Youhei Yamaji received his Dr. Eng. from The University of Tokyo, in February 2010. After holding research positions at institutions including the Quantum-Phase Electronics Center, The University of Tokyo, he was appointed Project Associate Professor in the Department of Applied Physics at the same university in December 2016. In April 2021, he joined the National Institute for Materials Science (NIMS) as a Senior Researcher, where he currently serves as a Group Leader.

In quantum materials, electrons exhibit not only charge and spin, but also orbital degrees of freedom, quantum entanglement, and quantum geometry, which play key roles in superconductivity, magnetism, and dielectric properties. Equally important is the fractionalization of these degrees of freedom.

Mott insulators in transition-metal compounds, where charge motion is frozen while spin degrees of freedom remain active, provide a paradigmatic platform for quantum magnetism. Of particular interest are quantum spin liquids, in which spin degrees of freedom become delocalized and fractionalized through nonlocal quantum entanglement, with potential relevance to fault-tolerant quantum computation.

We have applied many-body wave functions based on superpositions of entangled electron pairs to first-principles studies of quantum materials, including cuprate superconductors [1]. Combined with neural networks [2], this approach enables high-accuracy calculations of spectroscopic responses directly comparable with ARPES, neutron scattering, EELS, and RIXS experiments.

Here, by simulating and analyzing inelastic neutron scattering spectra using entangled ab initio many-body wave functions, we demonstrate the nonlocal and partially fractionalized character of spin excitations in the Mott-insulating parent compounds of Bi-based cuprate superconductors.

1. M. T. Schmid, J.-B. Morée, R. Kaneko, Y. Yamaji, and M. Imada, Phys. Rev. X 13, 041036 (2023) DOI:_10.1103/PhysRevX.13.041036
2. Y. Nomura, A. S. Darmawan, Y. Yamaji, and M. Imada, Phys. Rev. B 96, 205152 (2017) DOI:_10.1103/PhysRevB.96.205152