Numéro
Rev. Phys. Appl. (Paris)
Volume 5, Numéro 1, février 1970
Page(s) 25 - 31
DOI https://doi.org/10.1051/rphysap:019700050102500
Rev. Phys. Appl. (Paris) 5, 25-31 (1970)
DOI: 10.1051/rphysap:019700050102500

Superconducting point contacts weakly connecting two superconductors

R. De Bruyn Ouboter et A. Th. A. M. De Waele

Kamerlingh Onnes Laboratorium der Rijksuniversiteit te Leiden, Leiden (Netherlands)


Abstract
The present review concerns dc and ac interference phenomena in superconducting point contacts weakly connecting two superconductors. First, the properties of a single superconducting point contact will be discussed. Special attention will be paid to the resistive superconductive region, and its current-voltage characteristic. The point contact will be treated using a two-fluid model in which the total current is the sum of an ideal Josephson supercurrent is( t) = i1 sin Δφ*(t) and a normal current in = V/Rn = - (ħ/2e) (∂ Δφ*(t)/∂t)/Rn (i 1 the maximum supercurrent ; Δφ* the gauge-invariant phase difference ; V the voltage across the point contact and Rn its normal resistance). It appears that small shunt capacitances and series inductances play an important role, even in its dc current-voltage characteristic. Primarily, the two cases for which either the total current i = is + in or the voltage V are constants of time will be discussed. This approach appears to be essential for understanding the modulation in the voltage V(B>) as a function of an external magnetic field at constant applied current i for a double point contact. Second, the external magnetic field dependence of the critical current (the dc Josephson effect) through a double point contact weakly coupling two superconductors is studied extensively, when the self-induced flux in the enclosed area is taken into account. Its relation with a strongly coupled double point contact, in which flux quantization holds, will be treated. Experimental results will be shown for the critical current of a symmetrical double point contact with such a small enclosed area O = 24 × 10-8 cm2 (L = 1.8 × 10-12 henry, i1 = 185 μA) that flux quantization is very incomplete : (h/2e)/L2i1 ~ 3 > 1) and will be compared with theory. Finally, the properties of double point contacts in the resistive-superconductive region, in relation with both the ac and dc Josephson effects, will be discussed. It is observed that the dc voltage across a double point contact oscillates as a function of the applied magnetic field when a constant current is applied through the double point contact. The complete i - V - B> dependence can be represented by a corrugated surface. Two interpretations will be given : when the self-induced flux in the enclosed area is ignored and when it is taken into account respectively. An extension is made by connecting two superconductors by more than two point contacts, which are adjusted on a straight line in such a way that an interference grating is formed.

PACS
4100 - Electricity and magnetism: fields and charged particles.
2180 - Electrical contacts.
3240 - Superconducting devices.

Key words
electrical contacts -- superconducting devices