Download Mr. SQUID User`s Guide

Mr. SQUID User’s Guide
More recently, more controlled junctions have been developed, enabling one to produce dc
SQUIDs with relatively predictable characteristics. Your Mr. SQUID® chip is the first
commercialization of this new technology.
5.14 The SQUID in Mr. SQUID®
The SQUID in the Mr. SQUID® probe is a thin film washer of YBCO superconductor broken in
two places by Josephson junctions. The YBCO is patterned into a washer in order to take
advantage of the Meissner Effect, which is the ability of a superconductor to expel magnetic flux
from its interior. Because most of the magnetic flux generated by either of the Mr. SQUID®
modulation coils (see Figure 3-6) is forced into the center hole of the SQUID, the inductive
coupling of each coil to the SQUID is similar, even though their physical sizes are different.
(This is not exactly true because the coils do not make completely closed loops.) This fluxfocusing can be used to allow multi-turn spiral coils to be fabricated over a SQUID washer yet
have each turn contribute the same amount of flux.
H
H
(b)
(a)
Figure 5-11 (a) Magnetic flux penetrating the body of the SQUID while in the normal state
above Tc. (b) Flux-focusing produced by the Meissner Effect when the SQUID is in the
superconducting state below Tc.
The HTS Josephson junctions made to date appear to fall into two categories: S–N–S junctions
and grain boundary junctions. Some of the best results – particularly in the context of SQUID
performance – have been obtained for grain boundary junctions. In these devices, the Josephson
effect occurs because of the existence of a crystallographic grain boundary in a thin film that
allows two grains to couple only weakly together. Examples of these are naturally occurring
grain boundary junctions, step-edge junctions (in which YBCO films develop grain boundaries
as they grow over sharp steps in a substrate) and bi-crystal junctions (in which grain boundaries
in a substrate are replicated in an overlaying film). Researchers at IBM first developed bi-crystal
grain boundary junctions.5 In these devices, a junction was created by growing YBCO films on
5“Noise
characteristics of single grain boundary junction DC SQUIDs in Y1Ba2Cu3O7-d films,”
R. Gross, P. Chaudhari, M. Kawasaki, M. Ketchen, A. Gupta, Physica C 170, 315 (1990).
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