Session S24: Matter at Extreme Conditions: Superconductivity and Metallic Hydrogen

Metallization of hydrogen

In previous works [1, 2] we showed that hydrogen metallizes in phase III at temperatures below ~200 K and pressures near ~350 GPa. Here, we perform a detailed study of electrical conductivity R(T) in phase III over a pressure range of 200-400 GPa and a temperature range of 80-300 K, and we show that hydrogen transforms from a semiconducting to a metallic state already at ~315 GPa. This transformation is also supported by Raman spectroscopy: the Raman signal intensity decreases with pressure following the appearance and increase of electrical conductivity. Moreover, the Raman and electrical measurements yield the same boundary between the hydrogen phases III and IV.   

Controlling phase stability of superhydrides by combining pressure and electrochemistry

Recently progresses have been made on synthesizing superhydrides with a variety of metals at very high pressures. In this work, we explore the possibility of controlling phase stability of superhydrides by uniquely combining electrochemistry and applied pressure. We predict possible crystal structures of the superhydrides and calculate their energies over a broad range of pressures and electrode potentials, using density functional theory and particle swarm optimization calculations. Based on a thermodynamic analysis, we construct pressure-potential phase diagrams and provide an alternate synthesis concept, pressure-potential (P^2), to access novel phases having high hydrogen content. Palladium-hydrogen is a widely-studied material system with the highest hydride phase being Pd3H4. Most strikingly for this system, at potentials above hydrogen evolution and about 300 MPa pressure, we find the possibility to make palladium superhydrides (e.g., PdH10).  We predict the generalizability of this approach for La-H, Y-H and Mg-H with orders of magnitude reduction in required pressure for stabilizing phases.  In addition, the P^2 strategy allows stabilizing new phases that cannot be done purely by either pressure or potential and is a general approach that is likely to work for synthesizing other hydrides at modest pressures.   

Superconductivity in Carbon-Boron Clathrates

We report experimental and computational results for superconductivity in carbon–boron clathrate structures. For the case of SrB₃C₃ in the bipartite sodalite structure, in situ electrical transport measurements, facilitated by a novel experimental design compatible with extreme synthesis conditions (i.e., >3000 K at 50 GPa), show non-hysteretic resistivity drops that track the calculated magnitude and pressure dependence of T_c calculated using the Allen-Dynes modified McMillan equation with Coulomb pseudopotential values (μ^*) near 0.15. The superconducting nature of the transition (T_c ≈ 20 K) was confirmed via electrical transport measurements collected under applied magnetic fields up to 18 T. Carbon-boron clathrates thus represent a new class of superconductors that are similar to covalent metals like MgB₂ and doped fullerenes. Carbon clathrates share structures similar with superconducting superhydrides, but covalent C–B bonds allow metastable persistence at ambient conditions. Different guest atom substitution schemes in various carbon clathrate structure types may enable conventional superconductivity with T_c approaching 100 K.   

Isotope quantum effects in the metallization transition in liquid hydrogen

Quantum effects in condensed matter are often associated with low temperatures. In our work we show a significant isotope effect in the liquid-liquid transition in high-pressure hydrogen. This transition between an insulating molecular liquid and an atomic metallic liquid occurs at temperatures on the order of thousands of Kelvin, beyond where quantum effects are often assumed to be relevant. We show that this transition nevertheless exhibits a significant isotope effect on the order of hundreds of Kelvin when compared to deuterium, arising from the large zero-point energy difference between hydrogen and deuterium.

Using ab-initio density functional theory combined with path integral molecular dynamics to accurately include nuclear quantum effects, we identify significant isotopic shifts in the dissociation, metallization and thermodynamic boundaries. This highlights shortcomings of the widely used Born-Oppenheimer molecular dynamics method, which implicitly treats the nuclei classically. Our results show that nuclear quantum effects remain significant in hydrogen even at these high temperatures, meaning that experiments performed on deuterated samples are likely to significantly overestimate transition pressures and temperatures.   

The Raman signal from a hindered hydrogen rotor

We present a method for calculation of Raman modes of hydrogen and deuterium in the solid phases. We use the mean-field assumption that the quantized excitations are localized on one molecule. This is done by explicit solution of the time-dependent Schroedinger equation in an angle-dependent potential, and direct calculation of the polarization. Our method generates the full Raman signal through Fourier transform of the time response of the system, using a single parameter for the decorrelation time that we obtain from AIMD. We show that in the free rotor limit, the H₂ and D₂ frequencies differ by a factor of 2, which evolves toward √2 as the modes acquire librational character due to stronger interactions. The ratio overshoots √2 if anharmonic terms weaken the harmonic potential. When the applied potential breaks the degeneracy of the free rotor states, new ‘re-orientational’ Raman active modes emerge from the Rayleigh line. The intensity of these modes is entirely suppressed in back scatter for a single crystal with C-axis parallel to the incident beam. We demonstrate good agreement between theory and experiment for phase I of hydrogen and deuterium.   

A Little Bit of Carbon Can do a Lot for Superconductivity in H3S

First-principles calculations were carried out to provide a chemical basis for proposed structures associated with the recently reported room-temperature superconductivity in a carbonaceous sulfur hydride material under pressure. Calculations were performed on supercells of H₃S doped with 1.85-25% carbon, corresponding to SH₃→CH₃ or SH₃→CH₄ substitutions, primarily at pressures of 270 GPa where the maximum critical temperature, Tc, has been reported. In the first type of substitution, the carbon atoms can be six-fold coordinated, stabilizing a CH₆ configuration within the cubic H3S framework structure that forms under pressure. In the second, the carbon can be four-fold coordinated as methane intercalated into the H-S lattice, with or without an additional hydrogen in the framework. The results indicate that unusual local bonding configurations with respect to carbon can be stabilized under pressure. The doping breaks degenerate bands, lowering the density of states at the Fermi level (NF), and localizing electrons in C-H bonds. Low levels of CH₄ doping do not increase NF to values as high as those calculated for Im3m H₃S, but they do result in a larger logarithmic average phonon frequency, and an electron-phonon coupling parameter comparable to that of R3m H₃S. The T_cs estimated for carbon doping levels ranging from 1.85-5.7% are compatible with experimental measurements for the C-S-H superconductor.   

Chemical templates that assemble the metal superhydrides

The recent discoveries of many metal superhydrides provide a new route to room temperature superconductors. However, their stability and structure trends and the large chemical driving force needed to dissociate H2 molecules and form H covalent network cannot be explained by direct metal-hydrogen bonds and volume effect. Here, we demonstrate that the understanding of superhydrides formation needs a perspective beyond traditional chemical bond theory. Using high-throughput calculations, we show that, after removing H atoms, the remaining metal lattices exhibit large electron localizations at the interstitial regions, which matches excellently to the H lattice like a template. Furthermore, H lattices consist of 3D aromatic building units that are greatly stabilized by chemical templates of metals close to s-d border. The chemical template theory can naturally explain the stability and structure trends of superhydrides and help predicting new materials such as two-metal superhydrides.   

High-pressure studies of lanthanum-yttrium ternary superhydrides

The discovery of near room temperature superconductivity in LaH₁₀ and YH₉ confirmed the theoretical predictions that these hydrogen-rich metal hydrides are superconductors with critical temperature T_c in the vicinity of room temperature. Recent experimental and theoretical results for ternary hydrides have opened further investigations of room-temperature superconductivity, including optimizing doping in La-based hydrides. In this study, the hydrogen-rich lanthanum-yttrium ternary hydrides (La_1-nY_nH_x) were synthesized using ammonia borane (NH₃BH₃) as the hydrogen source. The results include documenting ~10% yttrium doped lanthanum superhydrides at 165 GPa by pulsed laser heating. Powder x-ray diffraction of the laser-heated samples has been used to confirm the formation of La-Y superhydrides.   

High-temperature superconductivity in boron-carbon clathrates XYB6C6

Inspired by the recent synthesized boron-carbon clathrate SrB­₃C₃ at 50 GPa with an estimated superconducting critical temperature (T_c) of 42 K, a number of the XYB₆C₆ systems are studied by substitution of Sr with other elements using first-principles density functional calculations. The dynamical stabilities, electronic structures and superconducting properties of these systems are analyzed at ambient conditions. Several XYB₆C₆ phases with a calculated T_c above 70 K are found. The phase stability is analyzed using the chemical pressure technique.   

Effects of Pressure on Superconductivity in Chemically Substituted MoB2

Recently, superconductivity was discovered in MoB₂ with a T_c of 3K at a pressure of 25 GPa which goes to 32 K upon increasing pressure to 100 GPa. Here, we report the results of studies to examine the superconducting behavior of chemically substituted variants of MoB₂, both at ambient and high pressure.  In particular, we have identified a variant in which superconductivity in the MgB₂-like structure occurs at ambient pressure with a T_c of ~8 K.  We will present the results of measurements of the superconducting critical temperature, critical field, and crystal structure under high pressure.    

Withdrawn

Dense high symmetry carbon framework structures containing alkali or alkaline earth metals inside the framework are expected to exhibit interesting electronic properties such as superconductivity analogous to those found in superconducting hydrides under pressure. For example, sodalite-like NaC₆ is predicted to stabilize at high-pressure and have superconductivity above 100 K. [1] However, detailed studies of the phases in carbon-based systems are both unexplored experimentally under pressure. Here, we present high-pressure studies on M-C compounds (e.g., M = Li, Na, Mg) using a variety of techniques including x-ray diffraction and Raman spectroscopy. Stability, structure and bonding, and electronic properties are also studied using first-principles density functional theory calculations.

References:

[1] S. Lu et al., Phys. Rev. B 2016, 93 104509.

[2] Z. Liu et al., Physical Review Letters 2021, 127, 157002.

[3] K.  Sano, Y. Masuda, and H. Ito, arXiv (2020).   

Observation of pressure-induced superconductivity in WB2

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB₂ during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, T_c of 17 K at 90 GPa [1]. Upon further compression up to 190 GPa, the T_c gradually decreases to 14 K at a rate of -0.024 K/GPa. We investigate the presence of MgB₂-like structural transition (hP3, space group 191, prototype AlB2) in WB₂ under pressures, which might be responsible for the relatively high T_c superconductivity as recently found in MoB₂ [2]. Synchrotron x-ray diffraction measurements up to 145 GPa at room temperature show that the ambient pressure hP12 structure (space group 194, prototype WB₂) continues to persist to this pressure without any structural transition. Thus, these experimental results indicate the novel origin of the abrupt appearance of superconductivity in WB₂ under pressures above 50 GPa. In the following talk, we show that electron-phonon mediated superconductivity in WB₂ originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling the MgB₂ structure.   

An explanation for pressure-induced superconductivity in WB2

Our recent experimental investigations into the hP12 phase of WB2 revealed that WB₂ superconducts above 50GPa with a maximum superconducting critical temperature Tc of 17 K at 90 GPa. High-pressure resistivity and XRD measurement revealed no structural phase transition. Calculated electron-phonon T_c for the hP12 phase fails to explain the observed superconductivity. In this talk, we present theoretical evidence that the formation of planar defects at high pressure explains the observed superconductivity. Enthalpy calculations indicate that stacking faults and twin boundaries can form during plastic deformation of the hP12 WB₂ phase at high pressure. Furthermore, the atomic arrangement at these planar defects resembles the arrangement of atoms in the MgB₂ P6/mmm structure. Our electron-phonon coupling calculations show that P6/mmm WB₂ has the highest T_c among the competing phases and this phase is thermodynamically stable above 130GPa. The formation of planar defects is also consistent with the observed change in c/a ratio above about 50 GPa. Thus, the formation and percolation of mechanically induced stacking faults and twin boundaries lead to filamentary superconductivity in WB₂ at high pressure. Our results may lead to an alternate route for designing superconducting materials.   

Accurate non-perturbation electron-vibration interactions of superconductivity under high pressures

An electron-vibration interaction has essential effects on many temperature-dependent phenomena, including superconductivity. Accurate non-perturbation calculations recently developed can provide unprecedented insights into what thermal vibrational motion does to band structures of superconductors under high pressures. It clearly shows that there are strictly necessary and sufficient conditions for electron-vibration interactions to take place. For a vibrational motion to generate electron-vibration interaction, a longitudinal mode to stretch chemical bonds is essential. Degenerate energy levels at electronic wave-vectors are also required. Only such vibration motion coupling to degenerate electronic states in energy band structures can generate electron-vibration interactions for superconductivity. These conditions reveal some unexpected physical and chemical phenomena operated by electron-vibration interactions under high pressures. Accurate non-perturbation calculations for superconductors include Nb, YBa₂Cu₃O₇, LaH₁₀, and K₃C₆₀.    

New High Temperature Superconductors in Extremely Compressed Matters

Pressure can dramatically change materials states such as to induce high temperature superconductivity. This originates from the unusually enhanced interactions or DOS or related pairing force that can enforce superconductivity. Here we introduce our recent progress on the topic of pressure generated superconductivity in the extremely compressed materials.