arXiv subjects

Pablo Roura-Bas

Publications and source records attributed to Pablo Roura-Bas.

12 recordsLinked to original sources

arXiv2026

First principles calculations of electric-field-driven topological phase transitions in silicene, germanene and stanene

The emergence of two-dimensional topological materials, particularly the group-14 monolayers known as silicene, germanene, and stanene has opened promising pathways for next-generation nanoelectronics and spintronics. Their buckled honeycomb structure and strong spin-orbit coupling allow for bandgap engineering via a perpendicular electric field, leading to topological phase transitions (TPTs) from non-trivial to trivial insulating states. However, precise determination of the critical electric field $E_z^{\text{cr}}$ at which these transitions occur remains challenging, with tight-binding models often underestimating these values. Here, we present a first-principles framework that combines density-functional theory (DFT), maximally localized Wannier functions, and evolution of the Wannier charge centers (WCC) to accurately characterize TPTs in silicene, germanene, and stanene through the $\mathbb{Z}_2$ topological invariant. In contrast to earlier work, at each electric-field strength we run fully self-consistent ab initio simulations to obtain the screened electronic structure, accounting for the material's dielectric response from both electrons and ions. From these converged results we construct a Wannier tight-binding Hamiltonian at each electric field strength, which then enables a gauge-invariant calculation of the $\mathbb{Z}_2$ topological invariant. This methodology yields significantly more accurate numerical predictions of $E_z^{\text{cr}}$, $0.020$ and $0.250$ V/\r{A} for silicene and germanene, respectively. Compared to previous approaches, our framework delivers a marked quantitative improvement for predicting topological phase boundaries, essential for guiding the design of topological field-effect transistors and electrostatically controlled quantum devices based on two-dimensional materials.

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arXiv2025

Thermoelectric Signatures of Kondo Physics in Geometry-Tunable Double Quantum Dots

The equilibrium thermoelectric and spectral properties of a double quantum dot system are investigated, with the geometry continuously tuned from series to parallel via a parameter $ p $. Within the non-crossing approximation in the infinite-$ U $ limit, the Kondo peak remains robust, while satellite features and the Kondo temperature show strong sensitivity to the geometry. The Seebeck coefficient exhibits sign reversals and non-monotonic behavior as a result of the interplay between Kondo and satellite peaks. These findings underscore the role of interference and coupling asymmetry in governing transport properties, suggesting routes for geometry-based optimization in nanoscale devices.

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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.

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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 $\rho_i(\omega)$, 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 $\rho_i(\omega)$. 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 $\rho_i(\omega)$ 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 $T$.

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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.

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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.

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arXiv2017

Enhanced thermoelectric response in the fractional quantum Hall effect

We study the linear thermoelectric response of a quantum dot embedded in a constriction of a quantum Hall bar with fractional filling factors nu=1/m within Laughlin series. We calculate the figure of merit ZT for the maximum efficiency at a fixed temperature difference. We find a significant enhancement of this quantity in the fractional filling in relation to the integer-filling case, which is a direct consequence of the fractionalization of the electron in the fractional quantum Hall state. We present simple theoretical expressions for the Onsager coefficients at low temperatures, which explicitly show that ZT and the Seebeck coefficient increase with m.

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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.

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arXiv2011

Anderson model out of equilibrium: conductance and Kondo temperature

We calculate conductance through a quantum dot weakly coupled to metallic contacts by means of Keldysh out of equilibrium formalism. We model the quantum dot with the SU(2) Anderson model and consider the limit of infinite Coulomb repulsion. We solve the interacting system with the numerical diagrammatic Non-Crossing Approximation (NCA). We calculate the conductance as a function of temperature and gate voltage, from differential conductance (dI/dV) curves. We discuss these results in comparison with those from the linear response approach which can be performed directly in equilibrium conditions. Comparison shows that out of equilibrium results are in good agreement with the ones from linear response supporting reliability to the method employed. The discussion becomes relevant when dealing with general transport models through interacting regions. We also analyze the evolution of the curve of conductance vs gate voltage with temperature. While at high temperatures the conductance is peaked when the Fermi energy coincides with the energy of the localized level, it presents a plateau for low temperatures as a consequence of Kondo effect. We discuss different ways to determine Kondo's temperature and compare the values obtained in and out of equilibrium.

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arXiv2011

Transition between SU(4) and SU(2) Kondo effect

Motivated by experiments in nanoscopic systems, we study a generalized Anderson, which consists of two spin degenerate doublets hybridized to a singlet by promotion of an electron to two conduction bands, as a function of the energy separation $δ$ between both doublets. For $δ$=0 or very large, the model is equivalent to a one-level SU(N) Anderson model, with N=4 and 2 respectively. We study the evolution of the spectral density for both doublets ($ρ_{1 σ}(ω)$ and $ρ_{2 σ}(ω)$) and their width in the Kondo limit as $δ$ is varied, using the non-crossing approximation (NCA). As $δ$ increases, the peak at the Fermi energy in the spectral density (Kondo peak) splits and the density of the doublet of higher energy $ρ_{2 σ}(ω)$ shifts above the Ferrmi energy. The Kondo temperature $T_K$ (determined by the half width at half maximum of the Kondo peak in density of the doublet of lower energy $ρ_{1 σ}(ω)$) decreases dramatically. The variation of $T_K$ with $δ$ is reproduced by a simple variational calculation.

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arXiv2011

Universal transport signatures in two-electron molecular quantum dots: gate-tunable Hund's rule, underscreened Kondo effect and quantum phase transitions

We review here some universal aspects of the physics of two-electron molecular transistors in the absence of strong spin-orbit effects. Several recent quantum dots experiments have shown that an electrostatic backgate could be used to control the energy dispersion of magnetic levels. We discuss how the generically asymmetric coupling of the metallic contacts to two different molecular orbitals can indeed lead to a gate-tunable Hund's rule in the presence of singlet and triplet states in the quantum dot. For gate voltages such that the singlet constitutes the (non-magnetic) ground state, one generally observes a suppression of low voltage transport, which can yet be restored in the form of enhanced cotunneling features at finite bias. More interestingly, when the gate voltage is controlled to obtain the triplet configuration, spin S=1 Kondo anomalies appear at zero-bias, with non-Fermi liquid features related to the underscreening of a spin larger than 1/2. Finally, the small bare singlet-triplet splitting in our device allows to fine-tune with the gate between these two magnetic configurations, leading to an unscreening quantum phase transition. This transition occurs between the non-magnetic singlet phase, where a two-stage Kondo effect occurs, and the triplet phase, where the partially compensated (underscreened) moment is akin to a magnetically "ordered" state. These observations are put theoretically into a consistent global picture by using new Numerical Renormalization Group simulations, taylored to capture sharp finie-voltage cotunneling features within the Coulomb diamonds, together with complementary out-of-equilibrium diagrammatic calculations on the two-orbital Anderson model. This work should shed further light on the complicated puzzle still raised by multi-orbital extensions of the classic Kondo problem.

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arXiv2010

Effects of vertex corrections on diagrammatic approximations applied to the study of transport through a quantum dot

In the present work, we calculate the conductance through a single quantum dot weakly coupled to metallic contacts. We use the spin-1/2 Anderson model to describe the quantum dot, while considering a finite Coulomb repulsion. We solve the interacting system using the non-crossing-approximation (NCA) and the one-crossing approximation (OCA). We obtain the linear response conductance as a function of temperature and energy position of the localized level. From the comparison of both approximations we extract the role of the vertex corrections, which are introduced in the OCA calculations and neglected in the NCA scheme. As a function of the energy position, we observe that the diagrams omitted within NCA are really important for appropriately describing transport phenomena in Kondo systems as well as in the mixed valence regime. On the other hand, as a function of temperature, the corrections introduced by OCA partly recover the universal scaling properties known from numerical approaches such as the Numerical Renormalization Group(NRG).

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