High-temperature superconductivity on the verge of a structural instability in lanthanum superhydride

Abstract

The possibility of high, room-temperature superconductivity was predicted for metallic hydrogen in the 1960s. However, metallization and superconductivity of hydrogen are yet to be unambiguously demonstrated and may require pressures as high as 5 million atmospheres. Rare earth based "superhydrides", such as LaH₁₀, can be considered as a close approximation of metallic hydrogen even though they form at moderately lower pressures. In superhydrides the predominance of H-H metallic bonds and high superconducting transition temperatures bear the hallmarks of metallic hydrogen. Still, experimental studies revealing the key factors controlling their superconductivity are scarce. Here, we report the pressure and magnetic field dependence of the superconducting order observed in LaH₁₀. We determine that the high-symmetry high-temperature superconducting Fm-3m phase of LaH₁₀ can be stabilized at substantially lower pressures than previously thought. We find a remarkable correlation between superconductivity and a structural instability indicating that lattice vibrations, responsible for the monoclinic structural distortions in LaH₁₀, strongly affect the superconducting coupling.

Fig. 1. Structural data for LaH₁₀ synthesized from La and excess H₂.

a, b Rietveld refinement for Fm-3m phase of LaH₁₀ at 138 GPa and C2/m phase of LaH₁₀ at 120 GPa, respectively. The peaks originating from the hcp-I (a = 3.668(4) Å; c = 5.914(11) Å; V = 68.9(1) ų at 138 GPa) and hcp-II (a = 3.750(3) Å; c = 5.561(7) Å; V = 67.7(1) ų at 138 GPa) impurity phases are indicated through blue and red dashes, respectively. The refined ratio between the main and the impurity phases is provided in the left bottom corner of each figure. The main structural building block, two connected LaH₃₂ polyhedra, are shown in the middle inserts for each phase. Large blue and small black spheres correspond to La and H atoms, respectively. c, d The original powder X-ray diffraction patterns at 138 and 120 GPa, respectively. New reflections appear at 120 GPa due to the monoclinic distortions.

Fig. 2. The superconducting transitions in LaH₁₀.

a The electrical resistance in LaH₁₀ after the synthesis under 138 GPa (red curve), after the abrupt decompression down to 120 GPa (brown curve), and upon a gradual increase in pressure from 120 to 136 GPa (blue, green, purple, and black curves). The data measured at 138 GPa on the upper panel are divided by 9 for better presentation. b Pressure dependence of T_c in LaH₁₀ measured in the present study (black symbols) and from a prior study (open red symbols). Insets: photos of the DAC loaded with a La flake and after the synthesis of LaH₁₀ through laser-assisted heating.

Fig. 3. Resistance of LaH₁₀ as a function magnetic field at different temperatures.

a DC field measurements for the C2/m phase of LaH₁₀ at 120 GPa. Two dashed lines extrapolate the slope of the high-temperature superconducting transition (left line) toward the asymptotic trace representing the high-field normal state magnetoresistance (right line) at 170 K, respectively. The intersection between two lines provides an estimation of the upper critical field (H_c2). The intersection of the first line with the horizontal axis indicates the irreversibility field (H*) for the high-temperature superconducting phase. b Pulsed field measurements for the Fm-3m phase of LaH₁₀ at 136 GPa. Both DC and pulsed field traces were recorded under isothermal conditions with no observation of eddy current-generated Joule heating due to the sweeping of the field. c Fits of the superconducting upper critical H_c2 to the Werthamer–Helfand–Hohenberg (WHH) formalism. Red and blue squares denote the loci of H_c2 of LaH₁₀ at 136 and 120 GPa, respectively. Lines of the same color correspond to the WHH fits of the experimental data.

Fig. 4. Fermi velocities for the different hydrides.

a Calculated Fermi velocities associated with the electronic states on the Fermi surfaces in the first Brillouin zone (frame) where the color scale goes from blue (slowest) to the red (fastest). The perspective comprises angles of rotation with respect to the x-, y-, z-axis of 0°, 0°, and 0° for the C2/m-LaH₁₀ and 13°, −13°, and 1° for the Fm-3m-LaH₁₀, respectively. b Extracted coherence lengths ξ (cyan circles, left axis) and BCS Fermi velocities v_F (magenta circles, right axis) for the different hydrides^,,,–. The labels associated with the C2/m-LaH₁₀ and Fm-3m-LaH₁₀ phases correspond to results from the present study.