Superconducting quantum interference devices based on YBaCuO nanobridges

Abstract

Direct current SQUIDs based on YBaCuO thin film nanobridges have been investigated. Critical current densities J c of the devices are up to 3ϫ10 6 A/cm 2 at Tϭ77 K and show a temperature dependence of (1ϪT/T c ) 1.6Ϯ0.1 . High values of the voltage-flux modulation are observed ͑8 V peak to peak at 77 K, 45 V at 4.2 K͒. The temperature dependence of the SQUID modulation is found to be essentially different from the one of the conventional weak link SQUID. We discuss our results by considering degradation of the nanobridge area during structuring, which leads to a transition from SNS to SSЈS type junction with decreasing temperature.Most high-T c SQUIDs are based on grain boundary weak links or ramp-type junctions. The techniques to prepare such structures have been developed extensively and often face technological problems caused by an extremely short coherence length in high-T c superconductors. Recent results show that nanobridges, although having dimensions significantly exceeding , manifest a true Josephson behavior and are suitable for SQUID fabrication. 1-3 The concept of the coherent motion of flux quanta has become a standard for nanobridge description, and explains a wide variety of effects. 4 However, the periodic supercurrent-phase relation, which is a principal element of SQUID operation, does not follow obviously from this model.We investigated SQUIDs based on high-T c thin film nanobridges systematically. We explain the Josephson nature of these devices by considering degradation of the bridge area during structuring, which leads to a transition from SNS-to SSЈS-type junctions with decreasing temperature.SQUIDs were structured in 50 nm thick YBa 2 Cu 3 O 7Ϫ␦ films by electron beam lithography ͑EBL͒ and direct focused ion beam milling ͑FIB͒. An inductively shunted dc SQUID geometry was chosen. 3 The T c of unstructured films is ͑90Ϯ1͒ K. The superconducting transition curves R(T) of the investigated devices show the presence of a ''foot'' that grows rapidly with the decrease of the bridge width w, with an onset at wϷ250 nm ͓Fig. 1͑b͔͒. For wider bridges no degradation of T c has been observed. The current voltage characteristics (I-V͒ of SQUIDs at zero external magnetic field ͓see Fig. 1͑a͔͒ are similar to those of single nanobridges discussed elsewhere. 5 The critical current I c is a linear function of the nanobridge width w for 50 nm ϽwϽ350 nm. The critical current density J c is up to 3ϫ10 6 A/cm 2 at 77 K and follows J c ϰ(1ϪT/T c ) 1.6Ϯ0.1 in a wide range of temperatures from T c down to at least T c /2.For wϽ300 nm, voltage-flux modulation was observed in our SQUIDs. For a device based on 250 nm bridges, the maximum measured peak-to-peak voltage modulation U mod is 8 V at 77 K ͑see Fig. 2͒. For a SQUID with 100 nm bridges (T c ϭ60 K͒, the maximum U mod is 45 V at 4.2 K.In Fig. 3 the experimental data for the temperature dependence of the voltage modulation U mod (T) are given. U mod (T) of the nanobridge SQUID is essentially different from the one of conventional weak link SQUIDs described by ...

Ultra low noise YBa2Cu3O7−δ nano superconducting quantum interference devices implementing nanowires

“…In contrast to previous works [11][12][13] our nanoSQUIDs show critical current modulations as a function of an externally applied magnetic flux in the full temperature range below the transition temperature, T C , of the devices. Both the modulation depth and the period in magnetic field are in good quantitative agreement with numerical computations.…”

“…Several attempts to fabricate HTS nanoSQUIDs, implementing YBCO Dayem bridges, have been made during the last few decades [11][12][13]. However a proper SQUID behavior, with a periodic modulation of the critical current in the full temperature range below T C has never been observed.…”

Superconducting quantum interference device based on MgB2 nanobridges

“…The procedures used for this are not foreseen to be suited for the realization of trilayer Josephson junctions because of the post-anneal step involved. Fortunately, aside from Josephson junctions that include a barrier-layer, also nanobridges can be employed as the weak links in a SQUID 14 . In the following, we present the observation of Josephson quantum interference in MgB 2 using two superconducting nanobridges of 150 nm by 70 nm incorporated in a superconducting ring.…”

Focused ion beam fabrication and properties of nanoscale Josephson junctions for sensors and other applications

“…We have demonstrated that, provided Ga-contamination issues are addressed, the FIB can be used in a wide range of geometries and on a range of materials which covers most technologically important superconductors. Ga-contamination is potentially serious for all systems, but particularly for HTS materials [33]. In this case the approach discussed in Section 2, of coating the film with a passive material prior to patterning seems to be essential.…”