Higher-magnesium-doping effects on the singlet ground state of the Shastry-Sutherland SrCu2(BO3)2

TL;DR

This study experimentally investigates how higher magnesium doping in SrCu2(BO3)2 affects its quantum ground state, revealing a systematic reduction in the spin gap and the emergence of new magnetic features due to impurity effects.

Contribution

It provides the first experimental verification of doping effects on the Shastry-Sutherland model, showing how Mg doping induces dimer breaking and modifies magnetic properties.

Findings

Mg doping causes lattice expansion and decreases the spin gap.

Increased Mg doping leads to more free Cu2+ spins, as shown by EPR.

Suppression of the 1/8 magnetization plateau and new anomalies appear at high fields.

Abstract

Doping of quantum antiferromagnets is an established approach to investigate the robustness of their ground state against the competing phases. Predictions of doping effects on the ground state of the Shastry-Sutherland dimer model are here verified experimentally on Mg-doped SrCu2(BO3)2. A partial incorporation of Mg2+ on the Cu2+-site in the SrCu2(BO3)2 structure leads to a subtle but systematic lattice expansion with the increasing Mg-doping concentration, which is accompanied by a concomitant decrease in the spin gap, the Curie-Weiss temperature and the peak temperature of the susceptibility. These findings indicate a doping-induced breaking of Cu2+ spin-1/2 dimers which is also corroborated by X-band EPR spectroscopy that points to a systematic increase in intensity of free Cu2+ sites with increasing Mg-doping concentration. Extending the Mg-doping up to nominal x = 0.10 or SrCu1.9Mg0.1(BO3)2, in the magnetisation measurements taken up to 35 T, a suppression of the pseudo-1/8 plateau is found along with a clear presence of an anomaly at an onset critical field H'C0 ~ 9 T. The latter, absent in pure SrCu2(BO3)2, emerges due to the coupling of liberated Cu2+ spin-1/2 entities in the vicinity of Mg-doping induced impurities.