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Register a new userQuantum Creep in Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7}$ Crystals: Magnetic Relaxation and Transport
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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.
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We investigated the charge distribution in Pr_{1+x}Ba_{2-x}Cu_3O_{6+y}-crystals at liquid-He temperature by means of ^{63,65}Cu- and ^{141}Pr-NMR and NQR. The electric field gradients determined for the different oxygen coordinations of Cu(1) on the chains confirm the model that the hole states are localized in the O 2p - pi-orbitals of the CuO_(2)-planes. The intensity and line-shape analysis of the Cu(1)- and Pr-resonances in the oxygen doping series (x approx 0, y=0..1) and in the Pr/Ba solid-solution series (at y=1) allows us to assign the Pr-resonance to Pr at RE-sites. Therefore, we can investigate the charge distribution in the CuO_(2)-Pr-CuO_(2)-layers by crystal field analysis of the Pr-signal. To consistently describe our results with neutron scattering data we propose that two Pr-states are present in the RE-layer, and ascribe them to Pr with and one without a hole localized in the oxygen coordination shell. NMR detects only the former, with a virtually nonmagnetic singlet ground state, while space-integral techniques are dominated by the latter, due to a quasi-doublet ground state and antiferromagnetism of its Pr-moments at low temperature.
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.
The scope of this article is to report very detailed results of the measurements of magnetic relaxation phenomena in the new Cu$_{0.5}$Fe$_{2.5}$O$_{4}$ nanoparticles and known CuFe$_{2}$O$_{4}$ nanoparticles. The size of synthesized particles is (6.5$pm $1.5)nm. Both samples show the superparamagnetic behaviour, with the well-defined phenomena of blocking of magnetic moment. This includes the splitting of zero-field-cooled and field-cooled magnetic moment curves, dynamical hysteresis, slow quasi-logarithmic relaxation of magnetic moment below blocking temperature. The scaling of the magnetic moment relaxation data at different temperatures confirms the applicability of the simple thermal relaxation model. The two copper-ferrites with similar structures show significantly different magnetic anisotropy density and other magnetic properties. Investigated systems exhibit the consistency of all obtained results.
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.
In this paper, the $Fe$-containing superconductors $Fe_{0.5}Cu_{0.5}Ba_2YCu_2O_{7+delta}$, $Fe_{0.5}Cu_{0.5}BaSrYCu_2O_{7+delta}$ and $Fe_{0.5}Cu_{0.5}Sr_2YCu_2O_{7+delta}$ were successfully prepared by common solid-state reaction followed with a procedure of high pressure synthesis. The structural change and superconducting properties in $(Fe_xCu_{1-x})BaSrYCu_2O_{7+delta}$ ($x$ = 0 $sim$ 1.0) systems were also investigated. Annealing experiments indicate that the occurrence of superconductivity in $Fe_{0.5}Cu_{0.5}(Ba_{1-x}Sr_{x})_2YCu_2O_{7+delta}$ ($x$ = 0, 0.5 and 1) systems is mainly induced by the procedure of high pressure synthesis, which causes the increase of oxygen content and the redistribution of $Fe$ atoms between $Cu(1)$ and $Cu(2)$ sites, but not from possible secondary phase of $YBa_2Cu_3O_{7-delta}$, $YBaSrCu_3O_{7-delta}$ or $YSr_2Cu_3O_{7-delta}$ superconductors.
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