Quantum Crystal Structure in the 250 K Superconducting Lanthanum Hydride

Abstract: The discovery of superconductivity at 200 K in the hydrogen sulfide system at large pressures [1] was a clear demonstration that hydrogen-rich materials can be high-temperature superconductors. The recent synthesis of LaH${10}$ with a superconducting critical temperature (T${\text{c}}$) of 250 K [2,3] places these materials at the verge of reaching the long-dreamed room-temperature superconductivity. Electrical and x-ray diffraction measurements determined a weakly pressure-dependent T${\text{c}}$ for LaH${10}$ between 137 and 218 gigapascals in a structure with a face-centered cubic (fcc) arrangement of La atoms [3]. Here we show that quantum atomic fluctuations stabilize in all this pressure range a high-symmetry Fm-3m crystal structure consistent with experiments, which has a colossal electron-phonon coupling of $\lambda\sim3.5$. Even if ab initio classical calculations neglecting quantum atomic vibrations predict this structure to distort below 230 GPa yielding a complex energy landscape with many local minima, the inclusion of quantum effects simplifies the energy landscape evidencing the Fm-3m as the true ground state. The agreement between the calculated and experimental T$_{\text{c}}$ values further supports this phase as responsible for the 250 K superconductivity. The relevance of quantum fluctuations in the energy landscape found here questions many of the crystal structure predictions made for hydrides within a classical approach that at the moment guide the experimental quest for room-temperature superconductivity [4,5,6]. Furthermore, quantum effects reveal crucial to sustain solids with extraordinary electron-phonon coupling that may otherwise be unstable [7].

• The paper demonstrates that including quantum fluctuations in calculations stabilizes the Fm-3m phase of LaH10, directly correlating with its 250 K superconductivity.
• Advanced computational methods like SSCHA, isotropic SCDFT, and anisotropic Migdal-Eliashberg frameworks accurately predict the colossal electron-phonon coupling and Tc.
• The study underscores that quantum corrections are crucial to overcoming phonon instabilities, paving the way for designing high-pressure, hydrogen-rich superconductors.

Quantum Crystal Structure in the 250 K Superconducting Lanthanum Hydride

The paper investigates quantum effects on lanthanum hydride (LaH$_{10}$) in contextualizing the 250 K superconductivity achieved in this compound under high pressures. This exploration is pertinent to the ongoing pursuit of room-temperature superconductivity through metallic hydrogen and hydrogen-rich compounds, as initially verified in hydrogen sulfide systems. The authors emphasize the core realization that quantum atomic fluctuations contribute to stabilizing the high-symmetry $Fm\overline{3}m$ crystal structure of LaH$_{10}$, a pivotal factor in achieving its exceptional superconducting properties.

The $Fm\overline{3}m$ phase of LaH$_{10}$ manifests colossal electron-phonon coupling ($\lambda \sim 3.5$), a cornerstone for high-temperature superconductivity. Even though traditional ab initio calculations disregarding quantum vibrations indicate structural distortion beneath 230 GPa, encompassing quantum effects considerably simplifies the energy landscape and portrays the $Fm\overline{3}m$ structure as the authentic ground state throughout the pressure range explored (137–218 GPa). This study shows a distinct alignment between calculated and experimental superconducting critical temperatures ($T_c$), conclusively attributing the 250 K superconductivity to this phase.

The quantum corrections in the energy landscape, particularly the zero-point energy (ZPE) using the stochastic self-consistent harmonic approximation (SSCHA), underscore the stabilizing role of quantum fluctuations in LaH$_{10}$. Such quantum effects are critically influential, prompting a reevaluation of structural predictions for superconducting hydrides, implicating limitations in classical approaches that guide experimental efforts toward room-temperature superconductivity. Significantly, phonon-band structure analyses in the $Fm\overline{3}m$ phase reiterate stability from 129 GPa upwards, with quantum fluctuation exclusion revealing inherent vibrational predicaments or imaginary frequencies, contradicting classical phonons.

Furthermore, the reported calculations of the superconducting critical temperature are notably consistent with experimental findings, affirming the pivotal role of the highly symmetric phase under study. Both isotropic SCDFT and anisotropic Migdal-Eliashberg theoretical frameworks are employed in solving the electron-phonon interaction, further elucidating the phonon-driven mechanism behind the superconductivity observed in LaH$_{10}$.

In practical and theoretical landscapes, this work illustrates the non-negligible significance of quantum effects in sustaining materials with formidable electron-phonon coupling. The suppression of instability typically expected due to colossal electron-phonon interactions, demonstrated by the unprecedented $\lambda \sim 3.6$ at 129 GPa in LaH$_{10}$, holds potential for unfolding new superconducting systems with tailored quantum mechanical treatments. This research promotes a paradigm shift toward integrating comprehensive quantum mechanical insights into the design and stabilization of advanced superconductive materials, potentially guiding future explorations and material synthesis in high-pressure regimes.