Superconductive Pressure Contacts
Authors: Gaulé G. K. , Breslin J. T. , Winter J. J. 
Published: February 2004

Superconductive contacts are nearly as old as superconductivity itself; in 1914, only three years after he had discovered superconductivity, Kamerlingh-Onnes1 found that a Pb–Pb contact carried supercurrents. Significantly, extreme cleaning of the contact surfaces was not necessary. Systematic studies were not undertaken until 1932, when Holm and Meissner2 investigated Pb–Pb, Sn–Sn, and Sn–Pb contacts at various temperatures and magnetic fields. Their contact supercurrents reached fractions of an ampere. By deliberately depositing insulating layers on Ta contact surfaces, Dietrich3 in 1952 found strong evidence for small supercurrents through thin (≃15 Å) layers of this kind. In a theoretical study in 1962, Josephson4 found that such currents can result from a special kind of weak coupling between the two wave functions representing the superelectrons in the two contact member. Josephson weak coupling may also take place via a normal conducting layer; in fact Meissner, in 1958 had shown experimentally that interposed normal layers up to 3000 Å (and also ferromagnetic layers up to 100 Å) do not prevent an otherwise superconducting contact from passing small supercurrents. The authors recently found that in a changing magnetic field, these currents oscillate like those through insulating layers.5 The magnitude of the critical current and its response to an external magnetic field usually places a superconductive pressure contact into one of several distinct empirical groups. While weak-coupling currents through nearly uniform layers are quenched by fields of a few gauss, metallic microbridges (formed by asperities on the contact surfaces) sustain small supercurrents in the fields of 1000 G and higher.6 ``Intermediate coupling'' is generally characterized by critical currents which are larger (≥100 mA) and less field sensitive than those from weak coupling. Some oxidized Pb–Pb contacts showed thus far unexplained hysteretic current peaks (up to 500 mA) near 40 G. ``Strong coupling'' may be defined as the case when a contact behaves like a single, but not necessarily like a singly connected, superconductor. Strong coupling may be attained by applying an extreme contact force which leads, at least in spots, to cold-welding. While this method is satisfactory for making permanent connections, it is not feasible for switches.7 The authors show, however, that large ``strong coupling'' currents (≈240 A) can be carried by releasable contacts with moderately clean surfaces under moderate force (≃10 kg). In comparison to other superconductive ``switches,'' those with pressure contacts offer the advantage of virtually infinite resistance and, thus, loss-free operation in the ``open'' mode. At the same time, the possibly undesirable heat flow across the contact is also interrupted. Remote operation by a superconductive relay mechanism has been demonstrated. Applications of strong coupling contacts in energy storage systems, for transient magnetic fields, and in low-loss high-frequency circuitry are predicted. Weak-coupling pressure contacts compete with analogous junction devices made by thin-film techniques. Here, the main advantage of the pressure contact is seen in its adjustability and in its superior coupling to microwave and ir electromagnetic fields.