arXiv subjects

Pierre Pujol

Publications and source records attributed to Pierre Pujol.

At least 19 recordsLinked to original sources

arXiv2026

Tuneable skyrmion and anti-skyrmion fluids via mechanical strain in chiral kagome lattice

Magnetic skyrmions are nanometric swirling spin textures that exhibit remarkable stability at finite temperatures, making them promising candidates for spintronic applications. Achieving controllable stability and transitions between distinct topological structures is crucial for practical implementations. In this work, we investigate the effect of uniaxial mechanical strain on a magnetic model on the kagome lattice, focusing on skyrmion stability and emergent topological phases. To this end, we consider a Heisenberg model that includes exchange interactions and both in-plane and out-of-plane Dzyaloshinskii-Moriya interactions. Using a combination of Spin-Lattice Dynamics and Monte Carlo simulations, we explore uniaxial strain variations in the range of $-10\%$ to $10\%$, showing important effects on the phase diagram. For compressive strain, we find that the density of skyrmions in the skyrmion gas (SkG) phase can be tuned and that the stability of this phase extends to higher temperatures. Tensile strain, in contrast, reduces the number of skyrmions and promotes transitions to other magnetic states. Within this regime, strain levels of about ($\sim4-6\%$) lead to a change in topological charge, turning skyrmions ($Q=-1$) into antiskyrmions ($Q=+1$). We also examine how strain affects other phases commonly appearing in skyrmion-hosting systems, such as the helical and fully polarized states, showing that mechanical deformation alters their stability and characteristic properties. Finally, we compare these results with the strain response of a more conventional skyrmion model, in order to clarify the role of the different interactions involved. Our results identify strain as an experimentally accessible route for engineering topological spin textures.

cond-mat.str-el↗

arXiv2025

Pinch-line spin liquids as layered Coulomb phases and applications to cubic models

Spin liquids form fluctuating magnetic textures which have to obey certain rules imposed by frustration. These rules can often be written in the form of a Gauss law, indicating the local conservation of an emergent electric field. In reciprocal space, these emergent Gauss laws appear as singularities known as pinch points, that are accessible to neutron-scattering measurements. But more exotic forms of electromagnetism have been stabilized in spin liquids, and in a few rare instances, these zero-dimensional singularities have been extended into one-dimensional pinch lines. Here we propose a simple framework for the design of pinch-line spin liquids in a layered structure of two-dimensional algebraic spin liquids. A plethora of models can be build within this framework, as exemplified by several concrete examples where our theory is confirmed by simulations, and where the rank of the tensorial gauge field is continuously varied along the pinch line, opening new avenues in fractonic matter. Then we use our framework to understand how the evolution of the singularity pinch point along the pinch line can be understood as the interference pattern of two emergent electric fields. Finally, we apply our intuition on these emergent electric fields in real space to generic pinch line models beyond our layered framework, and revisit the recently proposed pinch line model on the octochlore lattice.

cond-mat.str-el↗

arXiv2023

Combined approach to analyze and classify families of classical spin liquids

Classical spin liquids have been a very active subject of research in the last few years. A very rich variety of cases have been shown to exist, including short-range and algebraic spin liquids displaying dipolar correlations at zero temperature. In this article, we investigate different families of classical spins liquids by combining analytical techniques and Monte Carlo simulations. Our study relies on the Luttinger-Tisza approximation (LTA), which is associated with the constraint vector function in momentum space, whose general properties allow for a classification of different spin liquids. We show that the general properties of the LTA provide a framework for identifying and accurately characterizing the different types of spin liquids in different geometries. We apply our approach to three different families of spin liquids defined on the checkerboard and kagome lattices, which exhibit a remarkable range of situations, including various cases of algebraic and short-range spin liquids. Additionally, we analyze the effective Gauss law emerging from different kinds of spin liquids and identify states that exhibit additional degeneracy lines. The presence of spin-liquid phases and pinch-point singularities are confirmed by Monte Carlo simulations validating our approach. Our study opens up avenues of research in the study of spin liquids, exploring algebraic spin liquids with higher-rank gauge fields and as critical points dividing different types of classical spin liquids.

cond-mat.str-el↗

arXiv2023

Revisiting the electronic properties of dislocated graphene sheets

The interplay between topological defects, such as dislocations or disclinations, and the electronic degrees of freedom in graphene has been extensively studied. In the literature, for the study of this kind of problems, it is in general used either a gauge theory or a curved spatial Riemannian geometry approach, where, in the geometric case, the information about the defects is contained in the metric and the spin-connection. However, these topological defects can also be associated to a Riemann-Cartan geometry where curvature and torsion plays an important role. In this article we study the interplay between a wedge dislocations in a planar graphene sheet and the properties of its electronic degrees of freedom. Our approach relies in its relation with elasticity theory through the so called elastic-gauge, where their typical coefficients, as for example the Poisson's ratio, appear directly in the metric, and consequently also in the electronic spectrum.

cond-mat.str-el↗

arXiv2022

Skyrmion fluid and bimeron glass protected by a chiral spin liquid on a kagome lattice

Skyrmions are of interest both from a fundamental and technological point of view, due to their potential to act as information carriers. But one challenge concerns their manipulation, especially at high temperature where thermal fluctuations eventually disintegrate them. Here we study the competition between skyrmions and a chiral spin liquid, using the latter as an entropic buffer to impose a quasi-vacuum of skyrmions. As a result, the temperature becomes a knob to tune the skyrmion density from a dense liquid to a diluted gas, protecting the integrity of each skyrmion from paramagnetic disintegration. With this additional knob in hand, we find at high field a topological spin glass made of zero- and one-dimensional topological defects (resp. skyrmions and bimerons).

cond-mat.stat-mech↗

arXiv2021

Chiral phase transition and thermal Hall effect in an anisotropic spin model on the kagome lattice

We present a study of the thermal Hall effect in the extended Heisenberg model with $XXZ$ anisotropy in the kagome lattice. This model has the particularity that, in the classical case, and for a broad region in parameter space, an external magnetic field induces a chiral symmetry breaking: the ground state is a doubly degenerate $q=0$ order with either positive or negative net chirality. Here, we focus on the effect of this chiral phase transition in the thermal Hall conductivity using Linear-Spin-Waves theory. We explore the topology and calculate the Chern numbers of the magnonic bands, obtaining a variety of topological phase transitions. We also compute the magnonic effect to the critical temperature associated with the chiral phase transition ($T_c^{SW}$). Our main result is that, the thermal Hall conductivity, which is null for $T>T_c^{SW}$, becomes non-zero as a consequence of the spontaneous chiral symmetry breaking at low temperatures. Therefore, we present a simple model where it is possible to "switch" on/off the thermal transport properties introducing a magnetic field and heating or cooling the system.

cond-mat.str-el↗

arXiv2017

Field theory of symmetry-protected valence bond solid states in (2+1) dimensions

This paper describes a semiclassical field-theory approach to the topological properties of spatially featureless Affleck-Kennedy-Lieb-Tasaki type valence bond solid ground states of antiferromagnets in spatial dimensions one to three. Using nonlinear sigma models set in the appropriate target manifold and augmented with topological terms, we argue that the path integral representation of the ground-state wave functional can correctly distinguish symmetry-protected topological ground states from topologically trivial ones. The symmetry-protection feature is demonstrated explicitly in terms of a dual field theory, where we take into account the nontrivial spatial structure of topological excitations, which are caused by competition among the relevant ordering tendencies. A temporal surface contribution to the action originating from the bulk topological term plays a central role in our study. We discuss how the same term governs the behavior of the so-called strange correlator. In particular, we find that the path integral expression for the strange correlator in two dimensions reduces to the well-known Haldane expression for the two point spin correlator of antiferromagnetic spin chains.

cond-mat.str-el↗

arXiv2016

$Z_2$ antiferromagnetic topological insulators with broken $C_4$ symmetry

A two-dimensional topological insulator may arise in a centrosymmetric commensurate Néel antiferromagnet (AF), where staggered magnetization breaks both the elementary translation and time reversal, but retains their product as a symmetry. Fang et al.[Phys. Rev. B 88, 085406 (2013)] proposed an expression for a $Z_2$ topological invariant to characterize such systems. Here, we show that this expression does not allow to detect all the existing phases if a certain lattice symmetry is lacking. We implement numerical techniques to diagnose topological phases of a toy Hamiltonian, and verify our results by computing the Chern numbers of degenerate bands, and also by explicitly constructing the edge states, thus illustrating the efficiency of the method.

cond-mat.str-el↗

arXiv2016

Identifying two-dimensional $Z_2$ antiferromagnetic topological insulators

We revisit the question of whether a two-dimensional topological insulator may arise in a commensurate Néel antiferromagnet, where staggered magnetization breaks both the elementary translation and time reversal, but retains their product as a symmetry. In contrast to the so-called $Z_2$ topological insulators, an exhaustive characterization of antiferromagnetic topological phases with the help of a topological invariant has been missing. We analyze a simple model of an antiferromagnetic topological insulator and chart its phase diagram based on a recently proposed criterion for centrosymmetric systems [Fang et al., Phys. Rev. B 88, 085406 (2013)]. We then adapt two methods, originally designed for paramagnetic systems, and make antiferromagnetic topological phases manifest. The proposed methods apply far beyond the particular example treated in this work, and admit straightforward generalization. We illustrate this by considering a non-centrosymmetric system, where there are no simple criteria to identify topological phases. We also present an explicit construction of edge states in an antiferromagnetic topological insulator.

cond-mat.str-el↗

arXiv2015

Protection against Spin Gap in 2-d Insulating Antiferromagnets with a Chern-Simons Term

We propose a novel mechanism for the protection against spin gapped states in doped antiferromagnets. It requires the presence of a Chern-Simons term that can be generated by a coupling between spin and an insulator. We first demonstrate that in the presence of this term the vortex loop excitations of the spin sector behave as anyons with fractional statistics. To generate such term, the fermions should have massive Dirac spectrum coupled to the emergent spin field of the spin sector. The Dirac spectrum can be realized by a planar spin configuration arising as the lowest-energy configuration of a square lattice antiferromagnet Hamiltonian involving a Dzyaloshinskii-Moriya interaction. The mass is provided by a combination of dimerization and staggered chemical potential. We finally show that for realistic parameters, anyonic vortex loop condensation will likely never occur and thus the spin gapped state is prevented. We also propose real magnetic materials for an experimental verification of our theory.

cond-mat.str-el↗

arXiv2015

Three-sublattice Skyrmion crystal in the antiferromagnetic triangular lattice

The frustrated classical antiferromagnetic Heisenberg model with Dzyaloshinskii-Moriya (DM) interactions on the triangular lattice is studied under a magnetic field by means of semiclassical calculations and large-scale Monte Carlo simulations. We show that even a small DM interaction induces the formation of an Antiferromagnetic Skyrmion crystal (AF-SkX) state. Unlike what is observed in ferromagnetic materials, we show that the AF-SkX state consists of three interpenetrating Skyrmion crystals (one by sublattice), and most importantly, the AF-SkX state seems to survive in the limit of zero temperature. To characterize the phase diagram we compute the average of the topological order parameter which can be associated to the number of topological charges or Skyrmions. As the magnetic field increases this parameter presents a clear jump, indicating a discontinuous transition from a spiral phase into the AF-SkX phase, where multiple Bragg peaks coexist in the spin structure factor. For higher fields, a second (probably continuous) transition occurs into a featureless paramagnetic phase.

cond-mat.str-el↗

arXiv2015

Diagnosing a strong topological insulator by quantum oscillations

We show how quantum oscillation measurements of surface states in an insulator may allow to diagnose a strong topological insulator and distinguish it from its weak or topologically trivial counterpart. The criterion is defined by the parity of the number of fundamental frequencies in the surface-state quantum oscillation spectrum: an even number of frequencies implies a weak or a topologically trivial insulator, whereas an odd number points to a strong topological insulator. We also discuss various aspects and issues related to applying this criterion in practice.

cond-mat.str-el↗

arXiv2015

Hole Properties On and Off Magnetization Plateaus in 2-d Antiferromagnets

The phenomenon of magnetization plateaus in antiferromagnets under magnetic field has always been an important topic in magnetism. We propose to probe the elusive physics of plateaus in 2-d by considering hole-doped antiferromagnet and studying the signatures of magnetization plateaus in terms of the properties of holes, coupled to an effective gauge field generated by the spin sector. The latter mediates interaction between the holes, found to be algebraically decaying long-ranged with both Coulombic and dipolar forms outside plateau and short-ranged (local) inside plateau. The resulting hole spectral weight is significantly broadened off-plateau, while it remains sharply-peaked on-plateau. We also extend the result obtained for 1-d system where finite hole doping gives rise to a shift in the magnetization value of the plateaus.

cond-mat.str-el↗

arXiv2007

Zero-temperature Kosterlitz-Thouless transition in a two-dimensional quantum system

We construct a local interacting quantum dimer model on the square lattice, whose zero-temperature phase diagram is characterized by a line of critical points separating two ordered phases of the valence bond crystal type. On one side, the line of critical points terminates in a quantum transition inherited from a Kosterlitz-Thouless transition in an associated classical model. We also discuss the effect of a longer-range dimer interactions that can be used to suppress the line of critical points by gradually shrinking it to a single point. Finally, we propose a way to generalize the quantum Hamiltonian to a dilute dimer model in presence of monomers and we qualitatively discuss the phase diagram.

cond-mat.str-el↗

arXiv2006

Strong disorder fixed points in the two-dimensional random-bond Ising model

The random-bond Ising model on the square lattice has several disordered critical points, depending on the probability distribution of the bonds. There are a finite-temperature multicritical point, called Nishimori point, and a zero-temperature fixed point, for both a binary distribution where the coupling constants take the values +/- J and a Gaussian disorder distribution. Inclusion of dilution in the +/- J distribution (J=0 for some bonds) gives rise to another zero-temperature fixed point which can be identified with percolation in the non-frustrated case (J >= 0). We study these fixed points using numerical (transfer matrix) methods. We determine the location, critical exponents, and central charge of the different fixed points and study the spin-spin correlation functions. Our main findings are the following: (1) We confirm that the Nishimori point is universal with respect to the type of disorder, i.e. we obtain the same central charge and critical exponents for the +/- J and Gaussian distributions of disorder. (2) The Nishimori point, the zero-temperature fixed point for the +/- J and Gaussian distributions of disorder, and the percolation point in the diluted case all belong to mutually distinct universality classes. (3) The paramagnetic phase is re-entrant below the Nishimori point, i.e. the zero-temperature fixed points are not located exactly below the Nishimori point, neither for the +/- J distribution, nor for the Gaussian distribution.

cond-mat.stat-mech↗

arXiv2005

High-Temperature Criticality in Strongly Constrained Quantum Systems

The exotic nature of many strongly correlated materials at reasonably high temperatures, for instance cuprate superconductors in their normal state, has lead to the suggestion that such behavior occurs within a quantum critical region where the physics is controlled by the influence of a phase transition down at zero temperature. Such a scenario can be thought of as a bottom-up approach, with the zero temperature mechanisms finding a way to manifest critical behavior at high temperatures. Here we propose an alternative, top-down, mechanism by which strong kinematic constraints that can only be broken at extremely high temperatures are responsible for critical behavior at intermediate but still high temperatures. This critical behavior may extend all the way down to zero temperature, but this outcome is not one of necessity, and the system may order at low temperatures. We provide explicit examples of such high-temperature criticality when additional strong interactions are introduced in quantum Heisenberg, transverse field Ising, and some bosonic lattice models.

cond-mat.str-el↗

arXiv2005

Quantum three-coloring dimer model and the disruptive effect of quantum glassiness on its line of critical points

We construct a quantum extension of the (classical) three-coloring model introduced by Baxter [J.Math.Phys.11, 784 (1970)] for which the ground state can be computed exactly along a continuous line of Rokhsar-Kivelson solvable points. The quantum model, which admits a local spin representation, displays at least three different phases; an antiferromagnetic (AF) phase, a line of quantum critical points, and a ferromagnetic (F) phase. We argue that, in the ferromagnetic phase, the system cannot reach dynamically the quantum ground state when coupled to a bath through local interactions, and thus lingers in a state of quantum glassiness.

cond-mat.str-el↗

arXiv2005

Random quantum Ising chains with competing interactions

In this paper we discuss the criticality of a quantum Ising spin chain with competing random ferromagnetic and antiferromagnetic couplings. Quantum fluctuations are introduced via random local transverse fields. First we consider the chain with couplings between first and second neighbors only and then generalize the study to a quantum analog of the Viana-Bray model, defined on a small world random lattice. We use the Dasgupta-Ma decimation technique, both analytically and numerically, and focus on the scaling of the lattice topology, whose determination is necessary to define any infinite disorder transition beyond the chain. In the first case, at the transition the model renormalizes towards the chain, with the infinite disorder fixed point described by Fisher. This corresponds to the irrelevance of the competition induced by the second neighbors couplings. As opposed to this case, this infinite disorder transition is found to be unstable towards the introduction of an arbitrary small density of long range couplings in the small world models.

cond-mat.dis-nn↗