That a superconducting substance will go normal when the current density in it exceeds a critical value is well known. The current needs to be of the order of 107 amp/sq.
That a superconducting substance will go normal when the current density in it exceeds a critical value is well known. The current needs to be of the order of 107 amp/sq.
So, 59 years ago, started an article in EW’s edition of October 5th 1960.
R. H. Parmenter of the RCA Laboratories, Princeton, New Jersey, last year advanced a theory that under suitable conditions, superconductivity will reappear for a range of current densities in the neighbourhood of 109 amp/sq cm, where the electron drift velocity approximates to the speed of sound.
Parmenter’s ideas are based on an extension of the superconductivity theory developed by Bar-deen, Cooper and Schrieffer.
This theory does not include the modification of the phonon spectrum in a moving co-ordinate system due to the Doppler effect which is crucial to the existence of the high-current range of superconductivity.
Because of this effeet the phonon induced attraction bet ween electrons rises rapidly as the electron drift velocity approaches the speed of sound.
In the high current region of superconductivity the effective energy gap may be several orders of magnitude greater than in the low current region.
It is therefore not unreasonable to expect superconduc tivity at room temperature in the high-current range.
To achieve high-current superconductivity, it is necessary to have a conductor possessing uniform characteristics over its entire cross–section.
A metallic whisker or a cylindrically shaped thin film would fulfil this requirement.
The whisker seems to be a more practical course, where the radius of the whisker is at least as small as the penetration depth.
