Twin formation and structural modulations in orthorhombic and tetragonal YBa2(Cu1-xCox)3O7-δ

Abstract

The microstructure of YBa₂(Cu_1?xCo_x)₃O_7?δ, prepared by the standard ceramic method, shows lamellar twin structures with decreasing spacings between twin walls with increasing Co content for x?0·02, developing into {110}-type cross-hatched ‘tweed’ modulation for x?0·02. Several wall junctions are found for x=0·02. The structural phase transition between macroscopically orthorhombic and tetragonal material occurs at x≈0·025; structural modulations (λ≈20Å) persist in the samples with high Co content (x>0·25). The modulations lead to a considerable broadening of the X-ray lines affected by orthorhombic splitting, and show maximum amplitude at the critical composition x≈0·025.     

Crystal structure of 1–2–3 superconductor precursor material: Vacancy distribution and lattice relaxation

Abstract

The crystal structure of tetragonal YBa₂Cu₃O_7-δ, with δ close to the critical composition for the tetragonal–orthorhombic transition, has been refined. The oxygen deficiency δ = 0·65(10) is accommodated by both the O(1) and O(2) sites which coordinate the ‘square planar’ Cu(1) site. Structural relaxation around the vacancies is indicated by large anisotropic Debye-Waller ellipsoids in the structural layer formed by these atomic sites. The Y, Ba, Cu(2) and O(3) sites are not involved in the relaxation mechanism. On comparing this structure with the δ ≈ 0 and δ ≈ 1 end-members, we find that with decreasing oxygen content of the structure the Cu(2) valence type transforms from square-pyramidal character towards square-planar character. This effect gives the main contribution to the increase of the c ₀ lattice constant with decreasing oxygen content.     

Failure mechanism in porous materials under compression: crackling noise in mesoporous SiO2

The failure mechanism of porous materials under uniaxial stress has been investigated experimentally. Microporous SiO₂, Vycor, has been subjected to slowly increasing compressive uniaxial stress with stress rates between 0.2 and 12.2?kPa/s. With increasing stress the strain changes stepwise with acoustic emission correlated with each volume collapse. The acoustic emission followed the characteristics of ‘crackling noise’ with a power law distribution over an exceptionally large interval of 6 decades at the slowest stress rate. The power law exponent is ?1.39. Possible applications in mining industry and others are discussed.     

Talking our way to systematicity

Do we think in a language-like format? Taking the marker of language-like formats to be the property of unconstrained systematicity, this paper considers the following master argument for the claim that we do: (1) language is unconstrainedly systematic, (2) if language is unconstrainedly systematic then so is thought, (3) so thought is unconstrainedly systematic. It is easy to feel that there is something right about this argument, that there will be some way of filling in its details that will vindicate the idea that our thought must be unconstrainedly systematic given that the language in which we express it is. Clearly, however, the second premise needs support—we need a principled reason for moving from the unconstrained systematicity of language to the unconstrained systematicity of thought. This paper gives three passes at formulating such a principle. This turns out to be much harder than it might seem. We should, I conclude, resist falling too easily for the lure of this master argument for the language-like format of thought.

     

Order-parameter saturation at low temperatures: Displacive phase transitions with coupled Einstein oscillators

Quantum saturation of displacive order parameters may be observed at temperatures higher than expected in a simple soft-mode model. The experimental results have been explained sometimes by coupling of the softphonon branch with hard mode branches. We analyse the effect of this coupling in the limit of coupled Einstein oscillators. It is shown that a significant change through this mechanism of the low-temperature quantum saturation of the order parameter requires abnormally large coupling terms. In any case, under favourable circumstances, the coupled hard-phonon spectrum can be derived from the second temperature derivative of the square of the measured order parameter.     

Phonon renormalization,specific-heat anomaly and why some aspects of mean-field theory can be relevant for high-T c superconductivity

Abstract

Phonon anomalies due to superconductivity in YBa₂Cu₃O₇ are correlated with the superconducting order parameter. The phonon part of the excess specific heat follows the same temperature evolution as the measured macroscopic excess heat capacity. The electronic part of the transformation entropy corresponds to about 10% of the total carrier concentration. No excess entropy was found at temperatures well above T_c. The effect of phonon contributions to the stabilization energy of the superconducting phase is discussed.