Search Results (4)

Superconductivity in highly pressurized hydrides has become the primary direction for the exploration of the fundamental upper limit of the superconducting transition temperature, $T_c$, after Drozdov et al. (Nature 2015, 525, 73) discovered a superconducting state with $T_c=203K$ in highly compressed sulfur hydride. To date, several dozen high-temperature superconducting polyhydrides have been discovered and, in addition, it was recently reported that highly compressed titanium and scandium exhibit record-high $T_c$ (up to 36 K). This exceeded the $T_c=9.2K$ value of niobium many times over, which was the record-high $T_c$ ambient pressure metallic superconductor. Here, we analyzed the experimental data for the recently discovered high-pressure superconductors (which exhibit high transition temperatures within their classes): elemental titanium (Zhang et al., Nature Communications 2022; Liu et al., Phys. Rev. B 2022), ${TaH}_3$ (He et al., Chinese Phys. Lett. 2023), ${LaBeH}_8$ (Song et al., Phys. Rev. Lett. 2023), black phosphorous (Li et al., Proc. Natl. Acad. Sci. 2018; Jin et al., arXiv 2023), and violet (Wu et al., arXiv 2023) phosphorous to reveal the nonadiabaticity strength constant $\frac{T_{\theta }}{T_F}$ (where $T_{\theta }$ is the Debye temperature, and $T_F$ the Fermi temperature) in these superconductors. The analysis showed that the $\delta$-phase of titanium and black phosphorous exhibits $\frac{T_{\theta }}{T_F}$ scores that are nearly identical to those associated with A15 superconductors, while the studied hydrides and violet phosphorous exhibit constants in the same ballpark as those of $H_{3}S$ and ${LaH}_{10}$. Full article

The classical Bardeen–Cooper–Schrieffer and Eliashberg theories of the electron–phonon-mediated superconductivity are based on the Migdal theorem, which is an assumption that the energy of charge carriers, $k_B{T}_F$, significantly exceeds the phononic energy, $\hslash {\omega }_D$, of the crystalline lattice. This assumption, which is also known as adiabatic approximation, implies that the superconductor exhibits fast charge carriers and slow phonons. This picture is valid for pure metals and metallic alloys because these superconductors exhibit $\frac{\hslash {\omega }_D}{k_B{T}_F}<0.01$. However, for n-type-doped semiconducting SrTiO₃, this adiabatic approximation is not valid, because this material exhibits $\frac{\hslash {\omega }_D}{k_B{T}_F}\cong 50$. There is a growing number of newly discovered superconductors which are also beyond the adiabatic approximation. Here, leaving aside pure theoretical aspects of nonadiabatic superconductors, we classified major classes of superconductors (including, elements, A-15 and Heusler alloys, Laves phases, intermetallics, noncentrosymmetric compounds, cuprates, pnictides, highly-compressed hydrides, and two-dimensional superconductors) by the strength of nonadiabaticity (which we defined by the ratio of the Debye temperature to the Fermi temperature, $\frac{T_{\theta }}{T_F}$). We found that the majority of analyzed superconductors fall into the $0.025\le \frac{T_{\theta }}{T_F}\le 0.4$ band. Based on the analysis, we proposed the classification scheme for the strength of nonadiabatic effects in superconductors and discussed how this classification is linked with other known empirical taxonomies in superconductivity. Full article

Hydrogen-rich superhydrides are promising high-$T_c$ superconductors, with superconductivity experimentally observed near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g., LaH${}_{10}$ at 170 GPa and CeH${}_9$ at 150 GPa. Superconductivity is believed to be closely related to the high vibrational modes of the bound hydrogen ions. Here, we studied the limit of extreme pressures (above 200 GPa) where lanthanide hydrides with large hydrogen content have been reported. We focused on LaH${}_{16}$ and CeH${}_{16}$, two prototype candidates for achieving a large electronic contribution from hydrogen in the electron–phonon coupling. In this work, we propose a first-principles calculation platform with the inclusion of many-body corrections to evaluate the detailed physical properties of the Ce–H and La–H systems and to understand the structure, stability, and superconductivity of these systems at ultra-high pressure. We provide a practical approach to further investigate conventional superconductivity in hydrogen-rich superhydrides. We report that density functional theory provides accurate structure and phonon frequencies, but many-body corrections lead to an increase of the critical temperature, which is associated with the spectral weight transfer of the f-states. Full article

Since the pivotal experimental discovery of near-room-temperature superconductivity (NRTS) in highly compressed sulphur hydride by Drozdov et al. (Nature 2015, 525, 73–76), more than a dozen binary and ternary hydrogen-rich phases exhibiting superconducting transitions above 100 K have been discovered to date. There is a widely accepted theoretical point of view that the primary mechanism governing the emergence of superconductivity in hydrogen-rich phases is the electron–phonon pairing. However, the recent analysis of experimental temperature-dependent resistance, R(T), in H₃S, LaH_x, PrH₉ and BaH₁₂ (Talantsev, Supercond. Sci. Technol. 2021, 34, accepted) showed that these compounds exhibit the dominance of non-electron–phonon charge carrier interactions and, thus, it is unlikely that the electron–phonon pairing is the primary mechanism for the emergence of superconductivity in these materials. Here, we use the same approach to reveal the charge carrier interaction in highly compressed lithium, black phosphorous, sulfur, and silane. We found that all these superconductors exhibit the dominance of non-electron–phonon charge carrier interaction. This explains the failure to demonstrate the high-T_c values that are predicted for these materials by first-principles calculations which utilize the electron–phonon pairing as the mechanism for the emergence of their superconductivity. Our result implies that alternative pairing mechanisms (primarily the electron–electron retraction) should be tested within the first-principles calculations approach as possible mechanisms for the emergence of superconductivity in highly compressed lithium, black phosphorous, sulfur, and silane. Full article