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Japanese Journal of Applied Physics

Controllable Superconducting Nb Long Microbridge by Low Frequency, and Its Resonant Characteristics

Keiji Tsukada, T. Morooka

Published Nov 1, 1992• 0 citations
Publisher

Abstract

A Nb superconducting microbridge is fabricated, 40-µm long, less than 0.25-µm wide and 0.1-µm thick. Four-terminal measurement of the I-V characteristics reveals hysterisis in the I-V curve. The microbridge has an excess current over its current in its normal state. Frequency resonance appears over the range of a few hundred kiloherz by electromagnetic wave radiation. The I-V characteristics show that critical current I c is depressed, and as radiation power increases I c decreases. This phenomenon is thought to be due to resonance of quasiparticles in the microbridge, and to resonance restricting the supercurrent.

DOI
10.1143/jjap.31.l1536

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Technical Physics Letters•Jul 1, 2018•7 citations
A Nb superconducting microbridge is fabricated, 40-µm long, less than 0.25-µm wide and 0.1-µm thick. Four-terminal measurement of the I-V characteristics reveals hysterisis in the I-V curve. The microbridge has an excess current over its current in its normal state. Frequency resonance appears over the range of a few hundred kiloherz by electromagnetic wave radiation. The I-V characteristics show that critical current I c is depressed, and as radiation power increases I c decreases. This phenomenon is thought to be due to resonance of quasiparticles in the microbridge, and to resonance restricting the supercurrent.
High resistance granular NbN microbridges with submicron dimensions have been fabricated and investigated. Devices with critical currents less than 5 microamps and bridge resistances of the order of 50 ohms have been studied as a function of temperature at both zero and finite voltages. The current‐voltage characteristics were measured with various levels of applied radiation in the frequency range 11–40 GHz. For these frequencies, constant voltage‐current steps were observed at the Josephson frequency, 2eV/h for low applied voltages, at higher voltages the radiation induced step spacing deviated from that of ideal Josephson behavior. Additional current steps were also observed at temperatures just above the bridge Tc where no finite critical current could be dected. These effects and other characteristics observed on the I‐V curves will be reported and discussed.
Experimental investigations on the proximity effect bridge (a Josephson device) at zero voltage and at finite voltages in the µV range are reported. The phase-super current relation at zero voltage was measured using an asymmetric superconducting quantum interferormeter circuit. The data are in agreement with the Josephson supercurrent-phase relation I_S=I_C sinδ with deviation less than 5% of the critical current I_c. The supercurrent density in the measured bridges reached as high as 50-100 µA/µm^2. Using microcircuitry techniques, proximity effect bridges were strongly coupled to superconducting microstrip resonators. Selfinduced steps in the I-V characteristics of bridges coupled to resonators were observed in the GHz region at voltages (frequencies) corresponding to the expected modes of the resonators. Two types of steps were seen depending on whether the resonator impedance on resonance was much higher or much smaller than the bridge resistance. A simple two fluid model of the bridge-resonator circuit was developed and the size and shape of self-induced steps were calculated for a generalized Josephson oscillator relation I_S = I_c(l-q + q sin∫ 2e/hV dt) where q = 1 corresponds to the original Josephson relation and q = 1/2 represents the phase slip regime. At low critical currents (I_c < 10 µA) and low voltages (V < 3µV) the size and shape of experimentally observed self-induced steps agree with the q = 1 model. At higher voltages and/or critical currents the step size increasingly deviates from the q = 1 model towards q = 1/2. These observations are interpreted to indicate a progressive reduction of the amplitude of the oscillating Josephson supercurrent in proximity effect bridges from I_c towards I_c /2 as the critical current and/or voltage are increased.
The temperature and field variation of the photoresponse of tin Josephson film microbridges are studied in experiments in which the microbridges are irradiated in the centimeter and millimeter ranges. Experiments have been carried out under conditions that correspond to narrow-band and wide-band detection. Certain features of the response which are due to stimulation of superconductivity in the microbridges by the microwave radiation and a direct current are studied. In heterodyne detection these features disappear at a high power level of the heterodyne oscillator, and a substantial increase is observed in the response, this is caused by a higher differential resistance of the microbridge. The maximum sensitivity (approx.10/sup 4/ V/W) is achieved with selective detection.
A high-quality superconducting resonator with a microbridge of hafnium film for use in a circuit for readout a terahertz-band imaging array with frequency division multiplexing is demonstrated experimentally. The variability of the impedance of the bridge at a frequency of 1.5 GHz, which is a key factor in the control of the quality of the resonator, is studied. The bridge, having a thickness of about 50 nm, a critical temperature T C ≈ 380 mK, and a plan size of 2.5 × 2.5 μm, was connected as a load of a resonator made of niobium film with a thickness of about 100 nm ( T C ~ 9 K). It is shown that the bridge smoothly changes its impedance proportionally to the bias power in the entire temperature range. The effective thermal insulation of the bridge was measured in a dilution cryostat at temperatures of 50–300 mK. Thermal conductivity G of the bridge was calculated and found to be ~4 × 10 –13 W/K, which gives an estimate of the sensitivity of the structure in the bolometric mode NEP ≈ 8 × 10 –19 W/Hz 1/2 at a temperature of 150 mK.