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Studies and application of quantum interference detectors.

Alistair A. Fife

Published Jan 1, 1971• 0 citations
OpenAlex

Abstract

c c e s s f u l l y developed and t h e i r p r o p e r t i e s s t u d i e d .The SQUIDS u t i l i s e f l u x q u a n t i s a t i o n and t h e Josephson e f f e c t s i n a r i n g ( o r r i n g s ) c o n t a i n i n g a weak l i n k s e c t i o n .The r i n g s may b e e i t h e r p u r e l y superconducting (M-SQUID) o r p a r t i a l l y r e s i s t i v e (R-SQUID) .The M-SQUIDS a r e u s e f u l a s extremely s e n s i t i v e magnetometers f o r measuring d c and a c f l u x .Versions o f t h e M-SQUID which use p o i n t -c o n t a c t weak l i n k s have been found t o b e r a t h e r s e n s i t i v e t o v i b r a t i o n s and t h e i r prop e r t i e s t e n d t o degrade o v e r a p e r i o d o f months.T h i s h a s l e d t o t h e development of reasonably s t a b l e , rugged d e v i c e s employing weak l i n k s o f NbSeZ, a superconducting l a y e r s t r u c t u r e .The R-SQUIDS r e p o r t e d h e r e employ Nb-Nb p o i n t c o n t a c t s , and have been used a s t h e superconducting i n p u t f o r a s e n s i t i v e superheterodyne r e c e i v e r f o r f r e q u e n c i e s i n t h e range l o l o t o 1011 Hz. S e v e r a l of t h e s e d e v i c e s w e r e cons t r u c t e d and t e s t e d i n t h e frequency ranges ( 2 6 .5 -40) GHz and (67 -73) GHz.The d e v i c e response i s complicated by t h e e f f e c t s on t h e AC Josephson c u r r e n t of d i s t r i b u t e d c i r c u i t elements and a range of complex behaviour i s p r e s e n t e d which i s o n l y p a r t i a l l y understood.T h e l o g i c behind developing t h e l a t t e r d e t e c t o r h a s been t o s e a r c h f o r s t i m u l a t e d emission of energy gap r a d i a t i o n from superconducting t u n n e l j u n c t i o n s .The R-SQUID was t o b e used f o r frequency and l i n e w i d t h measurements of t h e expected energy gap r a d i a t i o n .Broad band s e n s i t i v i t y w a s provided by a helium cooled S i l i c o n bolometer.D e s p i t e e x t e n s i v e s t u d i e s u s i n g a l a r g e v a r i e t y of t u n n e l j u n c t i o n p a r a m e t e r s , t h e s e a r c h h a s t u r n e d o u t t o b e completely n e g a t i v e i n c o n t r a s t t o t h e measurements of o t h e r a u t h o r s .A thorough a n a l y s i s o f t h e e a r l i e r t h e o r y and experiments h a s l e d t o t h e c o n c l u s i o n t h a t a s t i m u l a t e d e m i s s i o n p r o c e s s i n superconductors i s r a t h e r u n l i k e l y , and e a r l i e r e x p e r iments have t o be i n t e r p r e t e d d i f f e r e n t l y .......................... 3.3 Detection Electronics 72 3.4 Flux Patterns and Sensitivity .................

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c c e s s f u l l y developed and t h e i r p r o p e r t i e s s t u d i e d .The SQUIDS u t i l i s e f l u x q u a n t i s a t i o n and t h e Josephson e f f e c t s i n a r i n g ( o r r i n g s ) c o n t a i n i n g a weak l i n k s e c t i o n .The r i n g s may b e e i t h e r p u r e l y superconducting (M-SQUID) o r p a r t i a l l y r e s i s t i v e (R-SQUID) .The M-SQUIDS a r e u s e f u l a s extremely s e n s i t i v e magnetometers f o r measuring d c and a c f l u x .Versions o f t h e M-SQUID which use p o i n t -c o n t a c t weak l i n k s have been found t o b e r a t h e r s e n s i t i v e t o v i b r a t i o n s and t h e i r prop e r t i e s t e n d t o degrade o v e r a p e r i o d o f months.T h i s h a s l e d t o t h e development of reasonably s t a b l e , rugged d e v i c e s employing weak l i n k s o f NbSeZ, a superconducting l a y e r s t r u c t u r e .The R-SQUIDS r e p o r t e d h e r e employ Nb-Nb p o i n t c o n t a c t s , and have been used a s t h e superconducting i n p u t f o r a s e n s i t i v e superheterodyne r e c e i v e r f o r f r e q u e n c i e s i n t h e range l o l o t o 1011 Hz. S e v e r a l of t h e s e d e v i c e s w e r e cons t r u c t e d and t e s t e d i n t h e frequency ranges ( 2 6 .5 -40) GHz and (67 -73) GHz.The d e v i c e response i s complicated by t h e e f f e c t s on t h e AC Josephson c u r r e n t of d i s t r i b u t e d c i r c u i t elements and a range of complex behaviour i s p r e s e n t e d which i s o n l y p a r t i a l l y understood.T h e l o g i c behind developing t h e l a t t e r d e t e c t o r h a s been t o s e a r c h f o r s t i m u l a t e d emission of energy gap r a d i a t i o n from superconducting t u n n e l j u n c t i o n s .The R-SQUID was t o b e used f o r frequency and l i n e w i d t h measurements of t h e expected energy gap r a d i a t i o n .Broad band s e n s i t i v i t y w a s provided by a helium cooled S i l i c o n bolometer.D e s p i t e e x t e n s i v e s t u d i e s u s i n g a l a r g e v a r i e t y of t u n n e l j u n c t i o n p a r a m e t e r s , t h e s e a r c h h a s t u r n e d o u t t o b e completely n e g a t i v e i n c o n t r a s t t o t h e measurements of o t h e r a u t h o r s .A thorough a n a l y s i s o f t h e e a r l i e r t h e o r y and experiments h a s l e d t o t h e c o n c l u s i o n t h a t a s t i m u l a t e d e m i s s i o n p r o c e s s i n superconductors i s r a t h e r u n l i k e l y , and e a r l i e r e x p e r iments have t o be i n t e r p r e t e d d i f f e r e n t l y .......................... 3.3 Detection Electronics 72 3.4 Flux Patterns and Sensitivity .................
Several resistive SQUID frequency converters (R-SQUID) have been constructed and their properties studied in the frequency regimes 26.5–40 and 67–73 GHz. A brief outline of the theory of the device is presented together with constructional details for a Nb–Nb point-contact version. The device response is complicated by the effects on the ac Josephson current of distributed circuit elements, and a broad range of complex behavior is presented which is only partially understood. A useful amplification effect for the detected signal has been discovered, which gives the device good broad range sensitivity (∼ 10−7–∼ 10−12 W/Hz1/2) and reasonably narrow linewidths (Q-values of ∼ 104 at 70 GHz).
We report on the fabrication and characterization of micrometer-sized superconducting quantum interference devices (SQUIDs) based on nanoscale vanadium/copper/vanadium Josephson weak links. Magnetically driven quantum interference patterns have been measured for temperatures in the 0.24–2 K range. As DC SQUIDs, these devices obtain flux-to-voltage transfer function values as high as 450 μV/Φ0 leading to promising magnetic flux resolution ΦN<3 μΦ0/Hz, being here limited by the room temperature preamplification stage. Significant improvement in the flux noise performance figures is expected with the adoption of cryogenic preamplification. The presented devices are suitable for operation as small-area SQUIDs at sub-Kelvin temperature, but their design can also be upscaled to include input coils enabling their use as sensitive magnetometers via the adoption of optimized electronic readout stages based on flux feedback schemes.
Three Nb/AlO/sub x//Nb SIS detectors, designed to operate in the 400-550, 550-700, and 600-750 GHz bands, have been studied in direct detection mode using a Fourier-transform spectrometer. All three detectors were of quasi-optical type and had on-chip-integrated-fixed tuned SIS junctions. The tuning ranges of the detectors were selected to cover the interesting region around the superconducting gap frequency of Nb (about 700 GHz). Measurements show detector responses at frequencies above the gap frequency, i.e., up to /spl ap/920 GHz, and that cooling the detectors to 3.1 K improved the direct detection responses about 15% below 700 GHz and about 50% for frequencies up to 800 GHz, compared to the responses at 4.2 K. The 500 GHz SIS detector was also studied in a 440-520 GHz heterodyne receiver set up. Good agreement between modeled tuning circuit characteristics, tuning range of the mixer and the direct detection response bandwidths were found. However, it is essential that the dispersion of the field penetration depth into the superconductor is included in the modeling of the tuning circuits when the detector is operated at frequencies above the superconducting gap.>