Welcome to IPA

• Véronique Terras (IPA Director)

Ultrafast generation of hidden phases via energy-tuned electronic photoexcitation in magnetite

• Siham Benhabib (Laboratoire Physique des Solides, Univ Paris-Saclay)

In the quest of controlling materials’ properties on demand, light presents a promising avenue due to its ability to provide ultrafast control and induce phases of matter that are otherwise inaccessible through traditional adiabatic means. This unlocks states exhibiting tailored emergent properties, named hidden phases. Here, we demonstrate how to reach in the same system two distinct metastable structural hidden phases associated with different electronic properties. This manipulation is made possible by harnessing the strongly correlated nature of magnetite and employing two specific photon energies to photoexcite the system. This study breaks ground for an approach to control matter at ultrafast timescale using tailored photon pulses.

Hydrodynamics and nonthermal fixed points

• Michal Heller (Ghent University and Jagiellonian University)

Nonthermal fixed points are far from equilibrium weak-coupling phenomena in which the distribution function of particles exhibit self-similarity in time in the absence of spatial dependence. In 2504.18754 with Berges, Denicol and Preis we proposed that introducing spatial gradients leads to the emergence of a novel form of hydrodynamics, defined intrinsically far from equilibrium. I will discuss how these new kinds of fluids fit into what we know about near-equilibrium fluid mechanics at weak and strong coupling and ideas about hydrodynamic attractors.

Introduction to viscous electron fluids

• Andrew Lucas (University of Colorado Boulder)

I will overview the theory of viscous electron fluids, describe why this regime is uncommon in real quantum materials (but still observable under the right conditions), and discuss some experimental signatures of viscous electron flow. I will explain why the detection of viscous electron flow reveals new information about electron interactions and scattering, that can be very difficult to deduce from bulk transport measurements alone. Electron liquids are in general anisotropic in real quantum materials, and I will review some of the potentially novel hydrodynamic phenomena that might be detected in future experiments in anisotropic metals.

Electron Transport Through Complex Geometries

• Jack Farrell (University of Colorado Boulder)

Ultra-clean materials now routinely access regimes where electron flow is neither purely Ohmic nor purely hydrodynamic, and where boundaries and device geometry are part of the physics rather than a complication. Toward quantitatively modelling the realities of electronic transport and correlated quantum systems more broadly, in this talk I develop geometry-aware transport models from two complementary viewpoints, kinetic theory and nonlinear continuum mechanics, before discussing how to combine them. I begin with a kinetic description of linear response transport across the ballistic–hydrodynamic crossover in 2D devices. Using an angular-harmonic expansion of the Boltzmann equation with a relaxation-time (BGK-like) collision model, the approach retains long-lived modes responsible for nonlocal and tomographic response while recovering viscous hydrodynamics in appropriate limits. Applied to complex geometries, it produces phase diagrams in momentum-conserving and momentum-relaxing scattering times and makes quantitative, space-resolved predictions for flow patterns and nonlocal voltages accessible to scanning-probe and multi-terminal measurements. In particular, I propose simple device geometries that delineate ballistic, hydrodynamic, and diffusive transport even without sophisticated local imaging. I then switch to a fluid-mechanical viewpoint, where the hydrodynamic limit provides analytic control over even nonlinear electronic flow. When driven toward the electronic sound speed, electron liquids should exhibit genuinely fluid-mechanical compressible flow phenomena such as choking and shock-like transitions that lie beyond Ohmic and purely ballistic descriptions. As one concrete example, I discuss bilayer graphene de Laval nozzle devices, where transport and local potential signatures are consistent with a viscous shock front, illustrating how compressibility opens a largely unexplored regime for electron fluids. Finally, I combine these threads by showing how the hydrodynamic limit constrains nonlinear kinetic theory across the crossover, where long-lived ballistic modes, boundaries, and dissipation compete in time-dependent and unstable dynamics. I use the 1D Dyakonov–Shur instability as a laboratory to discuss what becomes of nonlinear hydrodynamics as electron flow evolves from viscous to ballistic behaviour.

AC Fingerprints of 2D Electron Hydrodynamics: Superdiffusion and Drude Weight Suppression

• Davis Thuillier (University of California, Irvine)

Clean two-dimensional Fermi liquids are now known to exhibit an intermediate tomographic regime, between ballistic and Navier-Stokes transport, caused by the anomalously slow relaxation of parity-odd multipolar deformations of the Fermi surface. Here we show that this anomaly extends to the dynamical realm. Starting from a microscopic numerical evaluation of the linearized electron-electron collision operator, we find that the finite-frequency nonlocal conductivity is controlled at low frequency by a single hydrodynamic pole with dynamical exponent z = 4/3. Remarkably, the pole residue itself is scale dependent and exhibits power-law decay with exponent 1/3, so the dynamical properties are described by two separate exponents rather than one. We interpret the residue suppression using a Krylov-chain description of current relaxation: as wavenumber increases, the longest-lived quasinormal mode ceases to be a nearly pure current excitation and spreads over higher odd angular harmonics. Finally, we show that AC transport in narrow channels provides a direct route to measuring the two exponents separately.

Witnessing the Quantum Spin Liquid in Herbertsmithite

• Felix Flicker (University of Bristol)

Herbertsmithite is a leading candidate to host a quantum spin liquid -- a long sought state of matter featuring long-range quantum entanglement and fractionalised 'spinon' excitations. However, despite two decades' work, definitive evidence remains lacking. One complicating factor is that the material features significant disorder in the form of magnetic impurities. I will outline recent work in which we utilise these impurities as 'witnesses' to probe the quantum spin liquid. Using spin noise spectroscopy to measure magnetization fluctuations originating from witnesses, we find unusual 1/f noise developing below a cusp in DC magnetic susceptibility at 260mK. Ageing effects confirm spin glass formation among witnesses.

Planckian bound on the local equilibration time

• Marvin Qi (University of Chicago)

The local equilibration time $\tau_{\rm eq}$ of quantum many-body systems is conjectured to be bounded below by the Planckian time $\hbar /T$. We formalize this conjecture by defining $\tau_{\rm eq}$ as the time scale at which a hydrodynamic description emerges for conserved densities. Drawing on analytic properties of real time thermal correlators, we establish a rigorous lower bound $\tau_{\rm eq} \geq \alpha \hbar /T$ on the onset of hydrodynamic behavior in a `regulated' thermal two-point function. The dimensionless coefficient $\alpha $ depends only on dimensionality and the type of hydrodynamic or diffusive behavior that emerges, and is independent of the thermalization mechanism or other microscopic details. This bound applies universally to local quantum many-body systems, with or without a quasiparticle description, including in the presence of inelastic scattering.

Revealing ultrafast electron dynamics in correlated low-dimensional materials with time-resolved ARPES

• Martin Lebrat (JILA, University of Colorado Boulder)

Strongly correlated and low-dimensional materials host exotic ground states whose out-of-equilibrium dynamics remain poorly understood and are notoriously difficult to predict from first principles. Pump-probe photoemission spectroscopy using high-harmonic-generated extreme ultraviolet pulses offers direct, momentum-resolved access to these dynamics on their natural femtosecond timescales. In this talk, we present time- and angle-resolved photoemission spectroscopy measurements shedding light on two model systems: transiently modulating the Luttinger liquid ground state in purple bronze (Li₀.₉Mo₆O₁₇), and resolving the full momentum-space exciton dynamics of the correlated van der Waals semiconductor CrSiTe₃. Finally, we discuss progress toward brighter vacuum ultraviolet sources, whose improved energy resolution and sensitivity will sharpen access to bound states and subtle spectral features in correlated materials.

Holographic description of first order phase transitions at strong coupling

• Carlos Hoyos (Universidad de Oviedo)

First order phase transitions are of interest in a wide range of systems, from condensed matter to cosmology, and typically proceed through bubble nucleation. While some properties are purely determined by thermodynamics, other quantities, like the bubble wall velocity, are more challenging to obtain. I will review recent results concerning the holographic description of first order phase transitions at strong coupling and their connection to the effective theory approach.

Gravitational horizons in quantum matter

• Corentin Morice (Laboratoire de Physique des Solides, Université Paris-Saclay)

We propose a class of lattice models, including Weyl semimetals, realizable in condensed matter systems whose low-energy dynamics exactly reduces to Dirac fields subjected to gravitational backgrounds. Wave-packets propagating on the lattice exhibit eternal slowdown, signalling the formation of black hole event horizons. We show that the semiclassical wave packets trajectories coincide with the geodesics on (1+1)D dilaton gravity, paving the way for new and experimentally feasible routes to mimic black hole horizons and realize (1+1)D spacetimes as they appear in certain gravity theories.

Thermalization from mode structures

• Brants Robbe (Ghent University)

The dynamics of systems close the equilibrium is captured by the modes found in the analytic structure of the retarded correlation function. Due to the universality of this description, these mode structures allow a direct comparison between various out of equilibrium systems in otherwise very different regimes. In this talk I will explain how to interpret mode structures emerging from quantum systems, and use them to directly extract information on its thermalization properties.

Disordered Charged Horizons

• Pau Garcia Romeu (Instituto de Física Teórica (IFT-UAM/CSIC))

We construct fully backreacted charged black brane solutions with a spatially disordered chemical potential in asymptotically AdS$_3$ and AdS$_4$, providing holographic duals of strongly coupled disordered systems. At intermediate temperatures these geometries display highly inhomogeneous horizons, though their geometric averages reproduce the clean BTZ and Reissner–Nordström solutions. The low temperature behavior, however, differs sharply between dimensions. In AdS$_3$, inhomogeneities decay and the horizon flows to the clean charged BTZ fixed point, rendering disorder irrelevant in the infrared. In AdS$_4$, horizon inhomogeneities persist: while the averaged geometry flows to the clean AdS$_2\times \mathbb{R}^2$ throat, the disordered horizon induces a finite residual resistivity. These results show that disorder can qualitatively alter the IR physics of holographic metals and indicate violations of the Harris criterion in strongly coupled systems.

Momentum-gapped states in disordered metals

• Miguel Ángel Sanchez Martinez (University of Bristol)

Nature hosts both massless particles with linear dispersions and massive particles whose energies scale quadratically with momentum and exhibit finite gaps. In condensed matter systems, analogous behaviour appears in the form of collective modes, i.e., measurable excitations with well-defined energy–momentum relations. A long-discussed third possibility is that of a superluminal tachyon, characterised by ill-defined energies at small momentum. In hydrodynamic descriptions of matter, an analogous excitation has been predicted: a mode with a purely imaginary energy at low momentum, reflecting a finite lifetime rather than a propagating excitation. Despite its theoretical anticipation, clear evidence for such a dispersion has remained elusive. By systematically bridging hydrodynamic theory and microscopic models of metallic systems, we show that this unconventional dispersion naturally emerges in correlated quantum matter when momentum relaxation is present, for instance due to impurity-induced breaking of electronic translational symmetry. As a concrete realisation, we analyse the recently reported acoustic plasmon — known as Pines’ demon — in Sr$_2$RuO$_4$. Its experimentally observed dispersion deviates markedly from the massless linear behaviour expected within the random phase approximation. We argue that this deviation is consistent with the first experimental manifestation of a momentum-gapped collective mode.

Z2 spin liquids in the Higher spin Kitaev Honeycomb model

• Han Ma (Ecole polytechnique)

The higher-spin Kitaev models possess an extensive set of local conserved quantities, much like the spin-1/2 Kitaev honeycomb model, although they are not exactly solvable. In this talk, I will present the exact gauge structure of the spin-S Kitaev honeycomb model and show that these conserved quantities are precisely the Z2 gauge fluxes. A striking even-odd effect emerges: the Z2 gauge charges are fermionic for half-integer spins, but bosonic for integer spins. We show that the fermionic charges are always deconfined, implying that half-integer-spin Kitaev models necessarily realize nontrivial spin-liquid ground states. In contrast, the bosonic charges in the integer-spin case can condense, leading to a trivial product state, as happens in the anisotropic limit. This distinction is closely tied to an anomaly of the 1-form symmetry. I will also discuss applications to the quadrupolar Kitaev model.

Chiral pairing in two-dimensional Wigner crystals

• Ilya Esterlis (University of Wisconsin - Madison)

Wigner crystals–phases in which itinerant electrons spontaneously crystallize–feature prominently in the phase diagrams of modern two-dimensional materials. Unlike conventional two-dimensional electron gases, these crystals may form in bands with substantial Berry curvature, which can qualitatively change the nature of the crystalline state. I will present variational calculations showing that the cooperative effects of Berry curvature and quantum zero-point motion can stabilize a novel spin-triplet “paired Wigner crystal.” In this state, chiral spin-triplet electron pairs occupy each unit cell.

Review on near-equilibrium experiments in strongly-correlated quantum matter

• Peter Abbamonte (University of Illinois, Urbana-Champaign)

Critical density fluctuations in the strange metal state of Bi2Sr2CaCu2O8+x

• Peter Abbamonte (University of Illinois, Urbana-Champaign)

The strange metal is thought to represent a universal limit on quantum entanglement in a many-body system, though this hypothesis remains unproven. In 1989, theorists proposed that universality might manifest in strange metals as scale-invariant behavior in the dynamic charge susceptibility, $\chi''(q, \omega)$, at finite momentum, $q$. In this talk I will describe the first measurements of $\chi''(q, \omega)$ in the thermal energy regime, $\hbar\omega \sim k_B T$, obtained via momentum-resolved inelastic electron scattering from the strange metal Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$. The susceptibility is independent of momentum and follows the universal scaling form $\chi''(q, \omega) = T^{-\nu} f(\omega/T)$, with exponent $\nu = 0.94$. Further, the response shows conformal invariance, meaning the dynamics behave as if they occur on a circle of radius $1/T$ in imaginary time, characterized by conformal dimension $\Delta = 0.06$. These findings suggest that the strange metal is a universal phenomenon potentially compatible with holographic theories of matter in which conformal invariance plays a central role.

Quantum Critical Theories in a Periodic Potential: Strange Metallic Thermoelectric and Magnetotransport

• Eric Nilsson (Chalmers University of Technology)

We study DC and AC thermoelectric and magnetotransport in 2D quantum critical theories with strong translational symmetry breaking due to a varying chemical potential lattice with zero average \barμ=0. The combination of quantum criticality and the absence of the average natural scale implies that such systems have idiosyncratic signatures that may apply more generally when the variance in the lattice potential far exceeds the average or for strong translational symmetry breaking in general. We model such theories holographically through near-extremal AdS black holes. We find that these systems (a) become *better* conductors. In a 2D lattice, this can be explained by currents flowing around obstacles; (b) exhibit bad-metal electrical transport with Drude-like thermal transport, though it is not Drude, and, notably, (c) display an approximately B-linear longitudinal magnetoresistance at large fields, similar to Effective Medium Theory. We comment on how these results may apply when \barμ≠0.

Nonlocal and temporally engineered THz light–matter interactions in surface plasmon cavities

• Yannis Laplace (Laboratoire des solides irradiés - Ecole Polytechnique)

I will present our work on the development of cavities and metamaterials operating in the Terahertz (THz) frequency range and using a surface plasmonic mechanism for light confinement. In the first part of the talk, I will discuss equilibrium properties of this system and how the plasmonic approach enables the confinement of THz photons to volumes up to 10⁻⁸ times smaller than the free-space diffraction limit [1]. In such an extreme regime, the cavities approach the ultimate spatial scale permitted by plasmonics, where phenomena such as nonlocal light–matter interactions begin to emerge and allow to probe finite momentum properties of the electron gas with free space radiation. In the second part, I will focus on the out-of-equilibrium dynamics of periodically driven plasmonic cavities through the recent realization of a photonic time crystal [2]. This approach enables the engineering of emergent photonic properties and light–matter interactions from a temporal perspective, complementing the traditional spatial approach. [1] I. Aupiais et al. Nature Communications, 14(1), 2023. [2] T. Guo et al. arXiv 2510.02845

Quasinormal modes of nonthermal fixed points

• Matisse De Lescluze (Ghent University)

Nonthermal fixed points are universal, attractive phenomena arising in far-from-equilibrium systems. They have been studied in different physical systems, such as theoretical models of heavy-ion conditions and cold-atom experiments. In analogy to strongly-coupled CFTs or black holes, one can study the attractive nature of nonthermal fixed points by calculating their quasinormal mode spectra. These give rise to a tower of powerlaw decaying excitations. By making the connection between this technique used in black-hole physics, I try to bridge the field of far-from-equilibrium dynamics and strongly-coupled thermalization

Low Temperature Energy Dynamics in SYK Models

• Angshuman Choudhury (CPHT, Ecole Polytechnique)

The system of N fermions, interacting randomly, q at a time is called the SYK dot. These dots are put on a 1D chain and allow for q body interactions between nearest neighbor sites. At large N, this model is known to have two channels of energy transport at very low temperatures: a diffusive (Drude like) mode and a set of critical gapped modes. While these models are not analytically tractable in general, interestingly they show non-fermi liquid behavior at low temperatures. To better understand this behavior, we study the energy retarded Green’s function in the large q limit, where the model becomes analytically solvable. By analyzing the poles, we study the two transport channels and the interactions between them by a phenomenon called ‘pole collisions’, where the dispersion relations display branch point singularities. At zero temperature, the critical poles coalesce into a logarithmic branch cut. Despite this, a gapless, non-hydrodynamic mode survives and continues to dominate energy transport. Our results clarify how energy transport changes across scales in strongly interacting quantum systems and agree with coresponding predictions from holographic theories inspired from string theory.

The importance of strong translation symmetry breaking in cuprate strange metals

• Koenraad Schalm (Leiden University)

We will show how hydrodynamics in the presence of a weak lattice and by extension in the presence of weak translational symmetry breaking cannot explain the observed T-squared scaling of the Hall angle in high Tc cuprates. Building on this we present numerical studies of SYK-AdS2 fixed points in the presence of strong translational symmetry breaking, showing promising signs of Drude-like transport without Drude-physics and different temperature scaling in magneto- and longitudinal transport.

Quantum Corrections in Near-Extremal Black Holes: Thermodynamics, Dynamics and Applications

• Leopoldo Pando Zayas (University of Michigan)

Near-extremal black holes are known to contain strong quantum fluctuations in their near-horizon near-AdS2 throat region governed by an effective action that includes Schwarzian modes. These fluctuations lead to one-loop corrections in the gravitational path integral that are essential in understanding the thermodynamics of near-extremal black holes at low temperatures where they become more dominant that the semi-classical answer. We discuss the implications of these quantum fluctuations for near-extremal asymptotically AdS4 black branes in the context of the AdS/CFT correspondence. We note that at one-loop level there is a coupling of the shear gravitational fluctuations to one of the would-be zero modes. This coupling affects the retarded Green’s function in a way that leads to new and exciting low temperature behavior of transport coefficients (shear viscosity and conductivity).

Towards a quantum lattice bootstrap

• Subham Dutta Chowdhury (Abdus Salam ICTP, Trieste)

Constraints on infrared (IR) physics arising from ultraviolet (UV) consistency have provided deep insights into the structure of relativistic quantum field theories and their IR phases. I am interested in how analogous constraints can be formulated for systems with spontaneously or explicitly broken Lorentz symmetry. One such system of interest is relativistic Fermi liquid which describes a wide range of systems at finite density such as neutron stars. Can symmetry and causality constrain their Wilson coefficients- the Landau parameters? The scope of such questions often goes beyond relativistic systems relevant for high energy physics. It is well known that hydrodynamics or non-fermi liquids can emerge as IR phases of lattice systems. Can such constraints be formulated for such IR phases possessing well-defined lattice UV completions—namely, those described by bounded and local Hamiltonians? In this program I hope to identify paradigms where such bootstrap methods can be implemented to give universal bounds that might be relevant for condensed matter physics. Expertise and topic of research How does nature modify gravity at very small (e.g., atomic) distances? This fundamental question must be answered to understand the nature of black holes in the universe. While it is possible to formulate detailed deductive theoretical models, I adopt an inductive approach where I focus on the observable and measurable macroscopic effects rather than trying to detail every microscopic element. The missing microscopic details can be addressed as unknown parameters, such as deviations from Einstein's theory of gravity. Are these unknown parameters constrained by some fundamental principles? I want to vary these parameters within a permissible range so that the changes to gravity still follow basic principles of the universe that we observe, like causality- the idea that causes should come before effects. The range of allowed parameters obtained in this first principle approach, could lead to important and noticeable effects, such as predicting the discovery of new particles in nature. More ambitiously, does this lead to an unique theory of quantum gravity, like string theory? In summary, I am primarily interested in how such abstract general constraints can lead to observable consequences via such ``effective" approach. In the long term, I aim to explore how other different fundamental principles of nature constrain similar effective descriptions and affect our understanding of various phenomena. For e.g., can studying boiling water provide insights into neutron stars? Why does nuclear matter at very low densities behave like cold atoms? My project aims to answer these interesting questions using these fundamental techniques.

Unraveling exciton dynamics in an atomically thin van der Waals magnet

• Niloufar Nilforoushan (Université Paris Cité)

Van der Waals (vdW) materials have revolutionized our understanding of correlated quantum phenomena, where structural confinement and reduced screening give rise to strongly bound excitons and a wide variety of emergent electronic and magnetic phases. Among these, magnetic vdW semiconductors open an entirely new frontier where Coulomb correlations can be tuned in situ by the material’s intrinsic magnetic order. In this talk, I will focus on CrSBr, a semiconducting vdW magnet that uniquely combines robust magnetism with excitonic resonances. I will first discuss how spin-controlled quantum confinement of magnetic excitons in CrSBr can be manipulated by directly probing their internal Rydberg-like transitions. I will then show how terahertz polarization spectroscopy, combined with near-field microscopy, captures the formation and ultrafast relaxation dynamics of these excitons in the monolayer limit. Together, these results provide a direct view of the dielectric response of quasi-one-dimensional magnetic excitons and establish CrSBr as a model system for exploring magnetically controlled excitonic physics. In future applications, excitons or even condensates may be interfaced with spintronics, enabling on-demand phase transitions through extrinsically switchable Coulomb correlations.

Review on far-from-equilibrium experiments in strongly-correlated quantum matter

• Peter Armitage (Johns Hopkins)

Planckian dissipation, anomalous high temperature THz non-linear response and energy relaxation in the strange metal state of the cuprate superconductors

• Peter Armitage (Johns Hopkins)

C3 symmetry breaking in the charge ordered phase of 1T -TiSe

• Esther Van Grondelle (University of Amsterdam)

The interplay between lattice and electron degrees of freedom can give rise to complex ordered states in quantum materials. Here, we use high-resolution optical spectroscopy to investigate TiSe2, a material whose charge-ordered phase has been debated for decades. We discover a previously unobserved pronounced splitting of a doubly degenerate Eu optical phonon at the charge-density-wave transition. This directly reveals the breaking of three-fold rotational symmetry of the unit cell in a one-step transition. The results rule out the previously proposed centrosymmetric structure and points to the possibility of a low-temperature chiral charge-ordered state. Our findings demonstrate that carefully tracking optical phonons as a function of temperature provides a sensitive probe of subtle symmetry breaking. In this talk, I will recapitulate the fundamentals of charge density wave physics and discuss why we should still expect to find unconventional charge-ordered phases in nature. I will then focus on the complex charge-ordered phase of TiSe2 and show how our novel high-resolution optical spectroscopy measurements provide new insights into its underlying nature

Review on gauge/gravity duality

• Aristomenis Donos (Durham University)

Critical dynamics of superfluids

• Kailidis Polydoros (Durham University)

Describing the transition between normal and superfluid phases requires an effective theory that captures the coupled dynamics of the order parameter and the conserved charges of the system. In this talk, I present a relativistic effective field theory for nearly critical superfluids, constructed through three complementary approaches. I first outline the derivation within the framework of hydrodynamics, followed by an alternative construction using the Keldysh-Schwinger effective action. Finally, I present a holographic computation that analytically confirms the transport properties identified by the first two techniques. This unified description provides a robust framework for superfluid critical behaviour and demonstrates complete agreement between the different formalisms.

Semi-holographic Mott insulators and poles/zeros duality

• Alessio Caddeo (University of Würzburg)

Mott insulators are phases in which strong electron–electron interactions suppress charge transport despite partially filled bands. In terms of single-particle observables, this physics is often associated with zeros of the Green’s function rather than poles. In this talk, I will present a semi-holographic model that exhibits a Mott–metal transition. The setup consists of an elementary fermion coupled to a strongly interacting sector that can be analysed holographically. In our model, the Green’s function of the elementary fermion develops zeros in the Mott phase and poles in the metallic phase. The two regimes are related by a poles/zeros duality that arises naturally within the semi-holographic framework.

QNM orthogonality products in hyperboloidal foliations

• Christiana Pantelidou (University College Dublin)

Black hole quasinormal modes (QNMs) are associated with a non-normal evolution problem and therefore do not form an orthogonal basis under natural choices of inner product. However, special bilinear products can be introduced that establish orthogonality relations and allow the definition of QNM excitation coefficients. In this talk, we study these constructions in the hyperboloidal framework, where QNM solutions are smooth and the description of non-modal physics becomes more natural. We discuss why the associated bilinear products nevertheless remain divergent, review regularisation procedures that produce finite and well-defined expressions, and examine alternative formulations involving flux terms.

Horizon symmetries, hydrodynamics, and chaos

• Natalia Pinzani-Fokeeva (Florence University)

A local symmetry is required to formulate fluid dynamics from a modern effective-field-theory perspective. Black hole horizons possess the same redundancy. In this talk, I will show how asymptotic symmetries of horizons can be interpreted as symmetries of a dual hydrodynamic theory in the context of holography. I will also show how the horizon structure implies additional symmetries and how these symmetries lead to chaotic behavior in the dual theory.

Schwinger-Keldysh hydrodynamics of the SYK lattice

• Akash Jain (University of Oxford)

Hydrodynamics provides a universal low-energy effective description of interacting many-body systems. The Schwinger–Keldysh effective field theory (SK EFT) offers a Wilsonian, action-based formulation of hydrodynamics that systematically incorporates fluctuations. In this approach, the effective action is generically complex, encoding macroscopic dissipation. The framework features a discrete KMS symmetry which is responsible for implementing fluctuation-dissipation relations and underlies the emergence of the second law of thermodynamics. In this talk, we will discuss the first-ever derivation of an SK EFT directly from a local, unitary microscopic Hamiltonian. Specifically, we will consider a one-dimensional chain of SYK dots with Gaussian-random interactions between nearest neighbours. Its low-energy dynamics is governed by an SK EFT for energy diffusion. We will identify the fundamental and emergent symmetries of this theory and derive the associated classical entropy current for SYK chains. Time permitting, we will also comment on applications to out-of-time-ordered correlators of energy fluctuations. The talk will be based on the recent paper: https://arxiv.org/pdf/2604.18675.

Disordered electronic interactions as a route to non-Fermi liquids and quantum critical superconductivity in strange metals

• Davide Valentinis (Karlsruhe Institute of Technology)

Transport and hydrodynamics in traversable wormholes

• Mihailo Cubrovic (Institute of Physics Belgrade)

We consider the fluctuation equations in the background of an eternal traversable wormhole. We first construct the appropriate boundary conditions for the corresponding two-boundary problem, then we solve for the correlation functions and find hydro-like quasiparticle poles, contrary to naive expectations. Then we discuss how to detect the onset of teleportation.

Optically driven quantum liquids in one dimension

• Matteo Mitrano (Harvard University)

A key frontier of modern condensed matter is to harness light–matter interaction to coherently engineer quantum states in materials. Under optical driving, quantum materials exhibit emergent many-body phenomena, from ultrafast switching to dynamical quantum states without equilibrium analogs. Progress hinges on using light to both uncover new nonequilibrium states and devise strategies to stabilize them far beyond the duration of the drive. One-dimensional Mott insulators are particularly compelling platforms for these goals. Electron fractionalization produces highly entangled ground states that are exceptionally responsive to optical perturbations, and theory predicts exotic ordering phenomena upon photoexcitation. In this talk, I will show how ultrafast X-ray spectroscopy provides a direct, microscopic view of these dynamics by resolving charge and spin responses of optically driven phases. I will discuss our observation of a photoinduced Tomonaga–Luttinger liquid state in the one-dimensional cuprate chain Sr2CuO3 [1]. I will then show how nonequilibrium states can be stabilized for nanosecond timescales by leveraging symmetry constraints in the parent ladder compound Sr14Cu24O41 [2,3]. References [1] A. K. Jones, S. F. R. TenHuisen, et al. forthcoming (2026). [2] H. Padma et al. Nat. Mater. 24, 1584–1591 (2025). [3] H. Padma et al. Phys. Rev. Lett., in press (2026).

Far-from-equilibrium dynamics and thermalization in two-dimensional correlated quantum materials

• Dragana Popovic (National High Magnetic Field Laboratory, Florida State University)

The mechanisms that quantum systems can use to avoid thermalization and retain information in their local degrees of freedom have been of great research interest for both new fundamental physics and novel applications. These mechanisms, for example, are of great importance for quantum technologies since they can be used to build quantum memory devices. In quantum materials, there has been a recent surge of interest in exploring nonequilibrium phenomena, but most studies have focused on driving a system out of equilibrium by supplying it with energy, resulting in transient behaviors on ultra-short time scales or nonequilibrium steady states. In contrast, the goal of our work is to investigate far-from-equilibrium phenomena in situations where thermalization involves many slow processes and when the externally supplied energy is insufficient to allow the quantum system to explore all states on experimental time scales. This talk will discuss transport measurements that probe far-from-equilibrium behavior and the approach to thermal equilibrium in an open two-dimensional electron system at different densities across the metal-insulator transition. We observe different types of far-from-equilibrium dynamics, including many-body localization (MBL). We establish the phase diagram of the dynamical behavior that shows the crossover between MBL and ergodic regimes as a function of density and bath temperature.

Incipient modulated phase in SrTiO3

• Benoît Fauqué (ESPCI, CNRS, LPEM)

Nanometer-scale modulations can spontaneously emerge in complex materials when multiple degrees of freedom interact. In this talk, I will demonstrate that SrTiO₃ lies near an incipient structurally modulated phase. I will then explore how Ca doping and an applied electric field influence this phase, revealing that it cooperates with—rather than competes against—the other lattice instabilities in SrTiO₃.

Surface states from the metamorphosis of plane-impurity states: application to surface states and QPI patterns in UTe2

• Cristina Bena (CEA)

We present an exact formalism (Phys. Rev. B 100, 081106 (2019)) to describe the formation of end, edge or surface states through the evolution of impurity-induced states. We propose a general algorithm that consists of finding the impurity states via the T-matrix formalism and showing that they evolve into boundary modes when the impurity potential goes to infinity. Thus, in order to compute the surface states and the surface Green's functions we start with an infinite 3D system and model the boundary using a plane-like infinite-amplitude potential (Phys. Rev. B 102, 165117 (2020)). We apply our method to UTe2 in the superconducting state to calculate the surface states and the corresponding QPI patterns, and we compare our results to experimental findings. Our predictions are most consistent with these findings if the bulk superconducting gap function of UTe2 is described by a B3u pairing (S. Wang et al, Nature Physics 2025, arXiv:2503.17761)

A Thermodynamically Consistent Framework for Polar Active Matter

• Andrea Amoretti (University of Genoa)

Active particles are self-propelled units that continuously convert energy into directed motion, making them paradigmatic examples of far-from-equilibrium systems. Their intrinsic non-equilibrium nature gives rise to a wide range of collective phenomena, including flocking, pattern formation, and non-equilibrium phase transitions. Despite significant progress, a fundamental understanding of the emergent large-scale behavior of active matter remains a central challenge in statistical physics. In this talk, I will present recent theoretical advances in the hydrodynamic description of polar active fluids and the role of thermodynamic constraints in shaping their collective dynamics. Building on the formalism of fluids without boost symmetry, I will show that polar active fluids can be mapped onto passive fluids supplemented by non-thermal noise and symmetry-breaking constraints, without modifying the underlying thermodynamic relations. This framework allows for the derivation of exact scaling exponents for transport coefficients under dynamical renormalization group flow in arbitrary spatial dimension d. The resulting critical exponents smoothly interpolate between mean-field predictions and recent high-precision numerical results for flocking models, providing a unified picture that reconciles analytical theory with simulations and experiments. Time permitting, I will discuss how this paradigm can be generalized to novel fixed points in more complex active systems. In particular, I will consider two examples: (i) active systems exhibiting autochemotaxis, where self-propelled particles move coherently while emitting an attractive chemical field, and (ii) active systems in which both activity and inertia play a crucial role. In the latter case, I will show how spin-hydrodynamic techniques provide a powerful framework to characterize the resulting phases and their collective dynamics.

Review on visualizing quantum matter

• Séamus Davis (University of Oxford)

Quantum spin liquid noise

• Séamus Davis (University of Oxford)

Hydrodynamics of superfluids with vortices

• Vaios Ziogas (University of Bristol)

Hydrodynamics is expected to govern the universal long-time and large-scale dynamics of interacting systems. However, its applicability to intrinsically non-equilibrium states with topological defects remains poorly understood. Superfluids containing vortices provide a paradigmatic example: such configurations exhibit singular phase structures, strong spatial inhomogeneities, and additional gapped degrees of freedom, all of which lie outside the standard hydrodynamic paradigm. Whether a controlled hydrodynamic description survives under these conditions is a priori unclear. In this work, we show that a predictive hydrodynamic framework can nevertheless be constructed for superfluids containing vortices. In the presence of a finite vortex density, the superfluid velocity is no longer conserved but instead relaxes over time, with a rate controlled by the density of vortices. This provides the key macroscopic input that encodes vortex-induced dissipation and leads to a closed set of effective equations governing the long-wavelength dynamics. We analyze the resulting collective excitations and show that the system exhibits collective modes that originate from the hydrodynamic sector but become time dependent and gapped due to vortex-induced relaxation, leading to deviations from conventional superfluid hydrodynamics, including modified dispersion relations and attenuation. Crucially, the system under consideration admits no stationary background, rendering standard linear response and quasi-normal mode analyses inapplicable. To overcome this, we perform fully nonlinear real-time evolution in a holographic superfluid model with vortex configurations and directly probe the emergent macroscopic dynamics. We find that the late-time evolution is quantitatively captured by the effective hydrodynamic description, providing nontrivial evidence that hydrodynamics can persist far beyond its traditional regime of validity. Our results establish a concrete framework for incorporating vortex-induced relaxation into superfluid hydrodynamics and offer a new perspective on non-equilibrium dynamics in strongly coupled systems with topological defects.

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

• Louk Rademaker (Université de Genève & Leiden University)

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.

Fluctuating hydrodynamics of driven open systems

• Jacome Armas (University of Amsterdam)

I’ll discuss the effective theory of hydrodynamics for classes of thermal driven open systems that have not yet received considerable attention from a symmetry-based perspective. These systems have weakly broken time-translation symmetry and host thermostated (two-temperature) steady states which are time independent states that constantly produce entropy. They lead to violations of conventional properties of typical systems studied in holography such as the fluctuation-dissipation theorem. I will discuss the various applications and interesting connections with non-equilibrium physics. If time permits I’ll also show fluctuating hydrodynamic simulations of simple open systems.

eta/s corrections from near-extremal, near-horizon quantum fluctuations

• Clément Supiot (CPHT, École Polytechnique)

Extremal black holes have the particularity to exhibit an emergent geometry close to their horizon that contains an AdS2 factor. For compact internal spaces, the theory can be dimensionally reduced to obtain a dilaton gravity in two dimensions. At the linearised level, this reduces to JT gravity. The low-energy effective theory is governed by the Schwarzian action, enabling corrections to the entropy and to correlators of the dual near CFT1 to be computed, arising from quantum fluctuations of near-horizon zero modes. By studying shear diffusion, it is in principle possible to compute the effect of these corrections on the ratio viscosity over entropy density, which is conjectured to have a positive lower bound for any fluid. Saturated in numerous holographic systems at the classical gravity level, we address whether such quantum corrections change the picture. Using a matching calculation, we focus on the hydrodynamic regime, for which only weak quantum fluctuations can be considered in order to avoid entering a sub-Planckian regime. We find that the results obtained from the transverse Kubo formula and the one from the location of the diffusive pole in the shear sector agree.

Linear response beyond hydrodynamic poles

• Jonas Rongen (University of Genoa)

In this talk, I address the question of how hydrodynamics can be extended to incorporate non-hydrodynamic excitations. To this end, I consider the construction of an effective linear-response theory that reproduces the Mittag–Leffler expansion of a charge current correlator with an arbitrary number of simple poles. The resulting framework is compatible with hydrostatics and does not require any modification of equilibrium thermodynamics. As an application, I study charge fluctuations in the D3/D5 probe brane system and quantify how transport coefficients evolve as quasihydrodynamic behavior emerges at large charge density.

Non-Hermitian Holography

• Daniel Areán (Universidad Autónoma de Madrid)

Holography can describe non-Hermitian PT-symmetric systems. I will present novel results about the phase diagram of these systems, focusing on the interior geometry of the dual black holes and its imprint on field-theoretic observables.

Viscous electron flow in cadmium, an elemental metal with non-trivial electronic structure

• Kamran Behnia (ESPCI-Paris)

When the electron mean-free-path exceeds the sample thickness in a metal, Sondheimer oscillations of electric conductivity, which are periodic in magnetic field, can emerge. Our study of longitudinal and transverse conductivity in cadmium single crystals with thicknesses ranging from 12.6 to 475 μm demonstrate that the amplitude of the first ten oscillations is determined by the quantum of conductance and a length scale that depends on the sample thickness, the magnetic length and the Fermi surface geometry [1]. In the same crystals, one observes a low temperature size-dependent resistivity upturn in a finite window sandwiched between ballistic and diffusive regimes [2]. Within this window, the electrical conductivity displays a simultaneous quadratic dependence on both sample size and temperature, which is a fingerprint of hydrodynamic flow. This leads us to quantify the amplitude and the temperature dependence of kinematic and dynamic viscosity of the electron fluid. The rate of momentum-conserving e-e collisions is not set by the main Fermi energy, but by Lilliputian energy scales and inter-valley bottlenecks. 1. Xiaodong Guo, Xiaokang Li, Lingxiao Zhao, Zengwei Zhu & Kamran Behnia, Communications Materials 7, 76 (2026). 2. Xiaodong Guo, Xiaokang Li, Benoît Fauqué, Alaska Subedi, Lingxiao Zhao, Zengwei Zhu, Kamran Behnia, arXiv:2604.19416 (2026).