Universal scaling laws can guide the understanding of new phenomena, and for cuprate high-temperature superconductivity the influential Uemura relation showed, early on, that the maximum critical temperature of superconductivity correlates with the density of the superfluid measured at low temperatures. Here we show that the charge content of the bonding orbitals of copper and oxygen in the ubiquitous CuO2 plane, measured with nuclear magnetic resonance, reproduces this scaling. The charge transfer of the nominal copper hole to planar oxygen sets the maximum critical temperature. A three-dimensional phase diagram in terms of the charge content at copper as well as oxygen is introduced, which has the different cuprate families sorted with respect to their maximum critical temperature. We suggest that the critical temperature could be raised substantially if one were able to synthesize materials that lead to an increased planar oxygen hole content at the expense of that of planar copper.

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The results of thermal conductivity study of epoxy-matrix composites filled with different type of powders are reported. Boron nitride and aluminum nitride micro-powders with different size distribution and surface modification were used. A representative set of samples has been prepared with different contents of the fillers. The microstructure was investigated by SEM observations. Thermal conductivity measurements have been performed at room temperature and for selected samples it was also measured as a function of temperature from 300 K down to liquid helium temperatures. The most spectacular enhancement of the thermal conductivity was obtained for composites filled with hybrid fillers of boron nitride-silica and aluminum nitride-silica. In the case of sample with 31 vol.% of boron nitride-silica hybrid filler it amounts to 114% and for the sample with 45 vol.% of hybrid filler by 65% as compared with the reference composite with silica filler. However, in the case of small aluminum nitride grains application, large interfacial areas were introduced, promoting creation of thermal resistance barriers and causing phonon scattering more effective. As a result, no thermal conductivity improvement was obtained. Different characters of temperature dependencies are observed for hybrid filler composites which allowed identifying the component filler of the dominant contribution to the thermal conductivity in each case. The data show a good agreement with predictions of Agari-Uno model, indicating the importance of conductive paths forming effect already at low filler contents. Figure 3. SEM images of the: a) the composite filled with 45 vol.% hybrid filler Millsil W12 and BN PT100 (90:10), b) filled with 45 vol.% hybrid filler Millsil W12 and BN PT100 (80:20).

Spin and orbital magnetic moments of rhenium in AAЈFeReO 6 double perovskites ͑A , AЈ = Ba, Sr, and Ca͒ have been directly probed employing x-ray magnetic circular dichroism at the Re L 2,3 edges. A considerable orbital magnetic moment is observed in all the compounds studied, which confirm theoretical predictions of unquenched Re orbital moment despite its octahedral coordination. Relative orbital-to-spin moment ratio alters with lattice distortion from m L / m S = −0.285 to − 0.337 from Ba 2 FeReO 6 to Ca 2 FeReO 6 , respectively. Moreover, the spin moment of Re ion scales with Curie temperature, the most relevant property in spin electronics application of the compounds studied. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2234292͔ Ordered double perovskites have recently attracted great interest due to their large spin polarization and Curie temperature ͑T C ͒ much higher than room temperature. These properties are strongly desired in order to realize reasonable magnetoresistance effects at room temperature, which is not only a challenging subject of fundamental science but also an important phenomenon for potential applications in spin electronics. Therefore the first observation of substantial magnetoresistance at room temperature in Sr 2 FeMoO 6 ͑Ref. 1͒ quickly led to production of magnetic tunnel junctions and magnetoresistive potentiometers.Currently, the other ordered double perovskites AAЈBBЈO 6 ͑A , AЈ = Ca, Sr, Ba, La, etc.; BBЈ = FeMo, FeRe, CrRe, CrW, etc.͒ are being intensively studied in order to find a material with optimal performance. 2-7 Among them the Re-based double perovskites are the most promising compounds in terms of high Curie temperature, e.g., 538 and 635 K in Ca 2 FeReO 6 and Sr 2 CrReO 6 , respectively.3 Moreover, Re-based double perovskites are magnetically hard 4,6,8 and reveal large magnetoelastic effects, 9 which can only be explained by a substantial magnetocrystalline anisotropy due to the anisotropy of an unquenched orbital moment of Re.