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Register a new userMagnetotransport properties of untwinned YBa_{2}Cu_{3}O_{y} single crystals: Novel 60-K-phase anomalies in the charge transport
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We present the result of our accurate measurements of the a- and b-axis resistivity (rho_a and rho_b), magnetoconductivity (Deltasigma / sigma), Hall coefficient R_H, and the a-axis thermopower S_a in untwinned YBa_{2}Cu_{3}O_{y} single crystals in a wide range of doping (6.45 < y < 7.0). The systematics of our data reveals a number of novel 60-K-phase anomalies in the charge transport: (i) Temperature dependences of rho_a show anomalous overlap below ~130 K for 6.65 < y < 6.80, (ii) Hall mobility mu_H shows an enhancement near y ~ 6.65, which is reflected in an anomalous y dependence of sigma_{xy}, (iii) With decreasing temperature R_H shows a marked drop upon approaching T_c only in samples with 6.70 < y < 6.85, (iv) Superconducting fluctuation magnetoconductivity is anomalously enhanced near y ~ 6.7, and (v) H_{c2} is anomalously reduced near y ~ 6.70. We discuss that the fluctuating charge stripes might be responsible for these anomalies in the charge transport.
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Magnetoresistance (MR) in the a-axis resistivity of untwinned YBa_{2}Cu_{3}O_{y} single crystals is measured for a wide range of doping (y = 6.45 - 7.0). The y-dependence of the in-plane coherence length xi_{ab} estimated from the fluctuation magnetoconductance indicates that the superconductivity is anomalously weakened in the 60-K phase; this gives evidence, together with the Hall coefficient and the a-axis thermopower data that suggest the hole doping to be 12% for y = 6.65, that the origin of the 60-K plateau is the 1/8 anomaly. At high temperatures, the normal-state MR data show signatures of the Zeeman effect on the pseudogap in underdoped samples.
We use a mapping of the multiband Hubbard model for $CuO_{3}$ chains in $RBa_{2}Cu_{3}0_{6+x}$ (R=Y or a rare earth) onto a $t-J$ model and the description of the charge dynamics of the latter in terms pf s spinless model, to study the electronic structure of the chains. We briefly review results for the optical conductivity and we calculate the quantum phase diagram of quarter filled chains including Coulomb repulsion up to that between next-nearest-neighbor $Cu$ atoms $V_{2}$, using the resulting effective Hamiltonian, mapped onto an XXZ chain, and the method of crossing of excitation spectra. The method gives accurate results for the boundaries of the metallic phase in this case. The inclusion of $V_{2}$ greatly enhances the region of metallic behavior of the chains.
hether the node in the order parameter characteristic of a $d-wave$ superconductor can or cannot be removed by an applied magnetic field has been a subject of debate in recent years. Thermal conductivity results on the high Tc superconductor $Bi_{2}Sr_{2}CaCu_{2}O_{8}$ originally explained by Laughlin in terms of such a node removal were complicated by hysteresis effects, and judged inconclusive. We present new tunneling data on $YBa_{2}Cu_{3}O_{7-x}$ that support the existence of the node removal effect, under specific orientations of the samples surfaces and magnetic field. We also explain the hysteretic behavior and other previous tunneling results so far not understood satisfactorily, attributing them to a combination of node removal and Doppler shift of low energy surface bound states.
We report transport and magnetic relaxation measurements in the mixed state of strongly underdoped Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7} crystals. A transition from thermally activated flux creep to temperature independent quantum flux creep is observed in both transport and magnetic relaxation at temperatures T * 5 K. Flux transformer measurements indicate that the crossover to quantum creep is preceded by a coupling transition. Based on these observations we argue that below the coupling transition the current is confined within a very narrow layer beneath the current contacts.
The effects of pressure generated in a liquid medium, clamp, pressure cell on the in-plane and c-axis resistance, temperature-dependent Hall coefficient and low temperature, magnetoresistance in CaFe2As2 are presented. The T - P phase diagram, including the observation of a complete superconducting transition in resistivity, delineated in earlier studies is found to be highly reproducible. The Hall resistivity and low temperature magnetoresistance are sensitive to different states/phases observed in CaFe2As2. Auxiliary measurements under uniaxial, c-axis, pressure are in general agreement with the liquid medium clamp cell results with some difference in critical pressure values and pressure derivatives. The data may be viewed as supporting the potential importance of non-hydrostatic components of pressure in inducing superconductivity in CaFe2As2.
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