Condensed Matter Physics in the Alps: Geometric Frustration, Topology, Flat Bands, and Correlation in Kagome and Van der Waals Systems

4 Jan 2026, 14:00 → 9 Jan 2026, 12:30 Europe/Zurich

Saas Fee

Sunday 4 January

Arrival

Dinner

Monday 5 January

Opening remarks

Monday Morning Session I, Chair G. Aeppli

1 Kagome spin ice and strange metal phases

In the talk, I will focus on two members of the ZrNiAl structure type with twisted magnetic kagome layers. HoAgGe represents the first crystalline realization of kagome spin ice, evidenced by neutron scattering, and displays striking fractionalized plateau states in magnetic and transport experiments [1-3].
CrNiAs is a kagome metal in which the ferromagnetic order of Cr moments can be continuously suppressed under hydrostatic pressure. Remarkably a broad pressure range over which strange-metal behavior persists beyond the critical pressure is found contrasting sharply with conventional quantum criticality [4].

[1] K. Zhao, H. Deng, H. Chen, K.A. Ross, V. Petricek, G. Günther, M. Russina, V. Hutanu, P. Gegenwart: Realization of the kagome spin ice state in a frustrated intermetallic compound, Science 367, 1218 (2020).
[2] K. Zhao, Y. Tokiwa, H. Chen, P. Gegenwart: Time-reversal-like degeneracies distinguished by the anomalous Hall effect in a metallic kagome ice compound, Nat. Phys. 20, 442 (2024).
[3] Kan Zhao, Hao Deng, Hua Chen, Nvsen Ma, Noah Oefele, Jiesen Guo, Xueling Cui, Chen Tang, Matthias J. Gutmann, Thomas Mueller, Yixi Su, Vladimir Hutanu, Changqing Jin, Philipp Gegenwart, Nonlinear time-reversal symmetry breaking in kagome spin ice HoAgGe, arXiv:2505.22544.
[4] Bin Shen, Feng Du, Franziska Breitner, Victoria A. Ginga, Ece Uykur, Alexander A. Tsirlin, Philipp Gegenwart, Pressure-induced strange metal phase in a metallic kagome ferromagnet, arXiv:2503.09524.

Speaker: Philipp Gegenwart (University of Augsburg)

2 Magnetism of the RT6Sn6 kagome metals

Kagome metals are known for their unique electronic band structure containing flat bands and Dirac cones with topological character. This has elevated interest in kagome metals as an adaptable system to study the interplay of band topology with superconductivity, itinerant magnetism, and other charge instabilities that are driven by electronic correlations. In the RT6Sn6 kagome metals, conduction electrons within T=Cr, V, Mn kagome bilayers interacts with the local magnetic moments of interleaved rare-earth (R) triangular layers. Here, I will describe experimental neutron scattering and high-field magnetization data outlining the competing magnetic interactions and magnetic fluctuations that lead to a variety of collinear and non-collinear magnetic phases, including the discovery of dual time-reversal symmetry-breaking in the Ising ferromagnet TbV6Sn6. The manifestations of chirality in RMn6Sn6, such as fluctuation-driven scalar spin chirality, will also be discussed.

Speaker: Rob McQueeney (Iowa State University and Ames Laboratory)

10:00 Coffee Break

Monday Morning Session II, Chair C. Broholm

3 Magnetism on the Shastry-Sutherland lattice

The Shastry-Sutherland lattice, also known as the orthogonal dimer lattice, is a paradigmatic geometry of frustrated magnetism both for classical and quantum spins. In this talk, I will report on extensive tensor network simulations of that model that have allowed us to understand the remarkably rich phase diagram of the spin-1/2 Shastry-Sutherland SrCu2(BO3)2 under pressure and magnetic field, and the intriguing magnetization plateaus of Er2Be2GeO7, a recent rare-earth based realization of the Ising model on the Shastry-Sutherland lattice.

Speaker: Frédéric Mila (Ecole Polytechnique Fédérale de Lausanne)

4 Dirac-like spinons and emergent symmetry in s=1/2 kagome antiferromagnets

Kagome antiferromagnets have garnered significant attention in condensed matter physics owing to their potential to host a diverse range of quantum spin liquids (QSLs) and fractional magnetization plateaus. In particular, the S=1/2 kagome antiferromagnet has been regarded as a holy grail for realizing QSLs, with candidates ranging from gapped Z2 to U(1) Dirac QSLs. Theoretical studies further predict that kagome lattices exhibit fractional magnetization plateaus, which involve both entangled and unentangled spin states. These intriguing phenomena highlight the need for experimental validations to elucidate the underlying quantum phases.
In this presentation, I will share our group's recent work on the ground-state and field-induced phases in s=1/2 kagome antiferromagnets. In the first part, I will present thermodynamic and spectroscopic signatures of disordered Dirac spinons in YCu3(OH)6+xBr3-x (x~0.5), which forms a nearly perfect kagome lattice with an exchange interaction of J~60 K.
In the second part, I will highlight our observation of the sought-after 1/9, 1/3, and 5/9 plateaus under magnetic fields up to 140 T. Notably, the nature of the m=1/9 plateau remains a topic of debate, with possibilities including a topological Z3 spin liquid or a valence bond crystal. Remarkably, the angular dependence of our thermodynamic and NMR data reveals the emergence of C3-symmetric, gapless excitations and a field-induced Lifshitz transition of the spinon Fermi surface. These findings suggest that the 1/9 plateau phase originates from a topological reorganization between Dirac-like and other gapless spinon bands.

Speaker: Kwang-Yong Choi (Sungkyunkwan Unversity)

11:45 Midday Break

Monday Afternoon Session, Chair D. Mihailovic

5 The puzzle of metallic and insulating phases at the surface of 1T-TaSe2

In the layered transition-metal dichalcogenide 1T-TaSe$_2$ the formation of a star-of-David charge density wave results in a half-filled band at the Fermi surface that is sensitive to electron correlations. Interestingly, while the bulk material remains a metal, the surface displays a mix of different phases, ranging from insulating to metallic, all with the same in-plane charge ordering.
We used microfocus ARPES to investigate the quasiparticle dispersion in these different spatial domains. Insulating areas show characteristics of a band insulator, while metallic regions exhibit a chiral Fermi surface. Additionally, within the metallic phase, we found a series of bands varying in number and energy position. A direct comparison to DMFT calculations considering slabs of different thickness allows us to reconcile this puzzle as the combined effect of stacking faults between the layers and quantum confinement.

Speaker: Michael Straub (University of Geneva)

6 Interlayer stacking controls the electronic properties of the van der Waals material 1T-TaS2

Given the small binding between layers, stacking of van der Waals materials is a powerful tool for exploring the physics of quantum condensed matter. However, exploitation for engineering will require taking advantage of thicker defective stacks, or an ability to control the stacking order via external stimuli, such as electrical or optical pulses.

I will present X-ray diffraction data of the equilibrium [1] and non-equilibrium [2] charge-density wave phases of the model material 1$T$-TaS$_2$, promising application as a highly efficient cryogenic phase-change memory platform. Comparison to a computational framework based on recursive Hendricks-Teller calculations and Monte Carlo simulations [3] reveals that layer stacking order and faults underly the rich electronic phase diagram of 1$T$-TaS$_2$. The experiments also identify charge rearrangement and concomitant lattice strain as the drivers of the metastable hidden phase transition. More generally, the results underscore the importance of domain sizes and layer stacking in defining electronic behaviors of van der Waals materials.

[1] C. Burri, H. G. Bell, F. Dizdarević, W. Hu, J. Ravnik, J. Vonka, Y. Ekinci, S.-W. Huang, S. Gerber & N. Hua. Three-dimensional electronic domain correlations in 1$T$-TaS$_2$. arXiv:2508.17839.

[2] C. Burri, N. Hua, D. Ferreira Sanchez, W. Hu, H. G. Bell, R. Venturini, S.-W. Huang, A. G. McConnell, F. Dizdarević, A. Mraz, D. Svetin, B. Lipovšek, M. Topič, D. Kazazis, G. Aeppli, D. Grolimund, Y. Ekinci, D. Mihailović & S. Gerber. Imaging of electrically controlled van der Waals layer stacking in 1$T$-TaS$_2$. arXiv:2411.04830 (Nat. Commun., accepted).

[3] N. Hua, F. Petocchi, H. G. Bell, G. Aeppli, P. Werner & S. Gerber. Interlayer stacking controls the electronic properties of the van der Waals material 1$T$-TaS$_2$. arXiv:2503.24124 (Phys. Rev. Research, accepted).

Speaker: Simon Gerber (PSI - Paul Scherrer Institut)

7 Magnetic order and interactions in two polytypes of V1/3NbS2

Magnetically intercalated transition metal dichalcogenides (TMDs) provide a versatile three-dimensional (3D) material platform to explore quantum phenomena and functionalities that emerge from an intricate interplay between magnetism, band structure, and correlations.
I shall describe the observation of a nearly magnetization-free anomalous Hall effect (AHE) in V1/3NbS2. Our single-crystal neutron diffraction measurements identify a commensurate, collinear AFM order formed by intercalated V moments. In the magnetically ordered state, the spontaneous AHE is tenfold greater than expected from empirical scaling with magnetization [1].
Through X-ray and magnetic neutron diffraction evidence is provide for two structural polytypes. Both have A type AFM order but different easy axes, which profoundly impacts their anomalous Hall response. Using inelastic magnetic scattering we detect coherent spin wave excitations from both polytypes. By comparing the experimental spectra to those calculate through spin wave theory, we infer oscillatory three-dimensional RKKY type interactions and relate these to the underlying electronic band structure [2].
* This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0024469 and by the Gordon and Betty Moore foundation through the EPIQS program GBMF9456.
[1] “Zero-field Hall effect emerging from a non-Fermi liquid in the collinear antiferromagnet V1/3NbS2,” Mayukh Kumar Ray Mingxuan Fu, Youzhe Chen, Taishi Chen, Takuya Nomoto, Shiro Sakai , Motoharu Kitatani, Motoaki Hirayama, Shusaku Imajo, Takahiro Tomita, Akito Sakai, Daisuke Nishio-Hamane, Gregory T. McCandless, MichiTo Suzuki, Zhijun Xu, Yang Zhao, Tom Fennell, Yoshimitsu Kohama, Julia Y. Chan, Ryotaro Arita, Collin Broholm, Satoru Nakatsuji, Nature Communications 16, 3532 (2025).
[2] “Stacking-dependent anisotropic altermagnetism in V1/3NbS2,” Chris J. Lygouras, Nathan Prouse, Jack H. Drouin, Youzhe Chen, Laura Garcia-Gassull, Aleksandar Razpopov, Zili Feng, Mingxuan Fu, Lü Fang, Alexander I. Kolesnikov, Christina Hoffman, Yiqing Hao, Huibo Cao, Maxime A. Siegler, Robert J. Birgeneau, Roser Valentí, Satoru Nakatsuji, and C. Broholm, unpublished (2025).

Speaker: Collin Broholm (Johns Hopkins University)

Dinner

Tuesday 6 January

Tuesday Morning Session I, Chair E. Demler

8 Interfacial electron-phonon coupling in 2D materials

Hexagonal boron nitride (hBN), a polar wide gap insulator, is the gate dielectric of choice in the field of 2D materials. However, surprisingly little is known about how hBN affects the electronic properties of 2D materials of interest. Here, we use nano-ARPES to study the prototypical systems of monolayer transition metal dichalcogenide semiconductors on hBN. Our data show two replica bands of the semiconductor  valley at energies close to hBN phonon modes. This is the fingerprint of long-range electron-phonon interaction across the interface. Our data is well reproduced by a generic model describing the propagation of a charged particle above a polar substrate, suggesting that interfacial electron-phonon coupling is universal for 2D materials encapsulated in hBN. Consistent with this interpretation, control experiments on non-polar graphite substrates do not show the replica bands observed on hBN.

Speaker: Felix Baumberger (University of Geneva & SLS, PSI)

9 Engineering quantum materials by moiré potentials and light fields

Two-dimensional materials and heterostructures exhibit correlated phenomena, whose electronic structures are fundamental to the underlying physics. In this talk, I will present our experimental progress on engineering the electronic structure of quantum materials using moiré superlattice potential and time-periodic light fields. In particular, topological flat bands in rhombohedral graphene and the enhancement of the topological flat bands by the moiré superlattice in aligned rhombohedral graphene superlattice (material exhibiting fractional quantum anomalous Hall effect) will be discussed.

Speaker: Shuyun Zhou (Tsinghua University)

Tuesday Coffee Break and Poster

10 A New Look on an Old Problem: Mass Enhancement in Fermi Liquids

Understanding how quasiparticle renormalization influences thermodynamic properties is a central question of Fermi-liquid theory. In this work, we analyze the mass enhancement arising from the reduction of quasiparticle weight and its impact on the electronic specific heat. A straightforward microscopic evaluation of the Migdal–Galitskii expression, using a $T$-independent spectral function, seems at first to disagree with Landau’s phenomenological prediction for the Sommerfeld coefficient. We demonstrate that this apparent discrepancy is not a failure of Landau theory, but rather a consequence of missing contributions in the microscopic treatment.

By carefully incorporating the temperature dependence of the single-particle spectral function $A(k,\omega,T)$, we derive an additional sum rule that restores full consistency with Landau’s result. This sum rule links the quasiparticle renormalizations $m^*$ (includes both energy and momentum contributions), and identifies a previously overlooked contribution coming from $\dfrac{\partial^2A}{\partial T^2}$ at the Fermi level.

Our analytical insights are benchmarked using Dynamical Mean-Field Theory (DMFT) applied to the Hubbard model on the Bethe lattice. The numerical results confirm the validity of the new sum rule and highlight the key role played by the temperature dependence of the self-energy in determining the specific heat.

Speaker: Anna Efimova (Université de Genève)

11 Exotic superconducting states in altermagnets

The interplay between magnetism and superconductivity is one of the central topics of condensed matter physics, which has recently been put into new light by the discovery of altermagnets. Here, we study this interplay from a fundamental symmetry perspective using irreducible co-representations of the altermagnetic spin-point groups. We construct and tabulate all symmetry-allowed pairing functions for altermagnets, which uncovers numerous exotic pairing states. We focus on three of them, namely: (i) a non-unitary superconductor with different spatial anisotropies for the spin-up and spin-down condensates, (ii) a half-and-half metal-superconductor where only electrons with one of the two spin components form Cooper pairs, and (iii) a spin chiral superconductor with spin-polarized edge states. Interestingly, the first of these three superconductors exhibits an unusual fractional ac Josephson current for only one of the two spin polarizations. We present phenomenological Ginzburg-Landau theories for these unconventional superconductors and show that they correspond to stable minima of the free energies. We examine their topological properties, study the effects of small spin-orbit coupling, consider possible material examples, and investigate their topological responses.

Speaker: Kirill Parshukov (Max Planck Institute for Solid State Research)

12 Exploring spin reorientation and structural stability in distorted kagome metal RAgGe (R = Tb, Dy)

Distorted kagome metals have emerged as fertile platforms for investigating the interplay between geometrical frustration, magnetoelastic coupling, and exotic electronic states. Rare-earth based kagome metal RAgGe (R = Tb, Dy) crystallize in a hexagonal structure that hosts a distorted kagome network of magnetic ions, providing an intriguing platform to study field-induced spin reorientation in frustrated systems. In this work, we investigate the magnetic anisotropy and spin reorientation behavior of TbAgGe and DyAgGe using X-ray magnetic circular dichroism (XMCD) and X-ray magnetic linear dichroism (XMLD) at the rare-earth M4,5 edges. XMCD reveals strong magnetocrystalline anisotropy arising from crystal field effects and highlights the gradual evolution of 4f moment orientation under applied magnetic field. Complementary XMLD measurements provide direct evidence of exchange-driven spin arrangement and local crystal field anisotropy. The distinct dichroic responses of Tb and Dy compounds indicate element-specific reorientation mechanisms governed by their 4f–ligand hybridization strength and single-ion anisotropy. The robustness of the hexagonal crystal framework down to low temperature, evidenced by Extended X-ray absorption fine structure (EXAFS) and Raman spectroscopy, revealing that the distorted kagome geometry remains structurally resilient, with no evidence of symmetry breaking despite noticeable shifts in bond angles and interlayer spacing. These results establish spin dynamics and structural stability in distorted kagome systems and pave the way for understanding magnetic frustration and anisotropy-driven transitions in rare-earth intermetallics.

Speaker: Jayjit Kumar Dey (Deutsches Elektronen-Synchrotron DESY)

Tuesday Morning Session II, Chair A. Imamoglu

13 Revealing Correlations and Orbital Textures in Quantum Materials with photoemission spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) provides direct momentum-resolved access to correlation effects and orbital textures in quantum materials. In this talk, I will show how a combined experimental and theoretical approach allows us to extract these key electronic properties in van der Waals and Kagome-related systems.

First, I will discuss our ARPES and dynamical-mean-field-theory analysis of Nb₃Br₈ [1]. By mapping the out-of-plane dispersion, we identify a clear momentum-space fingerprint of a dimerized Mott insulator—namely a 2π/d periodicity of the valence-band maxima—demonstrating how correlations reshape the spectral function of a weakly dimerized layered material.

Second, I will present how circular-dichroism ARPES, supported by first-principles and Wannier-based photoemission simulations, reveals monopole-like orbital angular momentum (OAM) textures in chiral topological semimetals such as PtGa and PdGa [2]. The ability to reverse the monopole polarity by switching structural chirality highlights new routes for controlling orbital polarization.
Together, these results illustrate how ARPES, in synergy with advanced modeling, can uncover both correlation-driven physics and complex OAM textures, offering new opportunities for designing correlated and orbitronic quantum materials.

[1] Date et al., Momentum-resolved fingerprint of Mottness in layer-dimerized Nb3Br8 Nat Commun 16 4037 (2025)
[2] Yen et al., Controllable orbital angular momentum monopoles in chiral topological semimetals Nat. Phys. 20 1912 (2024)

Speaker: Michael Schüler (Paul Scherrer Institute)

14 Coulomb blockade in superconducting graphene

Superconductivity in graphene can arise when 2,3 or 4 layers are twisted with respect to each other at the so-called magic angle. We experimentally demonstrate Josephson Junctions and SQUIDS that behave like devices fabricated from conventional superconductors and insulators. We also observe time-resolved vortex trapping and detect quantum tunneling of vortices at low temperatures. For samples with nano-fabricated gates we can confine carriers in regimes that are superconducting (or normaler conducting) and are separated by insulating barriers to leads with are normal conducting (or superconducting). This way we build a Cooper pair box that can be tuned to a standard single electron transistors by applying magnetic fields.
This work was done in collaboration with Marta Perego, Alexandra Mestre Tora, Artem Denisov, Clara Galante, Elias Portoles and Thomas Ihn.

Speaker: Klaus Ensslin (ETH Zurich)

11:30 Midday Break

Tuesday Afternoon Session, Chair R. McQueeney

15 Terahertz-frequency magnons and chiral phonons in a kagome ferromagnetic Weyl semimetal

Kagome lattice provides a rich platform for exploring novel quantum states, emerging from the interplay between its frustrated corner-sharing triangular geometry and intriguing electronic structure. Co3Sn2S2 is a kagome lattice ferromagnet, exhibiting a unique interplay between its electronic wavefunction topology and magnetic spin configuration. This interaction results in several intriguing properties, including Weyl points, a colossal anomalous Hall effect, and a pronounced magneto-optical response.
In the first part of the talk, I will discuss our recent ultrafast study of Co3Sn2S2 [1]. To our surprise, we directly observe two magnon modes in the terahertz range in the time domain. These frequencies exceed typical ferromagnetic resonance frequencies by 1-2 orders of magnitude. These dual modes originate from the strong coupling of localized spin and orbital magnetic moments. These findings unveil an unconventional category of magnons in a ferromagnet stemming from orbital magnetic moments, and position Co3Sn2S2 as a promising candidate for high-speed terahertz spintronic applications.
In the second part, I will report the discovery of chiral phonon modes in Co3Sn2S2 [2]. Using helicity-resolved magneto-Raman spectroscopy, we observe the spontaneous splitting of the doubly degenerate in-plane Eg modes into two distinct chiral phonon modes of opposite helicity when the sample is zero-field cooled below the Curie temperature, in the absence of an external magnetic field. As we sweep the out-of-plane magnetic field, this Eg phonon splitting exhibits a well-defined hysteresis loop directly correlated with the material’s magnetization. Our findings highlight the role of the magnetic order in inducing chiral phonons, paving the way for novel methods to manipulate chiral phonons through magnetization and vice versa.
References:
[1] M. Che et al., Discovery of terahertz-frequency orbitally-coupled magnons in a kagome ferromagnet, Science Advances 11, eadw1182 (2025).
[2] M. Che et al., Magnetic order induced chiral phonons in a ferromagnetic Weyl semimetal, Physical Review Letters 134, 196906 (2025).

Speaker: Luyi Yang (Tsinghua University)

16 Unconventional magnetism in kagome magnet Co3Sn2S2

Shandite Co3Sn2S2 has been synthesized and studied extensively, but there is still a lack of consensus regarding the magnetic ground state. Since its discovery, it has been considered a ferromagnet with c-axis as its easy axis. However, recently, there has been reports of exchange bias based on magnetometry and anomalous Hall effect attributed to spin glass and presence of antiferromagnetism at magnetic domains walls. Separately, muon spin rotation has reported an antiferromagnetic phase coexisting with a ferromagnetic phase. On the other hand, neutron scattering and non-linear optics experiments in Co3Sn2S2 have not detected phase separation between antiferromagnetic and ferromagnetic phases and instead suggest a homogenous c-axis ferromagnetic phase or a canted c-axis ferromagnetic phase, respectively. Conventional local probe techniques such as Magnetic Force Microscopy (MFM) and Magneto Optic Kerr Effect (MOKE) have not detected any antiferromagnetic phase either.

In this talk, I will present our studies on the magnetism of this material, using XMCD-PEEM and MOKE to image the magnetic domains, as well as spatially resolved Angular Resolved Photoemission Spectroscopy (ARPES) combined with Density Functional Theory (DFT) calculations to probe any electronic phase other than the ferromagnetic phase (1). If time allows, I will also present the magnetic ground state and magnetic Hamiltonian that we have derived based on neutron scattering.

1. S. Ekahana et al., Inhomogeneity in electronic phase and flat band in magnetic kagome metal Co3Sn2S2. Commun. Mater. 6, (2025).

Speaker: Yona Soh

17 Neutron scattering studies of two kagome magnets

In a first part I will summarise results of an experimental study to unravel the magnetic ground state and interactions in Co$_3$Sn$_2$S$_2$, conducted with colleagues from PSI (Yona Soh) and the ILL (David Tam). By exploring the magnetic interactions to moderately high energies and using non-invasive probes to measure the magnetic ground state, we discover that the spin wave spectrum and low-energy spin gap can be completely characterized with a simple Hamiltonian dominated by one in-plane nearest-neighbor interaction, containing a Heisenberg component and a large antisymmetric Dzyaloshinskii-Moriya (DM) interaction. Our results can explain many crucial features of previous experimental reports without the need for additional phases or exotic exchange terms. Our results significantly simplify the basis of future modelling efforts in this prototypical kagome magnet.
In a second part I will introduce a new quaternary intermetallic compound discovered with colleagues from IJL (Thomas Mazet). In this material, Mn atoms form a perfect kagome lattice, but the shortest Mn-Mn distance is interlayer. I will show how this structure can lead to a peculiar hierarchy of interactions and to a new type of frustrated magnetic order. Our results suggest a general route by which frustrated interlayer couplings stabilize in-plane noncollinear order even in dominantly ferromagnetic kagome metals.

Speaker: Romain Franck Sibille (PSI - Paul Scherrer Institut)

Dinner

Wednesday 7 January

Wednesday Morning Session I, Chair A. Balatsky

18 Orbital magnetism and flat bands in magnetic field

I will discuss recent theoretical progress in understanding the role played by the electron-electron interactions in orbital magnetism, and highlight results obtained for the flat bands of the magic angle twisted bilayer graphene, as well as for some twisted transition metal dichalcogenides. I will also discuss the effect of the external out-of-plane magnetic field on the correlated ground states and their excitations, and compare the ensuing Landau level degeneracies with the experimental observations.

Speaker: Oskar Vafek (William I. Fine Theoretical Physics Institute, University of Minnesota)

19 Kinetic magnetism in triangular lattices

This talk will review kinetic magnetism in the Fermi Hubbard model in triangular lattices. Focus will be on the regime of strong interactions, $t<<U$. In such systems for densities close to $n=1$ dominant magnetic interactions arise from magnetic polaron dressing of charge carrier propagating in a spin incoherent Mott insulator. In the case of hole doping, antiferromagnetic polarons originate from kinetic frustration of individual holes in a triangular lattice. In the case electron doping, Nagaoka type ferromagnetic correlations are induced by propagating doublons. I will discuss how this mechanism explains some of the surprising magnetic phenomena observed in moire systems of transition metal dichalcogenides. I will also review recent experimental studies of this phenomena in experiments with quantum gas microscopes and Rydberg atom arrays.

Speaker: Eugene Demler (ETH Zurich)

Wednesday Coffee Break & Poster

20 Buckling of NbSe2 thin films

Thin films under strain and with low interfacial adhesion are known to buckle and delaminate from their substrates [1,2]. While buckling has previously been reported in MoS₂ films grown on x-cut SiO₂, there are few reports of this phenomenon in other TMDCs or in films only a few tens of nanometers thick. Here, we present the first observation of buckling in MBE-grown NbSe₂ thin films (10–15 nm) on c-cut SiO₂ substrates. When exposed to high humidity in the ambient air, the epitaxial NbSe₂ films tend to buckle and form quasi-periodic structures valleys and ridges with heights of 150–200 nm and an average spacing of 2.6 μm. This causes a color change from gray to blue, presumably due to collective interference effect similar to structural colors in photonic crystals. The buckled films were analyzed using AFM, XRD and SEM, and their electrical transport properties were measured before and after buckling. Because the NbSe₂ is highly strained at the top of the buckled structures, these features may offer a platform for probing local strain-dependent changes in electronic properties such as superconductivity and charge density wave formation.

[1] Wang, E., Chen, Z., Shi, R., Xiong, Z., Xin, Z., Wang, B., Guo, J., Peng, R., Wu, Y., Li, C., Ren, H., Li, X., & Liu, K. (2022). Humidity-Controlled Dynamic Engineering of Buckling Dimensionality in MoS2Thin Films. ACS Nano, 16(9), 14157–14167. https://doi.org/10.1021/acsnano.2c04203
[2] Ren, H., Xiong, Z., Wang, E., Yuan, Z., Sun, Y., Zhu, K., Wang, B., Wang, X., Ding, H., Liu, P., Zhang, L., Wu, J., Fan, S., Li, X., & Liu, K. (2019). Watching Dynamic Self-Assembly of Web Buckles in Strained MoS 2 Thin Films. ACS Nano, 13(3), 3106–3116. https://doi.org/10.1021/acsnano.8b08411

Speaker: Nicolas Brunner (University of Fribourg)

21 Charge-density-wave quantum critical point in 2H-TaSe2

Superconductivity often emerges as a dome around a quantum critical point (QCP) where long- range order is suppressed to zero temperature, e.g., in high-temperature superconductors. However, the presence of a putative charge-density-wave (CDW) QCP and its impact on emerging superconductivity received increased scientific attention only after the discovery of CDW order in copper oxides. By now, it is known that various metallic transition-metal dichalcogenides feature emergent superconductivity when CDW order is suppressed, e.g., by pressure or intercalation and researchers are trying to understand the relevance of a CDW QCP for superconductivity in these systems.
Here, I will present our study of the CDW compound 2$H$-TaSe$_2$. We used inelastic x-ray scattering (IXS) and x-ray diffraction (XRD) at ambient [1] and high pressures [2] up to 25 GPa to determine the CDW phase diagram and study its lattice dynamical properties. Our experimental results are complemented by $ab-initio$ lattice dynamical calculations based on density-functional perturbation theory. Results at ambient pressure provide first evidence for a full phonon softening at the CDW transition in 2$H$-TaSe$_2$ and reveal a novel precursor region above the CDW transition temperature $T_{CDW}$, which is characterized by an overdamped phonon mode without long range order. Our high-pressure XRD defines the critical pressure $p_c\approx 20\,\rm{GPa}$ at which CDW order is fully suppressed. IXS identifies the presence of a CDW soft phonon mode at $p_c$ which is strong evidence for a continuous nature of the CDW transition near zero temperature, and, thus, a CDW QCP. Calculations show that electron-phonon coupling in 2$H$-TaSe$_2$ is mostly carried by the CDW soft phonon mode and can rationalize the reported superconducting transition temperatures at high pressures.

[1] Shen, X., Heid, R., Hott, R., Haghighirad, A.-A., Salzmann, B., dos Reis Cantarino, M., Monney, C., Said, A. H., Frachet, M., Murphy, B., Rossnagel, K., Rosenkranz, S. & Weber, F. Precursor region with full phonon softening above the charge-density-wave phase transition in 2H-TaSe2. Nat Commun 14, 7282, doi:10.1038/s41467-023-43094-5 (2023).
[2] Tymoshenko, Y., Haghighirad, A.-A., Heid, R., Lacmann, T., Ivashko, A., Merritt, A., Shen, X., Merz, M., Garbarino, G., Paolasini, L., Bosak, A., Diekmann, F. K., Rossnagel, K., Rosenkranz, S., Said, A. H. & Weber, F. Charge-density-wave quantum critical point under pressure in 2H-TaSe2. Communications Physics 8, 352, doi:10.1038/s42005-025-02254-3 (2025).

Speaker: Frank Weber (Karlsruhe Institute of Technology)

22 Investigating magnetism in 1T-NbSe2

NbSe$_2$ is a transition metal dichalcogenide (TMDC) well known for having the highest superconducting transition temperature among all pristine TMDCs. While bulk NbSe$_2$ is only found in its 2H superconducting phase, it has been shown that 1T-NbSe$_2$ can be stabilized on a substrate through molecular beam epitaxy growth, with extremely different properties from the 2H phase. 1T-NbSe$_2$ is not superconducting, but a Mott insulator that exhibits localized magnetic moments as seen through Kondo resonances, and has recently been characterized as a spin liquid. However, no evidence of long-range magnetic order has been shown to date. In this work, we grown mixed phase 2H/1T-NbSe$_2$ thin films on YAlO$_3$ substrates using molecular beam epitaxy. We observe the onset of a sizeable magnetoresistance around 75 K, and a pronounced magnetic hysteresis at low temperatures, implying the existence of a long-range magnetic order. However, muon spin rotation experiments showed no presence of a strong magnetic transition, limiting the possible magnetic volume fraction of the film to be less than ~5 %.

Speaker: Ryan Thompson (University of Fribourg)

23 Probing the magnetic order in a few-layer CrPS₄

Understanding magnetism in two-dimensional (2D) van der Waals (vdW) magnets requires nanoscale local probes. Scanning SQUID microscopy (SSM), with ~100 nm resolution, enables direct imaging of magnetic structures in mono- and few-layer materials. Using SSM, we investigate the magnetic behavior of CrPS₄, a weakly anisotropic vdW interlayer antiferromagnet. Monolayer CrPS₄ shows no remanence and a zero coercive field, unlike thicker odd layers that exhibit 20–50 mT coercivity and nearly 100% remanence. Layer-dependent studies reveal that interlayer coupling and dimensionality govern magnetic responses. These findings provide insight into mechanisms driving long-range magnetic order in 2D.

Speaker: Katharina Kress (Department of Physics, University of Basel, Basel, Switzerland)

Wednesday Morning Session II, Chair S. Zhou

24 Optical pumping of topological states in twisted MoTe2

I will describe recent experiments where we used optical pumping techniques to manipulate the Chern number of integer and fractional Chern insulators in twisted MoTe2 homobilayer.

Speaker: Atac Imamouglu (ETH)

25 Moire materials - Twisted graphene and 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: Dima EFETOV (Ludwig-Maximilians-Universität München)

11:30 Midday Break

Wednesday Afternoon Session, Chair L. Yang

26 Soft-phonon CDW formation in Kagome metals

Kagome metals host van Hove points and flat bands, with the resulting nesting effects potentially leading to unconventional CDW states. Using inelastic X-ray scattering, we show that the CDW in KV3Sb5 forms via a continuous softening of phonons to zero energy, similar to transition metal dichalcogenides (such as NbSe2). The soft phonons exhibit a prominent in-plane anisotropy that mirrors the electron-phonon coupling (EPC) strength, suggesting that the CDW in KV3Sb5 is EPC-driven. While the bilayer Kagome metals ScV6Sn6 and LuNb6Sn6 exhibit first-order CDW transitions, soft phonons are also observed prior to their CDW transitions. However, the softest phonons occur at wavevectors distinct from the CDW ordering vector, suggesting the presence of competing ordered phases in these systems. These findings show that the EPC plays a prominent role in driving the CDW of Kagome metals, and should be considered in understanding their CDW and superconducting states.

Speaker: Yu SONG (Zhejiang University)

27 Altermagnetism and Flat band Enhanced AFM Fluctuation in Kagome CsCr3Sb5

The CsCr3Sb5 exhibits superconductivity in close proximity to a density-wave (DW) like ground state at ambient pressure, however details of the DW are still elusive. Using first-principles density-functional calculations, we found its ground state to be a 4×2 altermagnetic spin-density-wave (SDW) at ambient pressure. The magnetic long range order is coupled to the lattice, generating 4a0 structural modulation. Multiple competing SDW phases are present and energetically close, suggesting strong magnetic fluctuation at finite temperature. The kagome flat bands are closer to the Fermi level, which enhances strong antiferromagnetic spin fluctuations. Our random phase approximation analysis reveals a sublattice-momentum-coupling mechanism, where the antiferromagnetic fluctuations enhanced from unoccupied flat bands give rise to a leading S± wave and a competing (dxy, dx2-y2) - wave superconducting order.

Speaker: Chenchao Xu (Hangzhou Normal University)

28 Unusual aspects of the coherent phonon in CsV3Sb5: its implication for the loop-current and chirality

The question of time-reversal symmetry breaking in CsV3Sb5 remains an open and debated topic, with no definitive consensus established to date. Conventional magnetic field-sensitive probes have not conclusively confirmed the presence of time-reversal symmetry-breaking fields. This presentation will provide new insights into this crucial aspect through a study into the optical coherent phonon modes. The observed conventionally symmetrical modes exhibit a pronounced unconventional contrast to the helicity of the circularly polarized light. I will discuss the underlying origins of this observed phenomenon and delineate its implications concerning the potential breaking of time-reversal symmetry in this material.

Speaker: Chennan Wang (Universität Freiburg)

29 Real-time twisting on a chip

Two-dimensional materials (2DM) and their heterostructures offer tunable electrical and optical properties, primarily modifiable through electrostatic gating and twisting. While electrostatic gating is a well-established method for manipulating 2DM, achieving real-time control over interfacial properties remains a frontier in exploring 2DM physics and advanced quantum device technology. Current methods, often reliant on scanning microscopes, are limited in their application scope, lacking the accessibility and scalability of electrostatic gating at the device level. In this work, we introduce an on-chip platform for 2DM with in situ adjustable interfacial properties, employing a microelectromechanical system (MEMS). This platform comprises compact and cost-effective devices capable of precise voltage-controlled manipulation of 2DM, including approaching, twisting, and pressurizing actions. We demonstrate this technology by creating synthetic topological singularities, such as half-skyrmions or merons, in the nonlinear optical susceptibility of twisted hexagonal boron nitride (h-BN). A key application of this technology is the development of integrated light sources with real-time and wide-range tunable polarization. Additionally, we envision a quantum analog capable of generating entangled photon pairs with adjustable entanglement properties. Our work extends the capabilities of existing technologies in manipulating low-dimensional quantum materials and paves the way for novel hybrid 2D-3D devices, with promising implications in condensed-matter physics, quantum optics, and related fields.

Speaker: Yuan Cao (UC Berkeley)

Dinner

Thursday 8 January

Thursday Morning Session I, Chair V. Dobrosavljevic

30 Signatures of unconventional superconductivity in the kagome superconductors Cs(V1-xTix)3Sb5

CsV3Sb5 exhibits several emergent phenomena, including superconductivity (TC ~ 3.2 K), charge-density wave (TCDW ~ 98 K), and nematicity (Tnem ~ 36 K). Despite tremendous investigations for several years, the nature of the superconductivity and multiple electronic orders, as well as the relationship among them, remain elusive. In this study, we have investigated nematic susceptibility and thermal conductivity in a broad doping and temperature range in high-quality single crystals of Cs(V1-xTix)3Sb5 (x = 0 – 0.06) where a double-dome superconducting phase diagram is realized. In most doping ranges, the nematic susceptibility exhibits the Curie‒Weiss behavior above Tnem and Ti doping systematically suppresses the Curie‒Weiss temperature from ~30 K for x = 0 to ~4 K for x=0.0075, resulting in a sign change at x = ~0.009, where the first superconducting dome exists. Furthermore, the Curie constant reaches a maximum at x = 0.01, suggesting a drastic enhancement of the nematic susceptibility near a putative nematic quantum critical point (NQCP) at x = ~0.009. Furthermore, we have investigated the temperature (T)- and magnetic field (H)-dependent thermal conductivity k of the Ti-substituted kagome superconductor Cs(V1-xTix)3Sb5 (x=0.0, 0.01, 0.015, and 0.06). In the samples belonging to the first superconducting dome (0.0 ≤ x ≤ 0.015), k( T ) revealed a set of excitations that indicated the existence of a small full gap and monotonic suppression of the gap minimum with increasing x. In the same sample set, most quasiparticles, as inferred from k( H ) were easily excited and delocalized when small H ≤ ~100 mT were applied along the c-axis, and the increasing behaviour of k( H ) was similar to that of the nodal superconductors. Moreover, the electronic k reached the normal state values under H well below the upper critical fields, uncovering anomalous gapless superconducting states under H. All these results strongly support the presence of unconventional chiral d + i d pairing inside the first superconducting dome, which develops a full gap that is quite fragile under time reversal symmetry breaking potential.
[1] Y. Sur at al Nat. Commun. 14, 3899 (2023)
[2] K. Nam et al. submitted (2024)

Speaker: KEE HOON KIM (Seoul National Univerisity)

31 Failed Superconductivity in a Geometrically Frustrated 2D Mott System

Unconventional superconductivity often emerges in complex materials in which competing orders and complicated band structure obscure its origin. In contrast, $\kappa$-organics are geometrically frustrated quasi-2D single-band systems in which superconductivity arises near the bandwidth-tuned Mott metal-insulator transition in the absence of other orders. We show that in chemically substituted $\kappa$-organics, superconductivity never achieves global coherence, even as temperature $T\rightarrow 0$. Instead, we reveal the presence of superconducting domains embedded in a percolating metallic background that undergo a magnetic field-tuned quantum superconductor-to-metal transition, followed by the surprising emergence of universal conductance fluctuations in macroscopic samples. Our findings demonstrate that failed superconductivity arises from the interplay of intrinsic inhomogeneity and quantum phase fluctuations, providing a new perspective on anomalous metallic states observed in cuprates, disordered thin films, and oxide interfaces.

Speaker: Dragana Popović

Thursday Coffee Break & Poster

32 Angle-dependent magnetoresistance revealing a structural transition in Mn3Ga thin films

As a kagome Weyl semimetal with a high Néel temperature, Mn3Ga has attracted interest due to its large anomalous Hall and topological transport properties. We performed angle-dependent magnetoresistance measurements on the Mn3Ga thin films and observed a distinct anomaly in the temperature evolution and angular dependence of the resistivity. The change in the angular profile indicates a modification of the electronic anisotropy, consistent with a hexagonal-to-orthorhombic structural transition expected for this material system, suggesting a strong coupling between lattice distortion and electronic properties.

Speaker: Zhen Tao (PSI - Paul Scherrer Institut)

33 Sub-50mK adiabatic demagnetization refrigeration with frustrated Yb-oxide magnets in the PPMS

Accessing milli-Kelvin temperatures is a prerequisite for quantum-matter research and applications in quantum technologies. Adiabatic demagnetization refrigeration (ADR) is a simple and sustainable alternative to 3He/4He dilution refrigeration. Geometrically frustrated triangular and kagome rare earth oxides feature important advantages compared to the traditionally utilized hydrated paramagnetic salts for mK-ADR, including higher entropy density, chemical stability and easier fabrication of cooling stages. I will present our recent work on materials optimization and customized ADR platforms for the Quantum Design Physical Property Measurement System.

Work in collaboration with Marvin Klinger, Jorginho Villar Guerrero, Anna Klinger, Paul Bittner, Tim Treu, Christian Heil, Anton Jesche, Arjun Unnikrishnan, Alexander Tsirlin, Yoshi Tokiwa and Kan Zhao. Financial support by the German Federal Ministry of Economic Affairs and Climate Action through project 03EFBY0321 and by the German Research Foundation (DFG) through Project 514162746 (GE 1640/11-1) is acknowledged.

Speaker: Philipp Gegenwart (University of Augsburg)

34 Unraveling Intertwined Orders in the Strongly Correlated Kagome Metal CsCr3Sb5

While correlated phenomena of flat bands have been extensively studied in twisted systems, the ordered states that emerge from interactions in the intrinsic flat bands of kagome lattice materials remain largely unexplored. The newly discovered kagome metal CsCr3Sb5 offers a unique and rich platform for this research, as its multi-orbital flat bands at the Fermi surface result in a complex interplay of pressurized superconductivity, antiferromagnetism, a structural phase transition, and density wave orders. Here, using ultrafast optical techniques, we provide strong spectroscopic evidence for a charge density wave transition in CsCr3Sb5, resolving previous ambiguities. Crucially, we identify rotational symmetry breaking that manifests as a three-state Potts-type nematicity. Our elastoresistance measurements directly demonstrate the electronic origin of this order, as the rotational-symmetry-breaking E2g component of the elastoresistance shows a divergent behaviour around the transition temperature. This exotic nematicity results from the lifting of degeneracy of the multi-orbital flat bands, akin to phenomena seen in certain iron-based superconductors. Our study pioneers the investigation of ultrafast dynamics in flat-band systems at the Fermi surface, offering new insights into the interactions between multiple elementary excitations in strongly correlated systems.

Speaker: Luyi Yang (Tsinghua University)

Thursday Morning Session II, Chair L. Rademaker

35 Evidence for electron fractionalization in a kagome metal

There are long-standing ideas and experiments concerning the emergence of unconventional quasiparticles in strongly interacting Fermi systems. The most dramatic are the fractional states originally observed for the two-dimensional electron gases in semiconductor heterostructures subjected to perpendicular magnetic fields, and explained shortly thereafter by Laughlin. Others found in zero field are more subtle in the sense that their peculiarity is reflected in their scattering rates which rise linearly together with their energies; these are the "marginal" fermions first conjectured for the layered cuprates also displaying high temperature superconductivity. Here we describe experiments revealing signatures of both types of anomalous quasiparticles in zero applied field. The material is Fe3Sn2, a ferromagnetic “kagome” metal, with numerous Weyl nodes near the Fermi level, and a high Curie temperature of ca. 640K. We investigated the compound using microfocused, laser-based angle-resolved photoemission, together with density functional theory (DFT) and machine learning-based analysis of images. Inelastic X-ray scattering measurements reveal that the correct starting point for understanding this material is not a single kagome layer, but rather a triangular lattice of Fe octahedra.

Refs.
1. S. A. Ekahana et al., Nature 627, 67-72 (2024)
2. S. A. Ekahana et al., Mach. Learn.: Sci. Technol. 4 035021 https://doi.org/10.1088/2632- 2153/aced7d (2023)
3. M. Yao et al. arXiv:1810.01514
4. W. Zhang et al., Nature Comm. 15, Article number: 8905 (2024)

Speaker: Gabriel Aeppli (EPFL)

36 Quantum criticality and emerging universality in kagome metals and twisted WSe2

Flat bands emerge in a diverse array of materials, spanning twisted heterostructures and compounds with geometrically-frustrated lattices. They feature strong correlations as well as non-trivial topology. Recent experiments on kagome and pyrochlore metals have uncovered a rich variety of strong-correlation phenomena [1], and the 2024 discovery of superconductivity in TMD moiré systems has likewise generated much excitement. Here, we theoretically investigate the correlation phenomena in d-electron-based metals on lattices that realize destructive kinematic interference [2], and discuss the similarities and differences with the physics of TMD moiré systems [3]. The shared methodology we have developed is in terms of the notion of compact molecular orbitals, which enable effective models in the form of Kondo lattice models: From the dissimilar bandwidth between the flat and dispersive bands, artificial heavy fermion metals arise. Accordingly, our approach allows for the understanding of strange metallicity and unconventional superconductivity. In the process, we advance the general notion that topology induces quantum fluctuations and thus leads to new correlation physics, a route that complements its converse of strong correlations giving rise to new topological states.

References:

[1] J. Huang et al., Nat. Phys. 20, 603 (2024); L S. A. Ekahana, Nature 627, 67 (2024); L. Ye et al., Nat. Phys. 20, 610 (2024); L. Liu et al., Nature 632, 1032 (2024); J. Huang et al., npj Quantum Materials 9, 71 (2024).

[2] L. Chen et al., Nat. Commun. 15, 5242 (2024); L. Chen et al., arXiv:2307.09431; H. Hu et al., Sci. Adv. 9, eadg0028 (2023); F. Xie et al., Phys. Rev. Research 7, L022061 (2025).

[3] F. Xie et al., Phys. Rev. Lett. 134, 136503 (2025); C. Li et al., arXiv:2507.21043

Speaker: Qimiao Si (Rice University)

11:30 Midday Break

Thursday Afternoon Session, Chair Q. Si

37 Fractional Chern insulator edges: crystalline effects and optical measurement

Edge states of chiral topologically ordered phases are commonly described by chiral Luttinger liquids, an effective theory that is exact only in the conformal limit; but in crystalline systems, deviations from simple power-law scaling of correlators generally emerge. Motivated by recent bulk observations of fractional Chern insulators in two-dimensional materials, we revisit this framework on lattices and quantify departures from the fractional quantum Hall case arising from lattice geometry. Using a combination of analytical arguments and numerics, including time-dependent matrix product state simulations, we separate universal and non-universal edge information. From correlation functions, we extract the anomalous boundary exponent, which tracks the bulk filling factor, and independently determine the non-universal edge velocity and associated energy scales from short-time dynamics. Applied across integer and fractional Chern bands with realistic Berry-curvature inhomogeneity, our procedure provides stable estimators that connect edge responses to bulk topology beyond the flat-band limit. We outline experimental probes in excitonic FCIs, including time-resolved edge spectroscopy, which directly access the predicted exponents and velocities.

Speaker: Johannes Motruk (University of Geneva)

38 Disorder is both friend and foe to melting of Wigner-Mott insulators

Wigner crystals are extremely fragile, which is shown to result from very strong geometric frustration germane to long-range Coulomb interactions. Physically, this is manifested by a very small characteristic energy scale for sheer density fluctuations, which are gapless excitations in a translationally invariant system. The presence of disorder, however, breaks translational invariance, thus suppressing gapless excitations and pushing them to higher energy. We illustrate this general principle by explicit microscopic model calculations, showing that this mechanism very effectively stabilizes disordered Wigner lattices to much higher temperatures and densities, then in the clean limit. On the other hand, we argue that in two dimensions disorder significantly "smears" the melting transition, producing spatial coexistence of solid-like and liquid-like regions - just as recently observed in STM experiments. Our results paint a new physical picture for for melting of Wigner-Mott solids in two dimensions, corresponding to a Mott-Hubbard model with spatially varying local electronic bandwidth.

Speaker: Vladimir Dobrosavljevic (Florida State University)

39 A New Perspective on Correlated Metals: from Concealed Mott Quantum Criticality to Disorder in Heavy Fermi Liquids

The band-structure picture of metals is very successful in many materials where the electron correlations are weak. On the other extreme, when correlations are very strong, one expects interaction-induced insulators – due to Mott localization or symmetry breaking. However, the intermediate regime where correlations are strong but the material remains gapless, harbors many open questions in our understanding of quantum materials.
In this talk, I will give an overview of three aspects of correlated metals. I will discuss the relation between quantum criticality at the Kondo breakdown and in doped charge-transfer insulators like the cuprates. These metal-to-metal transitions can be viewed as exhibiting concealed Mott criticality.
Near a Mott critical point, large effective mass enhancements are observed. The famous Landau relation between mass enhancement and specific heat requires a new sum rule for the temperature-dependence of the electron self-energy.
In such heavy Fermi liquids, the interplay between correlations and disorder cannot be ignored. Inspired by new experiments on organic compounds, we show that contrary to textbooks, the residual resistivity is affected by the mass enhancement.

Speaker: Louk Rademaker (Université de Genève)

Dinner

Friday 9 January

Friday Morning Session, Chair S. Gerber

40 Controling solid state phases through vacuum field

While the effect of vacuum fields on atomic system (such as through the Lamb shift) is well known, there is a recent interest in controlling the many-body phases of solid-state systems using vacuum fluctuations strongly coupled to such a system inside a microcavity. In such a system, the strength of the electric field caused by the vacuum fluctuations, to which the strength of the light-matter coupling depends, scales inversely with the square root of the cavity volume. One very interesting feature of the circuit-based resonators is the fact that this volume can be scaled down to deep subwavelength values in all three dimension of space[1]. We have used transport to probe the ultra-strong light-matter coupling[2] and shown that the latter can induce a breakdown of the integer quantum Hall effect[3]. We have also explored the effect of a cavity on the fractional quantum Hall effect using a hovering cavity experiments[4].
In similar experiments performed in slot-antenna cavities, we have also observed that the cavity is able to dramatically suppress the longitudinal conductance between two successive plateau for some relatively high filling factors corresponding to magnetic fields 1-3T. These magneto-transport experiments are interpreted as a transport anisotropy that arises from ordering of the stripe phase through the vacuum fluctuations of the resonator[5].

1. Scalari, G. et al. Ultrastrong Coupling of the Cyclotron Transition of a 2D Electron Gas to a THz Metamaterial. Science 335, 1323–1326 (2012).
2. Paravicini-Bagliani, G. L. et al. Magneto-Transport Controlled by Landau Polariton States. Nat. Phys. 15, 186–190 (2019).
3. Appugliese, F. et al. Breakdown of topological protection by cavity vacuum fields in the integer quantum Hall effect. Science 375, 1030–1034 (2022).
4. Enkner, J. et al. Tunable vacuum-field control of fractional and integer quantum Hall phases. Nature 1–6 (2025) doi:10.1038/s41586-025-08894-3.
5. Graziotto, L. et al. Cavity QED Control of Quantum Hall Stripes. arXiv.org https://arxiv.org/abs/2502.15490v1 (2025).

Speaker: Jerome Faist (ETH Zurich)

41 Quantum Printing and Rectification

I introduce the concept of quantum printing [1]-- the imprinting of quantum states from photons and phonons onto quantum matter. The discussion is focusing on charged fluids (metals, superconductors, Hall fluids) and neutral systems (magnets, excitons). I demonstrate how structured light can generate topological excitations, including vortices in superconductors and skyrmions in magnets. I also discuss how quantum printing induces magnetization in quantum paraelectrics and strain-mediated magnetization in Dirac materials. Finally, I propose future applications, such as printing entangled photon states, creating entangled topological excitations, and discuss applications of quantum printing to light induced quantum turbulence in a charged fluid.

[1] Quantum Printing
arXiv:2509.16792

Speaker: Alexander Balatsky (Nordita)

42 Phantom quasiparticle dynamics in real time at a metastable Wigner crystal vertex.

The self-assembly of matter in the aftermath of a phase transition results in emergent states that are the origin of everything around us. Here we investigate the dynamics of emergent mesoscopic topologically non-trivial domain states formed after an polaronic Wigner crystal is quenched by an external perturbation using a fast STM. In particular, we study the detailed single-electron (polaron) dynamics in real space and in real time at a domain wall Y junction. The observed state is stabilised by global topological constraints and is exceptionally robust against local perturbations. The observed two-level system (TLS) behaviour of the emergent quasiparticle state is suggested to be driven by a hybridized Higgs-Goldstone collective mode that becomes soft at the Y junction. The observations points to a new class of topologically protected local vertex excitations that may be created and manipulated by STM tip.
We speculate that such excitations can be quite common in complex adaptive matter that forms under non-equilibrium conditions. A particular important technologically important example is AlOx, where the same symmetries are broken in the quasi-amorphous junction layer of transmon qubits that can lead to the generation of TLS noise recently observed on a very similar timescale.

Speaker: Dragan Mihailovic (Jozef Stefan Institute)

Closing remarks

Departure