Solving the Phase Diagram of the Model Quantum Magnet SrCu2(BO3)2
Low dimensional quantum magnets provide a framework for exotic phase behavior in new materials, with high temperature superconductivity being the most appreciated example. SrCu2(BO3)2 (SCBO), is a rare example of a quasi two-dimensional quantum magnet for which an exact theoretical solution exists. It serves as an experimental realization of the Shastry-Sutherland model for interacting S=1/2 dimers. The ratio of the intra and inter-dimer exchange interactions in this compound is close to a quantum critical point, where the ground state is predicted to transform from a gapped, non-magnetic singlet state to a gapless long-range ordered antiferromagnetic state as a function of the ratio of the strength of the magnetic interactions.
We conducted high resolution neutron scattering measurements on SCBO in its singlet ground state which identified the most prominent features of the spin excitation spectrum, including the presence of one and two triplet excitations and weak dispersion characteristic of sub-leading terms in the spin Hamiltonian. Additionally, we investigated the pressure-driven quantum phase transition in SCBO using synchrotron X-ray diffraction and neutron scattering. In these studies we were able to investigate the evolution of both the magnetic and structural properties of SCBO up to pressures of 6 Gpa, following the development and evolution of long-range magnetic order. Moreover, the resemblance between the spin gap behavior in the Mott insulator SCBO and that associated with high temperature superconductors motivated us to explore the significance of doping on the phase diagram.