arXiv subjects

Julián Faúndez

Publications and source records attributed to Julián Faúndez.

8 recordsLinked to original sources

arXiv2026

Real-space topology and charge order in the Haldane-Holstein Model

We study the half-filled Haldane-Holstein model, where a paradigmatic Chern insulator is coupled to fully dynamical phonons, and provide an unbiased characterization of how retarded electron-phonon interactions destabilize Chern topology. Using determinant quantum Monte Carlo, we find that increasing the coupling drives an abrupt, first-order transition from a Chern insulator to a staggered charge-density wave that acts as a dynamical sublattice (Semenoff) mass. The transition is simultaneously signaled by a nearly quantized many-body Bott index and a real-space local Chern marker constructed from the interacting Green's function, both of which collapse as the charge order parameter becomes extensive. Spectral and open-boundary calculations reveal concomitant gap closing and the loss of boundary spectral weight at the critical coupling. Despite the generic phase problem induced by broken time-reversal symmetry, we show that it remains mild in the low-frequency regime and that the average phase factor sharply tracks the Chern insulator-charge density wave boundary. Our results establish a concrete route by which electron-phonon coupling can trigger a discontinuous collapse of Chern topology and provide experimentally relevant signatures for correlated topological platforms.

cond-mat.str-el↗

arXiv2026

Inverse determination of light-matter coupling in disordered systems from transmittance spectra

We investigate quantum inverse problems in one-dimensional (1D) electronic disordered systems strongly coupled to optical cavities. More specifically, we consider the Anderson and the Aubry-Andre-Harper models connected to electronic reservoirs and embedded in a single-mode optical cavity. The light-matter interaction enables photon-assisted hopping processes that significantly modify the transmittance spectrum. Within the nonequilibrium Green's function formalism, we implement an inversion-based approach capable of accurately extracting the electron-photon coupling strength directly from transmittance spectra. While cavity coupling acts as a minor perturbation within the Anderson model, yielding broad yet precise parameter estimates, its influence is markedly different in the Aubry-André-Harper model. The latter exhibits a sharp metal-insulator transition in 1D, thus resulting in more pronounced cavity-induced spectral changes. This renders even more accurate inverse solutions, offering unparalleled precision in the characterization of low-dimensional disordered systems. Altogether, our results demonstrate that the quantum inverse problem provides a robust diagnostic tool for quantum materials, particularly effective for systems exhibiting metal-insulator transitions.

cond-mat.mes-hall↗

arXiv2026

Quantum percolation in honeycomb lattices under random spin-orbit coupling

We investigate quantum percolation in a honeycomb lattice with site dilution and random spin-orbit coupling. Using exact diagonalization combined with finite-size scaling analysis, we study the metal-insulator transition, extracting the quantum percolation threshold $p_q$, and the correlation-length exponent, $ν$. In the absence of spin-orbit coupling, we find that $p_q$ remains finite and demonstrate that the quantum threshold is significantly higher than the classical site-percolation threshold $p_c$ of the honeycomb lattice. When spin-orbit coupling is present, the spectral statistics exhibit a crossover from the Gaussian orthogonal ensemble to the Gaussian symplectic ensemble, reflecting the change in symmetry class. Simultaneously, the quantum percolation threshold shifts systematically to lower occupation probabilities, indicating that the spin-orbit coupling favors delocalization. For sufficiently strong spin-orbit coupling, $p_q$ tends to saturate, while the critical exponent approaches the expected one of the two-dimensional symplectic universality class.

cond-mat.dis-nn↗

arXiv2025

Spin-orbit coupled periodic Anderson model: Kondo-Dirac semimetal and orbital-selective antiferromagnetic semimetal

We investigate the periodic Anderson model composed of an itinerant $c$-band and a strongly localized $f$-band, featuring on-site electron-electron interactions in the $f$-orbitals. The two bands interact via a hybridization term with spin-orbit coupling, which enables spin-flip processes. In the non-interacting limit, these profoundly alter the electronic structure, leading to the emergence of flat bands, van Hove singularities, and, most notably, Dirac cones within a single Kondo-Dirac semimetal order. The strongly interacting regime is explored via the determinant quantum Monte Carlo method, in the absence of the sign problem, where we unveil a complete ground-state phase diagram revealing two distinct phases, the Kondo-Dirac semimetal phase and a novel antiferromagnetic semimetal phase. Their characterization by the spectral functions establishes an orbital-selective Mott transition in the antiferromagnetic semimetal phase, marked by the opening of a gap exclusively in the $f$-orbital while Dirac cones persist in the $c$-orbital. Conversely, in the Kondo-Dirac semimetal phase, both $c$- and $f$-orbitals sustain robust Dirac cones. We establish that spin-orbit coupling in the hybridization term gives rise to Dirac cones, which, combined with additional symmetry-breaking conditions, can generate novel topological states.

cond-mat.str-el↗

arXiv2025

The Two-Dimensional Rashba-Holstein Model: A Quantum Monte Carlo Approach

In this work, we investigate the impact of Rashba spin-orbit coupling (RSOC) on the formation of charge-density wave (CDW) and superconducting (SC) phases in the Holstein model on a half-filled square lattice. Using unbiased finite-temperature Quantum Monte Carlo simulations, we go beyond mean-field approaches to determine the ground state order parameter as a function of RSOC and phonon frequency. Our results reveal that the Rashba metal is unstable due to particle-hole instabilities, favoring the emergence of a CDW phase for any RSOC value. In the limit of a pure Rashba hopping, the model exhibits a distinct behavior with the appearance of four Weyl cones at half-filling, where quantum phase transitions are expected to occur at strong interactions. Indeed, a quantum phase transition, belonging to the Gross-Neveu Ising universality class between a semi-metal and CDW emerges at finite phonon frequency dependent coupling $λ_c$. In the antiadiabatic limit we observe an enhanced symmetry in the infrared that unifies SC and CDW orders. These results advance our understanding of competing CDW and SC phases in systems with spin-orbit coupling, providing insights that may help clarify the behavior of related materials.

cond-mat.str-el↗

arXiv2025

Multifractal critical phase driven by coupling quasiperiodic systems to electromagnetic cavities

We theoretically investigate criticality and multifractal states in a one-dimensional Aubry-Andre-Harper model coupled to electromagnetic cavities. We focus on two specific cases where the phonon frequencies are $ω_{0}=1$ and $ω_{0}=2$, respectively. Phase transitions are analyzed using both the average and minimum inverse participation ratio to identify metallic, fractal, and insulating states. We provide numerical evidence to show that the presence of the optical cavity induces a critical, intermediate phase in between the extended and localized phases, hence drastically modifying the traditional transport phase diagram of the Aubry-Andre-Harper model, in which critical states can only exist at the well-defined metal-insulator critical point. We also investigate the probability distribution of the inverse participation ratio and conduct a multifractal analysis to characterize the nature of the critical phase, in which we show that extended, localized, and fractal eigenstates coexist. Altogether, our findings reveal the pivotal role that the coupling to electromagnetic cavities plays in tailoring critical transport phenomena at the microscopic level of the eigenstates.

cond-mat.dis-nn↗

arXiv2025

Finite temperatures and flat bands: the Hubbard model on three-dimensional Lieb lattices

We investigate some thermodynamic and magnetic properties of the Hubbard model on two three-dimensional extensions of the Lieb lattice: the perovskite Lieb lattice (PLL) and the layered Lieb lattice (LLL). Using determinant quantum Monte Carlo (DQMC) simulations alongside Hartree-Fock and cluster mean-field theory (CMFT) approaches, we analyze how flat-band degeneracy, connectivity, and lattice anisotropy influence the emergence of magnetic order. Our results show that both geometries support finite-temperature magnetic transitions, namely ferromagnetic (FM) on the PLL, and antiferromagnetic (AFM) on the LLL. Further, we have established that the critical temperature, $T_c$, as a function of the uniform on-site coupling, $U$, displays a maximum, which is smaller in the AFM case than in the FM one, despite the absence of flat bands in the LLL. We also provide numerical evidence to show that flat bands in the PLL rapidly generate magnetic moments, but a small interorbital coordination suppresses the increase of $T_c$ at large interaction strength $U/t$. By contrast, the LLL benefits from higher connectivity, favoring magnetic order even in the absence of flat bands. The possibilities of anisotropic interlayer hoppings and inhomogeneous on-site interactions were separateley explored. We have found that magnetism in the PLL is hardly affected by hopping anisotropy, since the main driving mechanism is the preserved flat band; for the LLL, by contrast, spectral weight is removed from $d$-sites, which increases $T_c$ more significantly. At mean-field level, we have obtained that setting $U=0$ on $p$ sites and $U=U_d\neq0$ on $d$ sites leads to a quantum critical point at some $U_d$; this behavior was not confirmed by our DQMC simulations.

cond-mat.str-el↗

arXiv2021

Magnetic transitions induced by pressure and magnetic field in a two-orbital $5f$-electron model in cubic and tetragonal lattices

We investigate the onset and evolution of under the simultaneous application of pressure and magnetic field of distinct itinerant Néel states using the underscreened Anderson Lattice Model (UALM) which has been proposed to describe $5f$-electron systems. The model is composed by two narrow $f$-bands (of either $α$ or $β$ character) that hybridize with a wide $d$-band and local $5f$-electron interactions. We consider both cubic and tetragonal lattices. The Néel order parameters $ϕ^β$ and $ϕ^α$ are assumed to be fixed by an Ising anisotropy. The applied magnetic field $h_z$ is parallel to the anisotropy axis. It has been assumed that the variation of the band width $W$ is sensitive to pressure. In the absence of a magnetic field, the increase of $W$ takes the system from the phase AF$_1$ to another phase AF$_2$. The phase AF$_1$ occurs when $ϕ^β>ϕ^α>0$ while in the AF$_2$ phase the gaps satisfy $ϕ^α>ϕ^β>0$. In the presence of a magnetic field $h_z$, the phase AF$_2$ is quickly suppressed and reappears again at intermediate values of the magnetic field while it is predominant at higher magnetic fields. The analysis of the partial density of states close to the phase transition between the phases AF$_1$ and AF$_2$, allows a better understanding the mechanism responsible whereby the transition is induced by an increase in the magnetic field. As a important general result, we found that the magnetic field $h_z$ favours the phase AF$_2$ while the phase AF$_1$ is suppressed. For the tetragonal lattice, the phase AF$_2$ is even more favored when $h_z$ and $c/a$ increases concomitantly, where $c$ and $a$ are the lattice parameters.

cond-mat.str-el↗