R. Driver

Josephson vortices and flux penetration in high temperature superconductors

Abstract We have investigated, experimentally and theoretically, the magnetic flux which threads a YBa 2 Cu 3 O 7− x cylinder sample at 77 K when a weak low frequency ac magnetic field is applied. We explain the highly nonlinear magnetic response in great detail quantitatively, using a critical state model where during each ac cycle, intergranular (Josephson) vortices sweep in and out of the cylinder. The Josephson vortex pinning force is found to be field independent and several orders of magnitude smaller than in conventional type II superconductors. This explains the weak shielding properties and low critical current density found in this material. At magnetic fields greater than about 50 Oe, vortices start to enter the superconducting grains.

Nonlinear magnetic flux response in high temperature superconductors

Abstract We have spectrum analysed the voltage induced in a pick-up coil wound around a cylindrical bulk sample of YBa 2 Cu 3 O 7−δ when driven by a weak low frequency AC magnetic field, superimposed on a weak DC magnetic field, at 77 K. The harmonics generated by the highly nonlinear response are explained quantitatively in great detail by employing a critical state model with a field independent pinning force density, where intergranular vortices sweep in and out of the Josephson weak link regions during each AC cycle. The symmetry relation for the local flux density, which relates the first and second half of an AC cycle, is broken in the presence of a DC magnetic field, causing even harmonics to appear. This suggests high-sensitivity applications in magnetometry.

Critical current density of ceramic high-temperature superconductors in a low magnetic field

Abstract The transport critical current densities, ←J c →, of bulk sintered YBaCuO and Bi(Pb)SrCaCuO samples in the shape of slabs have been measured at 77 K as a function of the slab thickness, d , and of a weak applied magnetic field, H a , ( H a ⩽20 Oe, H a ← J c →). It was found that the transport critical current density ← J c → decreases rapidly with increasing applied field, H a , and that the self-field generated by the transport current causes ← J c → to increase significantly with decreasing slab thickness. A critical state model which describes the intergranular field distribution inside the slab samples is used to calculate the transport critical current density ← J c → ( d , H a ). Different local critical current densities J c ( H ) are assumed to account for the pinning of Josephson vertices in a weak-link network of grain boundaries. Good agreement with the experimental data was found if J c ( H )= J 0 /(1+(| H |/ H 0 ) β ) where β ≅2 for our YBaCuO samples and gb =1.3 for our Bi(Pb)SrCaCuO sample. A poorer agreement is obtained if an exponential form for J c ( H ) is assumed. It is speculated that the high ← J c →-value found in thin films is partly due to the small thickness of the films.

Specific heat and resistivity of the high-Tc superconductor Bi2.1Sr2CaCu2O8+x from 0.4 to 20 K: Evidence for a t-term

Abstract The specific heat, C p has been measured in the range of 0.4 to 20 K and the electrical resistance in the range 20 to 120 K in magnetic fields up to 7 T of Bi 2.1 Sr 2 CaCu 2 O 8+ x . Below 4 K the effects of lattice dispersion are more pronounced than in YBa 2 Cu 3 O 7 , there is an upturn in C p T , and evidence for the presence of a T -term ( γ = 4 ± mJ/mol. K 2 ). A value for the Debye temperature of 260 ± 6 K is obtained. The upper magnetic critical field, B c 2 (0), for the Bi 2,1 Sr 2 CaCu 2 O 8+x , specimen is estimated to be 19 T.

Flux pinning in tubes of a ceramic superconductor

Abstract The onset of magnetic flux penetration through the wall of a tube of sintered YBa2Cu3O7 has been measured in the temperature range of 4 to 90 K. These measurements have been used to calculate the temperature dependence of the intergranular critical transport current employing three critical-state models. Good agreement was obtained between this calculated critical current and direct critical current measurements on rod-shaped samples of the same material. The critical-state magnetic field profiles were used to explain the observed voltage waveforms induced in a pick-up coil inside the bore of tubular samples when excited by an AC field. Best agreement was obtained using a model in which the critical current (jc) has a magnetic field dependence given by j c = j 0 H 2 0 (H 2 +H 2 0 ) where j0 and H0 depend only on temperature.

Specific heat of (BiPb)2Sr2Ca2Cu3O10−y from 78 to 300 K

Abstract The specific heat anomaly of a polycrystalline sample of BPSCCO (2223 phase) was found to have a peak height δC(T c )/T c ≈ 24 mJ/mole K 2 at T c ≈ 107 K. The anomaly provides evidence for a fluctuation contribution above T c , whereas below T c it is not possible to separate fluctuation and strong-coupling effects. A second, considerably smaller, anomaly appears at T ≈ 101 K when the sample is cooled rapidly from 140 K to 78 K.

Specific heat of the ceramic superconductor Bi2Sr2CuO6 from 0.4 to 20 K

Abstract Specific heat, C p , data in the range 0.4 to 20 K for three polycrystalline specimens of Bi 2 Sr 2 CuO 6 (2:2:0:1 phase, T c ≈ 8 K) are reported and compared with similar measurements made on Bi 2 Sr 1.33 Ca 0.67 CuO 6 ( T c ≈ 20 K) which is also of the 2:2:0:1 phase, but with partial substitution of Ca for Sr. No anomaly in C p at T c is observed in any of the specimens. Below ≈ 2 K, an upturn in C p / T is observed in all specimens. The coefficient of the linear term in C p , γ 0 , is non-zero in all specimens, and is in the range 3 to 10 mJ / (mol K 2 ). A limiting value for the Debye temperature, θ 0 D , of 220±8 K is obtained.

Intergranular flux pinning in high temperature superconductors

Abstract A critical state model has been employed with an intergranular pinning force density which is independent of the local magnetic field, to calculate the critical transport current and the ac susceptibility of sintered Y Ba Cu O as a function of temperature. The assumed temperature dependence of the pinning force density is consistent with measurements on long tubular samples. The flux creep term of the effective intergranular pinning force causes a weak frequency dependence of the ac susceptibility.

The heat capacity of YBa2Cu3O7 and YBa2Cu3O6 in the range 0.4 to 20 K: Evidence for an intrinsic T-term

Abstract The heat capacity of a number of YBa 2 Cu 3 O 7 samples has been measured in the range 0.4 to 20 K to investigate the effect of paramagnetic chemical and phase impurities. Values for the T -term and Debye temperature are in the range 5.63–11.5 mJ/mol K 2 and 413–439 K, respectively. The role of various impurity phases, including BaCuO 2 , is discussed. The removal of the labile oxygen from YBa 2 Cu 3 O 7 to form the non-superconducting YBa 2 Cu 3 O 6 has enabled the determination of a value of ∼1.5 mJ/mol K 2 for the coefficient of the T -term that is intrinsic to the superconducting state. The intrinsic T -term is discussed in terms of the resonating-valence-bond- model and tunnelling due to two-level-systems.

Specific heat of the high-Tc superconductor (Bi, Pb)2Sr2Ca2Cu3O10, and related phases Ca2CuO3 and (Ca0.86Sr0.14)CuO2 from 0.4 to 20 K

Abstract Specific heat, Cp, data from 0.4 to 20 K for four polycrystalline specimens of (Bi, Pb)2Sr2Ca2Cu3O10 are reported, and compared with similar measurements made on Bi2Sr2CaCu2O8. Both the upturn in Cp/T and the coefficient of the linear term, γ0, are sensitive to the phase composition of the specimen. γ0 decreases from 10.8 mJ/(mol K2) in the specimen that has roughly equal proportions of the low-Tc (2:2:1:2) and high-Tc (2:2:2:3) phases to zero in the specimen that is close to 100% 2:2:2:3 phase. The variability of γ0 with phase purity suggests that it is extrinsic to the superconducting state of (Bi, Pb)2Sr2Ca2Cu3O10. Above 4 K, Cp of (Bi, Pb)2Sr2Ca2Cu3O10 shows a rapid departure from simple T3 (Debye) behavior due to dispersion and the excitation of low-lying optic modes. Values obtained for the Debye temperature, θD0, are in the range 276–292 K for (Bi, Pb)2Sr2Ca2 Cu3O10 (cf. 253 ± 7 K for Bi2Sr2CaCu2O8). To better understand how other copper-oxide compounds, which may appear as impurity phases in this system, contribute to Cp of (Bi, Pb)2Sr2Ca2Cu3O10 we have carried out Cp measurements between 0.4 and 20 K on Ca2CuO3 and on the structural parent phase (Ca0.86Sr0.14)CuO2. For Ca2CuO3, no upturn in Cp/T is observed and values of 1.63 mJ/(mol K2) and 516 K are obtained for γ0 and θD0, respectively. (Ca0.86Sr0.14)CuO2 behaves somewhat differently with an upturn in Cp/T, γ0=0 mJ/(mol K2) and θD0=496 K.