arXiv subjects

A. Bergara

Publications and source records attributed to A. Bergara.

12 recordsLinked to original sources

arXiv2020

$β$-As$_2$Te$_3$: Pressure-Induced 3D Dirac Semi-Metal

We report a theoretical \textit{ab-initio} study of $β$-As$_2$Te$_3$ ($R\bar{3}m$ symmetry) at hydrostatic pressures up to 12 GPa. We have systematically characterized the vibrational and electronic changes of the system induced by the pressure variation. The electronic band dispersions calculated at different pressures using \textit{QS}GW show an insulator-metal transition. At room pressure the system is a semiconductor with small band-gap, and the valence and conduction bands present a parabolic conventional dispersion. However around 2 GPa the parabolic shape of the bands become linear and touch at the Fermi level. This means that this compound undergoes a pressure-induced topological phase transition to a 3D analog of graphene, known as a 3D Dirac semi-metal, with gapless electronic excitations. At increasing pressures the gap reopens and variation of the character of the electronic band-gap from direct to indirect is evidenced. At 7 GPa we observe the formation of a negative band-gap character, which persists for pressures up to 12 GPa. Topological insulating features are evidenced from 2 to 12 GPa with a Z$_4$=3 topological index. Moreover by investigating the lattice thermal-conductivity at different pressures, we observe an ultra low value of $κ_\textrm{L}$ at 300 K for 0.5 GPa (0.294 and 0.486 Wm$^{-1}$K$^{-1}$ for the $x$-$y$-axis and for the $z$-axis, respectively) which is the result of existing low-frequency optical modes. At 2 GPa $κ_\textrm{L}$ increases to 1.170 and 0.669 Wm$^{-1}$K$^{-1}$, for the $x$-$y$-axis and for the $z$-axis, respectively. At 4 GPa the thermal-conductivity values between the two distinct crystallographic axis tend to approximate, with 1.495 and 1.433 Wm$^{-1}$K$^{-1}$, along the $x$-$y$-axis and the $z$-axis, respectively.

cond-mat.mtrl-sci↗

arXiv2013

Ab-initio analysis of plasmon dispersion in sodium under pressure

We present an ab-initio study of the electronic response function of sodium in its 5 known metallic phases from 0 to 180 GPa at room temperature. The considered formalism is based on a interpolation scheme within time-dependent density functional theory that uses maximally localized Wannier functions, providing an accurate sampling of the reciprocal space. Besides showing an excellent agreement with inelastic X-ray scattering experiments, our calculations reveal that the drastic decrease of the optical reflectivity recently measured in the high pressure phases oP8 and tI19 is associated to a new low-energy plasmon arising from collective interband excitations. Additionally, our calculations predict the existence of an anisotropic interband plasmon in the stability pressure range of fcc Na (65 to 105 GPa).

cond-mat.mtrl-sci↗

arXiv2010

Exotic behavior and crystal structures of calcium under pressure

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc \rightarrow bcc \rightarrow simple cubic \rightarrow Ca-IV \rightarrow Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The β-tin (I41/amd) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33-71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching ~20 K at 120 GPa, in good agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2007

Lindemann Criterion and the Anomalous Melting Curve of Sodium

Recent reports of the melting curve of sodium at high pressure have shown that it has a very steep descent after a maximum of around 1000K at 31 GPa. This is not due to a phase transition. According to the Lindemann criterion, this behaviour should be apparent in the evolution of the Debye temperature with pressure. In this work, we have performed an "ab-initio" analysis of the behaviour of both the Debye temperature and the elastic constants up to 102 GPa, and find a clear trend at high pressure that should cause a noticeable effect on the melting curve.

cond-mat.other↗

arXiv2006

Nesting Induced Peierls-type Instability for Compressed Li-CI16

Alkalies are considered to be simple metals at ambient conditions. However, recently reported theoretical and experimental results have shown an unexpected and intriguing correlation between complex structures and an enhanced superconducting transition temperature in lithium under pressure. In this article we analyze the pressure induced Fermi surface deformation in bcc lithium, and its relation to the observed cI16 structure. According to our calculations, the Fermi surface becomes increasingly anisotropic with pressure and develops an extended nesting along the bcc [121] direction. This nesting induces a phonon instability of both transverse modes at N, so that a Peierls-type mechanism is proposed to explain the stability of Li-cI16.

cond-mat.mtrl-sci↗

arXiv2005

Dynamical response function of a compressed lithium monolayer

Since recent both theoretical and experimental results have proved that the simple behaviour light alkaline metals present at equilibrium breaks when high pressures are applied, they have become an important object of study in Condensed Matter Physics. On the other hand, development of new techniques in the atomic manipulation allows the growth of atomic monolayers (ML's), therefore rising the interest to analyze low dimensional systems under different conditions. In particular, new \textit{ab initio} calculations performed for a lithium ML show that its electronic properties experience important modifications under pressure, which could lead to significant modifications in its dynamical response function. In this article we perform \textit{ab initio} calculations of the dynamical response function of a lithium ML analyzing its evolution with increasing applied pressure. We show that besides the well known intraband and interband plasmons, rising electronic density induces characteristic features of acoustic plasmons related to the presence of two types of carriers at the Fermi level.

cond-mat.mtrl-sci↗

arXiv2005

Pressure Induced Fermi Surface Deformation in Lithium

Recently reported structural complexity and superconducting transition in lithium under pressure has increased the interest in light alkalis, otherwise considered as simple metals and well known systems under normal conditions. In this work we present an analysis of the pressure induced Fermi surface deformation in lithium with increasing pressure. According to our calculations, under pressure the Fermi surface becomes highly anisotropic and around 30 GPa develops an extended nesting, which could be the origin of the complex phase transitions observed at this pressure via a phonon instability. On the other hand, the phonon softening induced by the observed distortion and nesting in the Fermi surface could also imply an increasing electron-phonon interaction with pressure, which might also allow a better understanding of the observed superconducting transition in lithium at around the same pressure range.

cond-mat.mtrl-sci↗

arXiv2005

Pressure induced complexity in a lithium monolayer

Light alkali metals have usually been considered as simple metals due to their monovalency and high conductivity. In these metals ionic pseudopotentials are weak and the nearly free electron model (NFE) becomes quite accurate at normal conditions. However, very recent experiments have shown that at high pressures their electronic properties deviate radically from the NFE model and even become unexpected good superconductors. In this work we present ab initio calculations to analyze the deviation from simplicity in a lithium monolayer (ML) when pressure is applied. We have seen that as a result of the increasing non-local character of the atomic pseudopotential with increasing pressure, the surprising half filling Hubbard-type nesting observed in the Fermi line can explain the interesting complex behavior in lithium ML, induced by its correlated structural, electronic and even magnetic properties.

cond-mat.mtrl-sci↗

arXiv2002

Ferromagnetic instabilities in atomically-thin lithium and sodium wires

Using density functional theory the ground state structural, electronic, and magnetic properties of monatomic lithium and sodium chains with low average density are investigated. A metallic, zigzag ground state structure is predicted but, most interestingly, stable equilibria for chains under tension are predicted to be {\it ferromagnetic}, which can be traced to exchange effects arising from occupation of the second subband as a function of the interatomic distance.

cond-mat.mtrl-sci↗

arXiv1999

Hydrodynamic approximation for the nonlinear response of a metal surface

We present semi-classical and quantized hydrodynamic models to obtain the quadratic electronic response of a plane-bounded electron gas. Explicit expressions for the dynamic image potential experienced by charged particles moving near a jellium surface are derived, up to third order in the projectile charge. These expressions are employed to compute the image potential at all distances outside the surface. Though nonlinear corrections are found to be more important far inside the solid than outside, our results indicate that the nonlinear image potential is enhanced with respect to the linear image potential by a factor that is for Al as large as $\sim 1.15$ near the surface in the case of a stationary particle ($v\to 0$) with positive unit charge $e$.

cond-mat.mtrl-sci↗

arXiv1999

Plasmon excitation by charged particles interacting with metal surfaces

Recent experiments (R. A. Baragiola and C. A. Dukes, Phys. Rev. Lett. {\bf 76}, 2547 (1996)) with slow ions incident at grazing angle on metal surfaces have shown that bulk plasmons are excited under conditions where the ions do not penetrate the surface, contrary to the usual statement that probes exterior to an electron gas do not couple to the bulk plasmon. We here use the quantized hydrodynamic model of the bounded electron gas to derive an explicit expression for the probability of bulk plasmon excitation by external charged particles moving parallel to the surface. Our results indicate that for each ${\bf q}$ (the surface plasmon wave vector) there exists a continuum of bulk plasmon excitations, which we also observe within the semi-classical infinite-barrier (SCIB) model of the surface.

cond-mat.mtrl-sci↗

arXiv1998

Quadratic electronic response of a two-dimensional electron gas

The electronic response of a two-dimensional (2D) electron system represents a key quantity in discussing one-electron properties of electrons in semiconductor heterojunctions, on the surface of liquid helium and in copper-oxide planes of high-temperature superconductors. We here report an evaluation of the wave-vector and frequency dependent dynamical quadratic density-response function of a 2D electron gas (2DEG), within a self-consistent field approximation. We use this result to find the $Z_1^3$ correction to the stopping power of a 2DEG for charged particles moving at a fixed distance from the plane of the 2D sheet, $Z_1$ being the projectile charge. We reproduce, in the high-density limit, previous full nonlinear calculations of the stopping power of a 2DEG for slow antiprotons, and we go further to calculate the $Z_1^3$ correction to the stopping power of a 2DEG for a wide range of projectile velocities. Our results indicate that linear response calculations are, for all projectile velocities, less reliable in two dimensions than in three dimensions.

cond-mat.mtrl-sci↗