Estimation of Inductance Value in SQUID

Takahiro Yamada, Masaaki Maezawa, Maruyama Michitaka +8 more

Derk Jan Adelerhof, H. Nijstad, J. Flokstra +1 more

Relaxation oscillation superconducting quantum interference devices (SQUIDs) (ROSs) and double relaxation oscillation SQUIDs (DROSs) have been fabricated and characterized. The SQUIDs are based on hysteretic Nb/Al,AlOx/Al/Nb Josephson tunnel junctions. The relaxation oscillations were simulated by computer. By these simulations the minimum and maximum values of the shunt inductance at which the relaxation oscillations persist have been determined for different values of the shunt resistance and of the critical current and capacitance of the junctions. The transfer from flux to dc voltage in ROSs increases linearly with the shunt resistance. The operation range of the shunt inductance, however, becomes smaller with increasing shunt resistance. In ROSs with a SQUID inductance of 20 pH, a maximum voltage modulation width of 400 μV and a flux-to-voltage transfer δV/δφ of 4 mV/φ0 have been obtained. Two ROSs have been operated in a simple flux-locked loop with direct voltage readout. The experimental white flux noise spectral density of about 3μφ0/√Hz is in good agreement with the theoretical model. The sensitivity of these SQUIDs is limited by so-called switching noise, which is noise due to the interaction between the relaxation oscillations and the Josephson frequency. In DROSs typical flux-to-voltage transfer values of 10–30 mV/φ0 and maximum values up to 80 mV/φ0 have been obtained. This transfer is determined by thermal noise on the critical currents of the SQUIDs. The effective flux noise density of DROSs in flux-locked-loop operation with direct voltage readout is in good agreement with the theoretical noise level which is mainly determined by thermal noise on the critical currents. In one of the DROSs with a SQUID inductance of 20 pH, a white flux noise of 1μφ0/√Hz has been obtained at an estimated relaxation frequency of about 80 MHz. This corresponds to an energy sensitivity of about 160 h.

Franco Bordoni, I. Modena, Romani G. L. +1 more

Victor I. Danilov, K. K. Likharev, O. Sniguiriev +1 more

The sensitivity of a two-Josephson-junction interferometer as a magnetic flux detector (dc SQUID) is estimated. The minimum value of the basic SQUID characteristic-noise equivalent temperature δT = (δΦ)2/kBL appears to be as low as for the best rf SQUID mode:(\deltaT)_{\min}/(\Deltaf)\cong 3 T/f_{o}. HereTis an operating temperature, fois a characteristic frequency of the Josephson junctions and\Deltafis a post-detector bandwidth. For a typical value of SQUID inductance L this limit corresponds to flux sensitivity of the order of 10-7Φo/Hz1/2.

Derk Jan Adelerhof, Jun Kawai, Gen Uehara +1 more

Double relaxation oscillation superconducting quantum interference devices (SQUIDs) (DROSs) have been fabricated with estimated relaxation frequencies up to 14 GHz. Both the intrinsic flux noise and the performance in a flux locked loop with direct voltage readout have been studied. In flux locked loop, a noise level of 0.55 μφ0/√Hz corresponding to an energy sensitivity of 34 h has been obtained for a DROS with a SQUID inductance of 29 pH. The intrinsic sensitivity improves with increasing relaxation frequency, leveling off to a value of 13 h at relaxation frequencies higher than about 3 GHz.

S.S. Tinchev, P. Gutmann