S.J. Collocott

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.

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.

Thermal properties of paratellurite (TeO2) at low temperatures

Measurements of the heat capacity, Cp, from 0.6 to 15 K can be represented by Cp=(0.32+or-0.02)T3 mJ mol-1 K-s ( theta D0=265+or-10 K) below 10 K. Linear thermal expansivities parallel ( alpha /sub ///) and perpendicular ( alpha perpendicular to ) to the symmetry axis from 2 to 120 K show marked anisotropy; alpha perpendicular to is negative below 10 K (while alpha /sub /// remains positive) but becomes positive and much larger than alpha /sub /// above 50 K. The Gruneisen parameter, gamma , increases from approximately=1 at 3 K to +1.4 at 30 K and is approximately=0.9 room temperature.

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.

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.

Thermal properties of high-Tc superconductors

Data are reported for the linear thermal expansivity, alpha , of one sample of La1.85Sr0.15CuO4 and four samples of YBa2Cu3O7 in the temperature range 2 to 1000 K. Values are calculated for the Gruneisen parameter, gamma approximately=0.7, using ultrasonic data for the elastic moduli. Published compressibility measurements give much higher values of the bulk modulus and of gamma .

AC susceptibility, resistivity and specific heat of the cubic superconductor Ba0.6K0.4BiO3

Abstract AC susceptibility, magnetotransport and specific heat measurements on single phase bulk polycrystalline Ba 0.6 K 0.4 BiO 3 , with T c ⋍31 K, are reported. The complex AC susceptibility, χ = χ ′ + iχ ″, measured in AC fields of 0.001–50 G rms, shows both intragrain and intergrain components characteristics of a granular superconductor, where the positions of the χ″ intra- and intergranular peaks are strongly field dependent. A value of 18 T is obtained for the upper critical field, H c2 (0), from measurements of the superconducting resistive transition as a function of DC magnetic field. The coherence length, ξ(0), is determined to be 43 A. Specific heat, C p , measurements from 0.5 to 20 K show no evidence of a term that has a linear temperature dependence or an upturn in C p / T .

The low-temperature heat capacity of the polymer polyethylene terephthalate, in the range 0.4 to 15 K

The heat capacity, Cp, of five samples of polyethylene terephthalate (PET) with volume fraction crystallinity varying from 0 to 0.58 has been measured in the range 0.4 to 15 K. The heat capacity exhibits a term roughly linear in temperature below 1.5 K, a broad peak in Cp/T3 at 3 K and a change from T3- to T-power dependence at temperatures approaching 15 K. Above 1 K, Cp decreases linearly, with increasing crystallinity; below 1 K, Cp increases with increasing crystallinity. The experimental data have been interpreted in terms of a Tarassov-like model for long-chain polymers with the addition of an Einstein term and T-term to account for the amorphous phase. The model gives a good numerical fit to the experimental data, but the physical significance of the Einstein term is unclear. The magnitude of the coefficient of the T-term for amorphous PET is consistent with that observed for other polymers, but increases with increasing crystallinity, becoming comparable with that observed in glass ceramics.

Specific heat of the high-Tc superconductors Bi2Sr2Can−1CunO2n+4 (n = 1, 2,3) from 0.4 to 6 K

Specific heat data, Cp, in the range 0.4 to 6 K for a polycrystalline specimen of Bi2Sr2Can−1CunO2n+4, n = 1, are reported and compared to specimens with n = 2 and 3. All specimens exhibit an upturn in Cp/T, which is least pronounced in the n = 1 specimen. The coefficient of the linear term, γ0 shows considerable variation from specimen to specimen, and ranges from 0 mJ/mol K2 to 3.2 mJ/mol K2 for n = 3 and 1, respectively. It would seem that γ0 is extrinsic in origin. Values obtained for the Debye temperature, θ0D, are 220 K, 246 K and 274 K for n = 1, 2 and 3, respectively.