# Quantum Crystal Structure in the 250 K Superconducting Lanthanum Hydride

## TL;DR

This paper demonstrates that quantum atomic fluctuations stabilize a high-symmetry crystal structure in LaH$_{10}$ across a wide pressure range, explaining its high-temperature superconductivity at 250 K and challenging classical predictions.

## Contribution

It shows that quantum effects are essential for accurately predicting the stable crystal structure and superconducting properties of LaH$_{10}$ hydrides, which classical methods fail to capture.

## Key findings

- Quantum fluctuations stabilize the Fm-3m structure across 137-218 GPa.
- The structure exhibits a colossal electron-phonon coupling of ~3.5.
- Quantum effects simplify the energy landscape, confirming the structure as the true ground state.

## 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].