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

通过对Tl_<1-x>K_xCuCl_3的比热测量数据和ESR谱线形状的分析，我们发现场致量子相变是三plons的玻色玻璃（BG）到BEC的相变，并且该相变场的温度依赖性的临界行为是BG-BEC相变的小指数特征。利用高频高场ESR研究了量子正弦-戈登（SG）自旋体系KCuGaF_6的初等激发。我们观察到许多共振模式，包括孤子共振和三阶呼吸子，这是量子SG模型的主要基本激发特征。我们发现它们的共振条件用量子SG场论很好地描述了。我们制备了自旋为1/2 kagome的反铁磁体Rb_2Cu_3SnF_<12>和Cs_2Cu_3SnF_<12>。我们发现Rb_2Cu_3SnF_<12>的基态是一个具有激发隙的自旋单重态。通过非弹性中子散射实验，我们观察到在基态中实现了一个风车形的价键-固体态。

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.