Landau theory predicts that there is no continuous quantum phase transition between two symmetry breaking states. However, in recent years, many-body computation based on some specifically designed models seems to support the existence of second-order phase transitions through deconfined quantum critical points (DQCP). The DQCP, if exists, should also be accompanied with enhanced symmetries and fractional excitations . Experimentally, however, such DQCP has not been found yet.
Here I report our experimental investigation of DQCP on a spin-frustrated Shastry-Sutherland material SrCu2(BO3)2 , through high-pressure, high-field, and ultra-low temperature NMR studies. In support of a phase transition from a dimerized state (DS) to a plaquette singlet (PS) state [3,4,5], we established spectroscopic evidence of a full-plaquette (FP) singlet state under pressure. At pressures of 2.1 GPa and 2.4 GPa, a field-induced, weak first-order quantum phase transition emerges from the PS to the antiferromagnetic (AFM) state, with the coexistence temperature of two phases as low as 0.07 K and decreasing with pressure. A duality in transition temperature of both phases by the same power-law scaling with field, and a quantum critical scaling behavior in low-energy spin dynamics are also established. Further numerical simulations also support an enhanced O(3) symmetry at the quantum phase transition. These results  reveal the first experimental existence of a proximate DQCP, which provides a concrete platform for further investigation on DQCP in the material under pressure.
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