Neutron scattering investigation of proposed Kosterlitz-Thouless transitions in the triangular-lattice Ising antiferromagnet TmMgGaO4

TL;DR

This study uses neutron scattering techniques to investigate the presence of a Kosterlitz-Thouless phase in TmMgGaO4, providing experimental evidence for vortex-antivortex pairs and phase transitions consistent with KT physics.

Contribution

The paper confirms the realization of KT physics in TmMgGaO4 through detailed neutron scattering experiments and analysis of spin correlations, advancing understanding of KT phases in dense spin systems.

Findings

Evidence of vortex-antivortex pairs around 5 K

Identification of low-energy effective Hamiltonian parameters

Observation of KT phase transitions via susceptibility and diffraction

Abstract

The transverse-field Ising model on the triangular lattice is expected to host an intermediate finite-temperature Kosterlitz-Thouless (KT) phase through a mapping of the spins on each triangular unit to a complex order parameter. TmMgGaO$_4$ is a candidate material to realize such physics due to the non-Kramers nature of Tm$^{3+}$ ion and the resulting two-singlet single-ion ground state. Using inelastic neutron scattering, we confirm this picture by determining the leading parameters of the low-energy effective Hamiltonian of TmMgGaO$_4$. Subsequently, we track the predicted KT phase and related transitions by inspecting the field and temperature dependence of the ac susceptibility. We further probe the spin correlations in both reciprocal space and real space via single crystal neutron diffraction and magnetic total scattering techniques, respectively. Magnetic pair distribution function analysis provides evidence for the formation of vortex-antivortex pairs that characterize the proposed KT phase around 5~K. Although structural disorder influences the field-induced behavior of TmMgGaO$_4$, the magnetism in zero field appears relatively free from these effects. These results position TmMgGaO$_4$ as a strong candidate for a solid-state realization of KT physics in a dense spin system.