Yu, Salamon, and Lu reply

The thermal conductivity anomaly in YBa2Cu307-„has been attributed [I] entirely to the phonon contribution tcL, due to reduced electron-phonon scattering below T,. As demonstrated in the Comment of Cohn et al. [2], with five or more parameters available the data can be fit equally well with or without a charge-carrier contribution x,. The goal of our analysis [3] was to bring the thermal conductivity data into agreement with the emerging picture of quasiparticle transport below T,. We made the simplifying assumption that quasiparticle and phonon systems are largely decoupled, and chose a particularly simple form for the underlying tcL. No assumption was made concerning the Wiedemann-Franz ratio beyond asserting that tc, should be temperature independent above T,

Thermal conductivity of an untwinned YBa2Cu3O7- single crystal and a new interpretation of the superconducting state thermal transport

We have measured the anisotropic thermal conductivity of an untwinned single crystal of YBa2Cu3O7- in the most conducting a-b plane from 14 to 200 K. Unlike previous analyses, ours attributes the observed rapid rise in thermal conductivity in the superconducting state to the electronic contribution of the Cu-O plane. We propose that strong suppression of the quasiparticle scattering rate with decreasing temperature is responsible for the large enhancement of the superconducting-state thermal conductivity

Magnetotransport properties of magnetic granular solids: The role of unfilled d bands

We calculate the magnetoresistance and magnetothermopower of magnetic granular solids. Contrary to previous theories of the giant magnetoresistance (GMR), we demonstrate that the unfilled d bands of magnetic grains play an essential role in the transport properties of these systems. Our results relate GMR and magnetothermopower to microscopic and geometric quantities, and provide a natural explanation for many experimentally observed features, such as the (M/Ms)2 dependence of the GMR, the giant magnetothermopower and its 1/ρ scaling behavior, and the absence of negative GMR in rare-earth-nonmagnetic structures

Anisotropic magnetic behavior in Dy/Y films and superlattices

By neutron diffraction we show that superlattices of Dy and Y grown by molecular-beam epitaxy along the hcp b axis exhibit little magnetic coupling between successive Dy layers, even for Y spacers as thin as 9 atomic planes (26). Previous studies of Dy/Y superlattices grown along the hcp c axis established that long-range three-dimensional helimagnetic ordering takes place even through Y spacer layers as thick as 120. This highly anisotropic coupling behavior is shown to have its origin in nearly-two-dimensional nesting features of the Y and Dy Fermi surfaces. Nesting along the c axis gives rise to sharp peaks along c in the wave-vector-dependent magnetic susceptibility, and causes the exchange coupling to exhibit long-range oscillations in real space. The lack of nesting features along the b axis leaves a rapid exponential decay of the exchange interaction with spin separation. From magnetic measurements by superconducting-quantum-interference-device magnetometry on b-axis superlattices and films, we deduce that the first-order ferromagnetic transition of Dy is suppressed, and that the critical field required to produce the ferromagnetic alignment is much higher than the c-axis counterpart. This difference arises from anisotropy of the energy balance of the system. The magnetic coherence in b-axis superlattices and films is anisotropic and exhibits an unusual temperature dependence

Conductivity Due to Classical Phase Fluctuations in a Model For High-T_c Superconductors

We consider the real part of the conductivity, \sigma_1(\omega), arising from classical phase fluctuations in a model for high-T_c superconductors. We show that the frequency integral of that conductivity, \int_0^\infty \sigma_1 d\omega, is non-zero below the superconducting transition temperature $T_c$, provided there is some quenched disorder in the system. Furthermore, for a fixed amount of quenched disorder, this integral at low temperatures is proportional to the zero-temperature superfluid density, in agreement with experiment. We calculate \sigma_1(\omega) explicitly for a model of overdamped phase fluctuations.Comment: 4pages, 2figures, submitted to Phys.Rev.

Critical scaling of the a.c. conductivity for a superconductor above Tc

We consider the effects of critical superconducting fluctuations on the scaling of the linear a.c. conductivity, \sigma(\omega), of a bulk superconductor slightly above Tc in zero applied magnetic field. The dynamic renormalization- group method is applied to the relaxational time-dependent Ginzburg-Landau model of superconductivity, with \sigma(\omega) calculated via the Kubo formula to O(\epsilon^{2}) in the \epsilon = 4 - d expansion. The critical dynamics are governed by the relaxational XY-model renormalization-group fixed point. The scaling hypothesis \sigma(\omega) \sim \xi^{2-d+z} S(\omega \xi^{z}) proposed by Fisher, Fisher and Huse is explicitly verified, with the dynamic exponent z \approx 2.015, the value expected for the d=3 relaxational XY-model. The universal scaling function S(y) is computed and shown to deviate only slightly from its Gaussian form, calculated earlier. The present theory is compared with experimental measurements of the a.c. conductivity of YBCO near Tc, and the implications of this theory for such experiments is discussed.Comment: 16 pages, submitted to Phys. Rev.

Probing the superconducting gap symmetry of PrRu4_{4}Sb12_{12}: A comparison with PrOs4_{4}Sb12_{12}

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of PrRu$_{4}$Sb$_{12}$ down to 0.1 K. Both $\lambda$ and superfluid density $\rho_{s}$ exhibit an exponential behavior for $T$ $<$ 0.5$T_{c}$, with parameters $\Delta$(0)/\textit{k}$_{B}$\textit{T}$_{c}$ = 1.9 and $\lambda(0)$ = 2900 \AA. The value of $\Delta$(0) is consistent with the specific-heat jump value of $\Delta C/\gamma T_{c}$ = 1.87 measured elsewhere, while the value of $\lambda(0)$ is consistent with the measured value of the electronic heat-capacity coefficient $\gamma$. Our data are consistent with PrRu$_{4}$Sb$_{12}$ being a moderate-coupling, fully-gapped superconductor. We suggest experiments to study how the nature of the superconducting state evolves with increasing Ru substitution for Os

Ground state properties of ferromagnetic metal/conjugated polymer interfaces

We theoretically investigate the ground state properties of ferromagnetic metal/conjugated polymer interfaces. The work is partially motivated by recent experiments in which injection of spin polarized electrons from ferromagnetic contacts into thin films of conjugated polymers was reported. We use a one-dimensional nondegenerate Su-Schrieffer-Heeger (SSH) Hamiltonian to describe the conjugated polymer and one-dimensional tight-binding models to describe the ferromagnetic metal. We consider both a model for a conventional ferromagnetic metal, in which there are no explicit structural degrees of freedom, and a model for a half-metallic ferromagnetic colossal magnetoresistance (CMR) oxide which has explicit structural degrees of freedom. The Fermi energy of the magnetic metallic contact is adjusted to control the degree of electron transfer into the polymer. We investigate electron charge and spin transfer from the ferromagnetic metal to the organic polymer, and structural relaxation near the interface. Bipolarons are the lowest energy charge state in the bulk polymer for the nondegenerate SSH model Hamiltonian. As a result electrons (or holes) transferred into the bulk of the polymer form spinless bipolarons. However, there can be spin density in the polymer localized near the interface.Comment: 7 figure

Thermal Conductivity Tensor in YBa2_2Cu3_3O7−x_{7-x}: Effects of a Planar Magnetic Field

We have measured the thermal conductivity tensor of a twinned YBa$_2$Cu$_3$O$_{7-x}$ single crystal as a function of angle $\theta$ between the magnetic field applied parallel to the CuO$_2$ planes and the heat current direction, at different magnetic fields and at T=13.8 K. Clear fourfold and twofold variations in the field-angle dependence of $\kappa_{xx}$ and $\kappa_{xy}$ were respectively recorded in accordance with the d-wave pairing symmetry of the order parameter. The oscillation amplitude of the transverse thermal conductivity $\kappa^0_{xy}$ was found to be larger than the longitudinal one $\kappa^0_{xx}$ in the range of magnetic field studied here ($0 T$$\le B \le 9$$T$). From our data we obtain quantities that are free from non-electronic contributions and they allow us a comparison of the experimental results with current models for the quasiparticle transport in the mixed state.Comment: 9 Figures, Phys. Rev. B(in press

Critical-point scaling function for the specific heat of a Ginzburg-Landau superconductor

If the zero-field transition in high temperature superconductors such as YBa_2Cu_3O_7-\delta is a critical point in the universality class of the 3-dimensional XY model, then the general theory of critical phenomena predicts the existence of a critical region in which thermodynamic functions have a characteristic scaling form. We report the first attempt to calculate the universal scaling function associated with the specific heat, for which experimental data have become available in recent years. Scaling behaviour is extracted from a renormalization-group analysis, and the 1/N expansion is adopted as a means of approximation. The estimated scaling function is qualitatively similar to that observed experimentally, and also to the lowest-Landau-level scaling function used by some authors to provide an alternative interpretation of the same data. Unfortunately, the 1/N expansion is not sufficiently reliable at small values of N for a quantitative fit to be feasible.Comment: 20 pages; 4 figure