Easy-plane antiferromagnet in tilted field: gap in magnon spectrum and susceptibility

TL;DR

This paper models an easy-plane antiferromagnet in a tilted magnetic field, analyzing the magnon spectrum gap and susceptibility, and explains experimental observations of non-monotonic gap behavior and dynamical diamagnetism.

Contribution

It provides an analytical study of the field dependence of magnon gap and susceptibility in a tilted field, connecting quantum fluctuations to experimental phenomena.

Findings

Magnon gap exhibits non-monotonic behavior due to quantum fluctuations.

Transverse susceptibility depends on optical magnon decay into two magnons.

Close to the ordered phase center, dynamical diamagnetism is observed.

Abstract

Motivated by recent experimental data on dichloro-tetrakis thiourea-nickel (DTN) [T.A. Soldatov $\textit{et al}$, Phys. Rev. B ${\bf 101}$, 104410 (2020)], a model of antiferromagnet on a tetragonal lattice with single-ion easy-plane anisotropy in the tilted external magnetic field is considered. Using the smallness of the in-plane field component, we analytically address field dependence of the energy gap in ``acoustic'' magnon mode and transverse uniform magnetic susceptibility in the ordered phase. It is shown that the former is non-monotonic due to quantum fluctuations, which was indeed observed experimentally. The latter is essentially dependent on the ``optical'' magnon rate of decay on two magnons. At magnetic fields close to the one which corresponds to the center of the ordered phase, it leads to experimentally observed dynamical diamagnetism phenomenon.