Spin-resonance modes of the spin-gap magnet TlCuCl_3

TL;DR

This study identifies three types of magnetic resonance signals in TlCuCl_3, revealing insights into its spin-gap behavior, triplet excitations, and field-induced antiferromagnetic phase, advancing understanding of quantum magnetic systems.

Contribution

The paper reports the first observation of microwave absorption, thermally activated resonance, and antiferromagnetic resonance in TlCuCl_3, elucidating its complex spin dynamics and phase transitions.

Findings

Detected microwave absorption related to singlet-triplet transitions.

Observed thermally activated resonance due to split triplet sublevels.

Identified anisotropic antiferromagnetic resonance above critical field.

Abstract

Three kinds of magnetic resonance signals were detected in crystals of the spin-gap magnet TlCuCl_3. First, we have observed the microwave absorption due to the excitation of the transitions between the singlet ground state and the excited triplet states. This mode has the linear frequency-field dependence corresponding to the previously known value of the zero-field spin-gap of 156 GHz and to the closing of spin-gap at the magnetic field H_c of about 50 kOe. Second, the thermally activated resonance absorption due to the transitions between the spin sublevels of the triplet excitations was found. These sublevels are split by the crystal field and external magnetic field. Finally, we have observed antiferromagnetic resonance absorption in the field-induced antiferromagnetic phase above the critical field H_c. This resonance frequency is strongly anisotropic with respect to the direction of the magnetic field.