arXiv subjects

S. Villa-Cortés

Publications and source records attributed to S. Villa-Cortés.

8 recordsLinked to original sources

arXiv2023

The Isotope Effect and Critical Magnetic Fields of Superconducting YH$_{6}$: A Migdal-Eliashberg Theory Approach

The emergence of near-ambient temperature superconductivity under pressure in the metal hydride systems has motivated a desire to further understand such remarkable properties, specifically critical magnetic fields. YH$_6$ is suggested to be a departure from conventional superconductivity, due to apparent anomalous behavior. Using density functional calculations in conjunction with Migdal-Eliashberg theory we show that in YH$_6$ the critical temperature and the isotope effect under pressure, as well as the high critical fields, are consistent with strong-coupling conventional superconductivity; a property anticipated to extend to other related systems. Furthermore, the strong-coupling corrections occur to the expected BCS values for the Ginzburg-Landau parameter ($κ_{1}(T)$), London penetration depth ($λ_{L}(T)$), electromagnetic coherence length ($ξ(T)$), and the energy gap ($Δ_{0}$).

cond-mat.supr-con↗

arXiv2022

A first-principles study of superconductivity on RbH by doping without applied pressure

The structural, electronic, lattice dynamics, electron-phonon coupling, and superconducting properties of the alkali-metal hydride RbH, metalized through electron-doping by the construction of the solid-solution Rb$_{1-x}$Sr$_x$H, are systematically analyzed as a function of Sr-content within the framework of density functional perturbation and Migdal-Eliashberg theories, taking into account the effect of zero-point energy contribution by the quasi-harmonic approximation. For the entire studied range of Sr-content, steady increments of the electron-phonon coupling constant and the superconducting critical temperature are found with progressive alkaline-earth metal content through electron-doping, reaching the values of $λ=1.92$ and $T_c=51.3(66.1)$~K with $μ^*$=0.1(0). The steady rise of such quantities as a function of Sr-content is consequence of the metallization of the hydride as an increase of density of states at the Fermi level is observed, as well as the softening of the phonon spectrum, mainly coming from H-optical modes. Our results indicate that electron-doping on metal-hydrides is an encouraging alternative to look for superconductivity without applied pressure.

cond-mat.supr-con↗

arXiv2021

Superconductivity on ScH$_{3}$ and YH$_{3}$ hydrides: Effects of applied pressure in combination with electron- and hole-doping on the electron-phonon coupling properties

The implementation of electron- and hole-doping, in conjunction to applied pressure, is analyzed as a mechanism to induce or enhance the superconducting state on fcc YH$_3$ and ScH$_3$. In particular, the evolution of their structural, electronic, and lattice dynamical properties, as well as the electron-phonon coupling and superconducting critical temperature ($T_c$) is presented and discussed, as a function of the electron- and hole-doping content as well as applied pressure. The study was performed within the density functional perturbation theory, taking into account the effects of zero-point energy through the quasi-harmonic approximation, while the doping was implemented by means of the construction of the Sc$_{1-x}$M$_{x}$H$_{3}$ (M=Ca,Ti) and Y$_{1-x}$M$_{x}$H$_{3}$ (M=Sr,Zr) solid solutions modeled with the virtual crystal approximation (VCA). We found that the ScH$_3$ and YH$_3$ hydrides shown a significant improvement of their electron-phonon coupling properties under hole-doping (M=Ca,Sr) and at pressure values close to dynamical instabilities. Instead, by electron-doping (M=Ti,Zr), the systems do not improve such properties, whatever value of applied pressure is considered. Then, as a result, $T_c$ rapidly increases as a function of $x$ on the hole-doping region, reaching its maximum value of $92.7(67.9)$~K and $84.5(60.2)$~K at $x=0.3$ for Sc$_{1-x}$Ca$_{x}$H$_{3}$ at $10.8$~GPa and Y$_{1-x}$Sr$_{x}$H$_{3}$ at $5.8$~GPa respectively, with $μ^{*}=0(0.15)$, while for both, electron- and hole-doping, $T_c$ decreases as a function of the applied pressure, mainly due to phonon hardening. By the thorough analysis of the electron-phonon properties as a function of doping and pressure, we can conclude that the tuning of the lattice dynamics is a promising path for improving the superconductivity on both systems.

cond-mat.supr-con↗

arXiv2021

Electron- and hole-doping on ScH$_{2}$ and YH$_{2}$: Effects on superconductivity without applied pressure

We present the evolution of the structural, electronic, and lattice dynamical properties, as well as the electron-phonon coupling and superconducting critical temperature ($T_c$) of ScH$_2$ and YH$_2$ metal hydrides solid solutions, as a function of the electron- and hole-doping content. The study was performed within the density functional perturbation theory, taking into account the effect of zero-point energy through the quasi-harmonic approximation, and the solid solutions Sc$_{1-x}M_{x}$H$_{2}$ ($M$=Ca,Ti) and Y$_{1-x}M_{x}$H$_{2}$ ($M$=Sr,Zr) were modeled by the virtual crystal approximation. We have found that, under hole-doping ($M$=Ca,Sr), the ScH$_2$ and YH$_2$ hydrides do not improve their electron-phonon coupling properties, sensed by $λ(x)$. Instead, by electron-doping ($M$=Ti,Zr), the systems reach a critical content $x \approx 0.5$ where the latent coupling is triggered, increasing $λ$ as high as $70\%$, in comparison with its $λ(x=0)$ value. Our results show that $T_c$ quickly decreases as a function of $x$ on the hole-doping region, from $x=0.2$ to $x=0.9$, collapsing at the end. Alternatively, for electron-doping, $T_c$ first decreases steadily until $x=0.5$, reaching its minimum, but for $x > 0.5$ it increases rapidly, reaching its maximum value of the entire range at the Sc$_{0.05}$Ti$_{0.95}$H$_2$ and Y$_{0.2}$Zr$_{0.8}$H$_2$ solid solutions, demonstrating that electron-doping can improve the superconducting properties of pristine metal hydrides, in the absence of applied pressure.

cond-mat.supr-con↗

arXiv2021

Effect of van Hove singularity on the isotope effect and critical temperature of H$_{3}$S hydride superconductor as a function of pressure

We have performed density functional calculations in conjunction with the linearized Migdal-Eliashberg equations and the functional derivative approach, which takes into account the energy-range dependence of the density of states at the Fermi level ($N(\varepsilon)$ variable on the scale of phonon energy), to determine the evolution of the critical temperature ($T_{c}$) and the isotope effect coefficient ($α$) of H$_{3}$S as a function of pressure on its $lm\bar{3}m$ crystal-structure range (from 162 to 250 GPa). Such approach, in comparison with $N(\varepsilon)$=$N(0)$=const., improves the agreement of $T_{c}$ with available experiments on the whole range of studied pressure. Considering for $α$ two main contributions: one positive coming from the electron-phonon (el-ph) and the other negative from the electron-electron interaction (el-el), we obtained a monotonic decrement as a function of pressure, independent of applied scheme ($N(\varepsilon)$ or $N(0)$). However, when $N(\varepsilon)$ is taken into account, an important renormalization occurs on both contributions, el-ph and el-el, improving the agreement with experimental data, specially for the high-pressure regime. The observed evolution of $T_{c}$ and $α$ as a function of pressure indicates, thus, the crucial role of the energy-dependence on $N(\varepsilon)$ for the proper analysis and description of the superconducting state on high-$T_{c}$ metal hydrides as H$_{3}$S, by considering the role of its van Hove singularities.

cond-mat.supr-con↗

arXiv2018

The Thermodynamics and the Inverse Isotope Effect of superconducting PdH and PdD under pressure

We present in this paper the thermodynamics of superconducting PdH and PdD under pressure. We make use of a general method to calculate the thermodynamics under pressure within the Migdal-Eliashberg theory. We have considered the crystal lattice to be zincblende taking into account the experimental evidence for both PdH and PdD at temperatures below 55 K. We have studied, in particular, the changes induced by pressure in the critical temperature, $T_c$, in the specific heat jump at $T_c$, in the energy gap at $T=0K$, in the deviation function $D(t)$ and in the isotope effect coefficient, $α$. We get a very good agreement with experiment where this data exist. This method represents a basis on which the thermodynamics of other hydrides under pressure can be calculated.

cond-mat.supr-con↗

arXiv2018

On the Calculation of the Inverse Isotope Effect in PdH(D): A Migdal-Eliashberg Theory Approach

Replacement of hydrogen by deuterium in palladium hydride results in higher superconducting temperatures and an anomalous isotope effect that has not been yet fully explained. In this work, we try a new approach to the explanation of the inverse isotope effect in PdH(D). Our approach introduces two new aspects. First, we took into account the experimental evidence that at temperatures below 50 K, the crystal structure of PdH and of PdD is zincblende. Second, we take into account not only the influence of the electron-phonon interaction but also the electron-electron interaction contribution to the isotope coeffiecient due tothe replacement of deuterium in the place of hydrogen. We used the Migdal-Eliashberg theory to perform our ab initio calculations. We found in this picture that the electron-electron interaction is considerably reduced by the isotope substitution and is the most important factor to explain the inverse isotope effect. We found $ΔT_c^{total}=2.224\:K$ and $α= -0.3134$ in excellent agreement with the values found experimentally.

cond-mat.supr-con↗

arXiv2017

Inverse Isotope Effect in PdH(D)

In this letter, we take a new approach to the explanation of the inverse isotope effect in PdH(D). Our approach introduces two new aspects. First, we took into account the experimental evidence that at temperatures below 50 K, the crystal structure of PdH and of PdD is zincblende. Second, we studied the contribution of both, the electron-phonon and the electron-electron interactions. We used the Migdal-Eliashberg theory to perform our ab initio calculations. We found that the electron-electron contribution is the most important one to explain the inverse isotope effect. We reproduced the experimentally found values for the critical temperature and the isotope coefficient. Our analysis represents a direct and simple explanation for the inverse isotope effect in PdH(D).

cond-mat.supr-con↗