arXiv subjects

Miquel Royo

Publications and source records attributed to Miquel Royo.

At least 19 recordsLinked to original sources

arXiv2026

Theory of phonon-magnon hybridization and angular momentum in CrI$_3$ and CrBr$_3$

In magnetic materials angular momentum can be mediated by different carriers, including electrons, magnons, and phonons. The magnons can interact with circularly polarized phonons which are close in energy, provided specific symmetry conditions are met. If the interaction is strong enough the phonons and magnons shift in frequency and start to mix to form hybrid magneto-elastic quasi-particles. In this paper, we develop a constrained Hamiltonian framework which incorporates hybrid stiffness matrices and Berry curvatures. We quantify the degree of hybridization between phonons and magnons (up to 8% in CrI$_3$ and 25% in CrBr$_3$) using a decomposition of the total energy, which is a generalization of the norm decomposition for atomic contributions to phonons used in the literature. We also explore how the total angular momentum is conserved but shared between the phononic and magnonic subsystems upon hybridization.

cond-mat.mtrl-sci↗

arXiv2026

Rototranslational sum rules for nuclear dynamics via traveling pseudopotentials

We establish a set of exact sum rules that relate the interatomic force constants to the frequency-dependent electromagnetic susceptibility of a solid or molecule, thereby generalizing the long-established principles of rototranslational symmetry to the nonadiabatic regime. Crucially, we show that in practical numerical implementations these sum rules are violated, unless special precautions are taken in the treatment of the atomic pseudopotentials. We solve these issues once and for all by correctly adapting the pseudopotential to the motion of the corresponding nucleus, with a velocity dependence of the nonlocal operator. This prescription restores the correct Galilean covariance of the Schrödinger equation, and the expected identity between mechanical rototranslations and electromagnetic perturbations. These results conclusively fix a number of worrisome inconsistencies that were pointed out over the years in the context of linear-response theory restoring, e.g., the validity of the Larmor theorem, and the equivalence between the inertial and electrical definitions of the Drude weight in metals.

cond-mat.mtrl-sci↗

arXiv2025

Dynamical response of noncollinear spin systems at constrained magnetic moments

Noncollinear magnets are notoriously difficult to describe within first-principles approaches based on density-functional theory (DFT) because of the presence of low-lying spin excitations. At the level of ground-state calculations, several methods exist to constrain the magnetic moments to a predetermined configuration, and thereby accelerate convergence towards self-consistency. Their use in a perturbative context, however, remains very limited. Here we present a general methodological framework to achieve parametric control over the local spin moments at the linear-response level. Our strategy builds on the concept of Legendre transform to switch between various flavors of magnetic functionals, and to relate their second derivatives via simple linear-algebra operations. Thereby, we can address an arbitrary response function at the time-dependent DFT level of theory with optimal accuracy and minimal computational effort. In the low frequency limit, we identify the leading correction to the existing adiabatic formulation of the problem [S. Ren \emph{et al.}, Phys. Rev. X {\bf 14}, 011041 (2024)], consisting in a renormalization of the phonon and magnon masses due to electron inertia. As a demonstration, we apply our methodology to the THz optical response of bulk CrI$_3$ and Cr$_2$O$_3$, where we identify contributions from hybrid (electro)magnons with mixed spin-lattice character.

cond-mat.mtrl-sci↗

arXiv2025

Abinit 2025: New Capabilities for the Predictive Modeling of Solids and Nanomaterials

Abinit is a widely used scientific software package implementing density functional theory and many related functionalities for excited states and response properties. This paper presents the novel features and capabilities, both technical and scientific, which have been implemented over the past 5 years. This evolution occurred in the context of evolving hardware platforms, high-throughput calculation campaigns, and the growing use of machine learning to predict properties based on databases of first principles results. We present new methodologies for ground states with constrained charge, spin or temperature; for density functional perturbation theory extensions to flexoelectricity and polarons; and for excited states in many-body frameworks including GW, dynamical mean field theory, and coupled cluster. Technical advances have extended abinit high-performance execution to graphical processing units and intensive parallelism. Second principles methods build effective models on top of first principles results to scale up in length and time scales. Finally, workflows have been developed in different community frameworks to automate \abinit calculations and enable users to simulate hundreds or thousands of materials in controlled and reproducible conditions.

cond-mat.mtrl-sci↗

arXiv2023

In-plane flexoelectricity in two-dimensional $D_{3d}$ crystals

We predict a large in-plane polarization response to bending in a broad class of trigonal two-dimensional crystals. We define and compute the relevant flexoelectric coefficients from first principles as linear-response properties of the undistorted layer, by using the primitive crystal cell. The ensuing response (evaluated for SnS$_{2}$, silicene, phosphorene and RhI$_{3}$ monolayers and for a hexagonal BN bilayer) is up to one order of magnitude larger than the out-of-plane components in the same material. We illustrate the topological implications of our findings by calculating the polarization textures that are associated with a variety of rippled and bent structures. We also determine the longitudinal electric fields induced by a flexural phonon at leading order in amplitude and momentum.

cond-mat.mtrl-sci↗

arXiv2023

Long-range electrostatic contribution to the electron-phonon couplings and mobilities of two-dimensional and bulk materials

Charge transport plays a crucial role in manifold potential applications of two-dimensional materials, including field effect transistors, solar cells, and transparent conductors. At most operating temperatures, charge transport is hindered by scattering of carriers by lattice vibrations. Assessing the intrinsic phonon-limited carrier mobility is thus of paramount importance to identify promising candidates for next-generation devices. Here we provide a framework to efficiently compute the drift and Hall carrier mobility of two-dimensional materials through the Boltzmann transport equation by relying on a Fourier-Wannier interpolation. Building on a recent formulation of long-range contributions to dynamical matrices and phonon dispersions [Phys. Rev. X 11, 041027 (2021)], we extend the approach to electron-phonon coupling including the effect of dynamical dipoles and quadrupoles. We identify an unprecedented contribution associated with the Berry connection that is crucial to preserve the Wannier-gauge covariance of the theory. This contribution is not specific to 2D crystals, but also concerns the 3D case, as we demonstrate via an application to bulk SrO. We showcase our method on a wide selection of relevant monolayers ranging from SnS2 to MoS2, graphene, BN, InSe, and phosphorene. We also discover a non-trivial temperature evolution of the Hall hole mobility in InSe whereby the mobility increases with temperature above 150 K due to the mexican-hat electronic structure of the InSe valence bands. Overall, we find that dynamical quadrupoles are essential and can impact the carrier mobility in excess of 75%.

cond-mat.mtrl-sci↗

arXiv2023

Accurate prediction of Hall mobilities in two-dimensional materials through gauge-covariant quadrupolar contributions

Despite considerable efforts, accurate computations of electron-phonon and carrier transport properties of low-dimensional materials from first principles have remained elusive. By building on recent advances in the description of long-range electrostatics, we develop a general approach to the calculation of electron-phonon couplings in two-dimensional materials. We show that the nonanalytic behavior of the electron-phonon matrix elements depends on the Wannier gauge, but that a missing Berry connection restores invariance to quadrupolar order. We showcase these contributions in a MoS$_2$ monolayer, calculating intrinsic drift and Hall mobilities with precise Wannier interpolations. We also find that the contributions of dynamical quadrupoles to the scattering potential are essential, and that their neglect leads to errors of 23% and 76% in the room temperature electron and hole Hall mobilities, respectively.

cond-mat.mtrl-sci↗

arXiv2021

Lattice-mediated bulk flexoelectricity from first principles

We present the derivation and code implementation of a first-principles methodology to calculate the lattice-mediated contributions to the bulk flexoelectric tensor. The approach is based on our recent analytical long-wavelength extension of density-functional perturbation theory [Royo and Stengel, Phys. Rev. X 9, 021050 (2019)], and avoids the cumbersome numerical derivatives with respect to the wave vector that were adopted in previous implementations. To substantiate our results, we revisit and numerically validate the sum rules that relate flexoelectricity and uniform elasticity by generalizing them to regimes where finite forces and stresses are present. We also revisit the definition of the elastic tensor under stress, especially in regards to the existing linear-response implementation. We demonstrate the performance of our method by applying it to representative cubic crystals and to the tetragonal low-temperature polymorph of SrTiO$_3$, obtaining excellent agreement with the available literature data.

cond-mat.mtrl-sci↗

arXiv2021

Direct and converse flexoelectricity in two-dimensional materials

Building on recent developments in electronic-structure methods, we define and calculate the flexoelectric response of two-dimensional (2D) materials fully from first principles. In particular, we show that the open-circuit voltage response to a flexural deformation is a fundamental linear-response property of the crystal that can be calculated within the primitive unit cell of the flat configuration. Applications to graphene, silicene, phosphorene, BN and transition-metal dichalcogenide monolayers reveal that two distinct contributions exist, respectively of purely electronic and lattice-mediated nature. Within the former, we identify a key $metric$ term, consisting in the quadrupolar moment of the unperturbed charge density. We propose a simple continuum model to connect our findings with the available experimental measurements of the converse flexoelectric effect.

cond-mat.mes-hall↗

arXiv2021

Exact long-range dielectric screening and interatomic force constants in quasi-2D crystals

We develop a fundamental theory of the long-range electrostatic interactions in two-dimensional crystals by performing a rigorous study of the nonanalyticities of the Coulomb kernel. We find that the dielectric functions are best represented by $2\times 2$ matrices, with nonuniform macroscopic potentials that are two-component hyperbolic functions of the out-of-plane coordinate, $z$. We demonstrate our arguments by deriving the long-range interatomic forces in the adiabatic regime, where we identify a formerly overlooked dipolar coupling involving the out-of-plane components of the dynamical charges. The resulting formula is exact up to an arbitrary multipolar order, which we illustrate in practice via the explicit inclusion of dynamical quadrupoles. By performing numerical tests on monolayer BN, SnS$_2$ and BaTiO$_3$ membranes, we show that our method allows for a drastic improvement in the description of the long-range electrostatic interactions, with comparable benefits to the quality of the interpolated phonon band structure.

cond-mat.mes-hall↗

arXiv2020

Phonon-limited electron mobility in Si, GaAs and GaP with exact treatment of dynamical quadrupoles

We describe a new approach to compute the electron-phonon self-energy and carrier mobilities in semiconductors. Our implementation does not require a localized basis set to interpolate the electron-phonon matrix elements, with the advantage that computations can be easily automated. Scattering potentials are interpolated on dense $\mathbf{q}$ meshes using Fourier transforms and ab initio models to describe the long-range potentials generated by dipoles and quadrupoles. To reduce significantly the computational cost, we take advantage of crystal symmetries and employ the linear tetrahedron method and double-grid integration schemes, in conjunction with filtering techniques in the Brillouin zone. We report results for the electron mobility in Si, GaAs, and GaP obtained with this new methodology.

cond-mat.mtrl-sci↗

arXiv2020

Electron-Phonon Beyond Fröhlich: Dynamical Quadrupoles in Polar and Covalent Solids

We include the treatment of quadrupolar fields beyond the Fröhlich interaction in the first-principles electron-phonon vertex in semiconductors. Such quadrupolar fields induce long-range interactions that have to be taken into account for accurate physical results. We apply our formalism to Si (nonpolar), GaAs, and GaP (polar) and demonstrate that electron mobilities show large errors if dynamical quadrupoles are not properly treated.

cond-mat.mtrl-sci↗

arXiv2020

Using high multipolar orders to reconstruct the sound velocity in piezoelectrics from lattice dynamics

Information over the phonon band structure is crucial to predicting many thermodynamic properties of materials, such as thermal transport coefficients. Highly accurate phonon dispersion curves can be, in principle, calculated in the framework of density-functional perturbation theory (DFPT). However, well-established techniques can run into trouble (or even catastrophically fail) in the case of piezoelectric materials, where the acoustic branches hardly reproduce the physically correct sound velocity. Here we identify the culprit in the higher-order multipolar interactions between atoms, and demonstrate an effective procedure that fixes the aforementioned issue. Our strategy drastically improves the predictive power of perturbative lattice-dynamical calculations in piezoelectric crystals, and is directly implementable for high-throughput generation of materials databases.

cond-mat.mtrl-sci↗

arXiv2019

First-principles theory of spatial dispersion: Dynamical quadrupoles and flexoelectricity

Density-functional perturbation theory (DFPT) is nowadays the method of choice for the accurate computation of linear and non-linear response properties of materials from first principles. A notable advantage of DFPT over alternative approaches is the possibility of treating incommensurate lattice distortions with an arbitrary wavevector, ${\bf q}$, at essentially the same computational cost as the lattice-periodic case. Here we show that ${\bf q}$ can be formally treated as a perturbation parameter, and used in conjunction with established results of perturbation theory (e.g. the "2n+1" theorem) to perform a long-wave expansion of an arbitrary response function in powers of the wavevector components. This provides a powerful, general framework to accessing a wide range of spatial dispersion effects that were formerly difficult to calculate by means of first-principles electronic-structure methods. In particular, the physical response to the spatial gradient of any external field can now be calculated at negligible cost, by using the response functions to $\mathit{uniform}$ perturbations (electric, magnetic or strain fields) as the only input. We demonstrate our method by calculating the flexoelectric and dynamical quadrupole tensors of selected crystalline insulators and model systems.

cond-mat.mtrl-sci↗

arXiv2017

A phononic switch based on ferroelectric domain walls

The ease with which domain walls (DWs) in ferroelectric materials can be written and erased provides a versatile way to dynamically modulate heat fluxes. In this work we evaluate the thermal boundary resistance (TBR) of 180$^{\circ}$ DWs in prototype ferroelectric perovskite PbTiO$_3$ within the numerical formalisms of nonequilibrium molecular dynamics and nonequilibrium Green's functions. An excellent agreement is obtained for the TBR of an isolated DW derived from both approaches, which reveals the harmonic character of the phonon-DW scattering mechanism. The thermal resistance of the ferroelectric material is shown to increase up to around 20%, in the system sizes here considered, due to the presence of a single DW, and larger resistances can be attained by incorporation of more DWs along the path of thermal flux. These results, obtained at device operation temperatures, prove the viability of an electrically actuated phononic switch based on ferroelectric DWs.

cond-mat.mtrl-sci↗

arXiv2017

The ferroelectric domain wall phonon polarizer

Modulating the polarization of a beam of quantum particles is a powerful method to tailor the macroscopic properties of the ensuing energy flux as it directly influences the way in which its quantum constituents interact with other particles, waves or continuum media. Practical polarizers, being well developed for electric and electromagnetic energy, have not been proposed to date for heat fluxes carried by phonons. Here we report on atomistic phonon transport calculations demonstrating that ferroelectric domain walls can operate as phonon polarizers when a heat flux pierces them. Our simulations for representative ferroelectric perovskite PbTiO$_3$ show that the structural inhomogeneity associated to the domain walls strongly suppresses transverse phonons, while longitudinally polarized modes can travel through multiple walls in series largely ignoring their presence.

cond-mat.mtrl-sci↗

arXiv2017

Electron and phonon transport in twisted graphene nanoribbons

We theoretically study the electrical, thermal and thermoelectric transport properties of graphene nanoribbons under torsional deformations. The modelling follows a nonequilibrium Green's function approach in the ballistic transport regime, describing the electrical and phononic properties through \textit{ab-initio} density functional theory and empirical interatomic potentials, respectively. We consider two different types of deformations, a continuous twist of a given angle applied to the nanoribbon, and two consecutive twists applied in opposite angular directions. The numerical results are carefully analysed in terms of spatially-resolved electron eigenchannels, polarization-dependent phonon transmission and thermoelectric figure-of-merit.

cond-mat.mes-hall↗

arXiv2016

Tailoring the core electron density in modulation-doped Core-Multi-Shell nanowires

We show how a proper radial modulation of the composition of core-multi-shell nanowires critically enhances the control of the free-carrier density in the high-mobility core with respect to core-single-shell structures, thus overcoming the technological difficulty of fine tuning the remote doping density. We calculate the electron population of the different nanowire layers as a function of the doping density and of several geometrical parameters by means of a self-consistent Schrödinger-Poisson approach: Free carriers tend to localize in the outer shell and screen the core from the electric field of the dopants.

cond-mat.mes-hall↗