Asymmetric nanowire SQUID: Linear current-phase relation, stochastic switching, and symmetries

Abstract

We study nanodevices based on ultrathin superconducting nanowires connected in parallel to form nanowire SQUIDs. The function of the critical current versus magnetic field, IC (B), is multivalued, asymmetric and its maxima and minima are shifted from the usual integer and half integer flux quantum points. The nanowire interference device is qualitatively distinct from conventional SQUIDs because nanowires do not obey the same current-phase relationship (CPR) as Josephson junctions. We demonstrate that the results can be explained assuming that (i) the CPR is linear and (ii) that each wire is characterized by a sample-specific critical phase, which is usually much larger than π/2. Our proposed model offers accurate fits to IC (B). It explains the single-valuedness regions where only one vorticity (i.e., the order parameter winding number) is stable as well as regions where multiple vorticity values are allowed for the SQUIDs. We also observe and explain regions in which the standard deviation of the switching current is independent of the magnetic field. We develop a technique that allows a reliable detection of hidden phase-slips. Using this technique we find that our model correctly predicts the boundaries of vorticity regions, even at low currents where IC (B) is not directly measurable.

Homogeneous superconductivity at the LaAlO3/SrTiO3 interface probed by nanoscale transport

“…However, the behavior of our nanorings is qualitatively differ- ent from classical DC SQUIDs: modulation is not sinusoidal, and the critical current is a multivalued periodic function of the magnetic field. A similar effect was previously reported in micro-SQUIDs with weak links formed by Dayem bridges [50,51], in YBCO nanowire SQUIDs [52], and asymmetric MoGe nanowire SQUIDs [53]. A long nanowire with L > ξ has very high kinetic inductance that results in a large phase gradient and nonsinusoidal current-phase relationship along the nanowire [40].…”

The impact of kinetic inductance on the critical current oscillations of nanobridge SQUIDs

“…The shape of the oscillations can be captured by the vorticity diamond model introduced in Ref. [21]. This model describes a SQUID containing two weak links, which both have a linear CΦR…”

“…Using this method, we conduct an experimental study of lithographically fabricated MoGe superconducting nanobridges and their current phase relation (CΦR). The latter is approximately given by [18][19][20][21][22]…”

“…Here, I s is the supercurrent through the nanobridge, L K is its kinetic inductance and ϕ is the phase difference across the nanobridge, which is limited by a critical value ϕ c above which the bridge transits to the normal state. We do this by imbedding the nanobridges in a SQUID * heleen.dausy@kuleuven.be [21,23,24]. The response of the SQUIDs used in this work is completely determined by their kinetic inductance, making the critical current versus magnetic field oscillations I c (B) of the SQUIDs directly reflect the CΦR and hence also the kinetic inductance of the nanobridge [21].…”

“…We do this by imbedding the nanobridges in a SQUID * heleen.dausy@kuleuven.be [21,23,24]. The response of the SQUIDs used in this work is completely determined by their kinetic inductance, making the critical current versus magnetic field oscillations I c (B) of the SQUIDs directly reflect the CΦR and hence also the kinetic inductance of the nanobridge [21].…”