Elementary Excitations and Ground States of Quantum Magnets Captured by Neutron Scattering

Abstract

The spin dimer system is a quantum magnet in which two S = 1/2 spins form a dimer by strong antiferromagnetic interaction, and weak interdimer interactions couple the dimers. We found a quantum phase transition from a singlet to an ordered state in a spin dimer magnet TlCuCl₃ induced by hydrostatic pressure through magnetization measurements and neutron scattering experiments. We verified that the excitation gap closes with increasing pressure. Our observation encouraged the theoretical study, which led to the discovery of the first Higgs mode in condensed matter. We also clarified the structures of magnetic excitations in Ba₃CoSb₂O₉ and Cs₂Cu₃SnF₁₂, described as S = 1/2 triangular lattice and kagome lattice antiferromagnets, respectively. In both systems, we found that single magnon excitation energies are significantly renormalized downwards and that an intense excitation continuum extends to high energies. These features strongly suggest spinon excitations as an elementary excitation. Combining our results with recent theories, we can deduce that the superposition of the ordered state and the quantum spin liquid state gives the ground states of these systems.