Quantum phases of SrCu2(BO3)2 from high-pressure thermodynamics.

In a quantum magnet, localized electronic spins can form unusual collective states at low temperature. While theoretical proposals for exotic states abound, many of the most intriguing scenarios of quantum phases and phase transitions beyond classical descriptions have been difficult to realize experimentally. In one class of hypothetical states, the spins entangle locally into dimer- or quadrumer-singlets, which order in patterns breaking some of the symmetries of the crystal lattice. Experimental signatures of such a state with four-spin singlets were only recently detected in an inelastic neutron scattering study of the quasi-two-dimensional quantum magnet SrCu2(BO3)2 under high pressure. The state remained incompletely characterized, however, and its existence has been questioned. Here we report heat capacity C(T) measurements along with simulations of relevant quantum spin models and map out the (P,T) phase diagram of the material. At pressures P between 1.7 and 2.4 GPa, the temperature dependence of C/T exhibits features - a hump followed by a smaller peak at lower T - characteristic of a paramagnet with strong quantum fluctuations undergoing a phase transition below T = 2 K into a plaquette-singlet state. We also observe a different transition at T ~ 2 - 3.5 K into what appears to be a previously missed antiferromagnetic state at P ~ 3 - 4 GPa. The possibility to tune SrCu2(BO3)2 between the plaquette-singlet and antiferromagnetic states at moderately high pressures opens opportunities for experimental tests of quantum field theories and lattice models involving fractionalized excitations, emergent symmetries, and gauge fluctuations.