Control of Interaction Parameters and Quantum Phase Transitions under Extreme Conditions in Spin Gap Systems

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Spin gap system is a spin system that has a singlet ground state with an excitation gap. In the present project, we studied quantum phase transitions in the gapped spin system induced by external magnetic field, hydrostatic pressure and disorder due to nonmagnetic impurities or ion substitution. First, we performed magnetization measurements on TlCuCl_3 down to 77 mK and in magnetic fields up to 7 T. The critical density of magnons as a function of temperature and the magnetic phase diagram were obtained. The magnon-magnon interaction constant was estimated. The phase boundary is expressed by the power law and agrees almost perfectly with the magnon BEC theory based on the Hartree-Fock approximation with realistic dispersion relations and interaction constant obtained. The critical exponent converges to 3/2 that is derived from the magnon BEC theory with decreasing fitting temperature range. These results strongly support the BEC description of the field-induced magnetic ordering in Tl … More CuCl_3. We performed magnetization measurements and neutron scattering experiments on KCuCl_3 under a hydrostatic pressure. A pressure-induced quantum phase transition from a gapped state to an antiferromagnetic state occurs at the critical pressure P_c=8.2 kbar. Above P_c magnetic Bragg reflections were observed at reciprocal lattice points that are equivalent to those for the lowest magnetic excitation at zero pressure. We also performed neutron inelastic scattering under pressure using newly designed pressure cell, and succeeded in observing the softening of triplet excitations. These results confirm that the pressure-induced magnetic ordering observed arises from the closing of the spin gap. We performed magnetic susceptibility, specific heat and ESR measurements on the S=1/2 one-dimensional Heisenberg antiferromagnet KCuGaF6 with the large exchange interaction. Due to the DM interaction with alternating D-vectors and/or the staggered g-tensor, the staggered magnetic field is induced when external magnetic field is applied. The effective magnetic model in magnetic field can be expressed by the quantum sine-Gordon (SG) model. It was found that most experimental results in KCuGaF_6 were beautifully described by the quantum SG field theory. We have found a new S=1/2 Kagome antiferromagnet Rb_2Cu_3SnF_<12>, which has a singlet ground state with the spin gap, as predicted from a recent theory. Less