J. L. Cohn

Thermal conductivity in the ab plane of untwinned YBa 2 Cu 3 O 7 − δ

We present measurements of thermal conductivity ({kappa}) in the {ital ab} plane of untwinned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} ({delta}{approx}0.10) for {ital T}{le}200 K. {kappa} has been measured with heat flow along both the {ital a} and {ital b} axes and for two oxygen configurations of the same crystal. The conductivity anisotropy has its maximum value at the conductivity peak ({ital T}{approx}40 K), where we find {kappa}{sub {ital a}}/{kappa}{sub {ital b}}{approx}1.15 before and after an oxygen anneal. Very little {ital ab} anisotropy in the magnitude of {kappa} is observed in the normal state, though near {ital T}{sub {ital c}} the temperature dependences are noticeably different for the two directions: {ital d}{kappa}/{ital dT}{gt}0 along the {ital b} axis and {ital d}{kappa}/{ital dT}{lt}0 along the {ital a} axis. Near {ital T}{sub {ital c}} ({ital T}{le}110 K) {kappa} is enhanced slightly for both transport directions, features we attribute to the effect of superconducting fluctuations.

Lattice thermal conductivity of YBa2Cu3O7−δ

Abstract We report a systematic study of the ab -plane thermal conductivity (κ) on single crystal and liquid-phase processed (LPP) specimens of YBa 2 Cu 3 O 7−δ (δ≤0.16) in the temperature range 10 K≤ T ≤300 K. From measurements of electrical conductivity on the same specimens and application of the Wiedemann-Franz law we estimate the relative contributions to the heat conduction from the carriers and the lattice. The normal-state phonon scattering mechanisms are quantified by calculations which employ the conventional theory of lattice heat conduction by longitudinal acoustic phonons. Differences in the magnitude and temperature dependence of κ for the LPP and crystal specimens are accounted for by differences in the relative weight of phonon-defect, phonon-carrier, and phonon-phonon scattering. For all specimens phonon-defect scattering predominates throughout most of the temperature range.

Influence of the Josephson junction on the impedance and noise of a resistive superconductive quantum interference device

The impedance and noise of a resistive superconductive quantum interference device (R‐SQUID) have been measured as a function of the dc and rf currents applied to it. The Josephson junction was adjustable so that data were also taken for several values of the junction critical current. The results were compared with the predictions of a resistively shunted junction (RSJ) model which takes into account the influence of the Josephson junction on the impedance and noise. The agreement was found to be quite good and demonstrates that the noise in the circuit is well understood. Use of the R‐SQUID as a noise thermometer below 1 K is assessed in terms of corrections due to the RSJ model. It is demonstrated how the dc and rf currents may be adjusted so that the total noise of the R‐SQUID is reduced to within 0.1% of the Johnson noise generated by the resistor alone. Under these conditions, the R‐SQUID may be used as a noise thermometer to determine thermodynamic temperature to this inaccuracy from 6 to 700 mK.

Thermal conductivity in theabplane of untwinnedYBa2Cu3O7−δ

We present measurements of thermal conductivity ({kappa}) in the {ital ab} plane of untwinned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} ({delta}{approx}0.10) for {ital T}{le}200 K. {kappa} has been measured with heat flow along both the {ital a} and {ital b} axes and for two oxygen configurations of the same crystal. The conductivity anisotropy has its maximum value at the conductivity peak ({ital T}{approx}40 K), where we find {kappa}{sub {ital a}}/{kappa}{sub {ital b}}{approx}1.15 before and after an oxygen anneal. Very little {ital ab} anisotropy in the magnitude of {kappa} is observed in the normal state, though near {ital T}{sub {ital c}} the temperature dependences are noticeably different for the two directions: {ital d}{kappa}/{ital dT}{gt}0 along the {ital b} axis and {ital d}{kappa}/{ital dT}{lt}0 along the {ital a} axis. Near {ital T}{sub {ital c}} ({ital T}{le}110 K) {kappa} is enhanced slightly for both transport directions, features we attribute to the effect of superconducting fluctuations.

Thermal conductivity in the ab plane of untwinned YBa sub 2 Cu sub 3 O sub 7 minus. delta

We present measurements of thermal conductivity ({kappa}) in the {ital ab} plane of untwinned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} ({delta}{approx}0.10) for {ital T}{le}200 K. {kappa} has been measured with heat flow along both the {ital a} and {ital b} axes and for two oxygen configurations of the same crystal. The conductivity anisotropy has its maximum value at the conductivity peak ({ital T}{approx}40 K), where we find {kappa}{sub {ital a}}/{kappa}{sub {ital b}}{approx}1.15 before and after an oxygen anneal. Very little {ital ab} anisotropy in the magnitude of {kappa} is observed in the normal state, though near {ital T}{sub {ital c}} the temperature dependences are noticeably different for the two directions: {ital d}{kappa}/{ital dT}{gt}0 along the {ital b} axis and {ital d}{kappa}/{ital dT}{lt}0 along the {ital a} axis. Near {ital T}{sub {ital c}} ({ital T}{le}110 K) {kappa} is enhanced slightly for both transport directions, features we attribute to the effect of superconducting fluctuations.

Characteristic H-T boundaries and dissipation in YBa2Cu3O7-δ crystals

Abstract Characteristic H-T boundaries of a YBa 2 Cu 3 O 7−δ crystal were determined form V(I) measurements for 0≤H≤6T with H ∥ ab and H ∥ c . These boundaries have the form H∼(T c −T) x where x can be consistent with predictions of the vortex-glass model. However, such a temperature dependence is also consistent with an alternate, granular superconductor model. Furthermore, magnetization and resistivity measurements suggest that a granular, percolating superconductor model is more appropriate than the vortex-glass model.