Evidence for superconductivity above 260 K in lanthanum superhydride at megabar pressures (1808.07695v3)

Abstract: Recent predictions and experimental observations of high Tc superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using pulsed laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200 GPa. We detail the synthesis and characterization, including four-probe electrical transport measurements, of lanthanum superhydride samples that display a significant drop in resistivity on cooling beginning around 260 K and pressures of 190 GPa. Additional measurements on two additional samples synthesized the same way show resistance drops beginning as high as 280 K at these pressures. The loss of resistance at these high temperatures is not observed in control experiments on pure La as well as in partially transformed samples at these pressures, and x-ray diffraction as a function of temperature on the superhydride reveal no structural changes on cooling. We infer that the resistance drop is a signature of the predicted room-temperature superconductivity in LaH10, in good agreement with density functional structure search and BCS theory calculations.

• The paper presents experimental evidence of superconductivity onset near 260 K in LaH10 synthesized under pressures of ~190-200 GPa.
• The research utilized pulsed laser heating with ammonia borane in a diamond anvil cell, with X-ray diffraction confirming an fcc structure.
• The findings validate theoretical predictions for high-Tc in hydrogen-rich compounds and suggest potential for practical near ambient superconductors.

Superconductivity in Lanthanum Superhydride at Megabar Pressures

The paper "Evidence for superconductivity above 260 K in lanthanum superhydride at megabar pressures" presents compelling results concerning the synthesis and characterization of lanthanum superhydride (LaH<sub\>10</sub>) under extreme pressures, targeting superconductivity near room temperature. This research extends the exploration of hydrogen-rich materials, which are theoretically predicted to exhibit high-temperature superconductivity due to their significant electron-phonon coupling that stems from the phonon modes associated with the stretching of H-H bonds within hydrogen clathrate structures.

Methodology and Experimental Results

To synthesize LaH<sub\>10±x</sub>, the authors employed a novel high-pressure preparation technique involving pulsed laser heating combined with ammonia borane (NH<sub\>3</sub>BH<sub\>3</sub>) as the hydrogen source. The synthesis was conducted in a diamond anvil cell, permitting the application of pressures up to 200 GPa. X-ray diffraction verified the structural characterization of the synthesized samples, which showed a face-centered cubic (fcc) structure at 170 GPa, closely resembling the predicted cubic metallic phase.

Electrical transport measurements conducted using a four-probe technique revealed a notable drop in resistivity, consistent with a superconducting transition. This drop began at temperatures around 260-280 K, with pressures of approximately 190-200 GPa, as predicted by BCS theory for these superhydrides. Importantly, control experiments confirmed that this phenomenon was not observed in pure lanthanum samples or partially transformed samples, supporting the inference that the drop is linked to superconductivity.

Theoretical and Practical Implications

The implications of these findings extend both theoretically and practically. The LaH<sub\>10</sub> system is yet another validation of the theoretical frameworks that predict high-T_c in hydrogen-rich compounds. It demonstrates the promise of utilizing heavy element stabilization to facilitate superconductivity under high-pressure conditions that are achievable with current technology. This has significant ramifications for the development of superconducting materials that can operate at temperatures closer to ambient conditions, thereby reducing the practical barriers of broadening the applications of superconductivity.

Future Prospects

While the experimental evidence supports the presence of superconductivity at remarkably high temperatures, further studies are necessary to unequivocally determine the superconducting nature of LaH<sub\>10</sub>. Future work should focus on measuring the Meissner effect and other magnetic properties to confirm superconductivity conclusively. Additionally, exploring the stoichiometry-dependence of T_c and understanding the phase behavior across different hydrogen incorporation levels will provide deeper insights into the superconducting mechanisms at play.

Moreover, extending such studies to other hydrogen-rich systems, potentially with different heavy atom stabilizers, could invite new avenues for discovering superconductors that might operate at even higher temperatures or under less extreme conditions. The combination of experimental advancements and theoretical predictions will continue to play a pivotal role in potentially heralding a new era in superconductor applications and research.

Overall, the findings contribute significantly to the field of high-pressure superconductivity, presenting a promising route toward realizing near room-temperature superconductors, with intriguing theoretical questions and practical applications on the horizon.