Magnetic penetration depth of Tl2Ba2CuO6+δ:Tc～σ0 in the overdoped region

We report transverse-field muon-spin-rotation experiments carried out on Tl₂Ba₂CuO₆₊ . This system spans the whole overdoped regime, andT _c is reduced by excess oxygen doping, which increases the normal-state carrier concentration. In the heavily overdoped regime(0) is found to scale linearly with the superconducting critical temperatureT _c , similar to the behavior previously observed for other cuprates in the underdoped regime. However, for the overdoped region one has to explain the reduction of ₀, thus the increase of the magnetic penetration depth, in spite of an increasing normal-state carrier concentration. We discuss some possible explanations for this behavior.     

Eliashberg Spectral Functions for Some High-Tc Superconductors

We used a simplified but more realistic form of a model spectral functions for some high-T _c superconductors (e.g., LSCO, YBCO, BSCCO), avoiding the elaborate process of solving the Eliashberg equations numerically. The resulting spectral functions, constructed using the phonon density of states (PDOS), obtained from INS experiments with the easy use of available expressions, reproduce the observed critical temperature, gap ratio, and several other parameters. T _c is found to have a stronger increase with the electron–phonon coupling constant compared to those obtained from several equations, constructed from numerical solution of the Eliashberg equations for lower values. The obtained functions show widely spreading spectra that inhibit lattice instability by yielding not too large values of (< 2.95).     

Aluminum. II. Derivation of C v0from C p and comparison to C v0 calculated from anharmonic models

The specific heat at constant pressure, C _p, of aluminum measured by Ditmars, Plint, and Shukla has been reduced to the volume V ₀ appropriate for 0 K employing the Murnaghan equation. The C _v0 thus obtained is compared with the theoretical C _v0 calculated in the harmonic and the lowest-order anharmonic approximation from three different pseudopotentials (Harrison, Ashcroft, and Dagens-Rasolt-Taylor) as well as a phenomenological Morse potential. The higher-order ( ⁴) anharmonic contributions are calculated from the same nearest-neighbor Morse potential as in the lowest-order anharmonic theory. The role of the vacancy and the higher-order anharmonic contributions to C _v0 has been examined and we conclude that the ⁴ contributions to C _v0 are much smaller than the vacancy contribution. After removal of the vacancy contribution, the reduced C _v0 is found to be in excellent agreement with the Ashcroft and Harrison pseudopotentials as well as the Morse potential including the ² and ⁴ contributions to C _v0.     

Effect of sample density on magnetic penetration depth in YBaCuO ceramic superconductors

We have measured magnetic penetration depth(0) of high-T _c YBCO samples of different density by the cavity resonant frequency shift method at 10, 16.65, and 22.3 GHz microwave frequencies. The value of(0) at 10 GHz is found to decrease from 5850 Å to 2550 Å as the density of the sample increases from 4.4 to 5.3 g/cm³. The results of the frequency response of the penetration depth show a fairly constant value of(0) for all the three samples in the frequency range 10–22.3 GHz. The wide variation observed in the value of(0) for different density samples has been explained in terms of varying Josephson coupling strength in these ceramics.     

Effect of Sintering Duration on the Thermal Conductivity of (Bi, Pb)-2223 Superconducting Pellets Between 10 and 150 K

The influence of sintering duration on the electrical resistivities and thermal conductivities of (Bi_0.8Pb_0.2)₂Sr₂Ca₂Cu₃O_9.8+ pellets with 0.11 < < 0.54 is reported between 10 and 150 K. The results indicate a gradual transformation of the 2212 phase to the 2223 phase and this transformation starts within 5 h of sintering in air at 840°C. The thermal conductivity of the pellets sintered for shorter durations display two maxima at T _c0 and around 110 K, respectively. The shape and magnitude of these maxima depend on the relative amount of the 2212 and 2223 phases present in the pellets. While the magnitude of the total thermal conductivity over the measured temperature range is strongly influenced by the duration of sintering, the phonon-dominant nature of heat transport is retained. The relative contribution of the electronic part (_E) to the total thermal conductivity () remains small and does not change appreciably with sintering time.     

Chemical Control of Underdoped and Overdoped States in Y(Ba2 ? ySry)Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba_2–y Sr _y )Cu₃O_{6 +} (0.9) compounds gives rise to high hole concentrations within both the CuO₂ planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.     

A note on the similarity solutions for free convection on a vertical plate

Summary The similarity solutions for free convection on a vertical plate when the (non-dimensional) plate temperature is x and when the (non-dimensional) surface heat flux is –x are considered. Solutions valid for 1 and 1 are obtained. Further, for the first problem it is shown that there is a value ₀, dependent on the Prandtl number, such that solutions of the similarity equations are possible only for >₀, and for the second problem that solutions are possible only for >–1 (for all Prandtl numbers). In both cases the solutions becomes singular as ₀ and as –1, and the natures of these singularities are discussed.     

Optical thermophysical characteristics of the condensed phase of combustion products and heatproof materials

An unconventional unit for experimentally studying the optical thermophysical properties of materials over a wide temperature range is described. Results are presented of studying the temperature function and dispersion of the absorption index for the condensed phase of the combustion products of a metalbearing fuel and of the emittance of fiber fireproof materials.Notation K spectral absorption coefficient - D spectral transmission coefficient - spectral absorption index - wavelength - d sample thickness - d_s surface layer thickness - I spectral intensity of radiation - , spectral and total emittances Kirov Polytechnical Institute. Kazan Chemical Engineering Institute, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 3, pp. 330–336. March, 1993