The effect of randomness on the quantum spin system Tl$_{1-x}$K$_x$CuCl$_3$ with ${\boldmath $x$}$ = 0.44 studied by the Zero-field Muon-Spin-Relaxation (ZF-$\mu$SR) method

TL;DR

This study investigates how randomness affects the magnetic properties of Tl$_{1-x}$K$_x$CuCl$_3$ with x=0.44 using zero-field muon-spin relaxation, revealing critical slowing down and possible spin freezing at very low temperatures.

Contribution

First application of ZF-μSR to this system showing the critical slowing down and spin freezing behavior due to randomness.

Findings

Muon relaxation rate increases rapidly at low temperatures.

Relaxation behavior suggests spatially-fixed fluctuating moments.

Evidence of spin freezing below 80 mK.

Abstract

Zero-field muon-spin-relaxation (ZF-{\boldmath $\mu$}SR) measurements were carried out down to 80 mK on the randomness bond system Tl$_{1-x}$K$_x$CuCl$_3$ with $x$ = 0.44. Time spectra are well fitted by the stretched exponential function $\exp (-\lambda t)^{\beta}$. The muon spin relaxation rate $\lambda$ increases rapidly with decreasing temperature, and $\beta$ tends to 0.5 at 80 mK. The divergent increase of $\lambda$ suggests the critical slowing down of the frequency of the Cu-3{\it d} spin fluctuations toward a spin frozen state below 80 mK, and the root-exponential-like behavior of the time spectrum indicates that the origin of the relaxation is possibly the spatially-fixed fluctuating dilute moments.