Band 3, Heft 1

Éliashberg theory is generalized to the electronphonon (EP) systems with the not constant density of electronic states. The phonon contribution to the anomalous electron Green’s function (GF) is considered. The generalized Éliashberg equations with the variable density of electronic states are resolved for the hydrogen sulphide SH 3 phase under pressure. The dependence of both the real and the imaginary part of the order parameter on the frequency in the SH 3 phase is obtained. The T c = 177 K value in the hydrogen sulfide SH 3 phase at the pressure P = 225 GPa has been defined. The results of the solution of the Eliashberg equations for the Im-3m (170 GPa), Im-3m (200 GPa) and R3m (120 GPa) phases are presented. A peak value T c = 241 K of the superconducting transition temperature has been predicted.

We propose a methodology that efficiently asseses major characteristics in the energy landscape for a given space of configurations (crystal structures) under pressure. In this work we study SH 3 and SeH 3 , both of fundamental interest due to their superconducting properties. Starting from the crystal fingerprint, which defines configurational distances between crystalline structures, we introduce an optimal one dimensional metric space that is used to both classify and characterize the structures. Furthermore, this is correlated to the electronic structure. Our analysis highlights the uniqueness of the Im − 3m phase of H 3 S and H3 3 Se for superconductivity. This approach is an useful tool for future material design applications.

A priori crystal structure prediction techniques have been used to explore the phase diagrams of hydrides of main group elements under pressure. A number of novel phases with the chemical formulas MHn, n > 1 and M = Li, Na, K, Rb, Cs; MHn, n > 2 and M= Mg, Ca, Sr, Ba; H n I with n > 1 and PH, PH 2 , PH 3 have been predicted to be stable at pressures achievable in diamond anvil cells. The hydrogenic lattices within these phases display a number of structural motifs including H 2 δ− , H − , H − 3 , as well as one-dimensional and three-dimensional extended structures. A wide range of superconducting critical temperatures, T c s, are predicted for these hydrides. The mechanism of metallization and the propensity for superconductivity are dependent upon the structural motifs present in these phases, and in particular on their hydrogenic sublattices. Phases that are thermodynamically unstable, but dynamically stable, are accessible experimentally. The observed trends provide insight on how to design hydrides that are superconducting at high temperatures.

The pressure-induced superconductivity in sulfur hydride has recently received tremendous amount of interest for its record-breaking transition temperature (T c ) of 203 K. We extensively discuss an infinite sequence of metastable structures of sulfur hydride under high pressure which has been recently proposed theoretically [R. Akashi et al., Phys. Rev. Lett. 117, 075503 (2016)]. The newly found structures are understood as the stacked layers of H2S and H3S, showing close similarity to the Magnéli phases formed in transition-metal oxides. The possible emergence of the “H x S Magnéli” phases are discussed as intermediate metastable phases in the formation of the high-T c H 3 S phase. Superconducting transition temperatures and the Xray diffraction patterns are simulated with the first-principles calculations. It is demonstrated that the H x S Magnéli phases could serve as the silver bullet to explain the complicated dependences of the observed T c s and diffraction patterns on the experimental protocol.

Recent computational studies confirmed by experiment have established the occurrence of superconducting temperatures, T c , near 200 K when the pressure is close to 200 GPa in the compound H3S. Motivated by these findings we investigate in this work the possibility of discovering high-temperature superconductivity in the material H 3 F. We performed linearized augmented plane wave(LAPW) calculations followed by the determination of the angular momentum components of the density of states, the scattering phase shifts at the Fermi level and the electron-ion matrix element known as the Hopfield parameter. Our calculated Hopfield parameters are much larger than those found in H 3 S suggesting that they may lead to large electron-phonon coupling constant and hence a large Tc similar or even larger than that of H 3 S. However, calculations of elastic constants are inconclusive regarding the stability of this material.