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

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.