Takao Itoh

Specific Heat and Superconductivity in BaPb1-xBixO3

Specific heat coefficient of BaPb 1- x Bi x O 3 at low temperatures was measured by the thermal relaxation method. Although the discontinuity in the specific heat C at the superconducting transition temperature T c could not be detected, it was clearly observed in the lowest experimental temperature region that C in the superconducting state at zero magnetic field was smaller than in the normal state at H =59 kOe. The electronic specific heat coefficient γ increased with x until x =0.25 and then decreased above, corresponding to the composition dependence of T c . However, the γ value, e.g. 1.6 mJ mole -1 K -2 at x =0.25, was one to two orders of magnitude smaller than those of the other typical high T c superconducting metals. The Debye temperature was in the range 166 K to 196 K for 0 ≤ x ≤0.3.

Electron-phonon coupling constant of BaPb1−xBixO3 as estimated from the McMillan equation

The McMillans electron-phonon coupling constant λ was calculated by the McMillan-Allen-Dynes equation from the available experimental results, including measurements on resistivity, Hall coefficient, Seebeck coefficient, specific heat and plasma reflectivity. A simple single band was assumed to obtain the electron-electron repulsion coefficient µ * by the Morel-Anderson equation. The coupling constant λ increases with bismuth content x , reaching 1.15 to 1.32 at x =0.2 where T c is 10.7 K, indicating that BaPb 1- x Bi x O 3 is a strong-coupling superconductor in spite of its reported low density of states N ( E F ). The value λ/ N ( E F ) of this material was found out to be extremely large if compared with conventional high T c superconductors. Furthermore the metal-semiconductor phase transition observed in this system is suggested to be induced as the result of the strong electron-phonon interaction.

Physical Properties of an 80 K-Superconductor: Bi-Sr-Ca-Cu-O Ceramics

The magnetization curve, tunneling spectra, and normal-state susceptibility were measured in well-characterized single-phase 80 K superconductor, Bi-Sr-Ca-Cu-O ceramic samples. The obtained result is very similar to that of other copper-oxide superconductors.

SPECIFIC HEAT AND MAGNETIC SUSCEPTIBILITY MEASUREMENTS OF YBA2CU4O8 : A STUDY OF THE DENSITY OF ELECTRONIC STATES

Abstract Specific heat and magnetic susceptibility measurements are reported for polycrystalline bulk samples of YBa 2 Cu 4 O 8 and DyBa 2 Cu 4 O 8 prepared at high oxygen pressure and for a companion sample of YBa 2 Cu 3 O 7 . The magnitude of the idealized specific heat jump ΔC p / T at T c (=80±0.5 K ) is 16±2 mJ /(mol K 2 ) and the transition width Δ T c is about 3 K for YBa 2 Cu 4 O 8 . The corresponding values for DyBa 2 Cu 4 O 8 are 75 K, 15±2 mJ/(mol K 2 ) and 3.3 K. The size of these jumps is greatly reduced compared to YBa 2 Cu 3 O 7 . The normal state spin susceptibility of YBa 2 Cu 4 O 8 is calculated to be only about 15% smaller than in YBa 2 Cu 3 O 7 , and it is temperature dependent. Both these results are consistent with a smaller electronic density of states for YBa 2 Cu 4 O 8 .

Bonding states in the CuO2-plane of the layered cuprates

The optical phonon spectra are investigated for various layered cuprates and the related materials. The quantitative analysis of the spectra for the single crystals has revealed the highly covalent character of the cuprates. By comparison of the spectra between T-, T*- and T’- structure for (214)-compounds, the role of apical oxygen in the Cu-O bonding state are discussed.

Superconducting properties of the oxygen vacancy phase of LnBa2Cu3Ox with Ln=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb

Abstract The temperature dependence of the resistivity for quickly quenched LnBa 2 Cu x ceramics, with with Ln=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb, has been investigated. It is shown that for the materials LnBa 2 Cu 3 O x , with Ln=Sm, Eu, Gd, Dy, Ho, Er and Tm, the oxygen vacancy phase with T c =60 K exists, although for the materials with Ln=Nd and Yb the existence of the T c =60 K phase is not clear. The onset T c of the T c =60 K phase changes slightly depending on Ln involved in LnBa 2 Cu 3 O x , Analysis of these data includes the determination of the lattice constants of the materials quenched at different temperatures.

Synthesis of a Tl-Ba-Ca-Cu-O Superconductor and Its Properties

Tl-Ba-Ca-Cu-O superconductor has been synthesized by means of the solid-state reaction, Tl2O3+BaCuO2+Ca2CuO3. The resultant bulk superconductor was found to show the onset Tc at 126 K, with the superconducting critical current density or over 100 A/cm2 at 105 K.

Electronic transport properties in 1T-TaS2

Abstract The temperature dependences of the Seebeck effect and transverse magnetoresistance are systematically observed on 1T-TaS 2 with various qualities. The Seebeck coefficient is positive in commensurate CDW state and its temperature dependence in 20 K ⪅ T ⪅ 60 K seems to indicate that the samples are classified into two groups: one is metallic and the other is non-metallic although they show all non-metallic behaviors in conductivity. In the non-metallic crystals we found a new anomaly at 20 K, as yet unknown. In the metallic crystals we observed small but negative magnetoresistance at 14.1 K, which suggests the variation of electronic states around this temperature. We discuss the sample dependence in relation to electronic localization and dimensionality.

Specific Heat of YBa2Cu4O8 near Tc

Specific heat measurements are reported for a polycrystalline, bulk sample of YBa2Cu4O8 prepared at high oxygen pressure. The magnitude of the idealized specific heat jump ΔCp/T at Tc (=80 ± 0.5K) is 15.5±2 mJ/K2-mole. The transition width ΔTc is about 3K. The size of the jump is greatly reduced compared to YBa2Cu3O7. The normal state magnetic susceptibility is somewhat smaller than in YBa2Cu3O7 and is temperature dependent. The results suggest a smaller electronic density of states for YBa2Cu4O8.