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Josephson Junction and SQUID

Josephson Junction, Circuit Josephson Junction, Mechanism The Josephson effect occurs in both bulk and 2-dimensional super-conductors when a piece of thin barrier (the junction) is inserted into the circuit. The weak link can be a thin insulating barrier (S-I-S) about 3 nm thick, a short section of non-superconducting metal (S-N-S) ~ 10 nm, or a physical constriction that weakens the superconductivity at the point of contact (S-s-S). The circuit consists of a biasing voltage V to control the current I flowing through the apparatus (Figure 13-08j).

Figure 13-08j Josephson Junction, Circuit

Figure 13-08k Josephson Junction, Mechanism [view large image]

Following is a summary on the mechanism of Josephson current (also see Figure 13-08k) :

SQUID SQUID is a very sensitive instrument for measuring magnetic flux which is closely related to magnetic field. Figure 13-08l,c shows the range of measurable objects below and above the non-SQUID limit. It is made of two Josephson junctions on each side of a loop. The flux through the hole is detected via its relationship with the output current or voltage as shown in detail below. The superconductive materials can be either type I or type II (Figure 13-08l,a,b). Type I superconductors require very low temperature for the transition. It depends on the formation of Cooper pairs to attain the superconductive state (see list of Type I Critical Temperatures). The type II super-components are the vortices (see 2-D Superconductivity). The critical temperature for type II materials in 2-D is even lower. However, many compounds exhibit superconductive at much high Critical Temperature (see list of type II superconductors).

Figure 13-08l SQUID
[view large image]

SQUID Flux-locked SQUID Mechanism The SQUID is a tiny equipment with a planar area of about 10-3m x 10-3m. It has to be kept inside a dewar with liquid helium (< 4.2K) or nitrogen (< 77K) to maintain the superconductive state (see Figure 13-08m,a for the basic components). It relies on the conversion of magnetic flux to voltage for performing the measurement.

Figure 13-08m SQUID Flux-locked Loop
[view large image]

Figure 13-08n SQUID, Operating Principle [view large image]

See a detailed description of a commercial SQUID system (as shown in Figure 13-08m,a).