Publications

Order by: publication date number of citations

• Phonon thermal Hall as a lattice Aharonov-Bohm effect

  Kamran Behnia

  SciPost Phys. Core 8, 061 (2025) · published 29 September 2025 | Toggle abstract · pdf

  In a growing list of insulators, experiments find that magnetic field induces a misalignment between the heat flux and the thermal gradient vectors. This phenomenon, known as the phonon thermal Hall effect, implies energy flow without entropy production along the orientation perpendicular to the temperature gradient. The experimentally-measured thermal Hall angle in various insulators does not exceed a bound and becomes maximal at the temperature of peak longitudinal thermal conductivity. The present paper aims to propose a scenario providing and explanation for these two experimental facts. It begins by noticing that at this temperature, $T_{max}$, Normal phonon-phonon collisions become most frequent in comparison with Umklapp and boundary scattering events. Furthermore, the Born-Oppenheimer approximated molecular wave functions are known to acquire a phase in the presence of a magnetic field. In an anharmonic crystal, in which tensile and compressive strain do not cancel out, this field-induced atomic phase gives rise to a phonon Berry phase and generates phonon-phonon interference. The rough amplitude of the thermal Hall angle expected in this picture is set by the phonon wavelength, $\lambda_{ph}$, and the crest atomic displacement, $\delta u_m$ at $T_{max}$. The derived expression is surprisingly close to what has been experimentally found in black phosphorus, germanium and silicon.

  1 citation

• Advancing tools for simulation-based inference

  Henning Bahl, Victor Bresó-Pla, Giovanni De Crescenzo, Tilman Plehn

  SciPost Phys. Core 8, 060 (2025) · published 29 September 2025 | Toggle abstract · pdf

  We study the benefit of modern simulation-based inference to constrain particle interactions at the LHC. We explore ways to incorporate known physics structures into likelihood estimation, specifically morphing-aware estimation and derivative learning. Technically, we introduce a new and more efficient smearing algorithm, illustrate how uncertainties can be approximated through repulsive ensembles, and show how equivariant networks can improve likelihood estimation. After illustrating these aspects for a toy model, we target di-boson production at the LHC and find that our improvements significantly increase numerical control and stability.

  2 citations

• Berry curvature and quantum oscillation from multi-orbital coadjoint-orbit bosonization

  Mengxing Ye, Yuxuan Wang

  SciPost Phys. 19, 078 (2025) · published 26 September 2025 | Toggle abstract · pdf

  We develop an effective field theory for a multi-orbital fermionic system using the method of coadjoint orbits for higher-dimensional bosonization. The dynamical bosonic fields are single-particle distribution functions defined on the phase space. We show that when projecting to a single band, Berry curvature effects naturally emerge. In particular, we consider the de Haas-van Alphen effect of a 2d Fermi surface, and show that the oscillation of orbital magnetization in an external field is offset by the Berry phase accumulated by the cyclotron around the Fermi surface. Beyond previously known results, we show that this phase shift holds even for interacting systems, in which the single-particle Berry phase is replaced by the static anomalous Hall conductance. Furthermore, we obtain the correction to the amplitudes of de Haas-van Alphen oscillations due to Berry curvature effects.

• Super-resolving normalising flows for lattice field theories

  Marc Bauer, Renzo Kapust, Jan Martin Pawlowski, Finn Leon Temmen

  SciPost Phys. 19, 077 (2025) · published 26 September 2025 | Toggle abstract · pdf

  We propose a renormalisation group inspired normalising flow that combines benefits from traditional Markov chain Monte Carlo methods and standard normalising flows to sample lattice field theories. Specifically, we use samples from a coarse lattice field theory and learn a stochastic map to the targeted fine theory. The devised architecture allows for systematic improvements and efficient sampling on lattices as large as $128 × 128$ in all phases when only having sampling access on a $4× 4$ lattice. This paves the way for reaping the benefits of traditional MCMC methods on coarse lattices while using normalising flows to learn transformations towards finer grids, aligning nicely with the intuition of super-resolution tasks. Moreover, by optimising the base distribution, this approach allows for further structural improvements besides increasing the expressivity of the model.

• Exact diagonalization, matrix product states and conformal perturbation theory study of a 3D Ising fuzzy sphere model

  Andreas M. Läuchli, Loïc Herviou, Patrick H. Wilhelm, Slava Rychkov

  SciPost Phys. 19, 076 (2025) · published 26 September 2025 | Toggle abstract · pdf

  Numerical studies of phase transitions in statistical and quantum lattice models provide crucial insights into the corresponding Conformal Field Theories (CFTs). In higher dimensions, comparing finite-volume numerical results to infinite-volume CFT data is facilitated by choosing the sphere $S^{d-1}$ as the spatial manifold. Recently, the fuzzy sphere regulator [Zhu et al, Phys. Rev. X 13, 021009 (2023)] has enabled such studies with exact rotational invariance, yielding impressive agreement with known 3D Ising CFT predictions, as well as new results. However, systematic improvements and a deeper understanding of finite-size corrections remain essential. In this work, we revisit the fuzzy sphere regulator, focusing on the original Ising model, with two main goals. First, we assess the robustness of this approach using Conformal Perturbation Theory (CPT), to which we provide a detailed guidebook. We demonstrate how CPT provides a unified framework for determining the critical point, the speed of light, and residual deviations from CFT predictions. Applying this framework, we study finite-size corrections and clarify the role of tuning the model in minimizing these effects. Second, we develop a novel method for extracting Operator Product Expansion (OPE) coefficients from fuzzy sphere data. This method leverages the sensitivity of energy levels to detuning from criticality, providing new insights into level mixing and avoided crossings in finite systems. Our work also includes validation of CPT in a 1+1D Ising model away from the integrable limit.

  1 citation

• Rainbow chains and numerical renormalisation group for accurate chiral conformal spectra

  Attila Szabó

  SciPost Phys. 19, 075 (2025) · published 26 September 2025 | Toggle abstract · pdf

  Based on the relationship between reduced and thermal density matrices in conformal field theory (CFT), we show that the entanglement spectrum of a conformal critical chain with exponentially decaying terms consists of conformal towers of the associated chiral CFT, with only weak finite-size effects. Through free-fermion and interacting examples, we show that these entanglement spectra present a reliable method to extract detailed CFT spectra from single wave functions without access to the parent Hamiltonian. We complement our method with a Wilsonian numerical renormalisation group algorithm for solving interacting, exponentially decaying chain Hamiltonians.

• The $\mathcal{W}$-algebra bootstrap of 6d $\mathcal{N}=(2,0)$ theories

  Mitchell Woolley

  SciPost Phys. 19, 074 (2025) · published 25 September 2025 | Toggle abstract · pdf

  6d (2,0) SCFTs of type $\mathfrak{g}$ have protected subsectors that were conjectured in [C. Beem et al., J. High Energy Phys. 05, 017 (2015)] to be captured by $\mathcal{W}_\mathfrak{g}$ algebras. We write down the crossing equations for mixed four-point functions $\langle S_{k_1} S_{k_2}S_{k_3}S_{k_4}\rangle$ of 1/2-BPS operators $S_{k_i}$ in 6d (2,0) theories and detail how a certain twist reduces this system to a 2d meromorphic CFT multi-correlator bootstrap problem. We identify the relevant 6d (2,0) $\mathcal{W}$-algebras of type $\mathfrak{g} = \{A_{N-1},D_N\}$ as truncations of $\mathcal{W}_{1+∞}$ and solve OPE associativity conditions for their structure constants, both using OPEdefs and the holomorphic bootstrap of [M. Headrick et al., J. High Energy Phys. 07, 059 (2015)]. With this, we solve the multi-correlator bootstrap for twisted 6d four-point correlators $\mathcal{F}_{k_1k_2k_3k_4}$ involving all $S_{k_i}$ up to $\{k_i\}=4$ and extract closed-form expressions for 6d OPE coefficients. We describe the implications of our CFT data on conformal Regge trajectories of the (2,0) theories and finally, demonstrate the consistency of our results with protected higher-derivative corrections to graviton scattering in M-theory on $AdS_7× S^4/\mathbb{Z}_\mathfrak{o}$.

  1 citation

• Feedback cooling of fermionic atoms in optical lattices

  Wenhua Zhao, Ling-Na Wu, Francesco Petiziol, André Eckardt

  SciPost Phys. 19, 073 (2025) · published 25 September 2025 | Toggle abstract · pdf

  We discuss the preparation of topological insulator states with fermionic ultracold atoms in optical lattices by means of measurement-based Markovian feedback control. The designed measurement and feedback operators induce an effective dissipative channel that stabilizes the desired insulator state, either in an exact way or approximately in the case where additional experimental constraints are assumed. Successful state preparation is demonstrated in one-dimensional insulators as well as for Haldane's Chern insulator, by calculating the fidelity between the target ground state and the steady state of the feedback-modified master equation. The fidelity is obtained numerically through exact diagonalization or via time evolution of the system with moderate sizes. For larger 2D systems, we compare the mean occupation of the single-particle eigenstates for the ground and steady state computed through mean-field kinetic equations.

• Supersymmetric grey galaxies, dual dressed black holes and the superconformal index

  Sunjin Choi, Diksha Jain, Seok Kim, Vineeth Krishna, Goojin Kwon, Eunwoo Lee, Shiraz Minwalla, Chintan Patel

  SciPost Phys. 19, 072 (2025) · published 25 September 2025 | Toggle abstract · pdf

  Motivated by the recent construction of grey galaxy and dual dressed black hole solutions in $AdS_5× S^5$, we present two conjectures relating to the large $N$ entropy of supersymmetric states in $\mathcal{N}=4$ Yang-Mills theory. Our first conjecture asserts the existence of a large number of supersymmetric states which can be thought of as a non interacting mix of supersymmetric black holes and supersymmetric "gravitons". It predicts a microcanonical phase diagram of supersymmetric states with eleven distinct phases, and makes a sharp prediction for the supersymmetric entropy (as a function of 5 charges) in each of these phases. The microcanonical version of the superconformal index involves a sum over states - with alternating signs - over a line in 5 parameter charge space. Our second (and more tentative) conjecture asserts that this sum is dominated by the point on the line that has the largest supersymmetric entropy. This conjecture predicts a large $N$ formula for the superconformal index as a function of indicial charges, and predicts a microcanonical indicial phase diagram with nine distinct phases. It predicts agreement between the superconformal index and black hole entropy in one phase (so over one range of charges), but disagreement in other phases (and so at other values of charges). We compare our predictions against numerically evaluated superconformal index at $N≤10$, and find qualitative agreement.

  3 citations

• Hydrodynamics without boost-invariance from kinetic theory: From perfect fluids to active flocks

  Kevin T. Grosvenor, Niels A. Obers, Subodh P. Patil

  SciPost Phys. 19, 071 (2025) · published 25 September 2025 | Toggle abstract · pdf

  We derive the hydrodynamic equations of perfect fluids without boost invariance [J. de Boer et al., SciPost Phys. 5, 003 (2018)] from kinetic theory. Our approach is to follow the standard derivation of the Vlasov hierarchy based on an a-priori unknown collision functional satisfying certain axiomatic properties consistent with the absence of boost invariance. The kinetic theory treatment allows us to identify various transport coefficients in the hydrodynamic regime. We identify a drift term that effects a relaxation to an equilibrium where detailed balance with the environment with respect to momentum transfer is obtained. We then show how the derivative expansion of the hydrodynamics of flocks can be recovered from boost non-invariant kinetic theory and hydrodynamics. We identify how various coefficients of the former relate to a parameterization of the so-called equation of kinetic state that yields relations between different coefficients, arriving at a symmetry-based understanding as to why certain coefficients in hydrodynamic descriptions of active flocks are naturally of order one, and others, naturally small. When inter-particle forces are expressed in terms of a kinetic theory influence kernel, a coarse-graining scale and resulting derivative expansion emerge in the hydrodynamic limit, allowing us to derive diffusion terms as infrared-relevant operators distilling different parameterizations of microscopic interactions. We conclude by highlighting possible applications.

  1 citation