Publications

Order by: publication date number of citations

• Visualizing near-coexistence of massless Dirac electrons and ultra-massive saddle point electrons

  Abhay Kumar Nayak, Jonathan Reiner, Hengxin Tan, Huixia Fu, Henry Ling, Chandra Shekhar, Claudia Felser, Tamar (Tami) Pereg-Barnea, Binghai Yan, Haim Beidenkopf, Nurit Avraham

  SciPost Phys. 15, 178 (2023) · published 25 October 2023 | Toggle abstract · pdf

  Strong singularities in the electronic density of states amplify correlation effects and play a key role in determining the ordering instabilities in various materials. Recently high order van Hove singularities (VHSs) with diverging power-law scaling have been classified in single-band electron models. We show that the 110 surface of Bismuth exhibits high order VHS with an usually high density of states divergence $\sim (E)^{-0.7}$. Detailed mapping of the surface band structure using scanning tunneling microscopy and spectroscopy combined with first-principles calculations show that this singularity occurs in close proximity to Dirac bands located at the center of the surface Brillouin zone. The enhanced power-law divergence is shown to originate from the anisotropic flattening of the Dirac band just above the Dirac node. Such near-coexistence of massless Dirac electrons and ultra-massive saddle points enables to study the interplay of high order VHS and Dirac fermions.

  1 citation

• A minimal tensor network beyond free fermions

  Carolin Wille, Maksimilian Usoltcev, Jens Eisert, Alexander Altland

  SciPost Phys. 18, 196 (2025) · published 19 June 2025 | Toggle abstract · pdf

  This work proposes a minimal model extending the duality between classical statistical spin systems and fermionic systems beyond the case of free fermions. A Jordan-Wigner transformation applied to a two-dimensional tensor network maps the partition sum of a classical statistical mechanics model to a Grassmann variable integral, structurally similar to the path integral for interacting fermions in two dimensions. The resulting model is simple, featuring only two parameters: one governing spin-spin interaction (dual to effective hopping strength in the fermionic picture), the other measuring the deviation from the free fermion limit. Nevertheless, it exhibits a rich phase diagram, partially stabilized by elements of topology, and featuring three phases meeting at a multicritical point. Besides the interpretation as a spin and fermionic system, the model is closely related to loop gas and vertex models and can be interpreted as a parity-preserving (non-unitary) circuit. Its minimal construction makes it an ideal reference system for studying non-linearities in tensor networks and deriving results by means of duality.

  1 citation

• Codebase release 1.0 for kdotpy

  Wouter Beugeling, Florian Bayer, Christian Berger, Jan Böttcher, Leonid Bovkun, Christopher Fuchs, Maximilian Hofer, Saquib Shamim, Moritz Siebert, Li-Xian Wang, Ewelina M. Hankiewicz, Tobias Kießling, Hartmut Buhmann, Laurens W. Molenkamp

  SciPost Phys. Codebases 47-r1.0 (2025) · published 28 January 2025 | Toggle abstract · src

  The software project kdotpy provides a Python application for simulating electronic band structures of semiconductor devices with k·p theory on a lattice. The application implements the widely used Kane model, capable of reliable predictions of transport and optical properties for a large variety of topological and non-topological materials with a zincblende crystal structure. The application automates the tedious steps of simulating band structures. The user inputs the relevant physical parameters on the command line, for example materials and dimensions of the device, magnetic field, and temperature. The program constructs the appropriate matrix Hamiltonian on a discretized lattice of spatial coordinates and diagonalizes it. The physical observables are extracted from the eigenvalues and eigenvectors and saved as output. The program is highly customizable with a large set of configuration options and material parameters.

  1 citation

• Weak vs. strong breaking of integrability in interacting scalar quantum field theories

  Bence Fitos, Gábor Takács

  SciPost Phys. 15, 137 (2023) · published 5 October 2023 | Toggle abstract · pdf

  The recently proposed classification of integrability-breaking perturbations according to their strength is studied in the context of quantum field theories. Using random matrix methods to diagnose the resulting quantum chaotic behaviour, we investigate the $\phi^4$ and $\phi^6$ interactions of a massive scalar, by considering the crossover between Poissonian and Wigner-Dyson distributions in systems truncated to a finite-dimensional Hilbert space. We find that a naive extension of the scaling of crossover coupling with the volume observed in spin chains does not give satisfactory results for quantum field theory. Instead, we demonstrate that considering the scaling of the crossover coupling with the number of particles yields robust signatures, and is able to distinguish between the strengths of integrability breaking in the $\phi^4$ and $\phi^6$ quantum field theories.

  1 citation

• The SU(2)$_{D}$ lepton portals for muon g - 2, W boson mass and dark matter

  Seong-Sik Kim, Hyun Min Lee, Adriana Guerrero Menkara, Kimiko Yamashita

  SciPost Phys. Proc. 12, 045 (2023) · published 4 July 2023 | Toggle abstract · pdf

  We propose a novel model which extends the Standard Model (SM) by introducing a $SU(2)_D$ gauge symmetry. In this model, a dark $SU(2)_D$ Higgs doublet and a Higgs bi-doublet can contribute to the muon g-2 anomaly and the $W$ boson mass, remaining in tune with the recent experimental results. At the same time, the isospin charged gauge boson of $SU(2)_D$ becomes a plausible candidate for Dark Matter(DM). We find that the resulting parameter space can fit the muon g-2, the W boson mass and the DM constraints simultaneously.

  1 citation

• Ruling out QCD phase transition as a PBH origin of LIGO/Virgo events

  Joaquim Iguaz Juan

  SciPost Phys. Proc. 12, 052 (2023) · published 5 July 2023 | Toggle abstract · pdf

  The best-motivated scenario for a sizable primordial black hole (PBH) contribution to the LIGO/Virgo binary black hole mergers invokes the QCD phase transition, which naturally enhances the probability to form PBH with masses of stellar scale. We reconsider the expected mass function assuming a CMB-like primordial spectrum and associated not only to the QCD phase transition proper, but also the $e^{+}e^{-}$ annihilation process, and analyze the constraints on this scenario from a number of observations. We find that the scenario is not viable, unless an ad hoc mass evolution for the PBH mass function and a cutoff in power-spectrum very close to the QCD scale are introduced by hand.

  1 citation

• Boson-fermion pairing and condensation in two-dimensional Bose-Fermi mixtures

  Leonardo Pisani, Pietro Bovini, Fabrizio Pavan, Pierbiagio Pieri

  SciPost Phys. 18, 076 (2025) · published 3 March 2025 | Toggle abstract · pdf

  We consider a mixture of bosons and spin-polarized fermions in two dimensions at zero temperature with a tunable Bose-Fermi attraction. By adopting a diagrammatic $T$-matrix approach, we analyze the behavior of several thermodynamic quantities for the two species as a function of the density ratio and coupling strength, including the chemical potentials, the momentum distribution functions, the boson condensate density, and the Tan's contact parameter. By increasing the Bose-Fermi attraction, we find that the condensate is progressively depleted and Bose-Fermi pairs form, with a small fraction of condensed bosons surviving even for strong Bose-Fermi attraction. This small condensate proves sufficient to hybridize molecular and atomic states, producing quasi-particles with unusual Fermi liquid features. A nearly universal behavior of the condensate fraction, the bosonic momentum distribution, and Tan's contact parameter with respect to the density ratio is also found.

  1 citation

• Three dimensional topological field theories and Nahm sum formulas

  Dongmin Gang, Heeyeon Kim, Byoungyoon Park, Spencer Stubbs

  SciPost Phys. 19, 128 (2025) · published 12 November 2025 | Toggle abstract · pdf

  It is known that a large class of characters of 2d conformal field theories (CFTs) can be written in the form of a Nahm sum. In [D. Zagier, in Frontiers in number theory, physics, and geometry II, Springer, Berlin (2007)], D. Zagier identified a list of Nahm sum expressions that are modular functions under a congruence subgroup of $SL(2,\mathbb{Z})$, which can be thought of as candidates for characters of rational CFTs. Motivated by the observation that the same formulas appear as the half-indices of certain 3d $\mathcal{N}=2$ supersymmetric gauge theories, we perform a general search over low-rank 3d $\mathcal{N}=2$ Abelian Chern-Simons matter theories which either flow to unitary TFTs or $\mathcal{N}=4$ rank-zero SCFTs in the infrared. These are exceptional classes of 3d theories, which are expected to support rational and $C_2$-cofinite chiral algebras on their boundary. We compare and contrast our results with Zagier's and comment on a possible generalization of Nahm's conjecture.

  1 citation

• Confinement and false vacuum decay on the Potts quantum spin chain

  Octavio Pomponio, Anna Krasznai, Gábor Takács

  SciPost Phys. 18, 082 (2025) · published 5 March 2025 | Toggle abstract · pdf

  We consider non-equilibrium dynamics after quantum quenches in the mixed-field three-state Potts quantum chain in the ferromagnetic regime. Compared to the analogous setting for the Ising spin chain, the Potts model has a much richer phenomenology, which originates partly from baryonic excitations in the spectrum and partly from the various possible relative alignments of the initial magnetisation and the longitudinal field. We obtain the excitation spectrum by combining semiclassical approximation and exact diagonalisation, and we use the results to explain the various dynamical behaviours we observe. Besides recovering dynamical confinement, as well as Wannier-Stark localisation due to Bloch oscillations similar to the Ising chain, a novel feature is the presence of baryonic excitations in the quench spectroscopy. In addition, when the initial magnetisation and the longitudinal field are misaligned, both confinement and Bloch oscillations only result in partial localisation, with some correlations retaining an unsuppressed light-cone behaviour together with a corresponding growth of entanglement entropy.

  1 citation

• Identifying quantum phase transitions with minimal prior knowledge by unsupervised learning

  Mohamad Ali Marashli, Ho Lai Henry Lam, Hamam Mokayed, Fredrik Sandin, Marcus Liwicki, Ho-Kin Tang, Wing Chi Yu

  SciPost Phys. Core 8, 029 (2025) · published 6 March 2025 | Toggle abstract · pdf

  In this work, we proposed a novel approach for identifying quantum phase transitions in one-dimensional quantum many-body systems using AutoEncoder (AE), an unsupervised machine learning technique, with minimal prior knowledge. The training of the AEs is done with reduced density matrix (RDM) data obtained by Exact Diagonalization (ED) across the entire range of the driving parameter and thus no prior knowledge of the phase diagram is required. With this method, we successfully detect the phase transitions in a wide range of models with multiple phase transitions of different types, including the topological and the Berezinskii-Kosterlitz-Thouless transitions by tracking the changes in the reconstruction loss of the AE. The learned representation of the AE is used to characterize the physical phenomena underlying different quantum phases. Our methodology demonstrates a new approach to studying quantum phase transitions with minimal knowledge, small amount of needed data, and produces compressed representations of the quantum states.

  1 citation