Incommensurate superlattice and 90° twist boundaries in the superconducting phase of Bi-Sr-Ca-Cu-O

We have observed an incommensurate superlattice structure in the Bi-Sr-Ca-Cu-O system using electron diffraction. The wave vector of the superlattice, q=(0,1/4.7,1), has an incommensurate component along the b axis and a commensurate component along the c axis. Twist boundaries perpendicular to the c axis, across which the crystallographic a and b axes are rotated 90°, are commonly observed in this system

X-Ray-Diffraction Study of Charge-Density-Waves and Oxygen-Ordering in YBa2Cu3O6+x Superconductor

We report a temperature-dependent increase below 300 K of diffuse superlattice peaks corresponding to q_0 =(~2/5,0,0) in an under-doped YBa_2Cu_3O_6+x superconductor (x~0.63). These peaks reveal strong c-axis correlations involving the CuO_2 bilayers, show a non-uniform increase below \~220 K with a plateau for ~100-160 K, and appear to saturate in the superconducting phase. We interpret this unconventional T-dependence of the ``oxygen-ordering'' peaks as a manifestation of a charge density wave in the CuO_2 planes coupled to the oxygen-vacancy ordering.Comment: 4 pages, 4 figure

To wet or not to wet: that is the question

Wetting transitions have been predicted and observed to occur for various combinations of fluids and surfaces. This paper describes the origin of such transitions, for liquid films on solid surfaces, in terms of the gas-surface interaction potentials V(r), which depend on the specific adsorption system. The transitions of light inert gases and H2 molecules on alkali metal surfaces have been explored extensively and are relatively well understood in terms of the least attractive adsorption interactions in nature. Much less thoroughly investigated are wetting transitions of Hg, water, heavy inert gases and other molecular films. The basic idea is that nonwetting occurs, for energetic reasons, if the adsorption potential's well-depth D is smaller than, or comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At the wetting temperature, Tw, the transition to wetting occurs, for entropic reasons, when the liquid's surface tension is sufficiently small that the free energy cost in forming a thick film is sufficiently compensated by the fluid- surface interaction energy. Guidelines useful for exploring wetting transitions of other systems are analyzed, in terms of generic criteria involving the "simple model", which yields results in terms of gas-surface interaction parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy

Superlattice modulation and epitaxy of Tl\u3csub\u3e2\u3c/sub\u3eBa\u3csub\u3e2\u3c/sub\u3eCa\u3csub\u3e2\u3c/sub\u3eCu\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e10\u3c/sub\u3e thin films grown on MgO and SrTiO\u3csub\u3e3\u3c/sub\u3e substrates

We compare the superlattice modulation, microstructure, and epitaxy of Tl2Ba2Ca2Cu3O10 thin films grown on MgO [100] and SrTiO3 [100] substrates by dc diode sputtering. Films grown on MgO were found to be quite clean with the c axis perpendicular to the substrate. However, no in-plane orientational relationship to the substrate was found. Films grown on SrTiO3, on the other hand, showed very good epitaxy to the substrate despite the presence of second phases. Films grown on MgO also exhibited a longer coherence length of the superlattice modulation than those grown on SrTiO3 under identical conditions