Reentrant Spin-Peierls Transition in Mg-Doped CuGeO_3

TL;DR

This study reveals a reentrant spin-Peierls transition in Mg-doped CuGeO_3, showing complex interplay between long-range order, impurities, and competing magnetic phases near a critical doping level.

Contribution

It uncovers the reentrant nature of the spin-Peierls transition in Mg-doped CuGeO_3 and highlights the role of impurity-induced interactions near the critical concentration.

Findings

Reentrant transition observed near critical doping level

Broadening of superlattice peaks indicates loss of long-range order

Hysteresis and relaxation times suggest complex phase competition

Abstract

We report a synchrotron x-ray scattering study of the diluted spin-Peierls (SP) material Cu_{1-x}Mg_xGeO_3. In a recent paper we have shown that the SP dimerization attains long-range order only for x < x_c = 0.022(0.001). Here we report that the SP transition is reentrant in the vicinity of the critical concentration x_c. This is manifested by broadening of the SP dimerization superlattice peaks below the reentrance temperature, T_r, which may mean either the complete loss of the long-range SP order or the development of a short-range ordered component within the long-range ordered SP state. Marked hysteresis and very large relaxation times are found in the samples with Mg concentrations in the vicinity of x_c. The reentrant transition is likely related to the competing Neel transition which occurs at a temperature similar to T_r. We argue that impurity-induced competing interchain interactions play an essential role in these phenomena.