Publications

Order by: publication date number of citations

• Pairing symmetry and fermion projective symmetry groups

  Xu Yang, Sayak Biswas, Shuangyuan Lu, Mohit Randeria, Yuan-Ming Lu

  SciPost Phys. 17, 161 (2024) · published 10 December 2024 | Toggle abstract · pdf

  The Ginzburg-Landau (GL) theory is very successful in describing the pairing symmetry, a fundamental characterization of the broken symmetries in a paired superfluid or superconductor. However, GL theory does not describe fermionic excitations such as Bogoliubov quasiparticles or Andreev bound states that are directly related to topological properties of the superconductor. In this work, we show that the symmetries of the fermionic excitations are captured by a Projective Symmetry Group (PSG), which is a group extension of the bosonic symmetry group in the superconducting state. We further establish a correspondence between the pairing symmetry and the fermion PSG. When the normal and superconducting states share the same spin rotational symmetry, there is a simpler correspondence between the pairing symmetry and the fermion PSG, which we enumerate for all 32 crystalline point groups. We also discuss the general framework for computing PSGs when the spin rotational symmetry is spontaneously broken in the superconducting state. This PSG formalism leads to experimental consequences, and as an example, we show how a given pairing symmetry dictates the classification of topological superconductivity.

• Time-dependent Schwinger boson mean-field theory of supermagnonic propagation in 2D antiferromagnets

  Martijn D. Bouman, Johan H. Mentink

  SciPost Phys. 17, 159 (2024) · published 9 December 2024 | Toggle abstract · pdf

  Understanding the speed limits for the propagation of magnons is of key importance for the development of ultrafast spintronics and magnonics. Recently, it was predicted that in 2D antiferromagnets, spin correlations can propagate faster than the highest magnon velocity. Here we gain deeper understanding of this supermagnonic effect based on time-dependent Schwinger boson mean-field theory. We find that the supermagnonic effect is determined by the competition between propagating magnons and a localized quasi-bound state, which is tunable by lattice coordination and quantum spin value $S$, suggesting a new scenario to enhance magnon propagation.

• Refined cyclic renormalization group in Russian doll model

  Vedant Motamarri, Ivan M. Khaymovich, Alexander Gorsky

  SciPost Phys. 17, 157 (2024) · published 6 December 2024 | Toggle abstract · pdf

  Focusing on Bethe-Ansatz integrable models, robust to both time-reversal symmetry breaking and disorder, we consider the Russian Doll Model (RDM) for finite system sizes and energy levels. Suggested as a time-reversal-symmetry breaking deformation of Richardson's model, the well-known and simplest model of superconductivity, RDM revealed an unusual cyclic renormalization group (RG) over the system size $N$, where the energy levels repeat themselves, shifted by one after a finite period in $\ln N$, supplemented by a hierarchy of superconducting condensates, with the superconducting gaps following the so-called Efimov (exponential) scaling. The equidistant single-particle spectrum of RDM made the above Efimov scaling and cyclic RG to be asymptotically exact in the wideband limit of the diagonal potential. Here, we generalize this observation in various respects. We find that, beyond the wideband limit, when the entire spectrum is considered, the periodicity of the spectrum is not constant, but appears to be energy-dependent. Moreover, we resolve the apparent paradox of shift in the spectrum by a single level after the RG period, despite the disappearance of a finite fraction of energy levels. We also analyze the effects of disorder in the diagonal potential on the above periodicity and show that it survives only for high energies beyond the energy interval of the disorder amplitude. Our analytic analysis is supported with exact diagonalization.

• Lack of near-sightedness principle in non-Hermitian systems

  Helene Spring, Viktor Könye, Anton R. Akhmerov, Ion Cosma Fulga

  SciPost Phys. 17, 153 (2024) · published 5 December 2024 | Toggle abstract · pdf

  The non-Hermitian skin effect is a phenomenon in which an extensive number of states accumulates at the boundaries of a system. It has been associated to nontrivial topology, with nonzero bulk invariants predicting its appearance and its position in real space. Here, we demonstrate that the non-Hermitian skin effect has weaker bulk-edge correspondence than topological insulators: when translation symmetry is broken by a single non-Hermitian impurity, skin modes are depleted at the boundary and accumulate at the impurity site, without changing any bulk invariant. Similarly, a single non-Hermitian impurity may deplete the states from a region of Hermitian bulk.

  1 citation

• Locality of topological dynamics in Chern insulators

  Anton Markov, Diana Golovanova, Alexander Yavorsky, Alexey Rubtsov

  SciPost Phys. 17, 152 (2024) · published 5 December 2024 | Toggle abstract · pdf

  A system having macroscopic patches in different topological phases has no well-defined global topological invariant. To treat such a case, the quantities labeling different areas of the sample according to their topological state are used, dubbed local topological markers. Here we study their dynamics. We concentrate on two quantities, namely the local Chern marker and the on-site charge induced by an applied magnetic field. The first one provides the correct information about the system's topological properties, the second can be readily measured in experiment. We demonstrate that the time-dependent local Chern marker is a much more non-local object than the equilibrium one. Surprisingly, large samples driven out of equilibrium lead to a simple description of the local Chern marker's dynamics by a local continuity equation. Also, we argue that the connection between the local Chern marker and magnetic-field induced charge known in static holds out of equilibrium in some experimentally relevant systems as well. This gives a clear physical description of the marker's evolution and provides a simple recipe for experimental estimation of the topological marker's value.

• Tunable second harmonic generation in 2D materials: Comparison of different strategies

  Simone Grillo, Elena Cannuccia, Maurizia Palummo, Olivia Pulci, Claudio Attaccalite

  SciPost Phys. Core 7, 081 (2024) · published 4 December 2024 | Toggle abstract · pdf

  Nonlinear optical frequency conversion, where optical fields interact with a nonlinear medium to generate new frequencies, is a key phenomenon in modern photonic systems. However, a major challenge with these techniques lies in the difficulty of tuning the nonlinear electrical susceptibilities that drive such effects in a given material. As a result, dynamic control of optical nonlinearities has remained largely confined to research laboratories, limiting its practical use as a spectroscopic tool. In this work, we aim to advance the development of devices with tunable nonlinear responses by exploring two potential mechanisms for electrically manipulating second-order optical nonlinearity in two-dimensional materials. Specifically, we consider two configurations: in the first, the material does not inherently exhibit second-harmonic generation (SHG), but this response is induced by an external field; in the second, an external field induces doping in a material that already exhibits SHG, altering the intensity of the nonlinear signal. In this work, we have studied these two configurations using a real-time ab-initio approach under an out-of-plane external field and including the effects of doping-induced variations in the screened electron-electron interaction. We then discuss the limitations of current computational methods and compare our results with experimental measurements.

• A $C^\ast$-algebraic view on the interaction of real- and reciprocal space topology in skyrmion crystals

  Pascal Prass, Fabian R. Lux, Emil Prodan, Duco van Straten, Yuriy Mokrousov

  SciPost Phys. Core 7, 080 (2024) · published 4 December 2024 | Toggle abstract · pdf

  Understanding the interaction of real- and reciprocal space topology in skyrmion crystals is an open problem. We approach it from the viewpoint of $C^\ast$-algebras and calculate all admissible Chern numbers of a strongly coupled tight-binding skyrmion system on a triangular lattice as a function of Fermi energy and texture parameters. Our analysis reveals the topological complexity of electronic states coupled to spin textures, and the failure of the adiabatic picture to account for it in terms of emergent electromagnetism. On the contrary, we explain the discontinuous jumps in the real-space winding number in terms of collective evolution in real-, reciprocal, and mixed space Chern numbers. Our work sets the stage for further research on topological dynamics in complex dynamic spin textures coupled to external fields.

  1 citation

• Superdiffusive transport in quasi-particle dephasing models

  Yu-Peng Wang, Chen Fang, Jie Ren

  SciPost Phys. 17, 150 (2024) · published 3 December 2024 | Toggle abstract · pdf

  Investigating the behavior of noninteracting fermions subjected to local dephasing, we reveal that quasi-particle dephasing can induce superdiffusive transport. This superdiffusion arises from nodal points within the momentum distribution of local dephasing quasi-particles, leading to asymptotic long-lived modes. By studying the dynamics of the Wigner function, we rigorously elucidate how the dynamics of these enduring modes give rise to Lévy walk processes, a renowned mechanism underlying superdiffusion phenomena. Our research demonstrates the controllability of dynamical scaling exponents by selecting quasi-particles and extends its applicability to higher dimensions, underlining the pervasive nature of superdiffusion in dephasing models.

  2 citations

• Unambiguous simulation of diffusive charge transport in disordered nanoribbons

  Henrique Pina Veiga, Simão Meneses João, João Manuel Alendouro Pinho, João Pedro Santos Pires, João Manuel Viana Parente Lopes

  SciPost Phys. 17, 149 (2024) · published 3 December 2024 | Toggle abstract · pdf

  Charge transport in disordered two-dimensional (2D) systems showcases a myriad of unique phenomenologies that highlight different aspects of the underlying quantum dynamics. Electrons in such systems undergo a crossover from ballistic propagation to Anderson localization, contingent on the system's effective coherence length. Between the extended and localized phases lies a diffusive crossover in which the charge conductivity is properly defined. The numerical observation of this regime has remained elusive because it requires fully coherent transport to be simulated in systems whose dimensions are sufficiently large to meaningfully split the mean-free path and localization length scales. To address this challenge, we employed a novel linear scaling time-resolved approach that enabled us to derive the dc-transport characteristics and observe the three expected 2D transport regimes — ballistic, diffusive, and localized.

• Duality and hidden symmetry breaking in the q-deformed Affleck-Kennedy-Lieb-Tasaki model

  Tyler Franke, Thomas Quella

  SciPost Phys. Core 7, 078 (2024) · published 2 December 2024 | Toggle abstract · pdf

  We revisit the question of string order and hidden symmetry breaking in the $q$-deformed AKLT model, an example of a spin chain that possesses generalized symmetry. We first argue that the non-local Kennedy-Tasaki duality transformation that was previously proposed to relate the string order to a local order parameter leads to a non-local Hamiltonian and thus does not provide a physically adequate description of the symmetry breaking. We then present a modified non-local transformation which is based on a recently developed generalization of Witten's Conjugation to frustration-free lattice models and capable of resolving this issue.