arXiv subjects

Idoia G. Gurtubay

Publications and source records attributed to Idoia G. Gurtubay.

11 recordsLinked to original sources

arXiv2022

Ab initio self-consistent many-body theory of polarons at all couplings

We present a theoretical framework to describe polarons from first principles within a many-body Green's function formalism. Starting from a general electron-phonon Hamiltonian, we derive a self-consistent Dyson equation in which the phonon-mediated self-energy is composed by two distinct terms. One term is the Fan-Migdal self-energy and describes dynamic electron-phonon processes, the other term is a new contribution to the self-energy originating from the static displacements of the atomic nuclei in the polaronic ground state. The lowest-order approximation to the present theory yields the standard many-body perturbation theory approach to electron-phonon interactions in the limit of large polarons, and the ab initio polaron equations introduced in [Sio et al., Phys. Rev. B 99, 235139 (2019); Phys. Rev. Lett. 122, 246403 (2019)] in the limit of small polarons. A practical recipe to implement the present unifying formalism in first-principles calculations is outlined. We apply our method to the Fröhlich model, and obtain remarkably accurate polaron energies at all couplings, in line with Feynman's polaron theory and diagrammatic Monte Carlo calculations. We also recover the well-known results of Fröhlich and Pekar at weak and strong coupling, respectively. The present approach enables predictive many-body calculations of polarons in real materials at all couplings.

cond-mat.mtrl-sci↗

arXiv2022

Unified approach to polarons and phonon-induced band structure renormalization

Ab initio calculations of the phonon-induced band structure renormalization are currently based on the perturbative Allen-Heine theory and its many-body generalizations. These approaches are unsuitable to describe materials where electrons form localized polarons. Here, we develop a self-consistent, many-body Green's function theory of band structure renormalization that incorporates localization and self-trapping. We show that the present approach reduces to the Allen-Heine theory in the weak-coupling limit, and to total energy calculations of self-trapped polarons in the strong-coupling limit. To demonstrate this methodology, we reproduce the path-integral results of Feynman and diagrammatic Monte Carlo calculations for the Fröhlich model at all couplings, and we calculate the zero point renormalization of the band gap of an ionic insulator including polaronic effects.

cond-mat.mtrl-sci↗

arXiv2021

Electron-phonon coupling of Fe-adatom electron states on MgO/Ag(100)

We study the strength of the electron-phonon interaction on Fe single adatoms on MgO/Ag(100) based on many-body \textit{ab-initio} spin collinear calculations. In particular, we analyze the relative importance of the substrate and, among other results, we conclude that the interface electron state of Ag(100) plays a prominent role in determining the electron-phonon coupling of localized Fe electron states. The analysis of the hybridization of the adatom with the substrate reveals qualitative differences for even or odd coverages of MgO, affecting significantly the spectral structure and strength of the electron-phonon coupling. Our calculations indicate that the electron-phonon interaction is very strong for $\le$~1 layers of MgO, while it is sharply suppressed for larger coverages, a trend that is consistent with recent experimental findings.

cond-mat.mes-hall↗

arXiv2020

Fully anisotropic superconductivity with few Helmholtz Fermi-surface harmonics

We present an alternative representation for the anisotropic Eliashberg equations of superconductivity, whose numerical solution yields an efficiency gain of several orders of magnitude with respect to the conventional representation in momentum space. Our method is a practical realization of a long-sought approach, whose essence is a linear transformation from regular ${\bf k}$ space to a set of orthonormal functions defined as the solutions of the Helmholtz equation on the Fermi surface. In this way, all the anisotropy of the problem can be described by a handful of coefficients with built-in symmetry. We perform benchmark calculations on the gap anisotropy of MgB$_2$, and reproduce previous results at a remarkably reduced computational cost. Furthermore, we apply our methodology to efficiently determine the transition temperature of the compressed YH$_6$ hydride, obtaining very good agreement with recent experimental measurements. The simplification introduced by our method enables the high-throughput exploration of superconducting materials without having to resort to the isotropic approximation, and opens up possibilities towards first principles calculations of more advanced theories of superconductivity.

cond-mat.supr-con↗

arXiv2020

Symmetric Helmholtz Fermi-surface harmonics for an optimal representation of anisotropic quantities on the Fermi surface: Application to the electron-phonon problem

We outline a numerical procedure to incorporate the crystal symmetries in the Helmholtz Fermi-surface harmonics basis set, which are the solutions of the Helmholtz equation defined on the Fermi surface. This improvement allows for an optimal representation of anisotropic quantities defined on the Fermi surface in terms of few symmetric elements of the set. We demonstrate the general validity of our approach by identifying the fully symmetric Helmholtz Fermi-surface harmonics subset for several representative systems with different crystal structures, namely, FCC-Cu, HEX-MgB$_2$, and BCC-YH$_6$. Furthermore, we illustrate the potential of the method applied to the electron-phonon problem, showing that the anisotropic electron-phonon mass-enhancement parameter $λ_{\bf k}$ can be represented to high accuracy by a handful of coefficients. This works as an effective filter, paving the way for a reduction of several orders of magnitude in the computation of superconductivity, impurity problems, or any other Fermi surface dependent property of metals from first principles.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic oscillations induced by phonons in non-magnetic materials

An unexpected finding two decades ago demonstrated that Shockley electron states in noble metal surfaces are spin-polarized, forming a circulating spin texture in reciprocal space. The fundamental role played by the spin degree of freedom was then revealed, even for a non-magnetic system, whenever the spin-orbit coupling was present with some strength. Here we demonstrate that similarly to electrons in the presence of spin-orbit coupling, the propagating vibrational modes are also accompanied by a well-defined magnetic oscillation even in non-magnetic materials. Although this effect is illustrated by considering a single layer of the WSe2 dichalogenide, the phenomenon is completely general and valid for any non-magnetic material with spin-orbit coupling. The emerging phonon-induced magnetic oscillation acts as an additional effective flipping mechanism for the electron spin and its implications in the transport and scattering properties of the material are evident and profound.

cond-mat.other↗

arXiv2019

Emergence of large non-adiabatic effects induced by the electron-phonon interaction on the complex vibrational quasi-particle spectrum of the doped monolayer MoS$_{2}$

We present a comprehensive first-principles analysis of the non-adiabatic effects due to the electron-phonon interaction on the vibrational spectrum of the electron-doped monolayer MoS$_{2}$. Deep changes in the Fermi surface upon doping cause the linewidth broadening of the normal modes governing the spin-conserving inter-valley electronic scattering, which become unstable with the population of all the spin-split conduction valleys. We find that the non-adiabatic spectral effects modify dramatically the adiabatic dispersion of the long-wavelength optical phonon modes, responsible for intra-valley scattering, as soon as inequivalent valleys get populated. These results are illustrated by means of a simple analytical model. Finally, we explain the emergence of an intricate dynamical structure for the strongly interacting out-of-plane polarized A 0 1 optical vibrational mode spectrum by means of a multiple-phonon quasi-particle picture defined in the full complex frequency plane, showing that this intriguing spectral structure originates from the splitting of the original adiabatic branch induced by the electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2019

Long living carriers in a strong electron-phonon interacting two-dimensional doped semiconductor

Carrier doping by the electric field effect has emerged recently as an ideal route for monitoring many-body physics in two-dimensional (2D) materials where the Fermi level is tuned in a way that -- indirectly -- the strength of the interactions can also be scanned. The possibility of systematic doping in combination with high resolution photoemission has allowed to uncover a genuinely many-body electron spectrum in single-layer MoS2 transition metal dichalcogenide, resolving three clear quasi-particle states, where only one state should be expected if the electron-phonon interaction vanished. Our analysis combines first-principles and consistent complex plane analytic approaches and brings into light the presence and the physical origin of two gaps and the three quasi-particle bands which are unambiguously present in the photoemission spectrum. One of these states, though being strongly interacting with the accompanying virtual phonon cloud, presents a notably long lifetime which is an appealing property when trying to understand and take advantage of many-body interactions to modulate the transport properties.

cond-mat.mes-hall↗

arXiv2018

Coupled spin and electron-phonon dynamics at the relativistic Tl/Si(111) surface

We investigate the role played by the electron spin and the spin-orbit interaction on the exceptional electronphonon coupling at the Tl/Si(111) surface. Our first-principles calculations demonstrate that the particular spin pattern of this system dominates the whole low-energy electron-phonon physics, which is remarkably explained by forbidden spin-spin scattering channels. In particular, we show that the strength of the electron-phonon coupling appears drastically weakened for surface states close to the K and K' valleys, which is unambiguously attributed to the spin polarization through the associated modulation due to the spinor overlaps. However, close to the Γ point, the particular spin pattern in this area is less effective in damping the electron-phonon matrix elements, and the result is an exceptional strength of electron-phonon coupling parameter λ ~ 1.4. These results are rationalized by a simple model for the electron-phonon matrix elements including the spinor terms.

cond-mat.mes-hall↗

arXiv2017

Relativistic response and novel spin-charge plasmon at the Tl/Si(111) surface

We present a comprehensive \textit{ab initio} analysis of the spin-charge correlations at the Tl/Si(111) surface, where the spin-orbit interaction is so strong that a detailed treatment of the non-collinear electron spin appears decisive for the correct description of the response properties. The relativistic limit enforces a unified treatment of the spin and charge densities as a four-vector, and the response function acquires then a 4$\times$4 tensor structure. Our all-electron implementation allows to resolve the real space structure of the possible collective modes, and demonstrates the emergence of a novel collective excitation combining transverse-spin and ordinary charge oscillations of a similar order of magnitude, whose spin character is strongly enhanced as we approach the $q$$\rightarrow$0 momentum limit.

cond-mat.mes-hall↗

arXiv2014

Helmholtz Fermi Surface Harmonics: an efficient approach for treating anisotropic problems involving Fermi surface integrals

We present a new efficient numerical approach for representing anisotropic physical quantities and/or matrix elements defined on the Fermi surface of metallic materials. The method introduces a set of numerically calculated generalized orthonormal functions which are the solutions of the Helmholtz equation defined on the Fermi surface. Noteworthy, many properties of our proposed basis set are also shared by the Fermi Surface Harmonics (FSH) introduced by Philip B. Allen [Physical Review B 13, 1416 (1976)], proposed to be constructed as polynomials of the cartesian components of the electronic velocity. The main motivation of both approaches is identical, to handle anisotropic problems efficiently. However, in our approach the basis set is defined as the eigenfunctions of a differential operator and several desirable properties are introduced by construction. The method demonstrates very robust in handling problems with any crystal structure or topology of the Fermi surface, and the periodicity of the reciprocal space is treated as a boundary condition for our Helmholtz equation. We illustrate the method by analysing the free-electron-like Lithium (Li), Sodium (Na), Copper (Cu), Lead(Pb), Tungsten (W) and Magnesium diboride (MgB2).

cond-mat.other↗