Crystal symmetry found to control hydrogen's quantum nature

A research group including Project Researcher Sudhansu Sekhar Das, Technical Specialist Taizo Kawauchi (at the time of research), Assistant Professor Takahiro Ozawa, and Professor Katsuyuki Fukutani of the Institute of Industrial Science at the University of Tokyo, along with Professor Hiroshi Nakanishi of the National Institute of Technology, Akashi College (at the time of research, currently a specially appointed professor at the Center for Quantum Information and Quantum Biology at the University of Osaka), has revealed that whether hydrogen in metal exhibits classical or quantum behavior depends on the symmetry of the crystal. The findings were published in Nature Communications.

Hydrogen atoms easily enter various materials, and in metals, they occupy interstitial sites surrounded by the host lattice. While it is known that they thermally diffuse as classical particles at high temperatures, because of their small mass, they exhibit wave properties similar to electrons. Quantum transport via the tunneling effect, by which they probabilistically pass through barriers, is also considered non-negligible. However, the conditions under which hydrogen exhibits quantum behavior were not fully understood, and guidelines for controlling it were also unclear.

The research group injected hydrogen ions into a thin film of vanadium, a hydrogen-storage metal, at low temperatures below 50 K and analyzed the depth distribution and lattice sites of hydrogen using resonant nuclear reaction analysis. The injected hydrogen mainly formed a high-concentration phase (β phase), and from its depth distribution and time evolution of electrical resistance, they clarified that hydrogen diffusion in the high-concentration phase is thermal and slow at low temperatures. On the other hand, a dilute, low-concentration phase (α phase) of around a few percent was already formed in deep regions immediately after hydrogen injection. It was suggested that hydrogen diffusion is fast in the low-concentration phase even at low temperatures.

To understand the mechanism by which the diffusion rate at low temperatures changes according to hydrogen concentration, they analyzed the eigenstates of the hydrogen nucleus using first-principles quantum dynamics calculations. It was found that in the low-concentration phase with a highly symmetric body-centered cubic structure (bcc structure), hydrogen moves between multiple energetically equivalent neighboring sites via the tunneling effect, entering a spatially extended quantum state spanning multiple sites.

On the other hand, in the high-concentration phase with a body-centered tetragonal structure (bct structure) having reduced symmetry, energy levels between neighboring sites become misaligned, suppressing the tunneling effect and causing hydrogen to localize at the most stable site. These results indicate that, via the alignment of energy levels between sites, hydrogen exhibits a quantum state due to the tunneling effect in a highly symmetric environment, whereas it can behave like a classical particle in a low-symmetry environment.

In this study, by combining advanced measurement of hydrogen behavior and quantum calculations of hydrogen states, the conditions under which hydrogen atoms behave like waves were clarified. This achievement provides design guidelines for controlling the quantum behavior of hydrogen through crystal structure. In the future, applications are expected in the development of new hydrogen-energy-related materials, control of catalytic reactions, and research on various materials containing light elements other than hydrogen.

Ozawa said, "Hydrogen is one of the most difficult atoms to observe directly. We have developed methods using ion beam analysis and electrical conductivity measurements, making it possible to directly observe its structure and diffusion. Harnessing these techniques, this study demonstrated that quantum tunneling of hydrogen is modulated by strains in the crystal structure. Expanding on this research, which presented guidelines for controlling the quantum nature of atomic nuclei, we will proceed with research aiming to control quantum tunneling using external fields toward the new development of hydrogen storage materials and hydrogen-driven devices."

Fukutani added, "Hydrogen is an element that plays a leading role in fields ranging from fundamental physics to chemical reactions, biological functions, and space science. In phenomena involving hydrogen, 'quantum nature' is often predicted to be important, but its actual state was not clear. In this research, we conducted studies to clarify under what circumstances the quantum nature of hydrogen manifests, demonstrating that crystal symmetry is important. Moving forward, while deepening our understanding of hydrogen's quantum nature, we hope to tackle the development of surface reactions and devices that utilize this quantum nature."

Journal Information
Publication: Nature Communications
Title: Impact of crystal symmetry lowering on proton tunneling
DOI: 10.1038/s41467-026-75020-w