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Divalent Eu (4f;{7}, J=7/2) possesses a strong local magnetic moment which suppresses superconductivity. Under sufficient pressure it is anticipated that Eu will become trivalent (4f;{6}, J=0) and a weak Van Vleck paramagnet, thus opening the door for a possible superconducting state, in analogy with Am metal (5f;{6}, J=0) which superconducts at 0.79 K. We present ac susceptibility and electrical resistivity measurements on Eu metal for temperatures 1.5-297 K to pressures as high as 142 GPa. At approximately 80 GPa Eu becomes superconducting at T_{c} approximately 1.8 K; T_{c} increases linearly with pressure to 2.75 K at 142 GPa. Eu metal thus becomes the 53rd known elemental superconductor in the periodic table.

In those cases where charge-stripe order has been observed in cuprates, the crystal structure is such that the average rotational symmetry of the CuO2 planes is reduced from fourfold to twofold. As a result, one could argue that the reduced lattice symmetry is essential to the existence of stripe order. We use pressure to restore the average fourfold symmetry in a single crystal of La1.875Ba0.125CuO4, and show by x-ray diffraction that charge-stripe order still occurs. Thus, electronically driven stripe order can spontaneously break the lattice symmetry.

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Silver zirconium nitride films deposited by unbalanced magnetron sputtering were studied by means of x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, spectroscopic ellipsometry, and nanoindentation. Coatings were deposited on silicon substrates at room temperatures with bias voltages in the −45 to −160 V range. The concentration of zirconium and silver was regulated by controlling the power to the sputtering guns. The nitrogen concentration was selected so that the nitrogen flow rate corresponded to the production of stoichiometric zirconium nitride. The films consisted of nanocrystals of zirconium nitride embedded in a silver matrix. The grain size was deduced from the width of the XRD peaks using the Scherrer formula and was found to decrease with the addition of silver. The chemical and phase composition was determined from XPS measurements. The optical constants were measured using spectroscopic ellipsometry. A correlation between film structure/composition and optical constants was established. The mechanical properties of the coatings were evaluated using nanohardness testing and were found to depend on composition and deposition parameters. Optimum mechanical properties were achieved for a silver content of 6% and a substrate bias of −160 V.

The thermal expansion coefficient α of MgB2 is revealed to change from positive to negative on cooling through the superconducting transition temperature Tc. The Grüneisen function also becomes negative at Tc followed by a dramatic increase to large positive values at low temperature. The results suggest anomalous coupling between superconducting electrons and low-energy phonons.PACS numbers: 74.70.Ad, 74.62.Fj Superconductivity in the binary compound MgB 2 near 39 K is a fascinating development. Over the last few years, scientists have argued that a lattice instability [1] and/or anomalous phonon behavior [2] might be responsible for the high transition temperature. Specific attention has focused on the E 2g phonon, a bond-stretching phonon within the plane of the hexagonal crystal structure [2,3,4]. In-plane tensile strain, induced by lattice mismatch through thin-film growth, increases the superconducting transition temperature T c to 41.8 K; this enhancement was attributed to a decrease in the E 2g phonon frequency [5]. Phonons can be studied with techniques such as Raman spectroscopy [3,4] and heat capacity [6,7,8]. Often neglected in the study of phonons is thermal expansion, partly because of the exceptional resolution needed to resolve the transition at T c .Thermal expansion from powder diffraction measurements [9, 10] of MgB 2 have revealed an anomalous volume expansion on cooling below T c . However, highresolution thermal expansion measurements (dilatometry) with a relative sensitivity approximately four orders of magnitude better than powder diffraction are required for meaningful thermodynamic analysis. Such measurements of polycrystalline MgB 2 were reported [11], but discrepancies with the diffraction data, such as the temperature at which the thermal expansion coefficient α changes from positive to negative, are apparent.In this Letter, high-resolution thermal expansion measurements of polycrystalline MgB 2 are presented. The results reveal a change in sign of α at T c , with negative thermal expansion below T c ; these data agree with diffraction investigations [9,10], but offer exceedingly greater detail. Analysis of the bulk Grüneisen function reveals anomalous behavior due to dominant low-energy phonon modes. The change in sign of α at precisely T c suggests a connection between these phonon modes and superconductivity.MgB 2 , synthesized with 11 B as described previously [12], was pelletized (diameter = 4.6 mm), placed in a boron nitride crucible and heated to 800• C for 30 min at 3 GPa using a cubic multi-anvil press. A very thin black layer, impurities from surface reaction with boron nitride, was removed, leaving behind a brilliant goldcolored MgB 2 sample with density 2.56 g/cm 3 (100% of theoretical density). Heat capacity was measured with a thermal-relaxation technique. T c versus pressure was determined inductively (0.12 Oe rms field at 1023 Hz) to 0.63 GPa using a helium pressure medium; a manganin sensor at room temperature served as a manometer [13].Thermal expansion was meas...

Whereas dhcp La superconducts at ambient pressure with T c ≃ 5 K, the other trivalent d-electron metals Sc, Y, and Lu only superconduct if high pressures are applied. Earlier measurements of the pressure dependence of T c for Sc and Lu metal are here extended to much higher pressures. Whereas T c for Lu increases monotonically with pressure to 12.4 K at 174 GPa (1.74 Mbar). T c for Sc reaches 19.6 K at 107 GPa, the 2nd highest value observed for any elemental superconductor. At higher pressures a phase transition occurs whereupon T c drops to 8.31 K at 111 GPa. The T c (P ) dependences for Sc and Lu are compared to those of Y and La. An interesting correlation is pointed out between the value of T c and the fractional free volume available to the conduction electrons outside the ion cores, a quantity which is directly related to the number of d electrons in the conduction band.

A search for superconductivity has been carried out on the hexagonal polymorph of Laves-phase CaLi(2), a compound for which Feng, Ashcroft, and Hoffmann predict highly anomalous behavior under pressure. No superconductivity is observed above 1.10 K at ambient pressure. However, high-pressure ac susceptibility and electrical resistivity studies to 81 GPa reveal bulk superconductivity in CaLi(2) at temperatures as high as 13 K. The normal-state resistivity displays a dramatic increase with pressure.