8.2.2.1 Flux Transformer and Gradiometer The magnetic field sensitivity of a SQUID depends on its effective area.. Using a superconducting flux transformer, one can increasethe magnetic
Trang 1High-Temperature SQUID Magnetometer
In spite of the commercial availability of low-T cSQUID for several years, theSQUID-based applications did not gain widespread acceptability This has beenmainly due to the inconvenience of its operation at liquid-helium temperature
The feasibility of the fabrication of high-T cSQUID operating at 77 K was
soon demonstrated after the discovery of high-T csuperconductors (HTS) Sincethen, there has been continuous progress in this area (4–6) Several novel ap-proaches have been conceived and applied for improving the performance of the
high-T cSQUIDs HTS SQUID magnetometers exhibiting magnetic field ity 10–50 fT/Hz1/2in the white-noise region at 77K have been demonstrated
sensitiv-(5) Several companies have started commercializing high-T cSQUIDs (7) HTS
Trang 2SQUID magnetometers and gradiometers have been successfully used in the tection of biomagnetic signals from the heart and brain (8,9), nondestructive test-ing of deep buried flaws in metallic specimens (10), geophysical applications (11),and several other novel applications such as in biological immunoassays (12), sen-tinels-lymph node biopsy (13), and so forth.
de-This chapter presents a review of the developments of high-T cSQUID netometers and discusses their applications in different areas
mag-8.2 SUPERCONDUCTING QUANTUM
INTERFERENCE DEVICE
The SQUID is an ultrasensitive magnetic flux sensor that converts magnetic fluxinto voltage There are two types of SQUIDs: dc-SQUID and rf-SQUID (1) A dc-SQUID consists of a superconducting loop interrupted by two Josephson junc-
F IGURE 8.1 (a) Schematic of a dc-SQUID Two Josephson junctions are shown by in the superconducting ring, (b) Variation of voltage of the dc- SQUID with the applied flux ( ) for a constant bias current, (c) Schematic of
an rf-SQUID and (d) variation of peak amplitude of Vrfwith the applied flux ( ) for a fixed rf bias current.
Trang 3tions, as shown in Figure 8.1a The prefix dc implies that it is biased with a directcurrent Both Josephson junctions in the dc-SQUID have identical characteristics.Critical current of the dc-SQUID is an oscillatory function of the applied flux with
a period of one flux quantum, 0 The value of one flux quantum, 0(h/2e), is
2 107G/cm2 When the dc-SQUID is biased with a dc current I B I c, where
I cis the critical current of the Josephson junction, the voltage across the SQUIDshows an oscillatory function of applied magnetic flux with the periodicity of 0(Fig 8.1b)
The rf-SQUID consists of one Josephson junction in the superconductingring as shown in Figure 8.1c The rf-SQUID is biased with an rf current applied tothe SQUID through a inductively coupled tank circuit Here also for an appropri-ate biasing, the rf voltage across the SQUID oscillates as a function of magneticflux having a periodicity of 0(Fig 8.1d)
8.2.1 Designing of SQUID
The performance of a SQUID depends on the characteristics of the Josephson
junction and inductance of the SQUID loop, LSQ For the Josephson junction,
im-portant parameters are the critical current (I c ), the capacitance (C), and the shunt resistance (R) Designing of a highly sensitive SQUID requires careful selection
of the appropriate values of LSQ, I c , R, and C (14,15).
Noise due to thermal fluctuation puts an upper limit on the selection of the
value of LSQ The thermal noise power in the SQUID is 12 k B T/Hz and the mean
en-ergy per Hertz in an inductor is 12 LSQI n ; thus I n k B T/LSQ The correspondingequivalent flux noise is
2 L2
where k B is the Boltzman constant and T is the operating temperature of the
SQUID Due to this constraint of the thermal fluctuation noise, the SQUID effectcan be observed only if nis less than 0/2, where ... schematics of some of the high- T ctions that have been used in the fabrication of HTS SQUIDs
junc-Soon after the discovery of high- T csuperconductors,... (74), fabrication of several narrow parallelloops in place of a wide solid pickup loop (75,76, 79) , creation of slits with a sep-aration of m in the pickup loop (78), or fabrication of slotted flux... a function of magneticflux having a periodicity of 0(Fig 8.1d)
8.2.1 Designing of SQUID
The performance of a SQUID depends on the characteristics of the Josephson