Strong-coupling character of superconducting phase in compressed selenium hydride

At present, metal hydrides are considered highly promising materials for phonon-mediated superconductors that exhibit high values of the critical temperature. In the present study, the superconducting properties of the compressed selenium hydride in its simplest form (HSe) are analyzed, toward quantitative characterization of this phase. By using the state-of-art Migdal-Eliashberg formalism, it is shown that the critical temperature in this material is relatively high ([Formula: see text][Formula: see text]K) and surpasses the level of magnesium diboride superconductor, assuming that the Coulomb pseudopotential takes value of [Formula: see text]. Moreover, the employed theoretical model allows us to characterize other pivotal thermodynamic properties such as the superconducting band gap, the free energy, the specific heat, and the critical magnetic field. In what follows, it is shown that the characteristic thermodynamic ratios for the aforementioned parameters differ from the predictions of the Bardeen-Cooper-Schrieffer theory. As a result, we argue that strong-coupling and retardation effects play important role in the discussed superconducting state, which cannot be described within the weak-coupling regime.

Volume dependence of superconducting state parameters of bismuth-based binary alloys: A pseudopotential approach

A comprehensive and systematic study of superconducting state parameters (SSPs) of Bi-based binary alloys of the form [Formula: see text] has been carried out in the framework of pseudopotential theory at different concentrations ([Formula: see text]) of impurity atom-[Formula: see text] in host atom Bi which varies from 0 to 1. Impurity atom [Formula: see text] has been replaced by In, Tl, Sb and Pb to form four alloy systems. The density-based local form of the pseudopotential proposed by Fiolhais et al. has been used in this study to compute SSPs such as electron–phonon coupling strength ([Formula: see text]), Coulomb pseudopotential ([Formula: see text]), transition temperature ([Formula: see text]), effective interaction strength ([Formula: see text] and isotopic effect parameter ([Formula: see text]) of Bi-based binary alloy systems. Computed results of SSPs of alloys under study are in good agreement with available experimental and other theoretical results. In order to investigate the effect of pressure on SSPs of alloys under study, a theoretical calculation of SSPs as a function of compressed volume is reported. The volume dependence of Debye temperature has been accounted by using Debye–Gruneisen model which involves the Gruneisen parameter. Thus, to describe the effect of pressure on SSPs accurately, the value of Gruneisen parameter should be as accurate as possible. But reported results of Gruneisen parameter in the literature are highly scattered. Due to this reason, we have used two sets of Gruneisen parameter obtained by different methods to understand its role in the computation of SSPs as a function of compressed volume. Further, critical volumes for all alloys at different concentration have been predicted at which electron–phonon coupling strength and Coulomb pseudopotential are equal ([Formula: see text]). At critical volume, superconductivity quenches where transition temperature, [Formula: see text], and effective interaction strength, [Formula: see text], become zero. It is observed that critical volumes predicted by all approaches used in this study are in excellent agreement with each other.

Characteristics of the s–Wave Symmetry Superconducting State in the BaGe3 Compound

Thermodynamic properties of the s–wave symmetry superconducting phase in three selected structures of the BaGe 3 compound ( P 6 3 / m m c , A m m 2 , and I 4 / m m m ) were discussed in the context of DFT results obtained for the Eliashberg function. This compound may enable the implementation of systems for quantum information processing. Calculations were carried out within the Eliashberg formalism due to the fact that the electron–phonon coupling constant falls within the range λ ∈ 0 . 73 , 0 . 86 . The value of the Coulomb pseudopotential was assumed to be 0 . 122 , in accordance with the experimental results. The value of the Coulomb pseudopotential was assumed to be 0 . 122 , in accordance with the experimental results. The existence of the superconducting state of three different critical temperature values, namely, 4 . 0 K, 4 . 5 K and 5 . 5 K, depending on the considered structure, was stated. We determined the differences in free energy ( Δ F ) and specific heat ( Δ C ) between the normal and the superconducting states, as well as the thermodynamic critical field ( H c ) as a function of temperature. A drop in the H c value to zero at the temperature of 4.0 K was observed for the P 6 3 / m m c structure, which is in good accordance with the experimental data. Further, the values of the dimensionless thermodynamic parameters of the superconducting state were estimated as: R Δ = 2 Δ ( 0 ) / k B T c ∈ { 3 . 68 , 3 . 8 , 3 . 8 } , R C = Δ C ( T c ) / C N ( T c ) ∈ { 1 . 55 , 1 . 71 , 1 . 75 } , and R H = T c C N ( T c ) / H c 2 ( 0 ) ∈ { 0 . 168 , 0 . 16 , 0 . 158 } , which are slightly different from the predictions of the Bardeen–Cooper–Schrieffer theory ( [ R Δ ] B C S = 3 . 53 , [ R C ] B C S = 1 . 43 , and [ R H ] B C S = 0 . 168 ). This is caused by the occurrence of small retardation and strong coupling effects.

Superconducting properties of under- and over-doped BaxK1−xBiO3 perovskite oxide

In this study, we investigate the thermodynamic properties of the Ba[Formula: see text]K[Formula: see text]BiO3 (BKBO) superconductor in the under- (x = 0.5) and over-doped (x = 0.7) regime, within the framework of the Migdal–Eliashberg formalism. The analysis is conducted to verify that the electron–phonon pairing mechanism is responsible for the induction of the superconducting phase in the mentioned compound. In particular, we show that BKBO is characterized by the relatively high critical value of the Coulomb pseudopotential, which changes with doping level and does not follow the Morel–Anderson model. In what follows, the corresponding superconducting band gap size and related dimensionless ratio are estimated to increase with the doping, in agreement with the experimental predictions. Moreover, the effective mass of electrons is found to take on high values in the entire doping and temperature region. Finally, the characteristic dimensionless ratios for the superconducting band gap, the critical magnetic field and the specific heat for the superconducting state are predicted to exceed the limits set within the Bardeen–Cooper–Schrieffer theory, suggesting pivotal role of the strong-coupling and retardation effects in the analyzed compound. Presented results supplement our previous investigations and account for the strong-coupling phonon-mediated character of the superconducting phase in BKBO at any doping level.

A First-Principles Study of Electron-Phonon Coupling of OsB2

We investigated the lattice dynamics and electron-phonon coupling (EPC) of superhard material OsB2by first-principles linear response calculations. The calculated EPC parameters for the optical phonon modes at Г indicate that the heavy Os atoms play the most important role in deciding the superconducting behavior, and there are sizeable contributions from lighter B atoms to EPC. Our calculated EPC constant is 0.42, and the estimated superconducting transition temperatureTcis 2.1 K using the Coulomb pseudopotentialμ*=0.125, in excellent agreement with the experimental ones.

Estimation of the superconducting parameters for silane at high pressure

The properties of the superconducting state in the Cmca phase of silane ( SiH4 ) at the pressure of 250 GPa have been considered. In particular, the critical temperature (TC), the free energy difference between the superconducting and normal state (ΔF ≡ FS - FN), the thermodynamic critical field (HC) and the specific heat jump (ΔC ≡ CS - CN) have been determined. It has been shown that the dimensionless ratios: [Formula: see text] and RC ≡ ΔC(TC)/CN(TC) assume the values: RH(μ⋆) ∈ 〈0.147, 0.154〉 and RC(μ⋆) ∈ 〈2.20, 1.79〉, where the symbol μ⋆ denotes the Coulomb pseudopotential and μ⋆∈ 〈0.1, 0.3〉.

INVESTIGATION OF THE SUPERCONDUCTING PHASE IN METALLIC HYDROGEN NEAR THE PRESSURE OF METALLIZATION

In this paper, the thermodynamic properties of the superconducting state in the metallic hydrogen under the pressure at 347 GPa have been determined. In the framework of the Eliashberg formalism, it has been shown that the characteristic thermodynamic parameters deviate from the predictions of the BCS theory. In particular, the ratio ΔC(TC)/CN(TC) decreases from 1.96 to 1.72 depending on the assumed value of the Coulomb pseudopotential (μ* ∈ 〈0.08, 0.15〉). On the other hand, the ratio [Formula: see text] increases from 0.150 to 0.158.