Study of New Ground and Excited States in Quantum Spin Systems under Extreme Conditions

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From the analysis of the phase transition data obtained from specific heat measurement and the lineshape of the ESR spectrum for Tl_<1-x>K_xCuCl_3, we found that the field-induced quantum phase transition is the Bose glass (BG) to BEC transition of triplons and that the critical behavior for the temperature dependence of the transition field is described by the small-exponent characteristic of the BG-BEC transition. Using high-frequency high field ESR, we investigated elementary excitations of a quantum sine-Gordon (SG) spin system KCuGaF_6. We observed many resonance modes including a soliton resonance and breathers up to the third order, which are main elementary excitations characteristic of the quantum SG model. We found that their resonance conditions are beautifully described by the quantum SG field theory. We developed spin 1/2 kagome antiferromagnets Rb_2Cu_3SnF_<12> and Cs_2Cu_3SnF_<12>. We found that the ground state of Rb_2Cu_3SnF_<12> is a spin singlet with an excitation gap. Using inelastic neutron scattering experiments, we observed that a pinwheel valence-bond-solid state is realized in the ground state.