Observation of hexagonally ordered flux quanta in YBA sub 2 CU sub 3 O sub 7.
22 October 1987
The high resolution Bitter pattern technique has been used to reveal the structure of the array of flux lines which is present when single crystal samples of the high T sub c superconductor YBa sub 2 Cu sub 3 O sub 7 are placed in a magnetic field. At 4.2K with the magnetic field parallel to the c hat axis, the patterns formed are strongly reminiscent of the analogous structures in ordinary type II superconductors.The structures typically consist of flux spots with short range hexagonal correlations. We interpret the patterns to consist of singly quantized vortices. Very uniform patterns are observed in thin, flat samples cooled in a constant field. Sampling the spot density yields a vortex density consistent with a flux quantum of Hc/2e.