Scanning SQUID Microscope and the Half-Integer Flux Quantum Effect

It is possible to design tricrystal substrates of SrTiO in such a way that when high-Tc superconductors are grown epitaxially on them, they spontaneously generate magnetic fields with half of the conventional total magnetic flux. This image shows a scanning SQUID microscope image of the magnetic fields above such a tricrystal with 4 rings of YBCO epitaxially grown on it, imaged at 4.2K. The center ring, which has 3 grain boundary weak links, has a half-flux quantum trapped in it. The outer 2-junction, and 0-junction control rings have no flux trapped in them. The 4-leaf clovers indicate the orientations of assumed d-wave superconducting gaps, aligned with the underlying substrate.

This is a 3-dimensional rendering of this image. The center peak is asymmetric because of stray pickup from the leads of the SQUID microscope.

The half-flux quantum effect can be turned on and off by changing the geometry of the underlying substrate. This indicates that it has to do with the symmetry of the gap. All of our experiments have been consistent with d-wave symmetry.

By cooling in different fields, the rings can be trapped in different states. In this image the right top ring has 3 flux quanta. , the center ring has 1 1/2 flux quanta, and the left and bottom rings have 2 flux quanta. By doing this experiment a number of times, one can map out the allowed flux states. The 3-junction ring always has half-integer multiples of the elementary flux quantum, while the control rings always have integer multiples in them.

The measured magnetic fields can be deconvoluted to give the circulating supercurrents in the rings. The image shows the supercurrents in the 3-junction ring in the half-flux quantum state. The currents tend to peak at the outside edges of the rings. The peaks are artifacts due to defects in the outside edge of the ring.

Since the half-flux quantum state is intrinsic to the tricrystal point, it happens for continuous coverage of YBCO as well as rings. The image shows 7 conventional vortices trapped in the bulk, 4 integer Josephson vortices trapped in the grain boundaries, and a half-flux quantum trapped at the tricrystal point of a blanket coverage of YBCO on a SrTiO tricrystal.

The same effect has been observed in thallium films. This indicates that this effect does not depend on the special properties of YBCO.

Superconductivity Home Page | T.J. Watson Research Center | Zurich Research Laboratory |
IBM home page | Order | Search | Contact IBM | Help | (C) | (TM) ]