Random Bond Effect in the Quantum Spin System (Tl$_{1-x}$K$_{x}$)CuCl$_3$

TL;DR

This study investigates how exchange bond randomness affects the ground state and magnetic ordering in a mixed quantum spin system, revealing that randomness induces magnetic states while preserving excitation gaps, and suppresses phase transition temperatures.

Contribution

It provides new insights into the effects of bond randomness on quantum spin systems, particularly on magnetic states and phase transition suppression.

Findings

Bond randomness induces magnetic states with finite susceptibility.

Excitation gaps remain despite bond randomness.

Bond randomness suppresses the phase transition temperature.

Abstract

The effect of exchange bond randomness on the ground state and the field-induced magnetic ordering was investigated through magnetization measurements in the spin-1/2 mixed quantum spin system (Tl$_{1-x}$K$_{x}$)CuCl$_3$ for $x<0.36$. Both parent compounds TlCuCl$_3$ and KCuCl$_3$ are coupled spin dimer systems, which have the singlet ground state with excitation gaps ${\Delta}/k_{\rm B}=7.7$ K and 31 K, respectively. Due to bond randomness, the singlet ground state turns into the magnetic state with finite susceptibility, nevertheless, the excitation gap remains. Field-induced magnetic ordering, which can be described by the Bose condensation of excited triplets, magnons, was observed as in the parent systems. The phase transition temperature is suppressed by the bond randomness. This behavior may be attributed to the localization effect.