  The study of interacting spin systems is of fundamental importance for modern
condensed matter physics. On frustrated lattices, magnetic exchange
interactions cannot be simultaneously satisfied, and often give rise to
competing exotic ground states. The frustrated 2D Shastry-Sutherland lattice
realized by SrCu2(BO3)2 is an important test to our understanding of quantum
magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet
ground state within a certain range of exchange parameters and frustration.
While the exact dimer state and the antiferromagnetic order at both ends of the
phase diagram are well known, the ground state and spin correlations in the
intermediate frustration range have been widely debated. We report here the
first experimental identification of the conjectured plaquette singlet
intermediate phase in SrCu2(BO3)2. It is observed by inelastic neutron
scattering after pressure tuning at 21.5 kbar. This gapped plaquette singlet
state with strong 4-spin correlations leads to a transition to an ordered Neel
state above 40 kbar, which can realize a deconfined quantum critical point.
