Proximate Deconfined Quantum Critical Point in a Shastry-Sutherland Compound SrCu₂(BO₃)₂

The deconfined quantum critical point (DQCP) describes a continuous quantum phase transition (QPT) beyond Landau paradigm, which takes place between two spontaneous symmetry-breaking states. The DQCP hosts novel phenomena such as emergent symmetries and fractionized excitations. Experimentally, however, DQCP has not been realized. Recently, a frustrated, quasi-2D compound SrCu₂(BO₃)₂ is found to be described proximately by the Shastry-Sutherland model¹. When tuned by pressure and field, its ground states transition from dimer singlet (DS) to plaquette singlet (PS) and long-range-ordered antiferromagnetism (AFM) ^2,3,4 , and therefore SrCu₂(BO₃)₂ may be a candidate material to search for a DQCP³.

Here I report our high-pressure and ultra-low temperature NMR studies on SrCu₂(BO₃)₂⁵. We first established microscopic evidences for a pressure-induced DS-PS phase transition at pressures above 1.8 GPa and zero field. At 2.1 and 2.4 GPa, a field-induced weakly first-order PS-AFM QPT is firmly identified with several key observations: the coexistence temperature for two phases is as low as 0.07 K at the transition; the (H, T ) phase boundaries of both PS and AFM phases follow the power-law scaling with a single power-law exponent at each pressure; with increasing pressure, the QPT goes toward a continuous type with further suppressed AFM order parameters at the transition; the spin dynamics at 2.4 GPa revealed by the spin-lattice relaxation rates exhibits a quantum critical scaling. These facts can be understood by an approximate DQCP, with a crossover from an emergent O(3) symmetry to an O(4) type with increasing pressure, and offer a concrete platform for studying the long-sought-for DQCP in a real material.

References:

1. H. Kageyama, et al., Phys. Rev. Lett. 82, 3168 (1999).

2. M. E. Zayed, et al., Nat. Phys. 13, 962 (2017).

3. J. Guo, et al., Phys. Rev. Lett. 124, 206602 (2020).

4. J. Larrea Jimenez, et al., Nature 592, 370 (2021).

5. Y. Cui et al., arXiv: 2204.08133.