Magnetic Superstructure in the Two-Dimensional Quantum Antiferromagnet SrCu2(BO3)2

TL;DR

This paper reports the discovery of a magnetic superstructure in SrCu2(BO3)2 during a magnetization plateau state, revealing a phase transition and triplet crystallization in a frustrated quantum spin system.

Contribution

It presents the first observation of a magnetic superstructure in SrCu2(BO3)2 and links experimental NMR data with theoretical models of triplet crystallization.

Findings

Discontinuous phase transition near 27 Tesla.

Observation of a 1/8 magnetization plateau.

Crystallization of triplets in a rhomboid unit cell.

Abstract

We report the observation of magnetic superstructure in a magnetization plateau state of SrCu2(BO3)2, a frustrated quasi-two-dimensional quantum spin system. The Cu and B nuclear magnetic resonance (NMR) spectra at 35 mllikelvin indicate an apparently discontinuous phase transition from uniform magnetization to a modulated superstructure near 27 tesla, above which a magnetization plateau at 1/8 of the full saturation has been observed. Comparison of the Cu NMR spectrum and the theoretical analysis of a Heisenberg spin model demonstrates the crystallization of itinerant triplets in the plateau phase within a large rhomboid unit cell (16 spins per layer) showing oscillations of the spin polarization. Thus we are now in possession of an interesting model system to study a localization transition of strongly interacting quantum particles.