arXiv subjects

Alberto García-Cristóbal

Publications and source records attributed to Alberto García-Cristóbal.

7 recordsLinked to original sources

arXiv2026

The Frequency-Dependent Spin Contribution to the Magnetoelectric Tensor of Cr$_2$O$_3$: A First-Principles Study

The magnetoelectric (ME) effect provides a promising pathway for controlling magnetic functionalities using electric fields. While first-principles methods for the static linear ME response are well established, comparable approaches for the frequency-dependent response remain less developed, despite experiments showing pronounced finite-frequency resonances. Here, we investigate the dynamical spin-induced linear ME response from first principles and systematically compare the independent-particle approximation (IPA), random-phase approximation (RPA), time-dependent density functional theory (TDDFT), and the Bethe-Salpeter equation (BSE). We apply these methods to the prototypical ME material Cr$_2$O$_3$ and compare the results with available experimental and theoretical studies. We find that the IPA and RPA fail to reproduce the previously reported finite static limit of the spin-induced response. Within the BSE framework, pronounced excitonic resonances emerge in the ME spectrum, in qualitative agreement with experiment. We also identify a magnon-like peak that coincides with a pole of the transverse spin susceptibility while remaining essentially dark in optical absorption, highlighting the sensitivity of the ME response to spin excitations. TDDFT places this mode closer to the expected low-energy magnonic regime and yields a sizable static spin-induced response. Our results show that these frameworks capture complementary aspects of the dynamical ME response. Low-energy collective spin excitations are required to recover the static limit, whereas electron-hole interactions are essential for reproducing the excitonic resonances.

cond-mat.mtrl-sci↗

arXiv2026

Coherent Ultrafast Excitonic Oscillations in Monolayer WS$_2$

Monolayer transition metal dichalcogenides are a suitable platform for studying excitonic coherence in the light-matter coupling regime. We present an ab initio time-dependent GW-Bethe-Salpeter equation (GW-BSE) investigation of coherent excitonic dynamics in monolayer WS$_2$. By solving the coherent coupling between the A, A$^{*}$, and B excitons under linearly polarized pump fields, we identify the microscopic origin of the resulting oscillatory dynamics and rationalize it using an effective theoretical model. Our results provide the interpretation of recently reported coherent excitonic phenomena in monolayer WS$_2$ (Nano Lett. 24, 8117 (2024)). Building on this first-principles time-resolved framework, we propose a tailored pump-probe scheme that enables the controlled generation and regeneration of coherent oscillations between excitonic states. These findings establish a predictive route for controlling excitonic coherence in two-dimensional materials, with direct relevance for ultrafast optoelectronic switches and solid-state quantum logic devices.

cond-mat.mtrl-sci↗

arXiv2025

Magnons in chromium trihalides from \emph{ab initio} Bethe-Salpeter equation

Chromium trihalides (CrX$_3$, with $\rm{X=I,Br,Cl}$) are layered ferromagnetic materials with rich physics and possible applications. Their structure consists of magnetic Cr atoms positioned between two layers of halide atoms. The choice of halide results in distinct magnetic properties, but their effect on spin-wave (magnon) excitations is not fully understood. Here we present first-principles calculations of magnon dispersions and wave functions for monolayer Cr trihalides using the finite-momentum Bethe-Salpeter equation (BSE) to describe collective spin-flip excitations. % We study the dependence of magnon dispersions on the halide species and resolve the small topological gap at the Dirac point in the magnon spectrum by including spin-orbit coupling. Analysis of magnon wave functions reveals that magnons are made up of electronic transitions with a wider energy range than excitons in CrX$_3$ monolayers, providing insight into magnon states in real and reciprocal space. We discuss Heisenberg exchange parameters extracted from the BSE and discuss the convergence of BSE magnon calculations. Our work advances the quantitative modeling of magnons in two-dimensional materials, providing the starting point for studying magnon interactions in a first-principles BSE framework.

cond-mat.mtrl-sci↗

arXiv2024

Unveiling exciton formation: exploring the early stages in time, energy and momentum domain

Resolving the early-stage dynamics of exciton formation following non-resonant photoexcitation in time, energy, and momentum is quite challenging due to their inherently fast timescales and the proximity of the excitonic state to the bottom of the conduction band. In this study, by combining time- and angle-resolved photoemission spectroscopy with \mathit{ab initio} numerical simulations, we capture the timing of the early-stage exciton dynamics in energy and momentum, starting from the photoexcited population in the conduction band, progressing through the formation of free excitons, and ultimately leading to their trapping in lattice deformations. The chosen material is bismuth tri-iodide ($BiI_3$), a layered semiconductor with a rich landscape of excitons in the electronic structure both in bulk and in monolayer form. The obtained results, providing a full characterization of the exciton formation, elucidate the early stages of the physical phenomena underlying the operation of the ultrafast semiconductor device.

cond-mat.mtrl-sci↗

arXiv2023

Physics of excitons in layered BiI$_{3}$. Effects of dimensionality and crystal anisotropy

We carry out a detailed theoretical study of the electronic and optical properties of bulk and monolayer bismuth triiodide (BiI$_{3}$), a layered metal halide, using the ab initio GW+BSE scheme with a full spinorial formulation. We discuss in detail the effects due to the change of dimensionality and the role of spin-orbit coupling. Moreover, we compute the exciton dispersion by solving the BSE at finite momentum, also analysing transverse (TE) and longitudinal (LE) excitons, and the L-T splitting at $\bf{q}\approxΓ$. The results provide a reference for future experimental measurements. In addition, the interplay between spin-orbit coupling and large binding energy, together with the role of quantum confinement, confirm that BiI$_{3}$ is an interesting material for opto-electronic applications and show that it is a good candidate for the study of exciton dynamics.

cond-mat.mtrl-sci↗

arXiv2020

Thermally Tunable Surface Acoustic Wave Cavities

We experimentally demonstrate the dynamical tuning of the acoustic field in a surface acoustic wave (SAW) cavity defined by a periodic arrangement of metal stripes on LiNbO3 substrate. Applying a DC voltage to the ends of the metal grid results in a temperature rise due to resistive heating that changes the frequency response of the device up to 0.3%, which can be used to control the acoustic transmission through the structure. The time scale of the switching is demonstrated to be of about 200 ms. In addition, we have also performed finite element simulations of the transmission spectrum of a model system which exhibit a temperature dependence consistent with the experimental data. The advances shown here enable easy, continuous, dynamical control and could be applied for a variety of substrates.

physics.app-ph↗

arXiv2013

Semiempirical pseudopotential approach for nitride-based nanostructures and {\it ab initio} based passivation of free surfaces

We present a semiempirical pseudopotential method based on screened atomic pseudopotentials and derived from \textit{ab initio} calculations. This approach is motivated by the demand for pseudopotentials able to address nanostructures, where \textit{ab initio} methods are both too costly and insufficiently accurate at the level of the local-density approximation, while mesoscopic effective-mass approaches are inapplicable due to the small size of the structures along, at least, one dimension. In this work we improve the traditional pseudopotential method by a two-step process: First, we invert a set of self-consistently determined screened {\it ab initio} potentials in wurtzite GaN for a range of unit cell volumes, thus determining spherically-symmetric and structurally averaged atomic potentials. Second, we adjust the potentials to reproduce observed excitation energies. We find that the adjustment represents a reasonably small perturbation over the potential, so that the ensuing potential still reproduces the original wave functions, while the excitation energies are significantly improved. We furthermore deal with the passivation of the dangling bonds of free surfaces which is relevant for the study of nanowires and colloidal nanoparticles. We present a methodology to derive passivant pseudopotentials from {\it ab initio} calculations. We apply our pseudopotential approach to the exploration of the confinement effects on the electronic structure of GaN nanowires.

cond-mat.mes-hall↗