arXiv subjects

S. M. João

Publications and source records attributed to S. M. João.

5 recordsLinked to original sources

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

Atomistic theory of hot carrier relaxation in large plasmonic nanoparticles

Recently, there has been significant interest in harnessing hot carriers generated from the decay of localized surface plasmons in metallic nanoparticles for applications in photocatalysis, photovoltaics and sensing. In this work, we present an atomistic approach to predict the population of hot carriers under continuous wave illumination in large nanoparticles. For this, we solve the equation of motion of the density matrix taking into account both excitation of hot carriers as well as subsequent relaxation effects. We present results for spherical Au and Ag nanoparticles with up to $250,000$ atoms. We find that the population of highly energetic carriers depends both on the material and the nanoparticle size. We also study the increase in the electronic temperature upon illumination and find that Ag nanoparticles exhibit a much larger temperature increase than Au nanoparticles. Finally, we investigate the effect of using different models for the relaxation matrix but find that qualitative features of the hot-carrier population are robust.

cond-mat.mtrl-sci↗

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↗

arXiv2021

Basis-Independent Spectral Methods for Non-linear Optical Response in Arbitrary Tight-binding Models

In this paper, we developed a basis-independent perturbative method for calculating the non-linear optical response of arbitrary non-interacting tight-binding models. Our method is based on the non-equilibrium Keldysh formalism and allows an efficient numerical implementation within the framework of the Kernel Polynomial Method for systems which are not required to be translation-invariant. Some proof-of-concept results of the second-order optical conductivity are presented for the special case of gapped graphene with vacancies and an on-site Anderson disordered potential.

cond-mat.other↗

arXiv2015

On the Stability of Fundamental Couplings in the Galaxy

Astrophysical tests of the stability of Nature's fundamental couplings are a key probe of the standard paradigms in fundamental physics and cosmology. In this report we discuss updated constraints on the stability of the fine-structure constant $α$ and the proton-to-electron mass ratio $μ=m_p/m_e$ within the Galaxy. We revisit and improve upon the analysis by Truppe {\it et al.} by allowing for the possibility of simultaneous variations of both couplings and also by combining them with the recent measurements by Levshakov {\it et al.} By considering representative unification scenarios we find no evidence for variations of $α$ at the 0.4 ppm level, and of $μ$ at the 0.6 ppm level; if one uses the Levshakov bound on $μ$ as a prior, the$α$ bound is improved to 0.1 ppm. We also highlight how these measurements can constrain (and discriminate among) several fundamental physics paradigms.

hep-ph↗