023).
[2] Wolfram Brenig, Olesia Krupnitska, J. Phys.: Condens. Matter 36 505806 (2024).
[3] Wolfram Brenig, arXiv:2504.07177, Phys. Rev. B accepted
[4] Wolfram Brenig, arXiv:2507.17823

Speaker: Wolfram Brenig (Institute for Theoretical Physics, Technical University Braunschweig, D-38106 Braunschweig, Germany)

Day 3: Lunch

Day 3: Free afternoon

Thursday 30 October

Day 4 Talk Session A

35 Topology and Chirality

Topology and Chirality

Claudia Felser

Max Planck Institute Chemical Physics of Solids, Dresden, Germany
(e-mail: felser@cpfs.mpg.de)

Topology, a well-established concept in mathematics, has nowadays become essential to describe condensed matter. At its core are chiral electron states on the bulk, surfaces and edges of the condensed matter systems, in which spin and momentum of the electrons are locked parallel or anti-parallel to each other. Magnetic and non-magnetic Weyl semimetals, for example, exhibit chiral bulk states that have enabled the realization of predictions from high energy and astrophysics involving the chiral quantum number, such as the chiral anomaly, the mixed axial-gravitational anomaly and axions. The potential for connecting chirality as a quantum number to other chiral phenomena across different areas of science, including the asymmetry of matter and antimatter and the homochirality of life, brings topological materials to the fore.
1. Topological Quantum Chemistry: Bradlyn et al., Nature 547 298, (2017), Vergniory, et al., Nature 566 480 (2019), Xu et al. Nature 586 (2020) 702,
2. New Femions: Bradlyn, et al., Science 353, aaf5037A (2016), Sanchez et al., Nature 567 (2019) 500, Schröter et al., Nature Physics 15 (2019) 759, Schröter Science 369 (2020) 179, Sessi et al, Nature Communications 11 (2020) 3507, Yao et al., Nature Communications 11 (2020) 2033, Y. Yen, et al., Nat. Physics 2024 online preprint arXiv:2311.13217
3. Catalysis: B. Yan, et al., Nature Com. 6 (2015) 10167, Guowei Li and Claudia Felser, APL 116 (2020) 070501. Qun Yang, et al., Advanced Materials 32 (2020) 1908518, G Li, et al., Angewandte Chemie 135 (2023), e202303296

Speaker: Claudia Felser

36 Emergent phases and new particles in Kitaev spin-orbital magnets

The Kitaev model provides an elegant framework for realizing quantum spin liquids, yet it is notoriously fragile since most perturbations fail to commute with the flux operators. A promising route to overcome this limitation lies in spin-orbital generalizations of the Kitaev model. In this talk, I will discuss the rich landscape of emergent phases stabilized in these systems, with a particular focus on bilayer structures[1,2,3]. I will further show that the coexistence of local and topological order enables the emergence of novel fractional excitations, including fractionalized Goldstone modes[4].

[1] A. Vijayvargia, E. Day-Roberts, A. Botana, O. Erten arXiv:2503.09705.
[2] M. Keskiner, M. Oktel, N. Perkins, O. Erten Mat. Today Quantum, 100038 (2025)
[3] M. Akram, A. Vijayvargia, H.-Y. Kee, O. Erten arXiv: 2507.21226
[4] A. Vijayvargia, E. Nica, R. Moessner, Y.-M. Lu, O. Erten PRR 5 2, L022062 (2023)

Speaker: Onur Erten (Arizona State University)

Day 4: Coffee Break 1

Day 4 Talk Session B

37 Origin of the CDW in kagome supercondutor CsV3Sb5

We investigate the pressure dependence of the charge density wave (CDW) in the kagome superconductor CsV₃Sb₅ [1] and find that it challenges nesting-based scenarios for CDW formation. Using high-resolution inelastic x-ray scattering, we probe the lattice dynamics and uncover pronounced phonon anomalies associated with the CDW—contrary to earlier reports [2]. These results point to strong electron–phonon coupling and significant anharmonic effects. Elucidating these behaviors is essential for understanding the microscopic mechanisms that give rise to exotic quantum phases in kagome materials.
[1] F. Stier, A.-A. Haghighirad et al. PRL 133,236503 (2024)
[2] P. McGuiness, F. Henßler et al. in preparation (2025)

Speaker: Matthieu Le Tacon

38 Beyond Quantum Hall ferromagnetism: a spin singlet quantum Hall phase at nu=1

In the integer quantum Hall effect, a two-dimensional electron gas under a strong out-of-plane magnetic field yields an incompressible quantum fluid when the number of electrons is equal to the number of magnetic flux quanta. In this nu=1 integer quantum Hall state, the Coulomb interaction between electrons favors the full polarization of electronic spins, even when the Zeeman energy vanishes, which is known as quantum Hall ferromagnetism. In this work, we numerically identify a spin-singlet ground state at filling factor nu = 1, stabilized by small modifications to the electronic interaction. Remarkably, this phase remains stable under a realistic interaction potential derived from a modified gate-screened Rytova-Keldysh model. The spin-singlet phase exhibits real-space charge oscillations, which are similar to charge density wave quantum Hall phases in higher Laudau Levels. These results may be especially relevant for the Chern insulators found in Moiré materials with topological flat bands, where the absence of a magnetic field makes the emergence of a spin singlet more likely.

Speaker: Cecile Repellin (LPMMC, CNRS, Grenoble)

Day 4: Lunch

Day 4 Talk Session C

39 Entanglement Randomness

The quantum spin liquid (QSL) is a state manifesting extraordinary many-body entanglement, and the material NaYbSe2 is thought to be one of the most promising candidates for its realization. Through low-temperature heat capacity and thermal conductivity measurements we identify an apparent contradiction familiar to many QSL candidates: while entropy is stored by apparently gapless excitations, the itinerant carriers of entropy are gapped. By studying the compositional series NaYbxLu1 − xSe2 across a percolation transition of the magnetic lattice, we suggest that this contradiction can be resolved by the presence of entanglement scales of random sizes. Moreover, as we truncate the scale of entanglement by magnetic dilution, we show that the itinerant magnetic entropy carrier in NaYbSe2 is not the result of long-range entanglement but rather depends on the propagation of the simplest entangled object of all—the spin dimer.

Speaker: James Analytis (University of california Berkeley)

40 Multipolar chiral phases

Understanding the intimate nature of unconventional electronic and magnetic orders and providing robust means for detecting them are key objectives in the field of condensed-matter physics. For instance, identifying non-standard types of magnetism with chiral electronic ordering has proven to be quite challenging. Here, we introduce and discuss symmetry-broken chiral phases which are based on nonstandard orbital and spin-orbital quadrupole currents [1]. We discuss the magnetic character of these phases and their properties [1,2]. Then, we consider which are the key signatures for their detection when considering circularly polarized, spin-selective, angular-resolved photoelectron spectroscopy. Focusing on the well-known quantum material Sr₂RuO₄, we uncover spectroscopic hallmarks that align with the presence of spin-orbital chiral currents at the material's surface [1]. Hence, we present another remarkable phenomenon associated with multipolar loop currents: spin handedness-selective signals that surpass conventional dichroic, spin, and spin-dichroic responses [3]. We observe this phenomenon in the kagome metal CsTi₃Bi₅ and we call it the anomalous spin-optical helical effect. This effect arises from the interplay of light' s helicity with spin-orbital electron correlations, providing a groundbreaking method to visualize multipolar loop currents in quantum materials. These findings not only enrich the debate surrounding loop currents but also paves the way for new strategies to exploit the electronic phases of quantum materials via light-matter interaction.

[1] F. Mazzola, W. Brzezicki, M. T. Mercaldo,…, M. Cuoco, A. Vecchione, “Signatures of a surface spin-orbital chiral metal”, Nature 626, 752–758 (2024).
[2] R. Fittipaldi, R. Hartmann, M. T. Mercaldo, ..., M. Cuoco, Z. Salman, A. Vecchione, A. Di Bernardo, "Unveiling unconventional magnetism at the surface of Sr₂RuO₄", Nature Communications 12, 5792 (2021).
[3] F. Mazzola, W. Brzezicki, C. Bigi, …, C. Ortix, M. Cuoco, “Anomalous spin-optical helical effect in Ti-based kagome metal”, arXiv:2502.19589 (2025).

We acknowledge support by Italian Ministry of University and Research (MUR) PRIN 2022 under the Grant No. 2022LP5K7 (BEAT) and from PNRR MUR Project No. PE0000023-NQSTI.

Speaker: Mario Cuoco (CNR SPIN)

Day 4

Day 4 Talk Session D

41 To be announced

Speaker: Andrew Boothroyd

42 Revealing electron-electron interactions in graphene at room temperature with the quantum twisting microscope

The Quantum Twisting Microscope (QTM) is a groundbreaking instrument that enables energy- and momentum-resolved measurements of quantum phases via tunneling spectroscopy across twistable van der Waals heterostructures. In this work, we significantly enhance the QTMs resolution and extend its measurement capabilities to higher energies and twist angles by incorporating hexagonal boron nitride (hBN) as a tunneling dielectric. This advancement unveils previously inaccessible signatures of the dispersion in the tunneling between two monolayer graphene (MLG) sheets, features consistent with a logarithmic correction to the linear Dirac dispersion arising from electron-electron (e-e) interactions with a fine-structure constant of alpha = 0.32. Remarkably, we find that this effect, for the first time, can be resolved even at room temperature, where these corrections are extremely faint. Our results underscore the exceptional resolution of the QTM, which, through interferometric interlayer tunneling, can amplify even subtle modifications to the electronic band structure of two-dimensional materials. Our findings reveal that strong e-e interactions persist even in symmetric, nonordered graphene states and emphasize the QTMs unique ability to probe spectral functions and their excitations of strongly correlated ground states across a broad range of twisted and untwisted systems.

Speaker: Dmitry Efetov (Ludwig-Maximilians-Universität München)

Day 4: Coffee Break 3

Day 4: Panel Discussion at IAS Faculty Club

Friday 31 October

Day 5: Optional "Weißwurst-Frühstück"

Day 5 Talk Session A

43 Spinon band flattening by its emergent gauge field

Speaker: Roderich Moessner

44 Critical spin fluctuations across the superconducting dome in a cuprate superconductor

Overdoped cuprate superconductors are strange metals above their superconducting transition temperature. In such materials, the electrical resistivity has a strong linear dependence on temperature (T) and electrical current is not carried by electron quasiparticles as in conventional metals. Here we demonstrate that the strange metal behaviour co-exists with strongly temperature-dependent critical spin fluctuations showing dynamical scaling across the cuprate phase diagram. Our neutron scattering observations and the strange metal behaviour are consistent with a spin density wave quantum phase transition in a metal with spatial disorder in the tuning parameter. Numerical computations on a Yukawa-Sachdev-Ye-Kitaev model yield an extended "Griffiths phase" with scaling properties in agreement with observations, establishing that low-energy spin excitations and spatial disorder are central to the strange metal behaviour.

Speaker: Stephen Hayden (University of Bristol)

Day 5: Coffee Break

Day 5 Talk Session B

45 Raman scattering spectroscopy as a probe of the magnetic Weyl semimetal state.

In magnetic Weyl semimetals the intrinsic anomalous Hall effect arises from non-zero Berry curvature near the Weyl nodes. However, to clearly establish Weyl semimetal evidence of the topological electronic structure and a particular form of a time reversal symmetry breaking order is needed.

In this presentation, I demonstrate how Raman scattering spectroscopy can be used to probe low-frequency electronic structure and magnetism in Weyl semimetals using examples of Nd2Ir2O7 and V1/3NbS2. Frequency dependence of electronic Raman scattering within Weyl cones is proportional do the density of states and depends on dimensionality, thus reveling the linear q-dependence of the density of states down to low frequencies unaccessible for the most part with other optical techniques. Simultaneously probing magnetic Raman scattering identifies a magnetic phase transition. In this way, polarization-dependent electronic and magnetic Raman scattering demonstrates the presence of the band structure and form of time reversal symmetry breaking necessary to realize a Weyl semimetal state.

Speaker: Natalia Drichko (Department of Physics and Astronomy, Johns Hopkins University)

46 Frustration & disorder: spin liquids, spn glasses, and beyond

Frustration & disorder: spin liquids, spn glasses, and beyond

Frustrated magnets are particularly susceptible to quenched disorder, leading to a plethora of non-trivial and intertwined phenomena. Here we highlight recent developments and insights. Part (i) deals with triangular-lattice magnets where disorder efficiently destroys antiferromagnetic long-range order and can lead to both spin-glass and valence-bond-glass phases, effectively mimicking spin-liquid behavior. Part (ii) is devoted to spiral spin liquids where disorder generically induces a spiral spin glass. We discuss how isolated impurities lead to long-ranged spin textures which can be used to diagnose the parent state, and we discuss neutron-scattering signatures of relevant disorder phenomena.

Speaker: Matthias Vojta (Technische Universität Dresden)

Day 5: Lunch