Youwen Xu

Direct observation of the atomic structure of a twin boundary layer in a superconducting Y-Ba-Cu-O ceramic

Abstract Direct structural images of a twin boundary in a superconducting Y–Ba–Cu–O ceramic have been obtained at 0·19 nm resolution by high-resolution transmission electron microscopy and an image processing technique. It was found that the a and b planes of a twin domain gradually change to the b and a planes of a neighbouring twin domain across a twin boundary. This involves a shifting of the respective planes by a distance (⅓∼½)d(110) along the boundary as well as the rotation of the planes by ⊘ ≃ 2(b−a)/(a + b) across the boundary. Also, this transition takes place within approximately four times the (110) interplanar spacing d (100), which is considered as the twin boundary layer thickness.

TEM studies on twin boundary in YBa 2 Cu 3 O 7 and YBa 2 (Cu 0.98 M 0.02 ) 3 O 7 (M = Zn, Al)

High resolution electron microscopy of the twin boundary layers in YBa{sub 2}(Cu{sub 1{minus}{ital x}}M{sub {ital x}}){sub 3}O{sub 7+{delta}} for {ital x}=0 and 0.02 and M=Zn and Al showed that the boundary widths are {similar to}1 nm for the pure and the Zn substituted YBa{sub 2}Cu{sub 3}O{sub 7} and {similar to}3 nm for the Al substituted oxide. From two beam contract studies and the high resolution images, it was concluded that the respective lattice plane across the boundary is shifted by (1/3{endash}{1/2}){center dot}2{ital d}{sub (110)} along the boundary. The broadening of the layer by the Al substitution is also thought to be a cause for the reduction of the twin boundary energy leading to an increased twin density.

TEM studies on twin boundary in YBa sub 2 Cu sub 3 O sub 7 and YBa sub 2 (Cu sub 0. 98 M sub 0. 02 ) sub 3 O sub 7 (M=Zn, Al)

High resolution electron microscopy of the twin boundary layers in YBa{sub 2}(Cu{sub 1{minus}{ital x}}M{sub {ital x}}){sub 3}O{sub 7+{delta}} for {ital x}=0 and 0.02 and M=Zn and Al showed that the boundary widths are {similar to}1 nm for the pure and the Zn substituted YBa{sub 2}Cu{sub 3}O{sub 7} and {similar to}3 nm for the Al substituted oxide. From two beam contract studies and the high resolution images, it was concluded that the respective lattice plane across the boundary is shifted by (1/3{endash}{1/2}){center dot}2{ital d}{sub (110)} along the boundary. The broadening of the layer by the Al substitution is also thought to be a cause for the reduction of the twin boundary energy leading to an increased twin density.

Superconductivity of La(2-x)BaxCuO4

Superconductivity near 30 K in the La-Ba-Cu-O system, first observed by Bednorz and Muller [1], was soon found to be attributable to the K 2 NiF 4 -type (at room temperature) compound La(2-x)B x xCuO 4 [2]. Substitution of Sr and Ca for Ba yielded superconducting systems, also with high superconducting transition temperature T c . Replacement of La by Y led to the discovery of the 90 K superconductors.