arXiv subjects

Claudio Garcia

Publications and source records attributed to Claudio Garcia.

4 recordsLinked to original sources

arXiv2026

Magnetic exchange interactions in the molecular orbital spin system NaV2O5 from a distributed moment point of view

The magnetism in NaV2O5 results from doping of the narrow split-off conduction band of V2O5, which becomes half-filled and leads to a Mott-insulating splitting of spin resolved bands with anti-ferromagnetic order of the spins along the zigzag chains. In the Pmmn structure the spin of this S = 1/2 system is equally shared between two vanadium atoms, residing in a molecular orbital type state. While below 34 K a charge disproportionation occurs into V4+ and V5+, the situation above this temperature is less clear and amounts to a fluctuating moment with equal probability of occupancy of each V. While traditionally described as a quarter-filled ladder system with electron spin localized on the rungs of the ladder, we here take a distributed moment approach in terms of the spin density lumped into individual atomic magnetic sites, including the small induced moments on the oxygen atoms. Exchange interactions are calculated between these sites using a linear response approach based on quasiparticle-self-consistent GW band structures. Surprisingly we find the exchange interactions between the small magnetic moments induced on the vanadyl and bridge oxygen sites to be of the same order of magnitude and even larger than the exchange interactions between vanadium atoms. Their role in the critical temperature is found to be crucial. The spin wave spectra obtained from this classical Heisenberg type Hamiltonian extracted from first-principles contains unusual optic spin wave type collective excitations of high energy.

cond-mat.str-el↗

arXiv2025

Optical absorption and luminescence of $\alpha$-LiV$_2$O$_5$ from the Bethe Salpeter Equation

$\alpha$-Li$_x$V$_2$O$_5$ is obtained by intercalating Li between the layers of V$_2$O$_5$. The partial filling of the split-off conduction band by electron donation from Li leads to significant changes in optical properties. Here we study the electronic band structure of $\alpha$-LiV$_2$O$_5$ using quasiparticle self-consistent (QS) $GW$ calculations and the optical dielectric function by means of the Bethe-Salpeter Equation (BSE). The half-filling of the narrow split-off band leads to a spin-splitting and formation of magnetic moments of 0.5 $\mu_B$ per V which order antiferromagnetically along the chain or $b$-direction. The imaginary part of the dielectric function shows a very strong optical absorption band near 2 eV for polarization along the $a$-direction. This absorptions stems from a localized transition between the occupied V-$d_{xy}$ derived band, which is odd with respect to the $a$-mirrorplane to the higher lying empty band formed from the same V-$d_{xy}$ orbitals but even with respect to that mirror-plane, which explains its polarization and large oscillator strength. We relate this to a recent experimental study of cathodoluminescence (CL) in which a suppression of the lowest CL peak was observed upon addition of Li. The lowest CL peak near 1.8 eV, which lies well below the indirect absorption onset of V$_2$O$_5$ is proposed to be related to recombination of a self-trapped electron polaron, resulting from oxygen vacancies, with a hole at the valence band maximum and is suppressed in LiV$_2$O$_5$ by the strong self--absorption from the Li induced occupied band to the higher empty bands at about the same energy.

cond-mat.mtrl-sci↗

arXiv2024

Quasiparticle band structure and excitonic optical response in V2O5 bulk and monolayer

The electronic band structure of V$_2$O$_5$ is calculated using an all-electron quasiparticle self-consistent (QS) $GW$ method, including electron-hole ladder diagrams in the screening of $W$. The optical dielectric function calculated with the Bethe-Salpeter equation exhibits excitons with large binding energy, consistent with spectroscopic ellipsometry data and other recent calculations. Sharp peaks in the direction perpendicular to the layers at high energy are found to be an artifact of the truncation of the numbers of bands included in the BSE calculation of the macroscopic dielectric function. The $\varepsilon_1(ω=0)$ gives indices of refraction in good agreement with experiment. The excitons are charge transfer excitons with the hole primarily on oxygen and electrons on vanadium, but depending on which exciton, the distribution over different oxygens changes. The exciton wave functions have a spread of about 5-15Å, with asymmetric character for the electron distribution around the hole depending on which oxygen the hole is fixed at. The monolayer quasiparticle gap increases inversely proportional to interlayer distance once the initial interlayer covalent couplings are removed which is thanks to the long-range nature of the self-energy and the reduced screening in a 2D system. The optical gap on the other hand is relatively independent of interlayer spacing because of the compensation between the self-energy gap shift and the exciton binding energy, both of which are proportional to the screened Coulomb interaction $\hat{W}$. Recent experimental results on very thin layer V$_2$O$_5$ obtained by chemical exfoliation provide experimental support for an increase in gap.

cond-mat.mtrl-sci↗

arXiv2018

Deep Haar Scattering Networks in Pattern Recognition: A promising approach

The aim of this paper is to discuss the use of Haar scattering networks, which is a very simple architecture that naturally supports a large number of stacked layers, yet with very few parameters, in a relatively broad set of pattern recognition problems, including regression and classification tasks. This architecture, basically, consists of stacking convolutional filters, that can be thought as a generalization of Haar wavelets, followed by non-linear operators which aim to extract symmetries and invariances that are later fed in a classification/regression algorithm. We show that good results can be obtained with the proposed method for both kind of tasks. We have outperformed the best available algorithms in 4 out of 18 important data classification problems, and have obtained a more robust performance than ARIMA and ETS time series methods in regression problems for data with strong periodicities.

eess.SP↗