Specific heat of Gd4Co3.

The specific heat (C(T)) of Gd4Co3 was measured in the temperature range 2-300 K and its magnetic contribution (C(m)(T)) was determined using a new method that fits the electronic specific heat coefficient (γ) and the Debye temperature (θ(D)) by constraining the resulting magnetic entropy (S(m)(T)) to saturate at temperatures far above the Curie temperature (T(C)). C(m)(T) exhibits a low-temperature bump originating from thermal excitation of gapped spin waves, which is responsible for pronounced peaks, at ≈35 K, in both C(m)/T and the temperature derivative of the magnetic contribution to electrical resistivity (dρ(m)/dT). Apart from in the vicinity of T(C), an excellent global correlation was found between C(m)/T and dρ(m)/dT. Our results provide strong support for the consistency of the new method proposed for the determination of C(m)(T) and rule out any major role of short-range order on Gd moments or d-electron spin fluctuation effects in the paramagnetic phase. A comparative analysis with other methods used in similar compounds points to the need for a better evaluation of C(m)(T) in such compounds, especially in the magnetically ordered phase, where a deficient evaluation of C(m)/T has a larger impact on the S(m)(T) curve.

Superhard semiconducting optically transparent high pressure phase of boron.

An orthorhombic (space group Pnnm) boron phase was synthesized at pressures above 9 GPa and high temperature, and it was demonstrated to be stable at least up to 30 GPa. The structure, determined by single-crystal x-ray diffraction, consists of B12 icosahedra and B2 dumbbells. The charge density distribution obtained from experimental data and ab initio calculations suggests covalent chemical bonding in this phase. Strong covalent interatomic interactions explain the low compressibility value (bulk modulus is K300=227 GPa) and high hardness of high-pressure boron (Vickers hardness HV=58 GPa), after diamond the second hardest elemental material.

Electronic band effects on the spin disorder resistivity of Gd(4)(Co(1-x)Cu(x))(3) compounds.

We present a study of the spin disorder resistivity ([Formula: see text]) and the electronic specific heat coefficient (γ) in Gd(4)(Co(1-x)Cu(x))(3) compounds, with x = 0.00, 0.05, 0.10, 0.20 and 0.30. The experimental results show a strongly nonlinear dependence of [Formula: see text] on the average de Gennes factor (G(av)) which, in similar intermetallic compounds, is usually attributed to the existence of spin fluctuations on the Co 3d bands. Values of γ were found around 110 mJ mol(-1) K(-2) for the Gd(4)(Co(1-x)Cu(x))(3) compounds, much larger than 38.4 mJ mol(-1) K(-2) found for the isostructural nonmagnetic Y(4)Co(3) compound. Using a novel type of analysis we show that the ratio [Formula: see text] follows a well-defined linear dependence on G(av), which is expected when appropriate dependencies with the effective electron mass are taken into account. This indicates that band structure effects, rather than spin fluctuations, could be the main cause for the strong electron scattering and γ enhancement observed in the Gd(4)(Co(1-x)Cu(x))(3) compounds. A discussion on relevant features of magnetization and electrical resistivity data, for the same series of compounds, is also presented.

Effect of chemical composition on superconductivity and magnetism in layered ruthenocuprates.

The ruthenium-based layered cuprates RuSr(2)GdCu(2)O(8) (RuGd1212) can be considered naturally occurring magnetic and superconducting multilayer systems. We have concentrated on the preparation of RuGd1212-type compounds with nominally stoichiometric composition under ambient pressure conditions. For small rare earth ions R  = Gd, Eu (and Sm), single phase compounds are obtained with the typical ordered layered structure and no significant changes of physical properties. With large rare earth ions (R  = Nd, Pr), multiphase samples are obtained. In these cases, no ordered layered structure was observed. The effect of substituting Sr(2+) with the smaller Ca(2+) and larger Ba(2+) is examined. A different number and different types of phases in equilibrium are found with different alkaline earths (A  = Ca, Sr, Ba) at the nominal RuA(2)NdCu(2)O(8) composition. The variation in the mismatch of the A/Nd size does not lead to the formation of an ordered layered RuA(2)NdCu(2)O(8) compound. Chemical transport in an open system was used to vary the Ru content in the RuGd1212 samples during the annealing step. With an increase of the Ru mass transport to the sample, the composition can be driven beyond the limit of the homogeneity range. Systematic changes in the phase composition of the resulting sample were observed. The magnetic and superconducting transition temperatures vary in a systematic way and are attributed to a variation of the Ru content in the RuR1212 phase.

An insight into what superconducts in polycrystalline boron-doped diamonds based on investigations of microstructure.

The discovery of superconductivity in polycrystalline boron-doped diamond (BDD) synthesized under high pressure and high temperatures [Ekimov, et al. (2004) Nature 428:542-545] has raised a number of questions on the origin of the superconducting state. It was suggested that the heavy boron doping of diamond eventually leads to superconductivity. To justify such statements more detailed information on the microstructure of the composite materials and on the exact boron content in the diamond grains is needed. For that we used high-resolution transmission electron microscopy and electron energy loss spectroscopy. For the studied superconducting BDD samples synthesized at high pressures and high temperatures the diamond grain sizes are approximately 1-2 mum with a boron content between 0.2 (2) and 0.5 (1) at %. The grains are separated by 10- to 20-nm-thick layers and triangular-shaped pockets of predominantly (at least 95 at %) amorphous boron. These results render superconductivity caused by the heavy boron doping in diamond highly unlikely.