# From order to randomness: Onset and evolution of the random-singlet state in bond-disordered BaCu$_2$(Si$_{1-x}$Ge$_x$)$_2$O$_7$ spin-chain compounds

## TL;DR

This study explores how introducing bond disorder in spin-chain compounds transitions the system from ordered to a random singlet state, revealing disorder's significant impact on magnetic properties and confirming theoretical predictions.

## Contribution

It provides experimental evidence of the evolution from ordered to random singlet states in disordered spin chains, using magnetometry and NMR techniques.

## Key findings

- Weak disorder reduces magnetic ordering temperature.
- Maximum disorder induces a random singlet state.
- Disorder influences NMR relaxation features.

## Abstract

Heisenberg-type spin-chain materials have been extensively studied over the years, yet not much is known about their behavior in the presence of disorder. Starting from BaCu$_2$Si$_2$O$_7$, a typical spin-1/2 chain system, we investigate a series of compounds with different degrees of bond disorder, where the systematic replacement of Si with Ge results in a re-modulation of the Cu$^{2+}$ exchange interactions. By combining magnetometry measurements with nuclear magnetic resonance studies we follow the evolution of the disorder-related properties from the well-ordered BaCu$_2$Si$_2$O$_7$ to the maximally disordered BaCu$_2$SiGeO$_7$. Our data indicate that already a weak degree of disorder of only 5% Ge, apart from reducing the 3D magnetic ordering temperature $T_\mathrm{N}$ quite effectively, induces a qualitatively different state in the paramagnetic regime. At maximum disorder our data indicate that this state may be identified with the theoretically predicted random singlet (RS) state. With decreasing disorder the extension of the RS regime at temperatures above $T_\mathrm{N}$ is reduced, yet its influence is clearly manifest, particularly in the features of NMR relaxation data.