Quantum Motion and Anharmonicity in Superconducting Hydrides

The recent discovery by Drozdov et al. [1] of superconductivity at 203 K in the hydrogen sulfide system compressed to about 150 GPa breaks the record of the cuprates and overturns the conventional wisdom that such a high critical temperatures cannot be obtained via phonon-mediated pairing. Exciting new prospects are now open to find room temperature superconductivity in other hydrogen-rich materials.

In this talk I will underline that the structural, chemical, and vibrational properties of the record hydrogen sulfide are largely influenced by the quantum nature of the protons. According to our ab initio calculations based on density-functional theory [2,3,4], in the record hydrogen sulfide superconductor the quantum nature of the proton and the consequent huge anharmonicity symmetrize the hydrogen bonds in a wide pressure range and make the solid adopt a crystal structure that is not the minimum of the static Born-Oppenheimer energy surface. At the same time, anharmonicity dramatically modifies the vibrational spectra. Interestingly, such strong quantum effects are necessary ingredients to understand the superconducting properties observed experimentally in this system.

Our results suggest, therefore, that the quantum nature of the proton needs to be included in theoretical calculations seeking for new hydrides with potential high-temperature superconductivity, not only for determining their vibrational spectra and superconducting properties, but even for determining the crystal structure and the hydrogen bonding.

[1] A. P. Drozdov et al., Nature 525, 73 (2015).
[2] I. Errea et al., Phys. Rev. Lett. 114, 157004 (2015).
[3] Y. Li et al., Phys. Rev. B 93, 020103(R) (2016).
[4] I. Errea et al., Nature 532, 81 (2016).