Atomic Scale Structure of Twin Boundary In Y-Ba-Cu-O Superconductors
- PDF / 2,418,857 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 68 Downloads / 123 Views
ATOMIC SCALE STRUCTURE OF TWIN BOUNDARY IN Y-Ba-Cu-O SUPERCONDUCTORS Yimei Zhu, M.Suenaga, Youwen Xu, and M.Kawasaki* Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973 *JEOL USA, Inc. Peabody, MA 01960 ABSTRACT High resolution electron microscopy of the twin boundary layers in YBa2(Cu,-Mx,)307-6 for x=0 and 0.02 and M=Zn, Fe and Al showed that the boundary widths are -1 nm for the pure and the Zn substituted YBa2Cu 30 7 , 2.5-3 nm for the Fe and Al substituted oxide. It was found that the lattice plane is shifted across the twin boundary by (1/3 - 1/2) -2d(10) along the boundary. The broadening of the layer for the cases of Fe and Al is also thought to be associated with a reduction of the twin boundary energy, which also leads to an increased twin density. INTRODUCTION Since the discovery of high T, superconductivity in YBa2Cu 3 0 7 , twinning has been presumed to be important in determining superconducting properties, particularly critical current density. For example, it has been argued that the twin boundary is an area of weak superconductivity", 2 or flux pinning sites.3,4 On the other hand, some reports suggest that the twin boundary may act as a diffusion path for oxygen.,"6 It is therefore of interest to study the twin boundary in detail to understand the correlation between the structure and superconducting properties. Our previous electron diffraction studies 7 on twin boundaries in YBa2(Cu-,,M,)307-6, where M=Zn, Al, Fe and Ni, indicated the existence of non-orthorhombic twin boundary layers. The layer thickness, which depends on the doping element and oxygen content, varied from ,-.1 nm for a pure YBa2Cu 3 0 7 and M=Al, and Zn and x=0.02, to -,2.5 nm for M=Ni and Fe and x=0.02. Furthermore, direct observation of a twin boundary by high resolution electron microscopy and optical diffraction from a single twin boundary supported the existence of such a twin boundary layer."' 9 In the present paper, we report a further investigation of the structure and of the nature of strain associated with the twin boundary in YBa2Cu30 7. In particular, the structure of the boundary for a pure oxide will be compared with those for which 2% of Cu is replaced with Zn, Fe and Al. EXPERIMENTAL High-quality bulk samples of Y-Ba-Cu-0 were produced by sintering the powders as described elsewhere."0 The HREM specimens were prepared by crushing bulk samples into fine fragments in acetone, and then depositng onto a holey carbon film. Electron microscopes employed were JEOL JEM-2000FX, JEM- 2000EX and JEM-2010, all operated at 200kv. RESULTS 1. Pure YBa2Cu3 07 Fig 1(a) is a typical multi-beam image of a twin boundary seen edge on in YBa2Cu30 7 at a direct magnification of 100,000 x. Thin lines of contrast at twin boundaries are clearly visible. Unless the twin boundary had been damaged (including the loss and disordering of oxygen at twin boundary) by the electron beam irradiation, such sharp and straight twin boundary contrast was always observed under multi-beam imaging conditions in YBa2Cu 3 0 7 6 (
Data Loading...
