YBa2Cu3O7−δ films: Calculation of the thermal conductivity and phonon mean‐free path

We estimate the phonon mean‐free path (mfp) in YBa2Cu3O7−δ by performing a theoretical fitting procedure on bulk single‐crystal data. This analysis indicates that the mfps of the phonons that are most responsible for the transport of heat are much longer than would be predicted from kinetic theory. These values are incorporated into a recently proposed treatment of the size effect to provide an estimate of the thermal conductivity of YBa2Cu3O7−δ films.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70503/2/JAPIAU-72-10-4788-1.pd

q-deformation of z→az+bcz+dz\to {az+b\over cz+d}

We construct the action of the quantum double of \uq on the standard Podle\'s sphere and interpret it as the quantum projective formula generalizing to the q-deformed setting the action of the Lorentz group of global conformal transformations on the ordinary Riemann sphere.Comment: LaTeX, 16 pages, we add a reference where an alternative construction of the q-Lorentz group action on the Podles sphere is give

Epitaxial strain, metastable structure, and magnetic anisotropy in Co‐based superlattices (invited)

We explore the relationship between interface structure and magnetic anisotropy in three types of Co‐based superlattices: Cohcp‐Au; Cofcc‐Cu; and Cohcp‐Cr, grown epitaxially on GaAs(110). For very thin layers of Co, Co‐Au, and Co‐Cu superlattices exhibit a perpendicular easy axis due to magnetoelastic contributions to the anisotropy energy. The magnetic anisotropy in Co‐Cr is found to be strongly dependent on growth conditions. At slow deposition rates of Co the interface between Co and Cr becomes diffuse as is evidenced by a low saturation moment and a shift toward perpendicular anisotropy whereas samples with abrupt interfaces show predominantly parallel anisotropy. The Cr layers grow in a metastable hcp phase which appears to be paramagnetic. The results illustrate the influence of the heterointerface on magnetic properties.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69828/2/JAPIAU-70-10-5775-1.pd

Thermal conductivity of Tl2Ba2Ca2Cu3O10 ceramics from 300 K down to 0.1 K

Thermal conductivity, [varkappa], of two ceramic samples of Tl2Ba2Ca2Cu3O10 has been measured over a temperature range from 300 K down to 0.1 K. At high temperatures, the data show features similar to the thermal conductivity of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O ceramics in both the magnitude and the temperature dependence. Specifically, a sudden increase in the thermal conductivity is observed at the onset of a superconducting transition near 120 K culminating in a pronounced maximum of [varkappa] around 75 K and an eventual rapid decrease of the thermal conductivity at lower temperatures. From 5 K down to 0.1 K we observe the thermal conductivity to decrease with an average power law exponent between 2.4 and 2.5. Such a temperature dependence is comparable with that for sintered Bi-Sr-Ca-Cu-O samples, but differs from the quadratic variation typical for Bi-Sr-Ca-Cu-O single crystals and the T-linear asymptotic behavior characteristic of Y-Ba-Cu-O and La-Sr-Cu-O ceramics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29278/1/0000337.pd

Thermoelectric enhancement in PbTe with K, Na co-doping from tuning the interaction of the light and heavy hole valence bands

The effect of K and K-Na substitution for Pb atoms in the rock salt lattice of PbTe was investigated to test a hypothesis for development of resonant states in the valence band that may enhance the thermoelectric power. We combined high temperature Hall-effect, electrical conductivity and thermal conductivity measurements to show that K-Na co-doping do not form resonance states but2 can control the energy difference of the maxima of the two primary valence sub-bands in PbTe. This leads to an enhanced interband interaction with rising temperature and a significant rise in the thermoelectric figure of merit of p-type PbTe. The experimental data can be explained by a combination of a single and two-band model for the valence band of PbTe depending on hole density that varies in the range of 1-15 x 10^19 cm^-3.Comment: 8 figure

D-branes in the Euclidean AdS3AdS_3 and T-duality

We show that D-branes in the Euclidean $AdS_3$ can be naturally associated to the maximally isotropic subgroups of the Lu-Weinstein double of SU(2). This picture makes very transparent the residual loop group symmetry of the D-brane configurations and gives also immediately the D-branes shapes and the $\sigma$-model boundary conditions in the de Sitter T-dual of the $SL(2,C)/SU(2)$ WZW model.Comment: 29 pages, LaTeX, references adde

Universal Heat Conduction in YBa_2Cu_3O_6.9

The thermal conductivity of YBa_2Cu_3O_6.9 was measured at low temperatures in untwinned single crystals with concentrations of Zn impurities from 0 to 3% of Cu. A linear term kappa_0/T = 0.19 mW/K^2.cm is clearly resolved as T -> 0, and found to be virtually independent of Zn concentration. The existence of this residual normal fluid strongly validates the basic theory of transport in unconventional superconductors. Moreover, the observed universal behavior is in quantitative agreement with calculations for a gap function of d-wave symmetry.Comment: Latex file, 4 pages, 3 EPS figures, to appear in Physical Review Letter

Promising bulk nanostructured Cu₂Se thermoelectrics via high throughput and rapid chemical synthesis

A facile and high yield synthesis route was developed for the fabrication of bulk nanostructured copper selenide (Cu2Se) with high thermoelectric efficiency. Starting from readily available precursor materials and by means of rapid and energy-efficient microwave-assisted thermolysis, nanopowders of Cu2Se were synthesized. Powder samples and compacted pellets have been characterized in detail for their structural, microstructural and transport properties. alpha to beta phase transition of Cu2Se was confirmed using temperature dependent X-ray powder diffraction and differential scanning calorimetry analyses. Scanning electron microscopy analysis reveals the presence of secondary globular nanostructures in the order of 200 nm consisting of <50 nm primary particles. High resolution transmission electron microscopy analysis confirmed the highly crystalline nature of the primary particles with irregular truncated morphology. Through a detailed investigation of different parameters in the compaction process, such as applied load, heating rate, and cooling profiles, pellets with preserved nanostructured grains were obtained. An applied load during the controlled cooling profile was demonstrated to have a big impact on the final thermoelectric efficiency of the consolidated pellets. A very high thermoelectric figure of merit (ZT) above 2 was obtained at 900 K for SPS-compacted Cu2Se nanopowders in the absence of the applied load during the controlled cooling step. The obtained ZT exceeds the state of the art in the temperature ranges above phase transition, approaching up to 25% improvement at 900 K. The results demonstrate the prominent improvement in ZT attributed both to the low thermal conductivity, as low as 0.38 W m(-1) K-1 at 900 K, and the enhancement in the power factor of nanostructured Cu2Se. The proposed synthesis scheme as well as the consolidation could lead to reliable production of large scale thermoelectric nanopowders for niche applications

Quantum Fluctuations and Curvature Singularities in Jackiw-Teitelboim Gravity

The Jackiw-Teitelboim gravity with the matter degrees of freedom is considered. The classical model is exactly solvable and its solutions describe non-trivial gravitational scattering of matter wave-packets. For huge amount of the solutions the scattering space-times are free of curvature singularities. However, the quantum corrections to the field equations inevitably cause the formation of (thunderbolt) curvature singularities, vanishing only in the limit $\hbar\to 0$. The singularities cut the space-time and disallow propagation to the future.The model is inspired by the dimensional reduction of 4-d pure Einstein gravity, restricted to the space-times with two commuting space-like Killing vectors. The matter degrees of freedom also stem from the 4-d ansatz. The measures for the continual integrations are judiciously chosen and one loop contributions (including the graviton and the dilaton ones) are evaluated. For the number of the matter fields $N=24$ we obtain even the exact effective action, applying the DDK-procedure. The effective action is nonlocal, but the semiclassical equations can be solved by using some theory of the Hankel transformations.Comment: 32 pages, LaTeX, PRA-HEP-93/

Thermal conductivity of single crystal lanthanum cuprates at very low temperature

We have undertaken a study of the thermal conductivity of single crystals of pure and strontium doped lanthanum copper oxide in the temperature range 0.1-6 K. For in plane conduction, all samples show approximately a T2.7 dependence, whereas for conduction across the planes for X = 0 we observe [kappa] ~ T2. This suggests that dimensionality may play an important role in the lattice heat conduction properties of these materials at very low temperature.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29444/1/0000526.pd