GB1137958A - Improvements in or relating to superconductive devices and to circuits including such devices - Google Patents

Abstract

1,137,958. Superconductive circuits. MULLARD Ltd. 13 Dec., 1967 [12 Oct., 1966], No. 45543/66. Heading H3B. [Also in Division H1] A superconductor storage device comprises a superconductive loop one arm of which contains a Josephson junction, and a control conductor arranged so that the junction can be switched between the superconducting and the normal states, information stored in the loop as a circulating current being read-out by switching the Josephson junctions to the normal state and sensing the voltage developed across the junction. As shown, Fig. 1, input and output conductors 2, 2 are joined by a storage loop 3 which comprises two Josephson junction 4, 5 connected in series by a bridge 6, and a parallel arm 7. A control conductor 1 crosses both arms of the loop and when energized drives the junctions 4, 5 from the superconducting state (tunnelling with no voltage drop) to the normal state (quasiparticle tunnelling). Information is stored by passing a writing current through the loop between conductors 2, 2, energizing control conductor 1 to drive the junctions 4, 5 normal thus diverting all the current through arm 7, removing the control current to return the junctions 4, 5 to the superconducting state, and then removing the writing current so that a circulating current is set up in the loop due to the trapped flux. The device is read by driving junctions 4 and 5 normal whereupon a voltage is developed across them with a polarity depending on the direction of the stored current. The device may be produced in the form of thin films of superconductor material on a substrate. The superconductor material may be niobium applied by vapour deposition or sputtering in vacuo through a stencil. The Josephson junction insulation may be of niobium oxide produced by admitting oxygen to the reactor after depositing the loop and is of the order of 10 Angstroms thick. The arm 7 and the bridge 6 are insulated from the control conductor 1 by thick films of niobium oxide or silicon monoxide (not shown). If difficulty is experienced in maintaining the thin layer of niobium oxide during deposition of niobium to form bridge 6, this member may be of lead, In the vapour deposition process the source of material may comprise a niobium rod the end of which is subjected to electron bombardment. A superconductive ground plane may be provided to reduce the control current required to switch the device. A plurality of such devices may be produced on a single substrate to form a computer storage matrix.