J. Takeya

Two-stage spin-flop transitions in the S = 1/2 antiferromagnetic spin chain BaCu(2)Si(2)O(7).

Two-stage spin-flop transitions are observed in the quasi-one-dimensional antiferromagnet BaCu(2)Si(2)O(7). A magnetic field applied along the easy axis induces a spin-flop transition at 2.0 T followed by a second transition at 4.9 T. The magnetic susceptibility indicates the presence of Dzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain neighboring spins. We discuss a possible mechanism whereby the geometrical competition between DM and interchain interactions, as discussed for the two-dimensional antiferromagnet La(2)CuO(4), causes the two-stage spin-flop transitions.

Magnetic-field-induced localization of quasiparticles in underdoped La2-xSrxCuO4 single crystals

Magnetic-field-induced ordering of electrons around vortices is a striking phenomenon recently found in high-T c . cuprates. To identify its consequence in the quasiparticle dynamics, the magnetic-field (H) dependence of the low-temperature thermal conductivity K of La 2 - x Sr x CuO 4 crystals is studied for a wide doping range. It is found that the behavior of K(H) in the subkelvin region changes drastically across optimum doping, and the data for underdoped samples are indicative of unusual magnetic-field-induced localization of quasiparticles; this localization phenomenon is probably responsible for the unusual insulating normal state under high magnetic fields.

Weak ferromagnetism of quasi-one-dimensional S = 1 2 antiferromagnet BaCu 2 Ge 2 O 7

Weak ferromagnetism of quasi-one-dimensional

Temperature- and magnetic-field-dependent thermal conductivity of pure and Zn-dopedBi2Sr2CaCu2O8+δsingle crystals

S=frac{1}{2}

Zn-doping effect on the magnetotransport properties ofBi2Sr2−xLaxCuO6+δsingle crystals

antiferromagnet

Quantum Phase Transitions in the Cuprate Superconductor Bi~2Sr~2~-~xLa~xCuO~6~+~d~e~l~t~a

{mathrm{BaCu}}_{2}{mathrm{Ge}}_{2}{mathrm{O}}_{7}

Novel anisotropy in the superconducting gap structure of Bi2Sr2CaCu2O(8+delta) probed by quasiparticle heat transport.

is studied by the magnetization measurement. The spontaneous magnetization appears along the b axis below 8.8 K. The local symmetry between the intrachain nearest-neighbor spins allows the presence of the Dzyaloshinskii-Moriya interaction, and the only possible spatial configuration of the weak ferromagnetic moment per spin definitely determines the sign of the interchain interaction. A weak a-axis magnetic field can change the direction of the magnetization to the a-axis direction, which shows that the spin chain forms a weakly coupled weak-ferromagnetic chain system.

Thermal conductivity of Mg-doped CuGeO 3

The thermal conductivity kappa of Bi-2212 is measured in pure and Zn-doped crystals as a function of temperature and magnetic field. The in-plane resistivity is also measured on the identical samples. Using these data, we make a crude estimate of the impurity-scattering rate Gamma of the pure and the Zn-doped crystals. Our measurement show that the plateau in the kappa(H) profile is not observed in the majority of our Bi-2212 crystals, including one of the cleanest crystals available to date. The estimated values of Gamma for the pure and Zn-doped samples allow us to compare the kappa(H) data with the existing theories of the quasiparticle heat transport in d-wave superconductors under magnetic field. Our analysis indicates that a proper inclusion of the quasiparticle-vortex scattering, which is expected to play the key role in the peculiar behavior of the kappa(H), is important for a quantitative understanding of the QP heat transport in the presence of the vortices.

Thermal conductivity of lightly Sr- and Zn-dopedLa2CuO4single crystals

We report the magnetotransport properties of Bi_{2}Sr_{2-x}La_{x}Cu_{1-z}Zn_{z}O_{6+delta} (Zn-doped BSLCO) single crystals with z of up to 2.2%. Besides the typical Zn-doping effects on the in-plane resistivity and the Hall angle, we demonstrate that the nature of the low-temperature normal state in the Zn-doped samples is significantly altered from that in the pristine samples under high magnetic fields. In particular, we observe nearly-isotropic negative magnetoresistance as well as an increase in the Hall coefficient at very low temperatures in non-superconducting Zn-doped samples, which we propose to be caused by the Kondo scattering from the local moments induced by Zn impurities.

Thermal conductivity of Mg-doped CuGeO 3 at very low temperatures: Heat conduction by antiferromagnetic magnons

To elucidate a quantum phase transition (QPT) in Bi(2)Sr(2-x)La(x)CuO(6+delta), we measure charge and heat transport properties at very low temperatures and examine the following characteristics for a wide range of doping: normal-state resistivity anisotropy under 58 T, temperature dependence of the in-plane thermal conductivity kappa(ab), and the magnetic-field dependence of kappa(ab). It turns out that all of them show signatures of a QPT at the 1/8 hole doping. Together with the recent normal-state Hall measurements under 58 T that signified the existence of a QPT at optimum doping, the present results indicate that there are two QPTs in the superconducting doping regime of this material.