Observation of a 4-spin Plaquette Singlet State in the Shastry-Sutherland compound SrCu2(BO3)2

TL;DR

This paper reports the first experimental observation of a 4-spin plaquette singlet state in the frustrated quantum magnet SrCu2(BO3)2, revealing a new intermediate phase and a pressure-induced quantum phase transition.

Contribution

It provides experimental evidence for the conjectured plaquette singlet phase in a Shastry-Sutherland lattice, advancing understanding of quantum magnetism in frustrated systems.

Findings

Identification of a gapped plaquette singlet state at 21.5 kbar pressure

Observation of a transition to a Neel ordered state above 40 kbar

Evidence for a deconfined quantum critical point in the phase diagram

Abstract

The study of interacting spin systems is of fundamental importance for modern condensed matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states. The frustrated 2D Shastry-Sutherland lattice realized by SrCu2(BO3)2 is an important test to our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated. We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu2(BO3)2. It is observed by inelastic neutron scattering after pressure tuning at 21.5 kbar. This gapped plaquette singlet state with strong 4-spin correlations leads to a transition to an ordered Neel state above 40 kbar, which can realize a deconfined quantum critical point.