Displacive Quantum Critical Point in Superconducting Hydrides: The Case of H_{3}S

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Mechanics

Background:

• Sulfur hydride (H3S) is a key precursor for superconducting hydrides.
• Its high superconducting critical temperature (Tc) of 203 K at 155 GPa is well-known.
• A comprehensive phase diagram including thermal and quantum effects is lacking.

Purpose of the Study:

• To determine the structural phase diagram of H3S over an extended temperature and pressure range.
• To investigate the role of thermal and quantum effects on the phase transitions.
• To understand the origin of high-Tc superconductivity in H3S.

Main Methods:

• Path integral molecular dynamics simulations.
• MACE neural network potential trained on BLYP density functional theory.
• Analysis of structural phase transitions and quantum fluctuations.

Main Results:

• The H3S phase diagram shows a displacive transition between centrosymmetric Im3̅m and polar R3m phases.
• A quantum critical point (QCP) is identified at approximately 134 GPa.
• High Tc superconductivity occurs in a region of significant nuclear quantum fluctuations above the QCP.
• The system near the QCP belongs to the 4D Ising universality class.

Conclusions:

• Quantum fluctuations play a crucial role in the high-Tc superconductivity of H3S.
• The identified QCP and universality class provide insights into the underlying physics.
• This work clarifies the complex phase behavior of H3S and its relation to superconductivity.