arXiv subjects

J. P. Santos Pires

Publications and source records attributed to J. P. Santos Pires.

13 recordsLinked to original sources

arXiv2025

Markov Inequality as a Tool for Linear-Scaling Estimation of Local Observables

We introduce a linear-scaling stochastic method to compute real-space maps of any positive local spectral operator in a tight-binding model. By employing positive-definite estimators, the sampling error at each site can be rigorously bounded relative to the mean via the Markov inequality, overcoming the lack of self-averaging and enabling accurate estimates even under strong spatial fluctuations. The approach extends to non-diagonal observables, such as local currents, through local unitary transformations and its effectiveness is showcased by benchmark calculations in the disordered two-dimensional (2D) $π$-flux model, where the LDoS and steady-state current maps are computed. This method will enable simulations of disorder-driven mesoscopic phenomena in realistically large lattices and accelerate real-space self-consistent mean-field calculations.

cond-mat.dis-nn↗

arXiv2024

Unambiguous Simulation of Diffusive Charge Transport in Disordered Nanoribbons

Charge transport in disordered two-dimensional (2D) systems showcases a myriad of unique phenomenologies that highlight different aspects of the underlying quantum dynamics. Electrons in such systems undergo a crossover from ballistic propagation to Anderson localization, contingent on the system's effective coherence length. Between the extended and localized phases lies a diffusive crossover in which the charge conductivity is properly defined. The numerical observation of this regime has remained elusive because it requires fully coherent transport to be simulated in systems whose dimensions are sufficiently large to meaningfully split the mean-free path and localization length scales. To address this challenge, we employed a novel linear scaling time-resolved approach that enabled us to derive the dc-transport characteristics and observe the three expected 2D transport regimes - ballistic, diffusive, and localized.

cond-mat.mes-hall↗

arXiv2023

Electron-mediated entanglement of two distant macroscopic ferromagnets within a nonequilibrium spintronic device

Using the nascent concept of quantum spin-transfer torque [A. Zholud et al., Phys. Rev. Lett. {\bf 119}, 257201 (2017); M. D. Petrović {\em et al.}, Phys. Rev. X {\bf 11}, 021062 (2021)], we demonstrate that a current pulse can be harnessed to entangle quantum localized spins of two spatially separated ferromagnets (FMs) which are initially unentangled. The envisaged setup comprises a spin-polarizer (FM$_p$) and a spin-analyzer (FM$_a$) FM layers separated by normal metal (NM) spacer. The injection of a current pulse into the device leads to a time-dependent superposition of many-body states characterized by a high degree of entanglement between the spin degrees of freedom of the two distant FM layers. The non-equilibrium dynamics are due to the transfer of spin angular momentum from itinerant electrons to the localized spins via a quantum spin-torque mechanism that remains active even for {\em collinear but antiparallel} arrangements of the FM$_p$ and FM$_a$ magnetizations (a situation in which the conventional spin-torque is absent). We quantify the mixed-state entanglement generated between the FM layers by tracking the time-evolution of the full density matrix and analyzing the build-up of the mutual logarithmic negativity over time. The effect of decoherence and dissipation in the FM layers due to coupling to bosonic baths at finite temperature, the use of multi-electron current pulses and the dependence on the number of spins are also considered in an effort to ascertain the robustness of our predictions under realistic conditions. Finally, we propose a ``current-pump/X-ray-probe'' scheme, utilizing ultrafast X-ray spectroscopy, that can witness nonequilibrium and transient entanglement of the FM layers by extracting its time-dependent quantum Fisher information.

cond-mat.str-el↗

arXiv2023

From Bloch Oscillations to a Steady-State Current in Strongly Biased Mesoscopic Devices

It has long been known that quantum particles moving in a periodic lattice and subject to a constant force field undergo an oscillatory motion that is referred to as Bloch Oscillations (BOs). However, it is also known that, under quite general conditions, a biased mesoscopic system connected to leads should settle in a steady-state regime characterized by a constant electric current (described by the Landauer formula). Since both effects are driven by a constant field, these two quantum transport phenomena appear to be at odds with each other. Here, we solve this apparent contradiction by theoretically demonstrating that BOs can actually be observed in biased two-terminal mesoscopic devices as a transient phenomenon, which relaxes for long times to a steady-state current that agrees with the Landauer formula. Furthermore, we also combine analytical and numerical time-evolution results for a one-dimensional tight-binding model of a biased two-terminal mesoscopic system, in order to characterize the decay times of the transient BOs and establish the conditions under which they can occur.

cond-mat.mes-hall↗

arXiv2022

Quantum Effects of Impurities and Lattice Defects in Topological Semimetals

Topological semimetals are a class of novel three-dimensional (3D) electronic phases that feature topologically protected conical band-touchings at the Fermi level. These band-touching points are monopoles of Berry curvature in momentum space and effectively realize (3+1)-dimensional Weyl fermions as emergent quasiparticles. Such features are robust to perturbations but not completely insensitive to them. In this thesis, we explore the yet fertile ground of disordered Weyl semimetals (WSMs), most notably by analysing the effects of on-site random fields, random smooth potential regions, point-like scalar impurities, and lattice point-defects in their electronic structure and electrodynamic properties.

cond-mat.mes-hall↗

arXiv2022

Nodal vacancy bound states and resonances in three-dimensional Weyl semimetals

The electronic structure of a cubic $\mathcal{T}$-symmetric Weyl semimetal is analysed in the presence of atomic-sized vacancy defects. Isolated vacancies are shown to generate nodal bound states with $r^{\scriptscriptstyle -2}$ asymptotic tails, even when immersed in a weakly disordered environment. These states show up as a significantly enhanced nodal density of states which, as the concentration of defects is increased, reshapes into a nodal peak that is broadened by inter-vacancy hybridisation into a comb of satellite resonances at finite energies. Our results establish point defects as a crucial source of elastic scattering that leads to nontrivial modifications in the electronic structure of Weyl semimetals.

cond-mat.mes-hall↗

arXiv2022

Anomalous Transport Signatures in Weyl Semimetals with Point Defects

We present the first theoretical study of transport properties of Weyl semimetals with point defects. Focusing on a class of time-reversal symmetric Weyl lattice models, we show that dilute lattice vacancies induce a finite density of quasi-localized states at and near the nodal energy, causing strong modifications to the low-energy spectrum. This generates novel transport effects, namely (i) an oscillatory behaviour of the dc-conductivity with the charge carrier density in the absence of magnetic fields, and (ii) a plateau-shaped dissipative optical response for photon frequencies below the inter-band threshold, $E_{F}\!\lesssim\!\hbarω\!\lesssim\!2E_{F}$.Our results provide a path to engineer unconventional quantum transport effects in Weyl semimetals by means of common point defects.

cond-mat.mes-hall↗

arXiv2021

Environmental Sensitivity of Fabry-Perot Microcavities Induced By Layered Graphene-Dielectric Hybrid Coatings

We propose a fiber-based environmental sensor that exploits the reflection phase shift tunability provided by the use of layered coatings composed of dielectric slabs spaced by conducting membranes. A transfer matrix study is done in a simplified theoretical model, for which an enhanced sensitivity of the reflection interference pattern to the output medium is demonstrated, in the typical refractive index range of liquid media. An experimental configuration using a cascaded Fabry-Perot microcavity coated by a graphene oxide/polyethylenimine (GO/PEI) multilayered structure is demonstrated. Its cost effective chemical production method makes graphene oxide-based hybrid coatings excellent candidates for future real-life sensing devices.

physics.optics↗

arXiv2021

A New Microscopic Look into Pressure Fields and Local Buoyancy

The concept of local pressure is pivotal to describe many important physical phenomena, such as buoyancy or atmospheric phenomena, which always require the consideration of space-varying pressure fields. These fields have precise definitions within the phenomenology of hydro-thermodynamics, but a simple and pedagogical microscopic description based on Statistical Mechanical is still lacking in present literature. In this paper, we propose a new microscopic definition of the local pressure field inside a classical fluid, relying on a local barometer potential that is built into the many-particle Hamiltonian. Such a setup allows the pressure to be locally defined, at an arbitrary point inside the fluid, simply by doing a standard ensemble average of the radial force exerted by the barometer potential on the gas' particles. This setup is further used to give a microscopic derivation of the generalized Archimedes's buoyancy principle, in the presence of an arbitrary external field. As instructive examples, buoyancy force fields are calculated for ideal fluids in the presence of: i) a uniform force field, ii) a spherically symmetric harmonic confinement field, and iii) a centrifugal rotating frame.

physics.class-ph↗

arXiv2021

Breakdown of universality in three-dimensional Dirac semimetals with random impurities

Dirac-Weyl semimetals are unique three-dimensional (3D) phases of matter with gapless electrons and novel electrodynamic properties believed to be robust against weak perturbations. Here, we unveil the crucial influence of the disorder statistics and impurity diversity in the stability of incompressible electrons in 3D semimetals. Focusing on the critical role played by rare impurity configurations, we show that the abundance of low-energy resonances in the presence of diluted random potential wells endows rare localized zero-energy modes with statistical significance, thus lifting the nodal density of states. The strong nonperturbative effect here reported converts the 3D Dirac-Weyl semimetal into a compressible metal even at the lowest impurity densities. Our analytical results are validated by high-resolution real-space simulations in record-large 3D lattices with up to 536 000 000 orbitals.

cond-mat.mes-hall↗

arXiv2020

Landauer transport as a quasisteady state on finite chains under unitary quantum dynamics

In this paper, we study the emergence of a Landauer transport regime from the quantum-mechanical dynamics of free electrons in a disordered tight-binding chain, which is coupled to finite leads with open boundaries. Both partitioned and partition-free initial conditions are analyzed and seen to give rise, for large enough leads, to the same spatially uniform quasi-steady-state current, which agrees with the Landauer value. The quasi-steady-state regime is preceded by a transient regime, which last for a time proportional to the length of the disordered sample, and followed by recursions, after a time that is proportional to the lead size. These theoretical predictions may be of interest to future experiments on transport of fermionic ultra-cold atoms across optical lattices. We also observe finite-size current oscillations, superimposed on the quasi-steady-state, whose behavior depends crucially on the conditions initially imposed on the system. Finally, we show how a time-resolved Kubo formula is able to reproduce this Landauer transport regime, as the leads grow bigger.

cond-mat.mes-hall↗

arXiv2019

Global Delocalization Transition in the de Moura-Lyra Model

The possibility of having a delocalization transition in the 1D de Moura-Lyra class of models (having a power-spectrum $\propto q^{-α})$ has been the object of a long standing discussion in the literature, filled with ambiguities. In this letter, we report the first numerical evidences that such a transition happens at $α=1$, where the localization length (measured from the scaling of the conductance) is shown to diverge as $(1-α)^{-1}$. The persistent finite-size scaling of the data is shown to be caused by a very slow convergence of the nearest-neighbor correlator to its infinite-size limit, and controlled by the choice of a proper scaling parameter. This last conclusion leads to the re-interpretation of the localization in these models to be caused by an effective Anderson uncorrelated model at small length-scales. Finally, the numerical results are confirmed by analytical perturbative calculations which are built on previous work.

cond-mat.dis-nn↗

arXiv2019

Spectral Functions of One-Dimensional Systems with Correlated Disorder

We investigate the spectral function of Bloch states in an one-dimensional tight-binding non-interacting chain with two different models of static correlated disorder, at zero temperature. We report numerical calculations of the single-particle spectral function based on the Kernel Polynomial Method, which has an $\mathcal{O}(N)$ computational complexity. These results are then confirmed by analytical calculations, where precise conditions were obtained for the appearance of a classical limit in a single-band lattice system. Spatial correlations in the disordered potential give rise to non-perturbative spectral functions shaped as the probability distribution of the random on-site energies, even at low disorder strengths. In the case of disordered potentials with an algebraic power-spectrum, $\propto\left|k\right|^{-α}$, we show that the spectral function is not self-averaging for $α\geq1$.

cond-mat.dis-nn↗