Thermodynamic Properties of the Kagome Lattice in Volborthite

TL;DR

This study investigates the thermodynamic properties of volborthite, a kagome lattice compound with S-1/2 spins, revealing gapless excitations, a phase transition at 1 K, and field-dependent changes in heat capacity.

Contribution

It provides detailed low-temperature heat capacity measurements of volborthite, identifying a thermodynamic phase transition and the nature of magnetic excitations in the kagome lattice.

Findings

Presence of dense gapless excitations above 1 K

Identification of a thermodynamic phase transition at 1 K

Field-induced suppression of the T-linear heat capacity component

Abstract

Thermodynamic investigations on volborthite (Cu3V2O7(OH)2 2H2O), which possesses a two-dimensional kagome structure of S-1/2 spins, are presented. The low-temperature heat capacities of volborthite and its Zn analogue compound were measured by the relaxation calorimetry technique between 0.8 and 45 K. The magnetic heat capacity of volborthite is characterized by two contributions of T-linear and T2 dependences, the former of which is large above 1 K, giving evidence of dense gapless excitations. We find a distinct kink in Cp T-1 at T* = 1 K, which demonstrates a thermodynamic phase transition of short-range nature to a novel ground state as reported in previous 51V-NMR experiments. The T-linear term becomes smaller but remains finite even in the low-temperature state below T*, while it is gradually reduced with increasing magnetic field and vanishes at 5 T, which is close to the critical field for the field-induced transition observed in the first magnetization step.