In the field of correlated electron materials, the relation between the
resonating spin singlet and antiferromagnetic states has long been an
attractive topic for understanding of the interesting macroscopic quantum
phenomena, such as the ones emerging from magnetic frustrated materials,
antiferromagnets and high-temperature superconductors. SrCu2(BO3)2 is a
well-known quantum magnet, and it is theoretically expected to be the candidate
of correlated electron material for clarifying the existence of a
pressure-induced deconfined quantum critical point (DQCP), featured by a
continuous quantum phase transition, between the plaquette-singlet (PS) valence
bond solid phase and the antiferromagnetic (AF) phase. However, the real nature
of the transition is yet to be identified experimentally due to the technical
challenge. Here we show the experimental results for the first time, through
the state-of-the-art high-pressure heat capacity measurement, that the PS-AF
phase transition of the pressurized SrCu2(BO3)2 at zero field is clearly a
first-order one. Our result clarifies the more than two-decade long debates
about this key issue, and resonates nicely with the recent quantum entanglement
understanding that the theoretically predicted DQCPs in representative lattice
models are actually a first-order transition. Intriguingly, we also find that
the transition temperatures of the PS and AF phase meet at the same
pressure-temperature point, which signifies a bi-critical point as those
observed in Fe-based superconductor and heavy-fermion compound, and constitutes
the first experimental discovery of the pressure-induced bi-critical point in
frustrated magnets. Our results provide fresh information for understanding the
evolution among different spin states of correlated electron materials under
pressure.Comment: 6 pages, 4 figure