Superconducting quantum interference proximity transistor

Abstract

When a superconductor is placed close to a non-superconducting metal, it can induce superconducting correlations in the metal [1][2][3][4][5][6][7][8][9][10] , known as the 'proximity effect' 11. Such behaviour modifies the density of states (DOS) in the normal metal [12][13][14][15] and opens a minigap 12,13,16 with an amplitude that can be controlled by changing the phase of the superconducting order parameter 12,15 . Here, we exploit such behaviour to realize a new type of interferometer, the superconducting quantum interference proximity transistor (SQUIPT), for which the operation relies on the modulation with the magnetic field of the DOS of a proximized metal embedded in a superconducting loop. Even without optimizing its design, this device shows extremely low flux noise, down to ∼10 −5 Φ 0 HzWb is the flux quantum) and dissipation several orders of magnitude smaller than in conventional superconducting interferometers [17][18][19] . With optimization, the SQUIPT could significantly increase the sensitivity with which small magnetic moments are detected.One typical SQUIPT fabricated with electron-beam lithography is shown in Fig. 1a. It consists of an aluminium (Al) superconducting loop interrupted by a copper (Cu) normal-metal wire in good electric contact with it. Furthermore, two Al electrodes are tunnel-coupled to the normal region to enable the device operation. A detailed view of the sample core (see Fig. 1b) shows the Cu region of length L 1.5 µm and width 240 nm coupled to the tunnel probes and the superconducting loop. The SQUIPTs were implemented into two different designs (see Fig. 1c), namely, the A-type configuration, where the loop extends into an extra third lead, and the B-type configuration, which contains only two tunnel probes. The ring geometry enables us to change the phase difference across the normal-metal/superconductor boundaries through the application of an external magnetic field, which gives rise to a total flux Φ through the loop area. This modifies the DOS in the normal metal, and hence the transport through the tunnel junctions.Insight into the interferometric nature of the SQUIPT can be gained by first analysing the theoretical prediction of its behaviour. Figure 2a shows the simplest implementation of the device in the A-type configuration, that is, that with just one junction tunnelcoupled to the normal metal. For simplicity, we suppose the tunnel probe (with resistance R T ) to be placed in the middle of the wire, and to feed a constant electric current I through the circuit while the voltage drop V is recorded as a function of Φ. In the limit that the kinetic inductance of the superconducting loop is negligible, the magnetic flux fixes a phase difference φ = 2πΦ/Φ 0 across the normal metal, where Φ 0 = πh/e is the flux quantum,h is the reduced Planck's constant and e is the electron charge. Figure 2b shows the low-temperature quasiparticle current-voltage (I -V ) characteristic of the SQUIPT calculated at a few selected values of Φ. The calculations were carried out f...

Pb/InAs Nanowire Josephson Junction with High Critical Current and Magnetic Flux Focusing

“…The coupling of superconducting electrodes by a weak link can result in the formation of a Josephson junction, where a dissipationless supercurrent is induced due to the proximity effect, i.e., the penetration of superconducting correlations in the non-superconducting link resulting from Andreev reflection [1][2][3]. Josephson junctions and coherent quantum transport have been attracting interest in recent years due to the development of modern nanofabrication techniques, which enable the fabrication of nanoscale hybrid superconducting devices in a broad range of design and materials.…”

Kinetic inductance magnetometer

“…Emerging sensing techniques include magnetometers combining the giant magnetoresistance effect and a superconducting receiver coil, the mixed sensors 11 and atomic magnetometers relying on the interaction of magnetic field and optically manipulated electronic spins of alkali vapour atoms 12 . Also, the magnetic manipulation of the density of states in normal metal-superconductor interfaces has been proposed, in a so-called superconducting quantum interference proximity transistor 13 .…”

The Stationary SQUID

“…The proposed circuit has close similarity with the normal-metal-insulator-superconductor interferometer [22], with the NS-QUID [23], and with the superconducting quantum interference proximity transistor [16][17][18]. The new feature is that the connector at the right is in clean contact with the normal metal.…”

“…It would not be possible to make justice to the vast literature on the subject; some examples are Refs. [6][7][8][9][10][11][12][13][14][15][16][17][18].…”