Colloquium : High pressure and road to room temperature superconductivity

Abstract

High pressure serves as a path finding tool towards novel structures, including those with very high Tc. The superconductivity in sulfur hydrides with record value of Tc =203 K (!) is caused by the phonon mechanism. However, the picture differs from the conventional one in important ways. The phonon spectrum in sulfur hydride is both broad and has a complex structure. High value of T c is mainly due to strong coupling to the high-frequency optical modes, although the acoustic phonons also make a noticeable contribution. New approach is described; it generalizes the standard treatment of the phonon mechanism and makes it possible to obtain an analytical expression for Tc . It turns out that, unlike in the conventional case, the value of the isotope coefficient varies with the pressure and reflects the impact of the optical modes. The phase diagram, that is the pressure dependence of Tc, is rather peculiar. A crucial feature is that increasing pressure results in a series of structural transitions, including the one, which yields the superconducting phase with the record Tc . In a narrow region near P≈150 GPa the critical temperature rises sharply from Tc ≈120K to Tc ≈200K. The sharp structural transition, which produces the high Tc phase, is a first-order phase transition caused by interaction between the order parameter and lattice deformations. A remarkable feature of the electronic spectrum in the high Tc phase is the appearance of small pockets at the Fermi level. Their presence leads to a two-gap spectrum, which 2 can, in principle, be observed, with the future use of tunneling spectroscopy. This feature leads to non-monotonic and strongly asymmetric pressure dependence of Tc. Other hydrides can be expected to display even higher values of Tc, up to room temperature. The fundamental challenge lays in creation a structure capable of displaying high Tc at ambient pressure.

Ab initio phonon self-energies and fluctuation diagnostics of phonon anomalies: Lattice instabilities from Dirac pseudospin physics in transition metal dichalcogenides

“…Different states of electronic quantum matter are often tightly linked to lattice degrees of freedom. Examples include superconductivity, periodic lattice distortions and charge-density waves (CDWs), metal-insulator transitions, and nematic, magnetic, "stripe", or excitonic order across vastly different material classes ranging from cuprate [1][2][3][4][5] and Fe-based high-temperature superconductors [6][7][8] to hydride compounds [9][10][11]. Disentangling the interplay of lattice and electronic degrees of freedom has remained a formidable challenge in many cases.…”

Unconventional superconductivity in highly-compressed unannealed sulphur hydride

“…From deduced (0) values ( Fig. 1 and Table 1), we calculated the Fermi temperature, TF, For H3S phase  = 3.53-4.7, where the lower bound was reported by Kaplan and Imry [34] and in our previous works [35][36][37], and the upper bond was reported by other authors [3,32]. Due to for all three models deduced Tc = 45.9 ± 0.1 K in Table I we show only the Tc/TF ratios.…”

“…It should be noted, that the discovery of the effect of pressure-induced superconductivity in non-superconducting materials is attributed to Jörg Wittig [8] who converted elemental cerium into superconductor at pressure of P = 5 GPa. During this, more than fifty years, long research journey, pressureinduced superconductivity was found in dozens of non-superconducting elements and compounds (details can be found in recent reviews [3,[9][10][11]).…”

“…Deduced and calculated parameters for unannealed H3S compressed at P = 155 GPa. Deduced critical temperature for all models (Eqs [2][3][4]. is Tc = 45.9 ± 0.1 K. Assumed charge effective mass is * = 2.76 •[32].…”

Nanoarchitecture: Toward Quantum‐Size Tuning of Superconductivity

“…This opens prospects of architecting superconducting nanostructures in order to improve and even tune their characteristics. The most important of them is the critical temperature T c .Main factors that can raise/lower the critical temperature of conventional superconducting materials are well-known since the early years of the BCS theory: modifications of the phonon spectrum [2,3] and disorder effects. [4] Therefore, when first results on the dependence of the critical temperature of nanodimensional superconductors on their geometry appeared, these two factors were immediately employed for explanations; see, for example, refs.…”