New quantum state formed by highly concentrated protons in superconducting palladium hydride

TL;DR

This study reveals a new quantum state in palladium hydride where highly concentrated protons exhibit tunneling behavior, affecting superconductivity and resistivity, thus providing a platform for exploring quantum many-body phenomena.

Contribution

It demonstrates the formation of a novel quantum state involving proton tunneling in superconducting palladium hydride, a phenomenon not previously observed in this material.

Findings

Resistivity drops sharply at superconducting transition temperature (~2 K).

Residual resistivity remains due to proton tunneling weakening superconducting coherence.

Zero resistivity occurs below 1 K, indicating long-range proton ordering.

Abstract

Hydrogen exhibits quantum phenomena, such as tunneling in materials. According to theory, the quantum properties of hydrogen change significantly in superconductors due to the emergence of an energy gap on the Fermi surface, which reduces the interaction between hydrogen nucleus (i.e., proton) and conduction electrons. This reduction is predicted to enhance the tunneling probability of protons. Here, we report the double transitions of the electrical resistivity in high-quality palladium hydride (PdHx) and deuteride (PdDx) prepared by low-temperature absorption below T = 180 K. After a sharp drop in the resistivity at T ~ 2 K owing to the superconducting transition of PdH(D)x, a large residual resistivity remained. Additionally, the resistivity dropped to zero below T = 1 K. The experimental results suggest that the quantum tunneling of highly concentrated protons (deuterons) in the superconducting state is responsible for the observed features: the residual resistivity arises from the weakening of the global coherence of superconductivity owing to the tunneling motion of protons (deuterons), while the zero resistivity is caused by long-range ordering of the protons (deuterons). This system offers a new platform for investigating the quantum many-body properties of tunneling particles.