Tc map and superconductivity of simple metals at high pressure

We calculate T_c map in region of weak electron–phonon coupling based on simple phonon spectrum. By using linear-response method and density functional theory, we calculate phonon spectra and Eliashberg functions of simple metals under pressure. Based on the evolutions of superconducting parameters of simple metals on the T_c map with increasing pressure, we find that there are two different responses to pressure for simple metals: (1) enhancing electron–phonon interaction λ such as for La and Li, (2) increasing phonon frequency such as for Pb, Pt. The λ threshold effect is found, which origins from the competition between electron–phonon interaction and electron–electron Coulomb interaction and is the reason why T_c of most superconductors of simple metals are higher than 0.1 K.     

A new mechanism of high Tc superconductivity

On the assumption that doping creates holes on the O rather than the Cu sites, a new model to describe the CuO₂ planes in the high T_c superconductor is proposed. It is shown that the RVB state of the holes on Cu sites at half-filling will lead to an effective attraction of the holes on the O sites which is responsible for superconductivity. Specific heat and susceptibility are discussed briefly and are consistent with experimental results.     

Phase transition in anyon superconductivity at finite temperature

The magnetic response of the charged anyon fluid at temperatures higher than the fermion energy gap (T ⪢ ω_c) is investigated in the self-consistent field approximation. In this temperature region a new phase, characterized by an inhomogeneous magnetic penetration, is found. The inhomogeneity is linked to the existence of an imaginary magnetic mass which increases with the temperature. It is explicitly proved that the electromagnetic field magnetic masses and rest-energies are different in the (T ⪢ ω_c)-phase.     

Bulk superconductivity in the Dirac semimetal TlSb

A feasible strategy for realizing the Majorana fermions is searching for a simple compound with both bulk superconductivity and Dirac surface states.In this paper,we perform calculations of electronic band structure,the Fermi surface,and the surface states,and measure the resistivity,magnetization,and specific heat of a TlSb compound with a CsCl-type structure.The band structure calculations show that TlSb is a Dirac semimetal when spin-orbit coupling is considered.TlSb is first determined to be a type-Ⅱsuperconductor with T_c=4.38 K,Hc1(0)=148 Oe,H_c2(0)=1.12 T,andκ_GL=10.6.We also confirm that TlSb is a moderately coupled s-wave superconductor.Although we cannot determine the band near the Fermi level EF that is responsible for superconductivity,its coexistence with topological surface states implies that the TlSb compound may be a simple material platform to realize the fault-tolerant quantum computations.     

Possible electron-exciton (E-E) interaction and high Tc superconductivity in bulk semimetals

In relation to the high T_c recently observed in YBaCuO systems, the possibility of formation of Cooper pairs by virtual exciton exchange in a single phase is analysed. Electron-Exciton coupling matrix elements and possible optimum condition of superconducting T_c and its upper limit are evaluated. It shows that this kind of superconductivity may display some different features compared with usual BCS superconducting phase.     

What is the highest Tc for phonon-mediated superconductivity?

We suggest that the high-temperature superconductivity can be attributed to the director-roles of the van Hove singularity between an electron-electron interaction and an electron-phonon interaction. The difference between the critical temperature and the pairing temperature is presented, and the Fermi arc, the d-wave symmetry and the poor conductivity, etc., are discussed. In particular, the non-s-wave symmetry is predicted to have the highest T _c for superconductors.      

二分量高温超导机理

用两种电子态,讨论了金属—绝缘相变、反铁磁性—金属相变,以及相分离、正常态—超导态相变、非费米液体行为、高Tc等 关键词： 两种电子态 格林函数 玻色化 高温超导     

On the antiferromagnetism of CuO at 230K and its relevance for high Tc superconductivity

The magnetic susceptibility of CuO shows an anomaly at 230K, which, together with reported neutron and specific heat data, can be interpreted as an indication of antiferromagnetic order. This finding is discussed in the context of the recently discovered high T_c superconductivity of CuO-derived compounds.     

Dyson-Schwinger approach to color superconductivity at finite temperature and density

We investigate the phases of dense QCD matter at finite temperature with Dyson-Schwinger equations for the quark propagator for N _f = 2 + 1 flavors. For the gluon propagator we take a fit to quenched lattice data and add quark-loop effects perturbatively in a hard-thermal-loop-hard-dense-loop approximation. We consider 2SC and CFL-like pairing with chiral up and down quarks and massive strange quarks and present results for the condensates and the phase diagram. We find a dominant CFL phase at chemical potentials larger than 500-600MeV. At lower values of the chemical potential we find a 2SC phase, which also exists in a small band at higher temperatures for larger chemical potentials. With values of 20–30 MeV, the critical temperatures to the normal phase turn out to be quite small.     

Relaxation of the electron temperature in an inert-gas afterglow plasma at elevated pressures

The relaxation of the electron temperature T _e in helium and neon afterglow at elevated pressures is studied theoretically and experimentally. It is shown that the processes in which fast electrons are produced are accompanied by the heating of thermal electrons. The high-energy part of the electron energy distribution function is studied in the intermediate regime (between the local and nonlocal regimes) of its formation. It is shown that, in this case, the calculated effective energy transferred from the fast electrons to the thermal electrons depends substantially on the wall potential of the discharge tube. Comparison of these calculations with experiments testifies to the reliability of the probe technique for measuring T _e in an afterglow at elevated pressures.     

High temperature superconductivity in sulfur and selenium hydrides at high pressure

Due to its low atomic mass, hydrogen is the most promising element to search forhigh-temperature phononic superconductors. However, metallic phases of hydrogen are onlyexpected at extreme pressures (400 GPa or higher). The measurement of the recordsuperconducting critical temperature of 203 K in a hydrogen-sulfur compound at 160 GPa ofpressure [A.P. Drozdov, M.I. Eremets, I.A. Troyan, arXiv:1412.0460[cond-mat.supr-con] (2014); A.P. Drozdov, M.I. Eremets, I.A. Troyan, V.Ksenofontov, S.I. Shylin, Nature 525, 73 (2015)], shows that metallization ofhydrogen can be reached at significantly lower pressure by inserting it in the matrix ofother elements. In this work we investigate the phase diagram and the superconductingproperties of the H-S systems by means of minima hopping method for structure predictionand density functional theory for superconductors. We also show that Se-H has a similarphase diagram as its sulfur counterpart as well as high superconducting criticaltemperature. We predict H₃Se to exceed 120 K superconductivity at 100 GPa. We showthat both H₃Se andH₃S, due to thecritical temperature and peculiar electronic structure, present rather unusualsuperconducting properties.     

高温超导机制研究近期重要进展

文章概述了近几年高温超导机理研究相关的一些重大进展.主要是：RVB\\|Plain Vanilla超导理论的提出及其争论;相图的补充,特别是第二个量子相变点存在的证据;还有超导态性质的新发现,特别是涉及超导态的磁性的一些发现;作者预言,高温超导机理研究定将进入一个理论研究的百花争艳新时期.     

High temperature superconductivity under pressure

Abstract

This review is a summary of the work concerned with the pressure studies of high temperature superconductors (HTSC) held in the LOW Temperature Department of the IHPP since the last Conference in Paderborn (July, 1989).