Publications

Order by: publication date number of citations

• Polariton condensation into vortex states in the synthetic magnetic field of a strained honeycomb lattice

  Cristóbal Lledó, Iacopo Carusotto, Marzena H. Szymańska

  SciPost Phys. 12, 068 (2022) · published 18 February 2022 | Toggle abstract · pdf

  Photonic materials are a rapidly growing platform for studying condensed matter physics with light, where the exquisite control capability is allowing us to learn about the relation between microscopic dynamics and macroscopic properties. One of the most interesting aspects of condensed matter is the interplay between interactions and the effect of an external magnetic field or rotation, responsible for a plethora of rich phenomena---Hall physics and quantized vortex arrays. At first sight, however, these effects for photons seem vetoed: they do not interact with each other and they are immune to magnetic fields and rotations. Yet in specially devised structures these effects can be engineered. Here, we propose the use of a synthetic magnetic field induced by strain in a honeycomb lattice of resonators to create a non-equilibrium Bose-Einstein condensate of light-matter particles (polaritons) in a rotating state, without the actual need for external rotation nor reciprocity-breaking elements. We show that thanks to the competition between interactions, dissipation and a suitably designed incoherent pump, the condensate spontaneously becomes chiral by selecting a single Dirac valley of the honeycomb lattice, occupying the lowest Landau level and forming a vortex array. Our results offer a new platform where to study the exciting physics of arrays of quantized vortices with light and pave the way to explore the transition from a vortex-dominated phase to the photonic analogue of the fractional quantum Hall regime.

  5 citations

• Entanglement entropy in quantum spin chains with broken parity number symmetry

  Arash Jafarizadeh, M. A. Rajabpour

  SciPost Phys. 12, 195 (2022) · published 14 June 2022 | Toggle abstract · pdf

  Consider a generic quantum spin chain that can be mapped to free quadratic fermions via Jordan-Wigner (JW) transformation. In the presence of arbitrary boundary magnetic fields, this Hamiltonian is no longer a quadratic Hamiltonian after JW transformation. Using ancillary sites and enlarging the Hamiltonian we first introduce a bigger quadratic Hamiltonian. Then we diagonalize this enlarged Hamiltonian in its most generic form and show that all the states are degenerate because of the presence of a zero mode. The eigenstates of the original spin chain with boundary magnetic fields can be derived after appropriate projection. We study in-depth the properties of the eigenstates of the enlarged Hamiltonian. In particular, we find: 1) the eigenstates in configuration bases, 2) calculate all the correlation functions, 3) find the reduced density matrices, 4) calculate the entanglement entropy. We show that the generic eigenstate of the enlarged Hamiltonian (including the eigenstates of the original spin chain) breaks the parity number symmetry and consequently one needs to take care of some technicalities regarding the calculation of the reduced density matrix and entanglement entropy. Interestingly we show that the entanglement structure of these eigenstates is quite universal and independent of the Hamiltonian. We support our results by applying them to a couple of examples.

  5 citations

• Annealed averages in spin and matrix models

  Laura Foini, Jorge Kurchan

  SciPost Phys. 12, 080 (2022) · published 3 March 2022 | Toggle abstract · pdf

  A disordered system is denominated 'annealed' when the interactions themselves may evolve and adjust their values to lower the free energy. The opposite ('quenched') situation when disorder is fixed, is the one relevant for physical spin-glasses, and has received vastly more attention. Other problems however are more natural in the annealed situation: in this work we discuss examples where annealed averages are interesting, in the context of matrix models. We first discuss how in practice, when system and disorder adapt together, annealed systems develop `planted' solutions spontaneously, as the ones found in the study of inference problems. In the second part, we study the probability distribution of elements of a matrix derived from a rotationally invariant (not necessarily Gaussian) ensemble, a problem that maps into the annealed average of a spin glass model.

  5 citations

• Quantum phases of hardcore bosons with repulsive dipolar density-density interactions on two-dimensional lattices

  J. A. Koziol, G. Morigi, K. P. Schmidt

  SciPost Phys. 17, 111 (2024) · published 10 October 2024 | Toggle abstract · pdf

  We analyse the ground-state quantum phase diagram of hardcore Bosons interacting with repulsive dipolar potentials. The bosons dynamics is described by the extended-Bose-Hubbard Hamiltonian on a two-dimensional lattice. The ground state results from the interplay between the lattice geometry and the long-range interactions, which we account for by means of a classical spin mean-field approach limited by the size of the considered unit cells. This extended classical spin mean-field theory accounts for the long-range density-density interaction without truncation. We consider three different lattice geometries: square, honeycomb, and triangular. In the limit of zero hopping the ground state is always a devil's staircase of solid (gapped) phases. Such crystalline phases with broken translational symmetry are robust with respect to finite hopping amplitudes. At intermediate hopping amplitudes, these gapped phases melt, giving rise to various lattice supersolid phases, which can have exotic features with multiple sublattice densities. At sufficiently large hoppings the ground state is a superfluid. The stability of phases predicted by our approach is gauged by comparison to the known quantum phase diagrams of the Bose-Hubbard model with nearest-neighbour interactions as well as quantum Monte Carlo simulations for the dipolar case on the square and triangular lattice. Our results are of immediate relevance for experimental realisations of self-organised crystalline ordering patterns in analogue quantum simulators, e.g., with ultracold dipolar atoms in an optical lattice.

  5 citations

• Density matrices in integrable face models

  Holger Frahm, Daniel Westerfeld

  SciPost Phys. 11, 057 (2021) · published 14 September 2021 | Toggle abstract · pdf

  Using the properties of the local Boltzmann weights of integrable interaction-round-a-face (IRF or face) models we express local operators in terms of generalized transfer matrices. This allows for the derivation of discrete functional equations for the reduced density matrices in inhomogeneous generalizations of these models. We apply these equations to study the density matrices for IRF models of various solid-on-solid type and quantum chains of non-Abelian ${SU(2)_3}$ or Fibonacci anyons. Similar as in the six vertex model we find that reduced density matrices for a sequence of consecutive sites can be 'factorized', i.e.\ expressed in terms of nearest-neighbour correlators with coefficients which are independent of the model parameters. Explicit expressions are provided for correlation functions on up to three neighbouring sites.

  5 citations

• Entanglement gap, corners, and symmetry breaking

  Vincenzo Alba

  SciPost Phys. 10, 056 (2021) · published 8 March 2021 | Toggle abstract · pdf

  We investigate the finite-size scaling of the lowest entanglement gap $\delta\xi$ in the ordered phase of the two-dimensional quantum spherical model (QSM). The entanglement gap decays as $\delta\xi=\Omega/\sqrt{L\ln(L)}$. This is in contrast with the purely logarithmic behaviour as $\delta\xi=\pi^2/\ln(L)$ at the critical point. The faster decay in the ordered phase reflects the presence of magnetic order. We analytically determine the constant $\Omega$, which depends on the low-energy part of the model dispersion and on the geometry of the bipartition. In particular, we are able to compute the corner contribution to $\Omega$, at least for the case of a square corner.

  5 citations

• A note on the identity module in $c=0$ CFTs

  Yifei He, Hubert Saleur

  SciPost Phys. 12, 100 (2022) · published 21 March 2022 | Toggle abstract · pdf

  It has long been understood that non-trivial Conformal Field Theories (CFTs) with vanishing central charge ($c=0$) are logarithmic. So far however, the structure of the identity module -- the (left and right) Virasoro descendants of the identity field -- had not been elucidated beyond the stress-energy tensor $T$ and its logarithmic partner $t$ (the solution of the "$c\to 0$ catastrophe"). In this paper, we determine this structure together with the associated OPE of primary fields up to level $h=\bar{h}=2$ for polymers and percolation CFTs. This is done by taking the $c\to 0$ limit of $O(n)$ and Potts models and combining recent results from the bootstrap with arguments based on conformal invariance and self-duality. We find that the structure contains a rank-3 Jordan cell involving the field $T\bar{T}$, and is identical for polymers and percolation. It is characterized in part by the common value of a non-chiral logarithmic coupling $a_0=-{25\over 48}$.

  5 citations

• Dirac fermions and topological phases in magnetic topological insulator films

  Kai-Zhi Bai, Bo Fu, Shun-Qing Shen

  SciPost Phys. 17, 146 (2024) · published 27 November 2024 | Toggle abstract · pdf

  We develop a Dirac fermion theory for topological phases in magnetic topological insulator films. The theory is based on exact solutions of the energies and the wave functions for an effective model of the three-dimensional topological insulator (TI) film. It is found that the TI film consists of a pair of massless or massive Dirac fermions for the surface states, and a series of massive Dirac fermions for the bulk states. The massive Dirac fermion always carries zero or integer quantum Hall conductance when the valence band is fully occupied while the massless Dirac fermion carries a one-half quantum Hall conductance when the chemical potential is located around the Dirac point for a finite range. The magnetic exchange interaction in the magnetic layers in the film can be used to manipulate either the masses or chirality of the Dirac fermions and gives rise to distinct topological phases, which cover the known topological insulating phases, such as the quantum anomalous Hall effect, quantum spin Hall effect and axion effect, and also the novel topological metallic phases, such as the half-quantized Hall effect, half quantum mirror Hall effect, and metallic quantum anomalous Hall effect.

  5 citations

• Quenching Factor estimation of Na recoils in NaI(Tl) crystals using a low-energy pulsed neutron beam measurement

  Mukund Raghunath Bharadwaj, G. Angloher, P. S. Barbeau, I. Dafinei, N. Di Marco, L. Einfalt, F. Ferroni, S. Fichtinger, A. Filipponi , T. Frank , M. Friedl , A. Fuss, Z. Ge , S. Hedges, M. Heikinheimo, K. Huitu, M. Kellermann, R. Maji, M. Mancuso, L. Pagnanini, F. Petricca, S. Pirro, F. Pröbst , G. Profeta, A. Puiu, F. Reindl , K. Schäffner, J. Schieck, D. Schmiedmayer , C. Schwertner , M. Stahlberg , A. Stendahl, M. Stukel , F. Wagner, S. Yue, V. Zema, Y. Zhu

  SciPost Phys. Proc. 12, 028 (2023) · published 4 July 2023 | Toggle abstract · pdf

  NaI(Tl) based scintillation detectors have become a staple in the field of direct dark matter searches, with the DAMA-LIBRA experiment being the standout for its reported observation of an annually modulating WIMP-like signal which is in direct contrast with other results. In order to accurately calibrate the energies of WIMP-induced nuclear recoil signals, precise measurements of the quenching factor of the NaI crystals are essential for each of these experiments, as it is well established that electron recoils and nuclear recoils have dissimilar scintillation light yields. In this contribution, we present first preliminary results of an ongoing systematic study that has been carried out by the COSINUS collaboration and Duke University to measure the quenching factor of Na recoils. Five ultra-pure NaI crystals, manufactured by the Shanghai Institute for Ceramics, each of which have varying Tl dopant concentrations, were irradiated with a mono-energetic neutron beam at the Triangle Universities National Laboratory, North Carolina, USA to extract the quenching factor values in the low recoil energies of 1-30keV$_{nr}$.

  5 citations

• Scattering Amplitudes and BCFW in $\mathcal{N}=2^*$ Theory

  Md. Abhishek, Subramanya Hegde, Dileep P. Jatkar, Arnab Priya Saha

  SciPost Phys. 13, 008 (2022) · published 25 July 2022 | Toggle abstract · pdf

  We use massive spinor helicity formalism to study scattering amplitudes in $\mathcal{N}=2^*$ super-Yang-Mills theory in four dimensions. We compute the amplitudes at an arbitrary point in the Coulomb branch of this theory. We compute amplitudes using projection from $\mathcal{N}=4$ theory and write three point amplitudes in a convenient form using special kinematics. We then compute four point amplitudes by carrying out massive BCFW shifts of the amplitudes. We find some of the shifted amplitudes have a pole at $z=\infty$. Taking the residue at $z=\infty$ into account ensures little group covariance of the final result.

  5 citations