G. P. Espinosa

A new family of ternary intermetallic superconducting/magnetic stannides

Abstract A new family of rare earth-rhodium-tin intermetallic compounds, with the representative formula (RE)Rh x Sn y , has been synthesized in single crystal form. The compounds containing the heavier rare earths are superconducting and those with the lighter rare earths are generally magnetic. The compound ErRh 1.1 Sn 3.6 exhibits reentrant superconductivity with T c = 0.97 K and T m = 0.57 K as determined from ac magnetic susceptibility measurements. The synthesis and X-ray characterization of the series are described and the results of electrical resistivity, upper critical magnetic field, magnetic susceptibility, specific heat and neutron scattering measurements on the Er compound are given.

Anisotropic critical current density in superconducting Bi2Sr2CaCu2O8 crystals

Critical current densities and resistivities were measured in a single crystal of the high Tc superconductor Bi2Sr2CaCu2O8 within the ab basal plane (∥) and along the c direction (⊥). A large anisotropy in critical current density is found, Jc∥/Jc⊥≊103, in quantitative agreement with the large normal‐state resistivity anisotropy near Tc. The data provide strong evidence for a two‐dimensional layered structure of metallic planes separated by semi‐insulating barriers.

Raman and optical absorption studies of the pressure-induced zircon to scheelite structure transformation in TbVO4 and DyV04

Abstract The pressure-induced phase transition in terbium vanadate (TbVO 4 ) and dysprosium vanadate (DyVO 4 ) from the zircon-type structure D 19 4 h to the scheelite-type structure C 6 4 h has been studied at room temperature by high-pressure Raman spectroscopy in a diamond anvil cell. The pressure dependencies of the Raman active modes are presented up to 18 GPa in TbVO 4 and 10 GPa in DyVO 4 . From these data the transition pressures are found to be 6.6 ± 0.6 GPa for TbVO 4 and 6.5 ± 0.3 GPa for DyVO 4 . X-ray powder diffraction studies confirm that the high-pressure phase, quenchable in polycrystalline form to atmospheric pressure in both cases, has the scheelite-type structure. The effect of pressure on the optical absorption edge was also investigated. It is found that d E g /d P is linear to 30 GPa, with slopes of −37 meV GPa -1 for TbVO 4 and −33meV GPa -1 for DyVO 4 . These results are discussed in terms of the phase transition kinetics, and changes in the electronic structure both at the transition and in the scheelite-type phase.

Crystal growth and crystal-chemical investigation of systems containing new superconducting and/or magnetic ternary stannides

Abstract Single crystals of compounds in the systems (RE)RhSn, (RE)IrSn, (RE)RuSn, (RE)CoSn, (M)RhSn, (M)IrSn, (M)RuSn and (M)CoSn where (RE) = rare earth and M = Mg, Ca, Sr, Sc, Y, Zn, Cd, In or Th have been grown from a tin solvent in vacuo. Phase relationships within and among these ternary systems are described for limited ranges of composition. The transition temperatures of several superconducting and/or magnetic compounds are reported.

Reversible reconstruction and crystallization of GeSe2 glass

Abstract Absorption of photons at low power levels below the band edge reversibly alters the molecular structure of GeSe 2 glass. Several stages of ordering, including the formation of microcrystallites, have been observed by Raman scattering. The absorbed photons systematically increase the chemical ordering of the glass by breaking like-atom bonds and replacing them with GeSe bonds.

Isomorphs of the superconducting/magnetic ternary stannides

Abstract Single crystals of some new ternary stannides in the systems (M)CoSn, (M)RuSn and (M)OsSn, where M = rare earth, Y. Sc, Ca, Sr, or Th have been grown from a tin solvent in vacuo. We find the new compounds to be isomorphous to the published rare earth rhodium tin compounds. The ternary, TmOs x Sn y , has been found to be a reentrant superconductor with T c = 1.1K and T m = 0.6K. Superconducting and/or magnetic transition temperatures and lattice parameters are given. A tabular summary of identified phases and their lattice constants for all the ternary stannides prepared so far is included for comparative purposes.

Crystal chemistry, growth and reentrant behavior of additional superconducting/magnetic stannides

Single crystals of compounds in the systems (M)-Os-Sn, (M)-Os-Pt-Sn, (M)-Pt-Sn; (M)-Ru-Sn, (M)-Ru-Pd-Sn and (M)-Pd-Sn, with M = Er, Y, La, Ca or Yb, have been grown from a tin solvent in vacuo. The compound ErOsxSny has been found to be a reentrant superconductor with Tc ⋍ 1.25 K and Tm ⋍ 0.45 K. Lattice parameters, superconducting and magnetic transition temperatures, and chemical analyses are given.

Distance distributions about Mn in icosahedral and crystalline Al Mn Si

Abstract We report EXAFS measurements on icosahedral and orthorhombic Al 6 Mn, icosahedral Al 74 Mn 20 Si 6 , and α-Al 100 Mn 24 Si 14 . We find close resemblance between α-phase and i -Al 74 Mn 20 Si 6 , and poorer agreement between α-phase and i -Al 6 Mn. Neither icosahedral phase showed much resemblance to the orthorhombic phase. We conclude that the icosahedral Al 74 Mn 20 Si 6 resembles a distorted α-phase, and so may be built of icosahedral glued together with Al as in the α structure. Icosahedral Al 6 Mn is perhaps a more disordered version of the above.

Magnetic and superconducting properties of rare earth osmium stannides

Abstract We present data on the magnetic and superconducting properties of rare earth osmium stannides. The compounds of Tb and Ho are superconducting only, those of Er and Tm are reentrant superconductors, and those of Gd and Dy appear to exhibit some type of short range magnetic order at low temperatures.

Raman scattering study of glass crystallization kinetics

Abstract Laser induced glass-crystalline transition is studied by light scattering. Three significant effects are observed depending on the incident laser energy density: (i) Spectral band narrowing indicating cluster enlargement constitutes a precursor effect , (ii) an intensity increase effect indicates a rapid rise of the density of clusters attaining microcrystalline size and (iii) a dynamical reversal effect indicative of glass-crystalline instability. Cluster volume and crystallization appear as separate but related threshold phenomena.