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Pablo García-Fernández

Publications and source records attributed to Pablo García-Fernández.

At least 19 recordsLinked to original sources
arXiv2025

Extension of Second-Principles Density Functional Theory into the time domain

We present an extension of the second-principles density functional theory (SPDFT) method to perform time-dependent simulations. Our approach, which calculates the evolution of the density matrix in real time and real space using the Liouville-von Neumann equation of motion, allows determining optical and transport properties for very large systems, involving tens of thousands of atoms, using very modest computational platforms. In contrast with other methods, we show that SPDFT can be applied to a wide variety of materials including both metals and insulators. In particular, we illustrate its capabilities by obtaining the spectra of SrTiO$_3$, diamond and metallic lithium. We find that, while SPDFT results in SrTiO$_3$ are quite similar to those obtained from DFT using linear perturbation theory, we observe significant improvements over this method in both diamond and metallic lithium. The inclusion of electron-electron interactions during the evolution of the density matrix in diamond allows the spectra to more closely resemble those obtained with the Bethe-Salpeter equation than from perturbation theory. In lithium time-dependent SPDFT not only predicts interband transitions but also the Drude peak, opening the possibility of detailed ab initio studies of transport properties beyond many of the usual approximations.

cond-mat.mtrl-sci↗
arXiv2024

Structural and electronic properties of bulk Li$_{2}$O$_{2}$: first-principles simulations based on numerical atomic orbitals

The development of advanced materials with high specific energy is crucial for enabling sustainable energy storage solutions, particularly in applications such as lithium-air batteries. Lithium peroxide (Li$_{2}$O$_{2}$) is a key discharge product in non-aqueous lithium-air systems, where its structural and electronic properties significantly influence battery performance. In this work, we investigate the atomic structure, electronic band structure, and Wannier functions of bulk Li$_{2}$O$_{2}$ using density functional theory. The performance of different basis sets of numerical atomic orbitals are compared with respect to a converged plane-wave basis results. We analyze the material's ionic characteristics, the formation of molecular orbitals in oxygen dimers, and the band gap discrepancies between various computational approaches. Furthermore, we develop a localized Wannier basis to model electron-vibration interactions and explore their implications for polaron formation. Our findings provide a chemically intuitive framework for understanding electron-lattice coupling and offer a basis for constructing reduced models that accurately describe the dynamics of polarons in Li$_{2}$O$_{2}$. These insights contribute to the broader goal of improving energy storage technologies and advancing the field of materials design.

cond-mat.mtrl-sci↗
arXiv2023

Understanding the local structure, magnetism and optical properties in layered compounds with d9 ions: Insight into silver fluorides and K2CuF4

Using first-principles DFT calculations, we analyze the origin of the different crystal structures, optical and magnetic properties of two basic families of layered fluoride materials with formula A2MF4 (M = Ag, Cu, Ni, Mn; A = K, Cs, Rb). On one hand, Cs2AgF4 and K2CuF4 compounds (both with d9 metal cations) crystallize in an orthorhombic structure with Cmca space group and MA - F - MB bridge angle of 180, and they exhibit a weak ferromagnetism (FM) in the layer plane. On the other hand, K2NiF4 or K2MnF4 compounds (with d8 and d5 metal cations, respectively) have a tetragonal I4/mmm space group with 180 bridge angle and exhibit antiferromagnetism (AFM) in the layer plane. Firstly, we show that, contrary to what is claimed in the literature, the Cmca structure of Cs2AgF4 and K2CuF4 is not related to a cooperative Jahn-Teller effect among elongated MF64- units. Instead, first-principles calculations carried out in the I4/mmm parent phase of these two compounds show that MF64- units are axially compressed because the electrostatic potential from the rest of lattice ions force the hole to lie in the 3z2 - r2 molecular orbital (z being perpendicular to the layer plane). This fact increases the metal-ligand distance in the layer plane and makes that covalency in the bridging ligand has a residual character (clearly smaller than in K2NiF4 or KNiF3) stabilizing for only a few meV (7.9 meV for Cs2AgF4) an AFM order. However, this I4/mmm parent phase of Cs2AgF4 is unstable thus evolving towards the experimental Cmca structure with an energy gain of 140 meV, FM ordering and orthorhombic MF64- units.

cond-mat.mtrl-sci↗
arXiv2023

Kittel law and domain formation mechanism in PbTiO$_3$/SrTiO$_3$ superlattices

We report second-principles simulations on the structural and energetic properties of domains in (PbTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_{n}$ superlattices. For the explored layer thickness ($n$ ranging between 8 and 16 unit cells) and lateral sizes of the domains, the most stable configuration corresponds to polar domains separated by a sequence of counter-rotating vortices (clockwise/counterclockwise) perpendicular to the stacking direction and acting as domain walls. The balance between the domain wall energy and the electrostatic energy yields to an optimal domain period $ω$ that is proportional to the square-root of the thickness of the PbTiO$_{3}$ layer, following the Kittel law. For a given lateral size of the simulation box, suboptimal domain structures (with a width larger than the one predicted by the Kittel law) can be obtained in a metastable form. However, at finite temperature, molecular dynamics simulations show the spontaneous change of periodicity, which implies the formation of new domains whose generation is initiated by the nucleation of vortices and antivortices at the interface between the SrTiO$_{3}$ and the PbTiO$_{3}$ layers. The vortices progressively elongate and eventually annihilate with the antivortices yielding the formation of new domains to comply the Kittel law via a topological phase transition.

cond-mat.mtrl-sci↗
arXiv2023

Emergent chirality in a polar meron to skyrmion phase transition

Polar skyrmions are predicted to emerge from the interplay of elastic, electrostatic and gradient energies, in contrast to the key role of the anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. With the discovery of topologically-stable polar skyrmions, it is of both fundamental and practical interest to understand the microscopic nature and the possibility of temperature- and strain-driven phase transitions in ensembles of such polar skyrmions. Here, we explore the reversible transition from a skyrmion state (topological charge of -1) to a two-dimensional, tetratic lattice of merons (with topological charge of -1/2) upon varying the temperature and elastic boundary conditions in [(PbTiO3)16/(SrTiO3)16]8 lifted-off membranes. This topological phase transition is accompanied by a change in chirality, from zero-net chirality (in meronic phase) to net-handedness (in skyrmionic phase). To map these changes microscopically required developing new imaging methods. We show how scanning convergent beam electron diffraction provides a robust measure of the local polarization simultaneously with the strain state at sub-nm resolution, while also directly mapping the chirality of each skyrmion. Using this, we demonstrate strain as a crucial order parameter to drive isotropic-to-anisotropic structural transitions of chiral polar skyrmions to non-chiral merons, validated with X-ray reciprocal space mapping and theoretical phase-field simulations. These results revealed by our new measurement methods provide the first illustration of systematic control of rich variety of topological dipole textures by altering the mechanical boundary conditions, which may offer a promising way to control their functionalities in ferroelectric nanodevices using the local and spatial distribution of chirality and order.

cond-mat.mes-hall↗
arXiv2022

Berezinskii-Kosterlitz-Thouless phases in ultra-thin PbTiO$_3$/SrTiO$_3$ superlattices

We study the emergence of Berezinskii-Kosterlitz-Thouless (BKT) phases in (PbTiO$_3$)$_3$/(SrTiO$_3$)$_3$ superlattices by means of second-principles simulations. Beyond a threshold tensile epitaxial strain of $ε= 0.25 \%$ the local dipole moments within the superlattices are confined to the film-plane, and thus the polarization can be effectively considered as two-dimensional. The analysis of the decay of the dipole-dipole correlation with the distance, together with the study of the density of defects and its distribution as function of temperature, supports the existence of a BKT phase in a range of temperature mediating the ordered ferroelectric (stable at low $T$), and the disordered paraelectric phase that appears beyond a critical temperature $T_{\rm BKT}$. This BKT phase is characterized by quasi-long-range order (whose signature is a power-law decay of the correlations with the distance), and the emergence of tightly bounded vortex-antivortex pairs whose density is determined by a thermal activation process. The proposed PbTiO$_{3}$/SrTiO$_{3}$ superlattice model and the imposed mechanical boundary conditions are both experimentally feasible, opening the door for the first experimental observation of these new topological phases in ferroelectric materials.

cond-mat.mtrl-sci↗
arXiv2022

Melting of Polarization Vortices Crystals and Chiral Phase Transitions in Oxide Superlattices

We study the equilibrium arrangements of polarization vortices in (PbTiO$_3$)$_n$/(SrTiO$_3$)$_n$ superlattices by means of second-principles simulations. We find that, at low temperatures, polarization vortices organize in a regular arrangement in which clockwise and counter-clockwise vortices alternate positions, leading to a crystal-like structure with well defined handedness. This chiral crystal melts at a critical temperature $T_\mathrm{M}$ into a chiral liquid, where long-range order is lost but handedness is preserved. At even higher temperatures, $T_\mathrm{C}$, a second phase transition occurs, at which the chiral liquid of polarization vortices loses its handedness. Both phase transitions can be readily identified by the adequate choices of order parameters.

cond-mat.mtrl-sci↗
arXiv2021

Order-disorder transitions in a polar vortex lattice

Order-disorder transitions are widely explored in various vortex structures in condensed matter physics, i.e., in the type-II superconductors and Bose-Einstein condensates. In this study, we have investigated the ordering of the polar vortex phase in the (PZT)n/(STO)n superlattice systems through phase-field simulations. An antiorder state is discovered for short periodicity superlattice on an SSO substrate, owing to the huge interfacial coupling between PZT and STO as well as the giant in-plane polarization in STO layers due to the large tensile strain. Increasing the periodicity leads to the anti-order to disorder transition, resulting from the loss of interfacial coupling and disappearance of the polarization in STO layers. On the other hand, for short periodicity superlattices, order-disorder-antiorder transition can be engineered by mediating the substrate strain, due to the delicate competition between the depoling effect, interfacial coupling, and strain effect. We envision this study to spur further interest towards the understanding of order-disorder transition in ferroelectric topological structures.

cond-mat.mtrl-sci↗
arXiv2020

Transferring Orbital Angular Momentum to an Electron Beam Reveals Toroidal and Chiral Order

Orbital angular momentum and torque transfer play central roles in a wide range of magnetic textures and devices including skyrmions and spin-torque electronics(1-4). Analogous topological structures are now also being explored in ferroelectrics, including polarization vortex arrays in ferroelectric/dielectric superlattices(5). Unlike magnetic toroidal order, electric toroidal order does not couple directly to linear external fields. To develop a mechanism that can control switching in polarization vortices, we utilize a high-energy electron beam and show that transverse currents are generated by polar order in the ballistic limit. We find that the presence of an electric toroidal moment in a ferro-rotational phase transfers a measurable torque and orbital angular momentum to the electron beam. Furthermore, we find that the complex polarization patterns, observed in these heterostructures, are microscopically chiral with a non-trivial axial component of the polarization. This chirality opens the door for the coupling of ferroelectric and optical properties.

cond-mat.mtrl-sci↗
arXiv2020

SIESTA: recent developments and applications

A review of the present status, recent enhancements, and applicability of the SIESTA program is presented. Since its debut in the mid-nineties, SIESTA's flexibility, efficiency and free distribution has given advanced materials simulation capabilities to many groups worldwide. The core methodological scheme of SIESTA combines finite-support pseudo-atomic orbitals as basis sets, norm-conserving pseudopotentials, and a real-space grid for the representation of charge density and potentials and the computation of their associated matrix elements. Here we describe the more recent implementations on top of that core scheme, which include: full spin-orbit interaction, non-repeated and multiple-contact ballistic electron transport, DFT+U and hybrid functionals, time-dependent DFT, novel reduced-scaling solvers, density-functional perturbation theory, efficient Van der Waals non-local density functionals, and enhanced molecular-dynamics options. In addition, a substantial effort has been made in enhancing interoperability and interfacing with other codes and utilities, such as Wannier90 and the second-principles modelling it can be used for, an AiiDA plugin for workflow automatization, interface to Lua for steering SIESTA runs, and various postprocessing utilities. SIESTA has also been engaged in the Electronic Structure Library effort from its inception, which has allowed the sharing of various low level libraries, as well as data standards and support for them, in particular the PSML definition and library for transferable pseudopotentials, and the interface to the ELSI library of solvers. Code sharing is made easier by the new open-source licensing model of the program. This review also presents examples of application of the capabilities of the code, as well as a view of on-going and future developments.

physics.comp-ph↗
arXiv2020

First-principles study of two-dimensional electron and hole gases at the head-to-head and tail-to-tail 180$^\circ$ domain walls in PbTiO$_{3}$ ferroelectric thin films

We study from first-principles the structural and electronic properties of head-to-head (HH) and tail-to-tail (TT) 180$^\circ$ domain walls in isolated free-standing PbTiO$_{3}$ slabs. For sufficiently thick domains ($n$ = 16 unit cells of PbTiO$_{3}$), a transfer of charge from the free surfaces to the domain walls to form localized electron (in the HH) and hole (in the TT) gases in order to screen the bound polarization charges is observed. The electrostatic driving force behind this electronic reconstruction is clearly visible from the perfect match between the smoothed free charge densities and the bound charge distribution, computed from a finite difference of the polarization profile obtained after the relaxation of the lattice degrees of freedom. The domain wall widths, of around six unit cells, are larger than in the conventional 180$^\circ$ neutral configurations. Since no oxygen vacancies, defects or dopant atoms are introduced in our simulations, all the previous physical quantities are the intrinsic limits of the system. Our results support the existence of an extra source of charge at the domains walls to explain the enhancement of the conductivity observed in some domains walls of prototypical, insulating in bulk, perovskite oxides.

cond-mat.mtrl-sci↗
arXiv2020

Comparison of band-fitting and Wannier-based model construction for WSe$_2$

Transition metal dichalcogenide materials $MX_2 (M=Mo,W;X=S,Se)$ are being thoroughly studied due to their novel two-dimensional structure, that is associated with exceptional optical and transport properties. From a computational point of view, Density Functional Theory simulations perform very well in these systems and are an indispensable tool to predict and complement experimental results. However, due to the time and length scales where even the most efficient DFT implementations can reach today, this methodology suffers of stringent limitations to deal with finite temperature simulations or electron-lattice coupling when studying excitation states: the unit cells required to study, for instance, systems with thermal fluctuations or large polarons would require a large computational power. Multi-scale techniques, like the recently proposed Second Principles Density Functional Theory, can go beyond these limitations but require the construction of tight-binding models for the systems under investigation. In this work, we compare two such methods to construct the bands of WSe$_2$. In particular, we compare the result of (i) Wannier-based model construction with (ii) the band fitting method of Liu and co-workers where the top of the valence band and the bottom of the conduction band are modeled by three bands symmetrized to have mainly Tungsten $d_{z^2}$, $d_{xy}$ and $d_{x^2-y^2}$ character. Our results emphasize the differences between these two approaches and how band-fitting model construction leads to an overestimation of the localization of the real-space basis in a tight-binding representation.

cond-mat.mtrl-sci↗
arXiv2019

Novel mechanisms to enhance the capacitance beyond the classical limits in capacitors with free-electron-like electrodes

The so-called negative electron compressibility refers to the lowering of the chemical potential of a metallic system when the carrier density increases. This effect has often been invoked in the past to explain the enhancement of the capacitance beyond the classical limits in capacitors with two-dimensional electron gases as electrodes. Based on experiments on strongly confined semiconductor quantum wells (QWs), it has been traditionally ascribed to the electron exchange energy as the main driving force. Recent research, however, has revealed that analogous effects can occur in other classes of materials systems, such as polar oxide interfaces, whose characteristics drastically depart from those of the previously considered cases. To rationalize these new results, it is necessary to revisit the established theory of confined electron gases, and test whether its conclusions are valid beyond the specifics of semiconductor-based QWs. Here we find, based on first-principles calculations of jellium slabs, that one must indeed be very careful when extrapolating existing results to other realistic physical systems. In particular, we identify a number of additional, previously overlooked mechanisms (e.g., related to the displacement of the electronic cloud and to the multiband structure of the delocalized gas), that enter into play and become new sources of negative capacitance in the weak-confinement regime. Our detailed analysis of these emerging contributions, supported by analytic models and multiple test cases, will provide a useful guidance in the ongoing quest for nanometric capacitors with enhanced performance.

cond-mat.mes-hall↗
arXiv2018

Theoretical guidelines to create and tune electric skyrmions

Magnetic skyrmions are mesmerizing spin textures with peculiar topological and dynamical properties, typically the product of competing interactions in ferromagnets, and with great technological potential [1-5]. Researchers have long wondered whether analogous electric skyrmions might exist in ferroelectrics, maybe featuring novel behaviors and possibilities for electric and mechanical control. The results thus far are modest, though: an electric equivalent of the most typical magnetic skyrmion (which would rely on a counterpart of the Dzyaloshinskii-Moriya interaction) seems all but impossible; further, the exotic ferroelectric orders observed or predicted to date [6-8] rely on very specific nano-structures (composites, superlattices), which limits the generality and properties (e.g., mobility) of the possible associated skyrmions. Here we propose an original approach to write electric skyrmions in simple ferroelectric lattices in a customary manner. Our second-principles simulations [9] of columnar ferroelectric nano-domains, in prototype compound PbTiO3 , show that it is possible to harness the Bloch-type internal structure of the domain wall [10] and hence create a genuine skyrmion. We check that the object thus obtained displays the usual skyrmion-defining features; further, it also presents unusual ones, including a symmetry-breaking skyrmion-skyrmion transition driven by strain, various types of topological transformations induced by external fields and temperature, and potentially very small sizes. Our results suggest countless possibilities for creating and manipulating electric textures with non-trivial topologies, using standard experimental tools and materials, effectively inaugurating the field of electric skyrmions.

cond-mat.mtrl-sci↗
arXiv2016

Efficient thermoelectric materials using nonmagnetic double perovskites with $d^0$/$d^6$ band filling

Efficient thermoelectric materials should present large Seebeck coefficient, high electrical conductivity and low thermal conductivity. An enhanced Seebeck coefficient can be obtained from materials where the Fermi level can be aligned with a large and narrow peak of the density of states, particularly when a substantial band valley degeneracy occurs. A high electrical conductivity comes as a consequence of large conductive hopping paths between the atoms of the material. Both physical quantities can be decoupled and optimized independently if their origins can be ascribed to different sets of bands. Based on these assumptions, double perovskites A$_2$BB'O$_6$ with $d^0/d^6$ filling for the B and B' metal cations, respectively, have been considered. They provide a desirable band structure with degenerate B-$t_{2g}$ / B'-$e_g$ bands above the Fermi level together with a low thermal conductivity. We have carried out first-principles simulations for various of these nonmagnetic double perovskites and showed that all of them present a large Seebeck coefficient (consequence of the localized and empty $t_{2g}$ states of the B-cation), and large electrical conductivity due to the more spread unoccupied $e_{g}$ band of the B' cation. We have seen that if they can be optimally doped, they could show a figure of merit comparable or even higher than the best $n$-type thermoelectric oxides, such as SrTiO$_{3}$. Different mechanisms to tune the band structure and enhance the thermoelectric figure of merit are explored, including epitaxial strain, hydrostatic pressure, chemical pressure, and external doping. A fully relaxed structure has also been studied, showing that a realistic calculation is necessary to make accurate predictions, but also proving that the main trends shown throughout the paper remain unchanged.

cond-mat.str-el↗
arXiv2015

Second-principles method including electron and lattice degrees of freedom

We present a first-principles-based (second-principles) scheme that permits large-scale materials simulations including both atomic and electronic degrees of freedom on the same footing. The method is based on a predictive quantum-mechanical theory, e.g., Density Functional Theory, and its accuracy can be systematically improved at a very modest computational cost. Our approach is based on dividing the electron density of the system into a reference part - typically corresponding to the system's neutral, geometry-dependent ground state - and a deformation part - defined as the difference between the actual and reference densities. We then take advantage of the fact that the bulk part of the system's energy depends on the reference density alone; this part can be efficiently and accurately described by a force field, thus avoiding explicit consideration of the electrons. Then, the effects associated to the difference density can be treated perturbatively with good precision by working in a suitably chosen Wannier function basis. Further, the electronic model can be restricted to the bands of interest. All these features combined yield a very flexible and computationally very efficient scheme. Here we present the basic formulation of this approach, as well as a practical strategy to compute model parameters for realistic materials. We illustrate the accuracy and scope of the proposed method with two case studies, namely, the relative stability of various spin arrangements in NiO and the formation of a two-dimensional electron gas at the interface between band insulators LaAlO$_3$ and SrTiO$_3$. We conclude by discussing ways to overcome the limitations of the present approach (most notably, the assumption of a fixed bonding topology), as well as its many envisioned possibilities and future extensions.

cond-mat.mtrl-sci↗
arXiv2012

Lattice screening of the polar catastrophe and hidden in-plane polarization in KNbO$_{3}$/BaTiO$_{3}$ interfaces

We have carried out first-principles simulations, based on density functional theory, to obtain the atomic and electronic structure of (001) BaTiO$_{3}$/KNbO$_{3}$ interfaces in an isolated slab geometry. We tried different types of structures including symmetric and asymmetric configurations, and variations in the thickness of the constituent materials. The spontaneous polarization of the layer-by-layer non neutral material (KNbO$_{3}$) in these interfaces cancels out almost exactly the "built-in" polarization responsible for the electronic reconstruction. As a consequence, the so-called polar catastrophe is quenched and all the simulated interfaces are insulating. A model, based on the modern theory of polarization and basic electrostatics, allows an estimation of the critical thickness for the formation of the two-dimensional electron gas between 42 and 44 KNbO$_{3}$ unit cells. We also demonstrate the presence of an unexpected in-plane polarization in BaTiO$_{3}$ localized at the $p$-type TiO$_2$/KO interface, even under in-plane compressive strains. We expect this in-plane polarization to remain hidden due to angular averaging during quantum fluctuations unless the symmetry is broken with small electric fields.

cond-mat.mtrl-sci↗
arXiv2012

First-principles modeling of the thermoelectric properties of SrTiO$_3$/SrRuO$_3$ superlattices

Using a combination of first-principles simulations, based on the density functional theory and Boltzmann's semiclassical theory, we have calculated the transport and thermoelectric properties of the half-metallic two dimensional electron gas confined in single SrRuO$_3$ layers of SrTiO$_3$/SrRuO$_3$ periodic superlattices. Close to the Fermi energy we find that the semiconducting majority spin channel displays a very large in-plane component of the Seebeck tensor at room temperature, $S$ = 1500 $μ$V/K, and the minority spin channel shows good in-plane conductivity $σ$ = 2.5 (m$Ω$cm)$^{-1}$. However, contrary to the expectation of Hicks and Dresselhaus model about enhanced global thermoelectric properties due to the confinement of the metallic electrons, we find that the total power factor and thermoelectric figure of merit for reduced doping is too small for practical applications. The reason for this failure can be traced back on the electronic structure of the interfacial gas, which departs from the free electron behaviour on which the model was based. The evolution of the electronic structure, electrical conductivity, Seebeck coefficient, and power factor as a function of the chemical potential is explained by a simplified tight-binding model. We find that the electron gas in our system is composed by a pair of one dimensional electron gases orthogonal to each other. This reflects the fact the physical dimensionality of the electronic system can be even smaller than that of the spacial confinement of the carriers.

cond-mat.mtrl-sci↗