Microscopic mechanism for the 1/8 magnetization plateau in SrCu_2(BO_3)_2

TL;DR

This paper proposes a simple microscopic mechanism involving quantum fluctuations and Dzyaloshinskii-Moriya interactions that explains the stable 1/8 magnetization plateau in SrCu_2(BO_3)_2, aligning well with experimental observations.

Contribution

It introduces a microscopic explanation for the 1/8 plateau in SrCu_2(BO_3)_2, emphasizing the role of quantum fluctuations and Dzyaloshinskii-Moriya interactions, which was not previously understood.

Findings

The 1/8 plateau with a diamond unit cell is stabilized by quantum fluctuations.

A stable magnetization plateau at 1/6 is predicted.

Stripe structures above 1/8 are suggested by the theory.

Abstract

The frustrated quantum magnet SrCu_2(BO_3)_2 shows a remarkably rich phase diagram in an external magnetic field including a sequence of magnetization plateaux. The by far experimentally most studied and most prominent magnetization plateau is the 1/8 plateau. Theoretically, one expects that this material is well described by the Shastry-Sutherland model. But recent microscopic calculations indicate that the 1/8 plateau is energetically not favored. Here we report on a very simple microscopic mechanism which naturally leads to a 1/8 plateau for realistic values of the magnetic exchange constants. We show that the 1/8 plateau with a diamond unit cell benefits most compared to other plateau structures from quantum fluctuations which to a large part are induced by Dzyaloshinskii-Moriya interactions. Physically, such couplings result in kinetic terms in an effective hardcore boson description leading to a renormalization of the energy of the different plateaux structures which we treat in this work on the mean-field level. The stability of the resulting plateaux are discussed. Furthermore, our results indicate a series of stripe structures above 1/8 and a stable magnetization plateau at 1/6. Most qualitative aspects of our microscopic theory agree well with a recently formulated phenomenological theory for the experimental data of SrCu_2(BO_3)_2. Interestingly, our calculations point to a rather large ratio of the magnetic couplings in the Shastry-Sutherland model such that non-perturbative effects become essential for the understanding of the frustrated quantum magnet SrCu_2(BO_3)_2.