Superconductivity in novel Ge-based skutterudites: {Sr,Ba}Pt_4Ge_{12}

Combining experiments and ab initio models we report on $\rm SrPt_4Ge_{12}$ and $\rm BaPt_4Ge_{12}$ as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge-atoms. Below $T_c=5.35$ K, and 5.10 K for $\rm BaPt_4Ge_{12}$ and $\rm SrPt_4Ge_{12}$, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-$p$ states dominate the electronic structure at the Fermi energy.Comment: 4 pages, 4 figures, accepted for publication in PR

How should we interpret the two transport relaxation times in the cuprates ?

We observe that the appearance of two transport relaxation times in the various transport coefficients of cuprate metals may be understood in terms of scattering processes that discriminate between currents that are even, or odd under the charge conjugation operator. We develop a transport equation that illustrates these ideas and discuss its experimental and theoretical consequences.Comment: 19 pages, RevTeX with 8 postscript figures included. To appear in ``Non Fermi Liquid Physics'', J. Phys:Cond. Matt. (1997

Structure inhomogeneities, shallow defects, and charge transport in the series of thermoelectric materials K2Bi8−xSbxSe13

The charge transport properties of the low-dimensional thermoelectric materials K2Bi8-xSbxSe13 (02Bi8-xSbxSe13 was analyzed on the basis of the classical semiconductor theory and discussed in the context of recent band calculations. The results suggest that the K2Bi8-xSbxSe13 materials possess coexisting domains with semimetallic and semiconducting characters whose ratio is influenced by the value of x and by local defects. The extent and relative distribution of these domains control the charge transport properties. Electron diffraction experiments performed on samples of K2Bi8-xSbxSe13 with x=1.6 show evidence for such domains by indicating regions with long range ordering of K+/Bi3+ atoms and regions with increased disorder. The semiconducting behavior is enhanced with increasing x (i.e., Sb/Bi ratio) in the composition through a decrease of the semimetallic fraction

Observation of Magnetic Flux Generated Spontaneously During a Rapid Quench of Superconducting Films

We report observations of spontaneous formation of magnetic flux lines during a rapid quench of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ films through T$_{c}$. This effect is predicted according to the Kibble-Zurek mechanism of creation of topological defects of the order parameter during a symmetry-breaking phase transition. Our previous experiment, at a quench rate of 20K/sec, gave null results. In the present experiment, the quench rate was increased to \TEXTsymbol{>} 10$^{8}$ K/sec. Within experimental resolution, the dependence of the measured flux on the cooling rate is consistent with the prediction

Evidence for field-induced excitations in low-temperature thermal conductivity of Bi_2Sr_2CaCu_2O_8

The thermal conductivity ,$\kappa$, of Bi_2Sr_2CaCu_2O_8 was studied as a function of magnetic field. Above 5 K, after an initial decrease, $\kappa(H)$ presents a kink followed by a plateau, as recently reported by Krishana et al.. By contrast, below 1K, the thermal conductivity was found to \emph{increase} with increasing field. This behavior is indicative of a finite density of states and is not compatible with the existence of a field-induced fully gapped $d_{x^{2}-y^{2}}+id_{xy}$ state which was recently proposed to describe the plateau regime. Our low-temperature results are in agreement with recent works predicting a field-induced enhancement of thermal conductivity by Doppler shift of quasi-particle spectrum.Comment: 4 pages including 4 eps figures, submitted to Phys. Rev. Let

Separation of Quasiparticle and Phononic Heat Currents in YBCO

Measurements of the transverse (k_{xy}) and longitudinal (k_{xx}) thermal conductivity in high magnetic fields are used to separate the quasiparticle thermal conductivity (k_{xx}^{el}) of the CuO_2-planes from the phononic thermal conductivity in YBa_2Cu_3O_{7-\delta}. k_{xx}^{el} is found to display a pronounced maximum below T_c. Our data analysis reveals distinct transport (\tau) and Hall (\tau_H) relaxation times below T_c: Whereas \tau is strongly enhanced, \tau_H follows the same temperature dependence as above T_c

Temperature-dependent Hc2H_{c2} anisotropy in MgB2_2 as inferred from measurements on polycrystals

We present data on temperature-dependent anisotropy of the upper critical field of MgB$_2$ obtained from the analysis of measurements on high purity, low resistivity polycrystals. The anisotropy decreases in a monotonic fashion with increase of temperature

Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3 / YBa2Cu3O7 superlattices

We study the interplay between magnetism and superconductivity in high quality YBa2Cu3O7 (YBCO) / La0.7Ca0.3MnO3(LCMO)superlattices. We find evidence for the YBCO superconductivity depression in presence of the LCMO layers. We show that due to its short coherence length superconductivity survives in the YBCO down to much smaller thickness in presence of the magnetic layer than in low Tc superconductors. We also find that for a fixed thickness of the superconducting layer, superconductivity is depressed over a thickness interval of the magnetic layer in the 100 nm range. This is a much longer length scale than that predicted by the theory of ferromagnetic/superconducting proximity effect.Comment: 10 pages + 5 figures, submitted to Phys. Rev.

Hole Localization in Underdoped Superconducting Cuprates Near 1/8th Doping

Measurements of thermal conductivity versus temperature over a broad range of doping in YBa$_2$Cu$_3$O$_{6+x}$ and HgBa$_2$Ca$_{n-1}$Cu$_n$O$_{2n+2+\delta}$ ($n$=1,2,3) suggest that small domains of localized holes develop for hole concentrations near $p$=1/8. The data imply a mechanism for localization that is intrinsic to the CuO$_2$-planes and is enhanced via pinning associated with oxygen-vacancy clusters.Comment: 4 pages, 4 eps fig.'s, to be published, Phys. Rev.