Investigations of noise and of quantum interference in proximityeffect bridges

Applied Physics, noise, quantum interference, proximity effect bridges

Decker, Stephen K. ; Mercereau, James E.

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: Applied Physics, noise, quantum interference, proximity effect bridges;   

Others  :  https://thesis.library.caltech.edu/8559/1/Decker-sk-1975.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

This work reports investigations upon weakly superconducting proximityeffect bridges. These bridges, which exhibit the Josephsoneffects, are produced by bisecting a superconductor with a short(<1µ) region of material whose superconducting transition temperatureis below that of the adjacent superconductors. These bridges arefabricated from layered refractory metal thin films whose transitiontemperature will depend upon the thickness ratio of the materialsinvolved. The thickness ratio is changed in the area of the bridgeto lower its transition temperature. This is done through novelphotolithographic techniques described in the text, Chapter 2.

If two such proximity effect bridges are connected in parallel,they form a quantum interferometer. The maximum zero voltage currentthrough this circuit is periodically modulated by the magnetic fluxthrough the circuit. At a constant bias current, the modulation ofthe critical current produces a modulation in the dc voltage acrossthe bridge. This change in dc voltage has been found to be the resultof a change in the internal dissipation in the device. A simple modelusing lumped circuit theory and treating the bridges as quantumoscillators of frequency ω = 2eV/h, where V is the time averagevoltage across the device, has been found to adequately describe theobserved voltagemodulation.

The quantum interferometers have been converted to a galvanometerthrough the inclusion of an integral thin film current path whichcouples magnetic flux through the interferometer. Thus a change insignal current produces a change in the voltage across the interferometerat a constant bias current. This work is described in Chapter3 of the text.

The sensitivity of any device incorporating proximity effectbridges will ultimately be determined by the fluctuations in theirelectrical parameters. He have measured the spectral power density ofthe voltage fluctuations in proximity effect bridges using a roomtemperature electronics and a liquid helium temperature transformerto match the very low (~ 0.1 Ω) impedances characteristic of thesedevices.

We find the voltage noise to agree quite well with that predictedby phonon noise in the normal conduction through the bridge plus acontribution from the superconducting pair current through the bridgewhich is proportional to the ratios of this current to the time averagevoltage across the bridge. The total voltage fluctuations are givenby = 4kTR^2_dI/V where R_d is the dynamic resistance, I thetotal current, and V the voltage across the bridge . An additionalnoise source appears with a strong 1/f^(n) dependence , 1.5 < n < 2, if thebridges are fabricated upon a glass substrate. This excess noise,attributed to thermodynamic temperature fluctuations in the volume ofthe bridge, increases dramatically on a glass substrate due to thegreatly diminished thermal diffusivity of the glass as compared to sapphire.