arXiv subjects

Vladimir Z. Kresin

Publications and source records attributed to Vladimir Z. Kresin.

10 recordsLinked to original sources

arXiv2021

Design Principles for High Temperature Superconductors with Hydrogen-based Alloy Backbone at Moderate Pressure

Hydrogen-based superconductors provide a route to the long-sought goal of room-temperature superconductivity, but the high pressures required to metallize these materials limit their immediate application. For example, carbonaceous sulfur hydride, the first room-temperature superconductor, can reach a critical temperature (Tc) of 288 K only at the extreme pressure of 267 GPa. The next recognized challenge is the realization of room-temperature superconductivity at significantly lower pressures. Here, we propose a strategy for the rational design of high-temperature superconductors at low pressures by alloying small-radius elements and hydrogen to form ternary hydride superconductors with alloy backbones. We identify a hitherto unknown fluorite-type backbone in compositions of the form AXH8, which exhibit high temperature superconductivity at moderate pressures. The Fm-3m phase of LaBeH8, with a fluorite-type H-Be alloy backbone, is predicted to be metastable and superconducting with a Tc ~ 191 K at 50 GPa; a substantially lower pressure than that required by the geometrically similar clathrate hydride LaH10 (170 GPa). Our approach paves the way for finding high-Tc ternary hydride superconductors at conditions close to ambient pressures.

cond-mat.supr-con↗

arXiv2020

High Tc superconductivity in heavy Rare Earth Hydrides: correlation between the presence of the f states on the Fermi surface, nesting and the value of Tc

Lanthanum hydrides, containing hydrogen framework structures under compression, display a superconducting state with a high observed critical temperature. However, this phenomenon has so far only been observed at very high pressures. Here, we computationally search for superconductors with very high critical temperatures, but at much lower pressures. We uncover two such sodalite-type clathrate hydrides, YbH6 and LuH6 (Tc =145 K at P=70 GPa for YbH6 and, especially, Tc =273 K at P=100 GPa for LuH6). These striking properties are a consequence of the strong interrelationship between the f states present at the Fermi surface, structural stability and Tc value. For example, TmH6, with unfilled 4f orbitals, is stable at 50 GPa, while it has a relatively low value of Tc of 25 K. As for the YbH6 and LuH6 compounds, they have filled f-shells and the decrease of the f-energy below the Fermi level leads to formation of the nesting regions on the Fermi surface and, as a result, to phonon "softening" and an increase in Tc.

cond-mat.supr-con↗

arXiv2020

Hydrogen "penta-graphene-like" structure stabilized by hafnium: a high-temperature conventional superconductor

The recent discovery of H3S and LaH10 superconductors with record high superconducting transition temperatures, Tc, at high pressure, has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we predict the existence of an unprecedented hexagonal HfH10, with an extraordinarily high Tc of around 213-234 K at 250 GPa. In HfH10, the H atoms are arranged in clusters to form a planar "penta-graphene-like" sublattice, in contrast to the covalent sixfold cubic structure in H3S and clathrate-like structure in LaH10. The Hf atom acts as a "precompressor" and electron donor to the hydrogen sublattice. This "penta-graphene-like" H10 structure is also found in ZrH10, ScH10 and LuH10 at high pressure, each material showing a high Tc ranging from 134 to 220 kelvin. Our study of dense superhydrides with "penta-graphene-like" layered structures opens the door to the exploration and exploitation of a new class of high Tc superconductors.

cond-mat.supr-con↗

arXiv2018

High Tc Hydrides: Interplay of Optical and Acoustic Modes and Comments Regarding the Upper Limit of Tc

Phonon spectrum of many superconducting compounds and, especially, high Tc hydrides, is broad and rather complicated ,because of presence of high frequency optical modes. In order to analyze an interplay of optical and acoustic phonon branches,it is convenient to introduce two coupling constants, λopt. and λac., along with to characteristic frequencies. The correlation between the value of Tc with this interplay is demonstrated for the family of tantal hydrides (TaH2/ TaH4/ TaH6). The problem of upper limit of Tc is discussed. The phenomenon of room temperature superconductivity can be provided by the electron-phonon interaction and described by the strong coupling theory.

cond-mat.supr-con↗

arXiv2017

Phase diagram of the sulfur hydride: transition into high Tc state

Remarkable feature of the phase diagram of sulfur hydrides is a sharp increase in critical temperature from 120K up to200K. This increase is a signature of the structural transition. The study described below is concerned with the nature of this phase transition. One can demonstrate that the symmetry analysis along with an analysis of the impact of lattice deformations lead to the conclusion that we are dealing with the first order transition. It leads to an abrupt appearance of small pockets on the Fermi surface and to the two-gap energy spectrum.

cond-mat.supr-con↗

arXiv2011

Ordering of dopants and potential increase in Tc to near room temperature

This paper describes a novel method to increase the resistive Tc of cuprate superconductors to values that might approach room temperature if the method is applied to the so-called underdoped regime. This involves ordering the dopants that provide the holes as well as provide pair breaking. The strategy is to separate the doped regions with lower Tc from metallic but undoped regions with the higher Tc.

cond-mat.supr-con↗

arXiv2011

Inhomogeneous Superconducting State and intrinsic Tc : Near Room Temperature Superconductivity in the Cuprates

Doped cuprates are inhomogeneous superconductors. The concept of an intrinsic critical temperature, Tcintr.$\equiv$ Tc*, whose value greatly exceeds that for the resistive Tcres.$\equiv$ Tc, is supported by a number of experimental studies, including those performed recently. These data are discussed in this review. The anomalous diamagnetism observed at Tcres.$\equiv$ <T< Tc* is a manifestation of the presence of superconducting clusters embedded into a normal metallic matrix. The value of intrinsic critical temperature in some cuprates reaches a value which is close to room temperature. The a.c. properties of such inhomogeneous systems are discussed.

cond-mat.supr-con↗

arXiv2008

Evidence for High Tc Superconducting Transitions in Isolated Al45- and Al47- Nanoclusters

Heat capacities measured for Al45- and Al47- nanoclusters have reproducible jumps at ~ 200 K. These jumps are consistent with theoretical predictions that some clusters with highly degenerate electronic states near the Fermi level will undergo a transition into a high Tc superconducting state. An analysis based on a theoretical treatment of pairing in Al45- and Al47- agrees well with the experimental data in both the value of the critical temperature and in the size and width of the jumps in the heat capacity.

cond-mat.supr-con↗

arXiv2006

Shell Structure and Strengthening of Superconducting Pair Correlation in Nanoclusters

The existence of shell structure and the accompanying high degeneracy of electronic levels leads to the possibility of strong superconducting pairing in metallic nanoclusters with N~100-1000 delocalized electrons. The most favorable cases correspond to (a) "magic" clusters with strongly degenerate highest occupied and lowest unoccupied shells and a relatively small energy spacing between them as well as to (b) clusters with slightly incomplete shells and small Jahn-Teller splitting. It is shown that realistic sets of parameters lead to very high values of Tc as well as to a strong alteration of the energy spectrum. The impact of fluctuations is analyzed. Spectroscopic experiments aimed at detecting the presence of pair correlations are proposed. The pairing should also manifest itself via odd-even effects in cluster spectra, similar to the case of nuclei.

cond-mat.supr-con↗

arXiv1998

Manganites at Low Temperatures and Light Doping:Band Approach and Percolation

A tight-band model is employed for the $e_{2g}$-orbitals in manganites. It is shown that a large intraatomic Hund's coupling, $J_H$ and the resulting double-exchange mechanism lead to antiferromagnetic ordering along one of the cubic axis stabilized by the cooperative JT effect which further decreases the band energy of electrons. As a result, LaMnO$_3$ is a band insulator built of 2D ferromagnetic layers. The critical concentration $x_c\simeq 0.16$, for onset of ferromagnetic and metallic behavior at low temperatures in La$_{1-x}$Sr$_x$MnO$_3$ and the phase transition are treated in the percolation approach.

cond-mat.str-el↗