Superconductivity at 253 K in lanthanum-yttrium ternary hydrides (2012.04787v1)

Abstract: Polyhydrides offer intriguing perspectives as high-temperature superconductors. Here we report the high-pressure synthesis of a series of lanthanum-yttrium ternary hydrides: cubic hexahydride $(La,Y)H_{6}$ with a critical temperature $T_{C}$ = 237 +/- 5 K and decahydrides $(La,Y)H_{10}$ with a maximum $T_{C}$ ~${253 K}$ and an extrapolated upper critical magnetic field $B_{C2(0)}$ up to ${135 T}$ at 183 GPa. This is one of the first examples of ternary high-$T_{C}$ superconducting hydrides. Our experiments show that a part of the atoms in the structures of recently discovered ${Im3m}$-$YH_{6}$ and ${Fm3m}$-$LaH_{10}$ can be replaced with lanthanum (~70 %) and yttrium (~25 %), respectively, with a formation of unique ternary superhydrides containing incorporated $La@H_{24}$ and $Y@H_{32}$ which are specific for ${Im3m}$-$LaH_{6}$ and ${Fm3m}$-$YH_{10}$. Ternary La-Y hydrides were obtained at pressures of 170-196 GPa via the laser heating of $P6_{3}$${/mmc}$ lanthanum-yttrium alloys in the ammonia borane medium at temperatures above 2000 K. A novel tetragonal $(La,Y)H_{4}$ was discovered as an impurity phase in synthesized cubic $(La,Y)H_{6}$. The current-voltage measurements show that the critical current density $J_{C}$ in $(La,Y)H_{10}$ may exceed $2500 A/mm^{2}$ at 4.2 K, which is comparable with that for commercial superconducting wires such as ${NbTi}$, $Nb_{3}$${Sn}$. Hydrides that are unstable in a pure form may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the same structure.

Superconductivity at High Pressure in Lanthanum-Yttrium Hydrides

The paper of high-temperature superconductivity in materials exhibiting this phenomenon at high pressures has seen significant advancements, particularly in the field of hydrides. This paper explores the synthesis of ternary lanthanum-yttrium hydrides, offering detailed experimental and theoretical analysis focused on their superconducting properties. Among the notable findings is the synthesis of cubic hexahydride $(La,Y)H_6$ and decahydride $(La,Y)H_{10}$, achieving superconductivity at remarkably elevated temperatures under extreme pressures.

Experimental Findings

The authors have synthesized various lanthanum-yttrium hydrides by laser heating lanthanum-yttrium alloys in a high-pressure ammonia borane medium. The reported superconducting transition temperature $T_c$ for the decahydride $(La,Y)H_{10}$ reaches approximately 253 K at pressures around 183 GPa. Additionally, the paper denotes an extrapolated upper critical magnetic field $B_{c2}(0) \approx 135 \: \text{T}$, setting a noteworthy benchmark for superconducting hydrides synthesized at such pressures.

Critical current density experiments show that $(La,Y)H_{10}$ compares favorably with established commercial superconductors like NbTi and Nb_3Sn. Notably, at 4.2 K, $(La,Y)H_{10}$ exhibits a critical current density exceeding 2500 A/mm², demonstrating robust superconducting characteristics.

Theoretical Implications

The paper’s theoretical investigations employ USPEX and density functional theory (DFT) for structural predictions, identifying that these hydrides can be stabilized in solid solutions, a critical aspect given their thermodynamic instability in pure forms at lower pressures. Phonon calculations and isotropic Migdal-Eliashberg equations provide insights into the electron-phonon coupling mechanisms contributing to enhanced superconductivity.

The research emphasizes anomalous impacts of Coulomb repulsion observed in lanthanum-yttrium hexahydrides. Within the superconducting density functional theory (SCDFT) framework, this anomaly indicates deviations from predictions, prompting further exploration into pairing interactions influencing critical temperature predictions.

Implications and Future Prospects

The paper breakthrough with superconductivity at 253 K advances the understanding of superconducting hydrides under substantial pressures, suggesting potential pathways for stabilizing materials which might normally be unstable. The substitution effect, replacing lanthanum or yttrium atoms in their respective hydrides, not only allows stabilization but also enhances superconducting properties, revealing opportunities for material design in superconductivity applications.

These findings are pivotal in the hunt for room-temperature superconductors, leveraging high-pressure synthesis techniques to overcome thermodynamic challenges inherent in these materials. Future exploitation of these high-T_c hydrides lies in refining synthesis techniques and further theoretical research to predict and stabilize novel superhydrides.

In conclusion, the advancement to 253 K in lanthanum-yttrium ternary hydrides marks a significant step in high-pressure superconductivity research, facilitating exploration for practical applications and theoretical modeling in complex hydride systems. This paper supplies valuable experimental evidence and theoretical modeling insights that will guide subsequent research directions and innovative approaches to superconductivity at ambient conditions.