arXiv subjects

Armando A. Aligia

Publications and source records attributed to Armando A. Aligia.

At least 19 recordsLinked to original sources

arXiv2026

Effect of interatomic repulsion and quasi-degenerate states on a Kitaev-transmon qubit based on double quantum dots

We investigate the effect of interatomic Coulomb repulsion $V$ and particular states disregarded previously on the Kitaev-transmon system proposed by Pino \textit{et al.} \cite{pino} which consists of a Josephson junction between two double quantum dots (DQDs) modeled by the spinless Kitaev Hamiltonian. For an isolated DQD, we demonstrate that a ``sweet spot'' hosting ``poor man's Majorana'' states persist in the presence of $V$, provided that system parameters are appropriately tuned. For the full system, we demonstrate that at the sweet spots of both DQDs, all eigenstates are doubly degenerate. This degeneracy arises from the existence of an operator that maps between two decoupled Hilbert subspaces. Away from the sweet spots, the microwave spectrum becomes sensitive to the choice of initial state. In our study, we consider transitions from the ground state (which depending on the flux alternates between the above mentioned subspaces) to all possible excited states. This scenario corresponds to a system initially in thermal equilibrium at low temperature.

cond-mat.mes-hall↗

arXiv2025

Probing Boundary Spins in the Su-Schrieffer-Heeger-Hubbard model

Studying boundary excitations provides a powerful approach to probe correlations in topological phases. We propose that localized spins near the ends of a Su-Schrieffer-Heeger-Hubbard chain embedded in an insulating environment can be detected experimentally using scanning tunneling microscopy (STM) combined with electron spin resonance (ESR). When the STM tip is in the contact regime, the tip-end-spin coupling realizes an effective Anderson impurity problem, giving rise to a Kondo peak at low bias. Spatially resolving the Kondo resonance width as the STM tip approaches the chain ends provides an indirect yet clear signature of these localized spins. To support this proposal, we use density-matrix renormalization group (DMRG) to calculate the spin gap and spin projection of end states for chains of various lengths and interaction strengths $U$ at half-filling. In the non-interacting limit ($U=0$), we derive simple analytical expressions that reproduce the numerical results for sufficiently long chains. We also discuss how the correlated phase of the isolated chain is characterized by boundary zeros in its single-particle Green's function, and briefly comment on their localization properties in relation to the boundary spins.

cond-mat.str-el↗

arXiv2025

Role of asymmetry in thermoelectric properties of a double quantum dot out of equilibrium

We investigate the thermoelectric properties of a double quantum dot system coupled to two metallic reservoirs, focusing on two main effects: (i) the influence of coupling asymmetry between the quantum dot and the reservoirs on the Seebeck coefficient, and (ii) the impact of asymmetry in the energy levels of the dots on current rectification. In the first case, we find that introducing moderate asymmetry significantly enhances the Seebeck coefficient. In the second case, while rectification vanishes when the energy levels are degenerate, substantial rectification is achieved when one energy level lies below and the other above the Fermi level. We further interpret the dependence of rectification magnitude and shape on system parameters using analytical results from a spinless model.

cond-mat.mes-hall↗

arXiv2024

Anisotropy-driven topological quantum phase transition in magnetic impurities

A few years ago, a topological quantum phase transition (TQPT) has been found in Anderson and Kondo 2-channel spin-1 impurity models that include a hard-axis anisotropy term $DS_z^2$ with $D > 0$. The most remarkable manifestation of the TQPT is a jump in the spectral density of localized electrons, at the Fermi level, from very high to very low values as $D$ is increased. If the two conduction channels are equivalent, the transition takes place at the critical anisotropy $D_c \sim 2.5\; T_K$, where $T_K$ is the Kondo temperature for $D=0$. This jump might be important to develop a molecular transistor. The jump is due to a corresponding one in the Luttinger integral, which has a topological non-trivial value $π/2$ for $D > D_c$. Here, we review the main results for the spectral density and highlight the significance of the theory for the interpretation of measurements conducted on magnetic atoms or molecules on metallic surfaces. In these experiments, where $D$ is held constant, the energy scale $T_K$ is manipulated by some parameters. The resulting variation gives rise to a differential conductance $dI/dV$, measured by scanning-tunneling spectroscopy, which is consistent with a TQPT at an intermediate value of $T_K$. We also show that the theory can be extended to integer spin $S>1$ and two-impurity systems. This is also probably true for half-integer spin and non-equivalent channels in some cases.

cond-mat.str-el↗

arXiv2024

Interactions enable Thouless pumping in a nonsliding lattice

A topological 'Thouless' pump represents the quantised motion of particles in response to a slow, cyclic modulation of external control parameters. The Thouless pump, like the quantum Hall effect, is of fundamental interest in physics because it links physically measurable quantities, such as particle currents, to geometric properties of the experimental system, which can be robust against perturbations and thus technologically useful. So far, experiments probing the interplay between topology and inter-particle interactions have remained relatively scarce. Here we observe a Thouless-type charge pump in which the particle current and its directionality inherently rely on the presence of strong interactions. Experimentally, we utilise a two-component Fermi gas in a dynamical superlattice which does not exhibit a sliding motion and remains trivial in the single-particle regime. However, when tuning interparticle interactions from zero to positive values, the system undergoes a transition from being stationary to drifting in one direction, consistent with quantised pumping in the first cycle. Remarkably, the topology of the interacting pump trajectory cannot be adiabatically connected to a non-interacting limit, highlighted by the fact that only one atom is transferred per cycle. Our experiments suggest that Thouless charge pumps are promising platforms to gain insights into interaction-driven topological transitions and topological quantum matter.

cond-mat.quant-gas↗

arXiv2023

Topological phases of strongly-interacting time-reversal invariant topological superconducting chains under a magnetic field

Using the density-matrix renormalization group, we determine the different topological phases and low-energy excitations of a time-reversal invariant topological superconducting (TRITOPS) wire with extended s-wave superconductivity, Rashba spin-orbit coupling (SOC) and on-site repulsion $U$, under an externally applied Zeeman field $J$. For the case in which $J$ is perpendicular to the SOC, the model describes a chain of Shiba impurities on top of a superconductor with extended superconductor pairing. We identify the different topological phases of the model at temperature $T=0$, and in particular study the stability of the TRITOPS phase against the Zeeman field $J$ and the chemical potential $μ$, for different values of $U$. In the case where the magnetic field $J$ is perpendicular to the SOC axis, the pair of Kramers-degenerate Majorana zero modes at the edges of the system that exist for $J=0$, remain degenerate until a critical value of the magnetic field is reached. For $J$ parallel to the SOC and up to moderate values of $U$, the fractional spin projection $\langle S_y \rangle=1/4$ at the ends, found for non-interacting wires at $U=0$, is recovered. In addition, the analytic expression that relates $\langle S_y \rangle$ with $J$ for finite non-interacting chains is shown to be universal up to moderate values of $U$.

cond-mat.supr-con↗

arXiv2023

Thermoelectric properties of a double quantum dot out of equilibrium in Kondo and intermediate valence regimes

We study a system composed of two quantum dots connected in series between two leads at different temperatures, in the limit of large intratomic repulsion. Using the non-crossing approximation, we calculate the spectral densities at both dots $ρ_i(ω)$, the thermal and thermoelectric responses, thermopower and figure of merit in different regimes. The interatomic repulsionleads to finite heat transport even if the hopping between the dots $t=0$. The thermopower can be very large compared to single-dot systems in several regimes. The changes in sign of the thermoelectric current can be understood from the position and magnitude of the Kondo and charge-transfer peaks in $ρ_i(ω)$. The figure of merit can reach values near 0.7. The violation of the Wiedemann-Franz law is much more significant than in previously studied nanoscopic systems. An analysis of the widths of $ρ_i(ω)$ indicates that the dots are at effective temperatures $T_i$ intermediate between those of the two leads, which tend to be the same for large $T$.

cond-mat.mes-hall↗

arXiv2023

Charge and spin gaps of the ionic Hubbard model with density-dependent hopping

We calculate the charge gap $ΔE_C$ and the spin gap $ΔE_S$ of the ionic Hubbard chain including electron-hole symmetric density-dependent hopping. The vanishing of $ΔE_C$ ($ΔE_S$) signals a quantum critical point (QCP) in the charge (spin) sector. Between both critical points, the system is a fully gapped spontaneously dimerized insulator (SDI). We focus our study in this region. Including alternation in the hopping, it is possible to perform an adiabatic Thouless pump of one charge per cycle, but with a velocity limited by the size of the gaps.

cond-mat.str-el↗

arXiv2023

Effective one-band models for the 1D cuprate Ba$_{2-x}$Sr$_x$CuO$_{3+δ}$

We consider a multiband Hubbard model $H_m$ for Cu and O orbitals in Ba$_{2-x}$Sr$_x$CuO$_{3+δ}$ similar to the tree-band model for two-dimensional (2D) cuprates. The hopping parameters are obtained from maximally localized Wannier functions derived from \textit{ab initio} calculations. Using the cell perturbation method, we derive both a generalized $t-J$ model $H_{tJ}$ and a one-band Hubbard model $H_{H}$ to describe the low-energy physics of the system. $H_{tJ}$ has the advantage of having a smaller relevant Hilbert space, facilitating numerical calculations, while additional terms should be included in $H_{H}$ to accurately describe the multi-band physics of $H_m$. Using $H_{tJ}$ and DMRG, we calculate the wave-vector resolved photoemission and discuss the relevant features in comparison with recent experiments. In agreement with previous calculations, we find that the addition of an attractive nearest-neighbor interaction of the order of the nearest-neighbor hopping shifts weight from the $3 k_F$ to the holon-folding branch. Kinetic effects also contribute to this process.

cond-mat.str-el↗

arXiv2023

Topological invariants based on generalized position operators and application to the interacting Rice-Mele model

We discuss different properties and the ability of several topological invariants based on position operators to identify phase transitions, and compare with more accurate methods, like crossing of excited energy levels and jumps in Berry phases. The invariants have the form $\text{Im} \text{ln} \left\langle \exp \left[ i(2π/L)Σ_{j}x_{j} \left( m_{_{\uparrow }}\hat{n}_{j\uparrow } +m_{\downarrow }\hat{n}_{j\downarrow }\right) \right] \right\rangle $, where $L$ is the length of the system, $x_{j}$ the position of the site $j$, $\hat{n}_{jσ}$ the operator of the number of particles at site $j$ with spin $σ$. We show that $m_{σ}$ should be integers, and in some cases of magnitude larger than 1 to lead to well defined expectation values. For the interacting Rice-Mele model (which contains the interacting Su-Schrieffer-Heeger and the Ionic Hubbard model as specific cases), we show that three different invariants give complementary information and are necessary and sufficient to construct the phase diagrams in the regions where the invariants are protected by inversion symmetry. We also discuss the consequences for pumping of charge and spin, and the effect of an Ising spin-spin interaction or a staggered magnetic field.

cond-mat.str-el↗

arXiv2021

Phase diagram of a model for topological superconducting wires

We calculate the phase diagram of a model for topological superconducting wires with local s-wave pairing, spin-orbit coupling $\vecλ$ and magnetic field $\vec{B}$ with arbitrary orientations. This model is a generalized lattice version of the one proposed by Lutchyn $\textit{et al.}$ [Phys. Rev. Lett. $\textbf{105}$ 077001 (2010)] and Oreg $\textit{et al.}$ [Phys. Rev. Lett. $\textbf{105}$ 177002 (2010)], who considered $\vecλ$ perpendicular to $\vec{B}$. The model has a topological gapped phase with Majorana zero modes localized at the ends of the wires. We determine analytically the boundary of this phase. When the directions of the spin-orbit coupling and magnetic field are not perpendicular, in addition to the topological phase and the gapped non topological phase, a gapless superconducting phase appears.

cond-mat.mes-hall↗

arXiv2020

Tomography of zero-energy end modes in topological superconducting wires

We characterize the Majorana zero modes in topological hybrid superconductor-semiconductor wires with spin-orbit coupling and magnetic field, in terms of generalized Bloch coordinates $φ, θ, δ$, and analyze their transformation under SU(2) rotations. We show that, when the spin-orbit coupling and the magnetic field are perpendicular, $φ$ and $δ$ are universal in an appropriate coordinate system. We use these geometric properties to explain the behavior of the Josephson current in junctions of two wires with different orientations of the magnetic field and/or the spin-orbit coupling. We show how to extract from there, the angle $θ$, hence providing a full description of the Majorana modes.

cond-mat.mes-hall↗

arXiv2019

Exact analytical solution of a time-reversal-invariant topological superconducting wire

We consider a model proposed before for a time-reversal-invariant topological superconductor (TRITOPS) which contains a hopping term $t$, a chemical potential $μ$, an extended $s$-wave pairing $Δ$ and spin-orbit coupling $λ$. We show that for $|Δ|=|λ|$, $μ=t=0$, the model can be solved exactly defining new fermion operators involving nearest-neighbor sites. The many-body ground state is four-fold degenerate due to the existence of two zero-energy modes localized exactly at the first and the last site of the chain. These four states show entanglement in the sense that creating or annihilating a zero-energy mode at the first site is proportional to a similar operation at the last site. By continuity, this property should persist for general parameters. Using these results we correct some statements related with the so called "time-reversal anomaly". Addition of a small hopping term for a chain with an even number of sites breaks the degeneracy and the ground state becomes unique with an even number of particles. We also consider a small magnetic field applied to one end of the chain. We compare the many-body excitation energies and spin projection along the spin-orbit direction for both ends of the chains with numerical results %for a small chain obtaining good agreement.

cond-mat.mes-hall↗

arXiv2019

Catalogue of Andreev spectra and Josephson effects in structures with time-reversal-invariant topological superconductor wires

We study all the possible different two terminal configurations of Josephson junctions containing wires of time-reversal invariant topological superconductors (TRITOPS) and ordinary superconductors, including combinations with an interacting quantum dot between both wires in the junction. We introduce simple effective Hamiltonians which explain the different qualitative behaviors obtained. We analyze a wide range of phenomena, including occurrence and quenching of the so called $0-π$ transition, anomalous periodicity and jumps of the Josephson current as a function of the phase difference, and finite Josephson current in the absence of magnetic flux.

cond-mat.mes-hall↗

arXiv2018

Relation between width of the zero-bias anomaly and Kondo temperaure in transport measurements through correlated quantum dots: Effect of asymmetric coupling to the leads

The zero-bias anomaly at low temperatures, originated by the Kondo effect when an electric current flows through a system formed by a spin-$1/2$ quantum dot and two metallic contacts is theoretically investigated. In particular, we compare the width of this anomaly $2T_{\rm NE}$ with that of the Kondo resonance in the spectral density of states $2T_{K}^ρ$, obtained from a Fano fit of the corresponding curves and also with the Kondo temperature, $T_K^G$, defined from the temperature evolution of the equilibrium conductance $G(T)$. In contrast to $T_K^G$ and $2T_{K}^ρ$, we found that the scale $2T_{\rm NE}$ strongly depends on the asymmetry between the couplings of the quantum dot to the leads while the total hybridization is kept constant. While the three scales are of the same order of magnitude, $2T_{\rm NE}$ and $T_{K}^ρ$ agree only in the case of large asymmetry between the different tunneling couplings of the contacts and the quantum dot. On the other hand, for similar couplings, $T_{\rm NE}$ becomes larger than $T_{K}^ρ$, reaching the maximum deviation, of the order of $30\%$, for identical couplings. The fact that an additional parameter to $T_{\rm NE}$ is needed to characterize the Kondo effect, weakenig the universality properties, points that some caution should be taken in the usual identification in experiments of the low temperature width of the zero-bias anomaly with the Kondo scale. Furthermore, our results indicate that the ratios $T_{\rm NE}/T_K^G$ and $T_{K}^ρ/T_K^G$ depend on the range used for the fitting.

cond-mat.str-el↗

arXiv2018

Magnetostriction Reveals Orthorhombic Distortion in Tetrahedral Gd-compounds

We report detailed thermal expansion and magnetostriction experiments on GdCoIn$_5$ and GdRhIn$_5$ single crystal samples that show a sudden change in the dilation at a field B$^\ast$ for temperatures below the Néel transition temperature TN. We present a first-principles model including crystal-field effects, dipolar and exchange interactions, and the dependence of the exchange couplings with lattice distortions in order to fully account for the magnetostriction and magnetic susceptibility data. The mean-field solution of the model shows that a transition between metastable states occurs at the field B$^\ast$. It also indicates that two degenerate phases coexist in the sample at temperatures below TN. This allows to explain the lack of observation, in high resolution x-ray experiments, of an orthorhombic distortion at the Néeel transition even though the magnetic structure breaks the tetragonal symmetry and the magnetoelastic coupling is significant. These conclusions could be extended to other tetragonal Gd-based compounds that present the same phenomenology.

cond-mat.str-el↗

arXiv2018

Enhancing of nonlinear thermoelectric response of a correlated quantum dot in the Kondo regime by asymmetrically coupling to the leads

We study the low temperature properties of the differential response of the current to a temperature gradient at finite voltage in a single level quantum dot including electron-electron interaction, non-symmetric couplings to the leads and non-linear effects. The calculated response is significantly enhanced in setups with large asymmetries between the tunnel couplings. In the investigated range of voltages and temperatures with corresponding energies up to several times the Kondo energy scale, the maximum response is enhanced nearly an order of magnitude with respect to symmetric coupling to the leads.

cond-mat.str-el↗

arXiv2017

Non-linear charge and energy dynamics of an adiabatically driven interacting quantum dot

We formulate a general theory to study the time-dependent charge and energy transport of an adiabatically driven interacting quantum dot in contact to a reservoir for arbitrary amplitudes of the driving potential. We study within this framework the Anderson impurity model with a local ac gate voltage. We show that the exact adiabatic quantum dynamics of this system is fully determined by the behavior of the charge susceptibility of the frozen problem. At $T=0$, we evaluate the dynamic response functions with the numerical renormalization group (NRG). The time-resolved heat production exhibits a pronounced feature described by an instantaneous Joule law characterized by an universal resistance quantum $R_0=h/(2 e^2)$ for each spin channel. We show that this law holds in non-interacting as well as in the interacting system and also when the system is spin-polarized. In addition, in the presence of a static magnetic field, the interplay between many-body interactions and spin polarization leads to a non-trivial energy exchange between electrons with different spin components.

cond-mat.mes-hall↗