No abstract

No abstract

No abstract

A theoretical study on the voltage response function V of a series array of dc SQUIDs is presented in which the elementary dc SQUID loops vary in size and, possibly, in orientation. Such series arrays of two-junction SQUIDs possess voltage response functions vs. external magnetic field B that differ substantially from those of corresponding regular series arrays with identical loop-areas, while maintaining a large voltage swing as well as a low noise level. Applications include the design of current amplifiers and quantum interference filters.Recently series arrays of dc SQUIDs have been successfully exploited as current amplifiers with wide bandwidth, large dynamic range and low noise level 1 . Using thin-film Nb-technology, amplifiers consisting out of up to 10 3 identical dc SQUID loops have been fabricated. Such serial devices are characterized by large voltage swings of several mV and current-to-voltage transfer functions of some V /mA so that a direct connection to a room temperature preamplifier is feasible.At present only series arrays consisting out of identical dc SQUID loops have been described in the literature 1 . The dc voltage response function V of such regular series arrays displays a Φ 0 -periodicity just as a single dc SQUID where Φ 0 = h 2e is the elementary flux quantum 2 . However, this periodicity may represent a serious limitation for the modes of operation of such devices. To get a linear flux-to-voltage conversion a feedback circuit is used in most applications in which the dc SQUID acts as a null-flux detector. Furthermore special electronic devices and efficient background shielding is often required 2 . Here theoretical studies on the voltage response of arithmetic and irregular series arrays of dc SQUID loops are presented, in which the individual loopareas are not all equal. For such devices the advantages of dc SQUID series arrays are preserved but limitations due to the periodicity of the voltage response can be circumvented.The arrays under consideration consist of N two junction SQUID loops connected in series. The bias current I b is fed into the array as indicated in Fig.(1). In general the areas of the N loops in a generic array differ in size and, possibly, in orientation. Let a n be the orientated area element of the n th dc SQUID loop. The magnetic flux threading a n is then Φ n = B, a n , where B is the total magnetic field. Taking into account all inductive effects in the dc SQUID array the total magnetic field B = B (p) + B (s) + B (c) is a superposition of the primary magnetic field B (p) one wants to measure, the secondary magnetic field B (s) induced by currents that flow in the dc SQUID array and, possibly, the magnetic compensation field B (c) induced by compensation current(s) I comp flowing through a set of suitable orientated compensation coils or wires.As a special case a generic planar dc SQUID array is shown in Fig.(1). The primary magnetic flux is coupled into the individual dc SQUID loops by the signal or input current I inp which flows through a common in...

Superconducting quantum interference filters (SQIFs) are arrays of superconducting loops, with Josephson junctions, of diverse loop sizes. The dependence of the SQIF voltage on external magnetic fields is non-periodic and shows only one unique peak at zero field. We present several kinds of SQIFs-serial arrays, parallel arrays and various combinations of both-which are all realized with high-T c superconductors. Compared to a single SQUID all SQIF types show improved magnetic field sensitivity and noise-limited field resolution. In order to realize really sensitive magnetometers the SQIFs were coupled to pickup coils. Consequently, the sensitivity and noise of the SQIF-based magnetometers are comparable with that of their SQUID-based counterparts. For a flip-chip configuration using a serial-parallel SQIF and a superconducting flux transformer, a noise-limited field resolution of 70 fT Hz −1/2 was achieved. This is only twice the value of that using a normal SQUID. Further reduction should be possible by a better adaptation of the complete design to the SQIF options.