D Squids as Radiofrequency Amplifiers and Application to the Detection of Nuclear Quadrupole Resonance

The high sensitivity of dc SQUID amplifiers is extended to the radio-frequency range. We perform a detailed investigation of the dynamic input impedance of tightly coupled dc SQUIDs and of the influence of parasitic capacitance between the SQUID and the input circuit on the SQUID characteristics. The reactive part of the dynamic input impedance is found to be determined by the inductive coupling, whereas the resistive part is found to be dominated by capacitive feedback. We also discuss the optimization of the input circuits for both tuned and untuned amplifiers and derive expressions for the optimum source resistance, gain and noise temperature for a given frequency, input coil and coupling. The performance of the amplifiers designed according to these prescriptions is measured. The gain of an untuned amplifier operated at 100 MHz at 4.2K is 16.5 (+OR-) 0.5dB with a noise temperature of 3.8 (+OR-) 0.9K; at 1.5K the gain increases to 19.5 (+OR-) 0.5dB while the noise temperature decreases to 0.9 (+OR-) 0.4K. A tuned amplifier operated at 93 MHz and 4.2K has a gain of 18.6 (+OR-) 0.5dB and a noise temperature of 1.7 (+OR-) 0.5K. These results are in good agreement with predicted values. The usefulness of these sensitive amplifiers for the detection of magnetic resonance is demonstrated. A SQUID system for pulsed nuclear quadrupole resonance at about 30 MHz is developed. At a bath temperature of 4.2K, a total system noise temperature of 6 (+OR-) 1K is achieved, with a quality factor Q of 2,500. A novel Q-spoiler, consisting of an array of Josephson tunnel junctions, reduces the ring-down time of the pick-up circuit after each pulse. The minimum number of nuclear Bohr magnetons observable after a single pulse is about 2 x 10('16) in a bandwidth of 10 kHz. Finally the low-noise SQUID amplifiers make it possible to use a novel technique for observing magnetic resonance in the absence of an externally applied radio-frequency field, by measuring the spectral density for the Nyquist noise current in a tuned circuit coupled to the sample. In thermal equilibrium, a dip is observed in the spectral density at the spin resonant frequency. For zero spin polarization, on the other hand, a bump in the spectral density is observed. This bump is due to temperature-independent fluctuations in the magnetization, and represents spontaneous emission from the spins into the circuit.