Preparation and physical properties of single crystals of Hg-based cupric oxide superconductors

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A novel cuprate Volborthite, Cu_3V_2O_7(OH)_2・2H_2O, containing an S-1/2 (Cu^<2+> spin) kagome like lattice is studied by magnetic susceptibility, specific heat, and V NMR measurements. Signs for neither long-range order nor spin-gapped singlet ground states are detected down to 1.8 K, in spite of large antiferromagnetic couplings of-100 K between Cu spins fomrning a two-dimensional Kagome like network. It is suggested that Volborthite represents a system close to a quantum critical point between classical long-range ordered and quantum disordered phases.BaCu_2(Si_<1-x>Ge_x)_2O_7 comprising corrugated Cu-O chains has been found to be a spin-1/2 1D Heisenberg system with a linearly tunable exchange interaction from 290 K (x=0) to 580 K (x=l). The interchain interaction also shows a remarkable composition dependence varying from ferromagnetic (x=0) to antifer-romagnetic (x=l). These strong composition dependences have been found to lead to a rich magnetic phase diagram.Holes doped on edge-sharing CuO_2 chains are almost localized because of nearly orthogonal Cu-O-Cu bonds. Consequently, only spin degrees of freedom survive at low temperature, which sometimes presents unusual quantum magnetism. Quasi-one-dimensional cupric oxides Ca_<1-x>CuO_<2+d> and Sr_<14>Cu_<24>O_<41>, both containing 25-50% hole-doped edge-sharing CuO2 chains, are studied by uniform magnetic susceptibility and specific heat measurements on a series of polycrystalline samples with controlled metal and oxygen contents. Most of spins are dimerized always in Sr_<14>Cu_<24>O_<41>, while, in contrast, antiferromagnetic chains made of nearly 50% spins exist with the remainder forming dimers of variable density in Ca_<1-x>CuO_<2+d>.A two-sublattice model is proposed by considering that the nearest-neighbor couplings are negligibly small, due to both geometrical frustration and the special Cu-O-Cu bond angle of-95.