Deconfined quantum critical point lost in pressurized SrCu2(BO3)2

Abstract

The deconfinement quantum critical point (DQCP), a paradigm beyond the Landau-Ginzburg-Wilson framework to classify states of matters, has been attracting extensive attention over the past two decades. Experimentally, SrCu2(BO3)2 plays key roles in verifying the DQCP between an antiferromagnetic (AF) Néel phase and a plaquette-singlet (PS) phase. However, the verification of the DQCP of the PS-AF transition lies in 2.4 - 3.1 GPa, which is unreachable previously due to technical limitations. Here, through the advanced high-pressure heat capacity measurements, we demonstrate that the PS-AF phase transition of SrCu2(BO3)2 at zero field is clearly first-order. Our result clarifies the two-decade-long debates about this key issue and resonates nicely with recent theoretical consensus that the previously predicted DQCPs in representative models are actually first-order transitions. Besides, the PS and AF phases transit at the same pressure-temperature point, a bi-critical point found in frustrated magnets.