arXiv subjects

A. A. Aligia

Publications and source records attributed to A. A. Aligia.

At least 19 recordsLinked to original sources

arXiv2026

Phase transitions through excited-state level crossings and topological indicators: the case of the XXZ chain with staggered Ising interaction

We combine two ways of determining the phase diagram of the spin-$1/2$ XXZ chain with a staggered Ising interaction and uniform transverse exchange, based on exact diagonalization. The model realizes a competition between Néel order and bond-dimerized phases generated by the alternating Ising interaction. The simplest approach to determine the phase boundaries is to use topological indicators based on generalized position operators (GPOs). We show that in general, the bosonized and numerical results for the topological indicators agree. The second is the method of crossings of excited energy levels (MCEL), which is justified by conformal field theory. Despite the partial loss of translational symmetry induced by the alternating Ising interaction, we show that, with the aid of the GPO to identify the relevant level crossings, the MCEL provides an accurate determination of the phase boundary between the Néel and dimerized phases. While the jumps of a topological indicator based on a GPO provide a qualitatively correct phase diagram, its accuracy is affected when the gap is very small (or the correlation length very large) at one side of the transition, as we show using field-theoretical arguments. The combination of both methods provides a more efficient way of calculating phase diagrams for correlated one-dimensional models than other widely used conventional approaches.

cond-mat.str-el↗

arXiv2026

Topological flat bands emerging at the inversion of stacking order in rhombohedral graphite

Motivated by the indications of high-Tc superconductivity in natural graphite enriched in the rhombohedral phase, we study the band structure of several stacking configurations that combine two of the three graphite structures as well as modifications of the rhombohedral sequence (from ABCABC... to CBACBA...), using first-principles calculations. We focus in particular on the possible emergence of flat bands near the Fermi level. When the two different rhombohedral orderings are combined, flat bands of topological origin emerge at the interface between the two domains, near the K and K' points of the Brillouin zone. Mapping a simple tight-binding model of a rhombohedral slab along the direction perpendicular to the graphene layers onto a Su-Schrieffer-Heeger chain provides a transparent understanding of the underlying physics.

cond-mat.mes-hall↗

arXiv2026

Exact analytical edge states in the extended Su-Schrieffer-Heeger model

We investigate the topology of the different phases of the extended Su-Schrieffer-Heeger (eSSH) model, which includes hopping processes between translationally inequivalent atoms beyond nearest neighbors. Exact analytical expressions for the edge states of a semi-infinite eSSH chain are derived, with wave functions that decay exponentially from the boundary with a unit-cell decay factor z. From the winding number of the bulk Hamiltonian under periodic boundary conditions, we determine the topological phase diagram and establish the bulk-boundary correspondence: changes in the winding number coincide with bulk gap closings and with the condition |z|=1 for the edge-state solutions. For finite chains, we further obtain analytical, approximate expressions for the low-energy edge states, which are shown to be highly accurate.

cond-mat.other↗

arXiv2026

Effect of spin-orbit coupling on spin and orbital ordering in Sr$_{n+1}$Cr$_n$O$_{3n+1}$, $n=1,2$

We incorporate spin-orbit coupling (SOC) into effective Kugel-Khomskii models for the $n=1$ and $n=2$ members of the Ruddlesden-Popper series Sr$_{n+1}$Cr$_n$O$_{3n+1}$. These model contain interacting spins 1 and pseudospins 1/2 at each site describing spin and orbitals degrees of freedom respectively. We solve the models at zero temperature using pseudospin bond operators and spin waves. We find that for realistic parameters, SOC dominates the physics of the compound Sr$_{2}$CrO$_{4}$ with almost decoupled single CrO$_2$ planes. The spin ordering is antiferromagnetic, with nearest-neighbor Cr spins aligned antiparallel. The corresponding orbital configuration is $d_{xy \uparrow }^{1}(d_{xz\uparrow }^{1}-id_{yz\uparrow }^{1})$ or $d_{xy \downarrow }^{1}(d_{xz\downarrow }^{1}+id_{yz\downarrow }^{1})$ depending on the spin of the site. In contrast, for the bilayer compound Sr$_{3}$Cr$_{2}$O$_{7}$ we find that the effect of the SOC is weak and the system prefers to form pseudospin singlets in the $z$ direction perpendicular to the planes. The spin order is antiferromagnetic within each plane and ferromagnetic between planes, in agreement with previous studies.

cond-mat.str-el↗

arXiv2026

Specific heat of Gd$^{3+}$ and Eu$^{2+}$-based magnetic compounds

We have studied theoretically the specific heat of a large number of non-frustrated magnetic structures described by the Heisenberg model for systems with total angular momentum $J=7/2$, corresponding to the 4f$^7$ configuration of Gd$^{+3}$ and Eu$^{+2}$. For a given critical temperature (determined by the magnitude of the exchange interactions), we find that, to a high degree of accuracy, the specific heat is governed by two primary parameters: the effective number of neighbors $z$, which dictates the extent of spatial and quantum fluctuations, and the axial anisotropy $K$. The universality of $z$ (its ability to describe specific heat across diverse lattices) holds robustly for systems where exchange interactions do not strongly increase with distance and in the absence of frustration. Otherwise, deviations from universality emerge. Using these two parameters we fit the specific heat of four Gd compounds and two Eu compounds, achieving a remarkable agreement. The present approach enables the extraction of magnetic interaction parameters not accessible through mean-field theory, offering a powerful tool for interpreting specific heat data in 4f$^7$ systems.

cond-mat.str-el↗

arXiv2024

Charge and heat pumping in the Rice-Mele chain at finite temperature

It is well known that quantized topological charge pumping takes place in the half filled Rice-Mele chain performing a closed cycle in parameter space. We extend previous studies to the case of charge and heat transport at arbitrary filling and temperature using the corresponding continuity equation with focus in the non-interacting case. The amount of charge and heat transported for any adiabatic time dependence of the parameters is given by a double integral of an analytical function. We find that quantized transport is lost except in trivial cases. In particular, for popular pumping circuits used which lead to quantized non-trivial charge transport at zero temperature, the heat transported in the cycle vanishes. For other pumping circuits, there is a heat transport among even and odd sites of the chain and the environment. As the temperature is increased, the transported charge and heat decrease and vanish at infinite temperature.

cond-mat.mes-hall↗

arXiv2024

Structural transition, spontaneous formation of strong singlet dimers and metamagnetism in $S=3/2$ magnetoelastic spin chains

We study a one-dimensional antiferromagnetic-elastic model with magnetic ions having spin $S=3/2$. By extensive DMRG computations and complementary analytical methods, we uncover a first-order transition from a homogeneous or weakly-dimerized phase (a situation that could be similar to the well known $S=1/2$ spin-Peierls effect) to a highly distorted phase, driven by the spin-phonon coupling $λ$. The striking characteristic of the second phase, present at large $λ$, is the appearance of weakly ferromagnetic (FM) couplings alternating with strong antiferromagnetic (AFM) ones (we dub it FM-AFM phase) with a ground state close to a direct-product state of singlet dimers sitting on the AFM bonds. The behavior of the spin gap in both phases is studied by DMRG computation and contrasted with bosonization predictions and perturbation theory around the direct product of dimers. In the FM-AFM phase robust magnetization plateaus and metamagnetic jumps show under magnetic fields. The novel phase could be realized in 5d oxides of current interest, with giant spin-phonon coupling. Potential applications of the transition would be associated to the possibility of tuning the transition by external parameters such as striction, magnetic or electric fields, or alloying.

cond-mat.str-el↗

arXiv2024

Phase diagram and topology of the XXZ chain with alternating bonds and staggered magnetic field

The XXZ spin-half chain has Heisenberg exchange interactions $J_z$ ($J_\perp$) in the $z$ ($x,y$) direction. The model has a transition from the spin-fluid phase for $-J_\perp < J_z < J_\perp$ to the Néel phase for $J_z > J_\perp >0$. When bond alternation $δ$ is included, the Néel phase transitions to the dimer phase for a finite value of $δ$. We determine the phase diagram using simple topological indicators related to the polarization of both spins. When a staggered magnetic field $B$ is included, a contour plot of these indicators as a function of $δ$ and $B$ determine the amount of topological quantized spin pumping around closed circuits in the $(δ,B)$ plane.

cond-mat.str-el↗

arXiv2024

Singlet polaron theory of low-energy optical excitations in NiPS$_3$

Light-matter interactions can be used as a tool to realize novel many-body states of matter and to study the interplay between electronic and magnetic degrees of freedom. In particular, tightly bound many-body states that behave as coherent quasi-particles are rare, and may lead to unconventional technological applications beyond the semi-conductor paradigm, particularly if these excitations are bosonic and can condense. Two-dimensional magnetic systems present a pristine platform to realize and study such states. We construct a theory that explains the low-energy optical excitations at 1.476 eV and 1.498 eV observed by photoluminiscence, optical absorption, and RIXS in the van der Waals antiferromagnet NiPS$_3$. Using \textit{ab initio} methods, we construct a two-band Hubbard model for two \textit{effective} Ni orbitals of the original lattice. The dominant effective hopping corresponds to third-nearest neighbours. This model exhibits two triplet-singlet excitations of energy near two times the Hund exchange. From perturbation theory, we obtain an effective model for the movement of the singlets in an antiferromagnetic background, that we solve using a generalized self-consistent Born approximation. These singlet excitations, dressed by a cloud of magnons, move coherently as polaronic-like quasi-particles, "singlet polarons". Our theory explains the main features of the observed spectra.

cond-mat.str-el↗

arXiv2024

Effect of interatomic repulsion on Majorana zero modes in a coupled quantum-dot-superconducting-nanowire hybrid system

We study the low-energy eigenstates of a topological superconductor wire modeled by a Kitaev chain, which is connected at one of its ends to a quantum dot through nearest-neighbor (NN) hopping and NN Coulomb repulsion. Using an unrestricted Hartree-Fock approximation to decouple the Coulomb term, we obtain that the quality of the Majorana end states is seriously affected by this term only when the dependence of the low-lying energies with the energy of the quantum dot shows a "diamond" shape, characteristic of short wires. We discuss limitations of the simplest effective models to describe the physics. We expect the same behavior in more realistic models for topological superconducting wires.

cond-mat.mes-hall↗

arXiv2023

Phase diagram of the ionic Hubbard model with density-dependent hopping

We obtain the quantum phase diagram of the ionic Hubbard model including electron-hole symmetric density-dependent hopping. The boundaries of the phases are determined by crossing of excited levels with particular discrete symmetries, which coincide with jumps of charge and spin Berry phases with a topological meaning. Reducing the magnitude of the hopping terms that do not change the total number of singly occupied sites with respect to the other one, the region of the phase diagram occupied by the fully gapped spontaneously dimerized insulator (which separates the band insulating and Mott insulating phases) is enlarged, particularly for small values of the alternating on-site energy. This result might be relevant for experiments in cold atoms in which topological charge pumping is observed when alternation in the hopping is included.

cond-mat.str-el↗

arXiv2023

Topological quantum phase transition in individual Fe Atoms on MoS$_2$/Au(111)

In a recent experiment [Trishin et al., Phys. Rev. Lett. 127, 236801 (2021)] a rich physics was observed for Fe atoms on MoS$_2$/Au(111), characterized by three different behaviors depending on the spectral density of the substrate $ρ_c$: one dominated by a single ion anisotropy, one by the Kondo effect, and an intermediate one. Based on symmetry and previous works, we show that the appropriate model to describe the system is the anisotropic two-channel spin-1 Kondo model (A2CS1KM), which has an underlying topological quantum phase transition (TQPT) between an ordinary Fermi liquid and a topological one with a non-trivial value of the Luttinger integral. Solving the model with the numerical renormalization group (NRG), we show that the different behaviors can be explained in a unified fashion as a function of $ρ_c$, and correspond to both Fermi liquids and an intermediate regime close to the TQPT in the topological phase. Further experiments should confirm this transition.

cond-mat.str-el↗

arXiv2023

Topological quantum phase transition of nickelocene on Cu(100)

Local quantum phase transitions driven by Kondo correlations have been theoretically proposed in several magnetic nanosystems; however, clear experimental signatures are scant. Modeling a nickelocene molecule on a Cu(100) substrate as a two-orbital Anderson impurity with single-ion easy-plane anisotropy coupled to two conduction bands, we find that recent scanning tunneling spectra measured at different microscope tip heights reveal the existence of a topological quantum phase transition from the usual local Fermi liquid with high zero-bias conductance to a non-Landau Fermi liquid, characterized by a non-trivial quantized Luttinger integral, with a small conductance. The effects of intermediate valence, finite temperature, and structural relaxation of the molecule position allow us to explain the different observed behaviors.

cond-mat.str-el↗

arXiv2022

Determination of superexchange interactions for the CuO$_2$ chains in LiCu$_2$O$_2$

Starting from \textit{ab-initio} calculations, we derive a five-band Hubbard model to describe the CuO$_2$ chains of LiCu$_2$O$_2$. This model is further simplified to a low-energy effective Heisenberg model with nearest-neighbor (NN) $J_1$, and next-nearest-neighbor (NNN) $J_2$ interactions, combining perturbation theory, exact diagonalization calculations and Density Matrix Renormalization Group results. For realistic parameters we find the corresponding values of these interactions. The obtained effective model is consistent with a spiral-magnetic ground state as experimentally observed. Using symmetry arguments, the spiral state is a sufficient condition for the ferroelectricity observed in the system.

cond-mat.str-el↗

arXiv2022

Josephson junctions of 2D time-reversal invariant superconductors: signatures of the topological phase

We determine the current-phase relation (CPR) of two-terminal configurations of Josephson junctions containing two-dimensional (2D) time-reversal invariant topological superconductors (TRITOPS), including TRITOPS-TRITOPS, as well as junctions between topological and non-topological superconductors (TRITOPS-S). We focus on long junctions for which several channels intervene in the tunneling coupling through the junction. We present a description of the topological edge modes for different TRITOPS models including $p$-wave pairing and the combination of $s$-wave pairing with spin-orbit coupling. We derive effective low-energy Hamiltonians to describe the Josephson junction, which can be solved analytically to explain the contribution of the edge states to the Josephson current as a function of the phase bias. We find that edge-modes yield singular corrections to the CPR for both junction types. The primary effects occur for the response of the Majorana zero-modes at half-flux quantum phase $ϕ\approx π$ in TRITOPS-TRITOPS junctions and for integer flux quantum phase $ϕ\approx 0$ for TRITOPS-S junctions, respectively. The former effect is particularly strong two-component nematic superconductors. The latter effect leads to a spontaneously broken time-reversal symmetry in the TRITOPS-S junction and to a breakdown of the bulk-boundary correspondence.

cond-mat.supr-con↗

arXiv2022

Splitting of topological charge pumping in an interacting two-component fermionic Rice-Mele Hubbard model

A Thouless pump transports an integer amount of charge when pumping adiabatically around a singularity. We study the splitting of such a critical point into two separate critical points by adding a Hubbard interaction. Furthermore, we consider extensions to a spinful Rice-Mele model, namely a staggered magnetic field or an Ising-type spin coupling, further reducing the spin symmetry. The resulting models additionally allow for the transport of a single charge in a two-component system of spinful fermions, whereas in the absence of interactions, zero or two charges are pumped. In the SU(2)-symmetric case, the ionic Hubbard model is visited once along pump cycles that enclose a single singularity. Adding a staggered magnetic field additionally transports an integer amount of spin while the Ising term realizes a pure charge pump. We employ real-time simulations in finite and infinite systems to calculate the adiabatic charge and spin transport, complemented by the analysis of gaps and the many-body polarization to confirm the adiabatic nature of the pump. The resulting charge pumps are expected to be measurable in finite-pumping speed experiments in ultra-cold atomic gases, for which the SU(2) invariant version is the most promising path. We discuss the implications of our results for a related quantum-gas experiment by Walter et al. [arXiv:2204.06561].

cond-mat.quant-gas↗

arXiv2022

Low-energy physics for an iron phthalocyanine molecule on Au(111)

The system of an iron phthalocyanine molecule on the Au(111) surface, has been studied recently due to its peculiar properties. In particular, several surprising results of scanning tunneling spectroscopy changing the position of the molecule and applying magnetic field can be explained by the {\it non-Landau} Fermi liquid state of a 2-channel spin-1 Kondo model with anisotropy. The localized orbitals near the Fermi level are three, one of symmetry $z^2$ and two (nearly) degenerate $π$ orbitals of symmetry $xz$ and $yz$. Previous studies using the numerical renormalization group neglected one of these orbitals to render the problem tractable. Here we investigate, using a slave-boson mean-field approximation, if the splitting $S$ between $π$ orbitals caused by spin-orbit coupling (SOC) justifies this approximation. We obtain an abrupt transition from a 3-band regime to a 2-band one at a value of $S$ which is about 1/3 of the atomic SOC for Fe, justifying the 2-band model for the system.

cond-mat.str-el↗

arXiv2021

Magnetic properties of chiral EuIr$_2$P$_2$

We present a minimal model that provides a description of the magnetic and thermodynamic properties of \Eu. The model contains two exchange coupling parameters, which are calculated using Density Functional Theory, and a local easy axis magnetic anisotropy term. The classical ground state of the system is a generalization of the well known 120$^\circ$ structure observed in triangular antiferromagnets. Monte Carlo simulations show two phase transitions as a function of the temperature. With increasing temperature, the system transitions from the ground state into a high-entropy collinear antiferromagnet, which in turn at higher temperatures presents a second order transition to a paramagnetic state. A high enough external magnetic field parallel to the anisotropy axis produces a spin-flop transition at low temperatures. The field also reduces the temperature range of stability of the collinear antiferromagnet phase and leads to a single phase transition as a function of the temperature. The reported behavior of the specific heat, the magnetization, and the magnetic susceptibility is in agreement with the available experimental data. Finally, we present the magnetic phase diagrams for magnetic fields parallel and perpendicular to the easy axis.

cond-mat.other↗