Phase diagram of doped spin-Peierls systems

TL;DR

This paper models the phase diagram of doped CuGeO$_3$, highlighting how impurities induce local moments and disorder, leading to an inhomogeneous antiferromagnetic phase with no true long-range order, consistent with experimental observations.

Contribution

It introduces a combined mean field and real space decimation approach to analyze disorder effects in doped spin-Peierls systems, revealing a Griffith antiferromagnetic phase at low doping.

Findings

Transition to inhomogeneous Néel phase at low doping

Appearance of large magnetic clusters at high temperatures

Qualitative agreement with experimental phase diagrams

Abstract

The phase diagram of a model describing doped CuGeO$_3$ is derived. The model emphasizes the role of local moments released by the impurities and randomly distributed inside the gaped singlet background. The phase diagram is investigated by two methods: (i) in a mean field treatment of the interchain coupling and (ii) in a real space decimation procedure in a two dimensional model of randomly distributed moments. Both methods lead to similar results, in a qualitative agreement with experiments. In particular, a transition to an inhomogeneous N\'eel phase is obtained for arbitrary small doping. From the decimation procedure, we interpret this phase at very low doping as a {\sl Griffith antiferromagnet}. Namely, it does not have a true long range order down to zero temperature. Nonetheless, large magnetically ordered clusters appear already at relatively high temperatures. This demonstrates the role of disorder in the theoretical description of doping in CuGeO$_3$. A detailed comparison with other approaches is also given.