C. Sułkowski

Thermal conductivity and electrical resistivity of the high-Tc superconductor YBa2Cu3O9−Δ

Abstract Thermal conductivity and electrical resistivity of single-phase YBa 2 Cu 2 O 9−Δ compound were measured in the superconducting ( T c =89.5 K) and normal states, in the temperature range 5–320 K. The electronic component of the total thermal conductivity was estimated to be 20%. The electrical resistivity changed linearly in the normal state up to highest measured temperature.

Effect of oxygen atom disordering on Tc OF YBa2Cu3O7 − x

Abstract The effects of oxygen deficiency and temperature-induced oxygen atom disordering on the critical temperature (Tc) of YBa2Cu3O7 − x are presented. Two possible influences are proposed and discussed to explain an extraordinary behaviour of Tc, observed in the oxygen-deficient region of the phase. The first influence is the effect of oxygen disordering on the b-axis Cu-O chains, which results in a state characteristic of a set of mutually perpendicular and inequivalently occupied Cu-O chains, which is possibly more favourable for superconductivity. The second is the influence of the Cu-O planes, which exist between the yttrium and the barium-ion layers, the contribution of which on Tc may exceed that of the Cu-O chains and, starting at a certain oxygen index, it may become dominating. Estimates on the upper value of Tc for 1:2:3 type compounds (165 K) and on the limit of superconductivity in the YBa2Cu3O7 − x phase have been made. They give 6.23, if determined by the oxygen index, or 3 × 10−3 if determined by the orthorombic distortion parameter p.

Superconductivity and magnetism of TmRh1.3Sn4.0 single crystals

Abstract The electrical resistivity, upper critical field and DC susceptibility of TmRh 1.3 Sn 4.0 single crystals have been studied versus temperature down to 0.3 K. The transition to the superconducting state was observed at about 2 K; however, the reentrant behavior in an applied magnetic field was found to be at lower temperatures. The weakness of the superconductivity is probably caused by antiferromagnetic order which appears at about 0.4 K. The suppression of H c2 as a function of temperature has been determined and reproduced by calculations using a multiple-pair-breaking formulation.

Heat capacity and thermopower coefficient of the carbon preform of sapele wood

This paper reports on measurements of the heat capacity at constant pressure Cp in the 80–300-K temperature interval and the thermopower coefficient S at 5–300 K of the carbon preform of sapele wood, which was prepared at the carbonization temperature of 1000°C. Measurements of Cp(T), our previous data on the phonon thermal conductivity, and literature information on the sound velocity have been used to calculate the phonon mean free path l(T) for this material. It has been shown that within the temperature interval 200–300 K, l is constant and equal to 11 Å, a figure matching the size of the nanocrystallites (“graphite fragments”) making up the carbon framework of the sapele carbon preform. The high-temperature parts of S(T) have been found to follow a linear course characteristic of diffusive thermopower for the degenerate state of charge carriers, with only one type of charge carriers present. The anisotropy of the thermopower coefficient has been estimated.

Thermal conductivity anomalies in GdBa2Cu3O7−x

Abstract The measurements of the thermal and electric conductivity of three GdBa 2 Cu 3 O 7−x samples with different orthorhombic distortions are presented. The electronic component of the total conductivity in the normal state is evaluated. Its value is connected with the magnitude of the change of the slope of the total conductivity λ for the critical temperature T c .

Thermopower of biomorphic silicon carbide

The thermopower coefficients of cubic bio-SiC, a high-porosity semiconductor with cellular pores prepared from the biocarbon template of white eucalyptus wood, and single-crystal β-SiC taken as a reference are measured in the temperature range 5–280 K. It is revealed that, in the low-temperature range, the samples are characterized by a thermopower contribution associated with the electron drag by phonons. The thermopower of the bio-SiC samples is measured both along and across the empty pore channels and is found to be anisotropic. Two models are proposed to account for the anisotropy of the thermopower in cubic bio-SiC.

Thermopower of the light heavy-fermion compound YbMgCu4 in the region of its homogeneity

The thermopower coefficient S of the light heavy-fermion system YbMgCu4 and, for comparison, the thermopower coefficient of metallic LuMgCu4 are measured in the temperature range 5–300 K. It is shown that the YbMgCu4 compound has a fairly broad region of homogeneity. The obtained data on the temperature dependence of the thermopower coefficient S for the YbMgCu4 compound confirm that this compound belongs to heavy-fermion systems. The Kondo temperature of YbMgCu4 is shown to depend on the unit cell parameter in the region of its homogeneity.

Electronic properties of YBa2Cu3O6.85 compound

Abstract Superconducting transition temperature T c , resistivity p, thermopower S, and lattice parameters have been measured for five samples of YBa 2 Cu 3 O 7−Δ where the oxygen concentration changes between 6.83–6.90 only. The Tcs of the samples are almost the same and equal to 91 K (±1 K), however, large differences in values of the ϱ (0.74–1.7 mΩcm) and the S (−1.6+5.9 μV/K) at between these parameters have been observed. We suppose, the differences are coused by ordering of the heavy ions in the unit cell rather than changes of the oxygen concentration. Growth of the ordering leads to observed decreasing of unit cell volume and reduction of the resistivity and thermopower.

Disordering of (Ba,Cu)-atoms as crucial determinant of the 90 K superconductivity in YBa2Cu3O7−x phase

It is shown that low temperature (150°C), long-time annealing of YBa 2 Cu 3 O 7−x (YBACUO) phase samples (x ∼ 0.5), affects the lattice parameters in the way that, no mater the sample oxygen index, the parameters undergo a steady change following the rule that; a(final) and c/3 (final)→ b(final), while b (final) ∼ b (starting). As the result, a new tetragonal pseudo-cubic_, non=superconducting form of the phase occurs. The results are explained in terms of the process of ordering of Ba-, and Cu(2)-atoms, consisting in change of the coordination parameter z , characteristic for these structure components (Pmmm),towards the ideal values equal to ⅙ and ⅓, respectively.

Interatomic distances as decisive factors of the 90 K superconductivity in YBa2Cu3O7 − x

Abstract It is shown that low temperature (150°C), lengthy annealing of YBa2-Cu3O7 − x samples (x ≈ 0.5) affects their cell parameters in such a continuous way that, regardless of their oxygen content, the parameters change according to the scheme a(final) and c 3 (final) → b (final) ≈ b (starting) As a result, a new, non-superconducting form of YBa2Cu3O7 − x of tetragonal (pseudo-cubic) symmetry occurs. Results are explained in terms of an ordering process of barium and Cu(2) atoms, consisting in a shift of the variable positional parameter z, characteristic for these structure constituents (space group Pmmm), towards ideal values of 1 6 and 1 3 , respectively.