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 ...
High T, superconductingYBazCusO, (YBCO) thin films have been prepared on (305) SrTiOs (STO) substrates. X-ray diffraction analysis and Rutherford backscattering experiments reveal that the c-axis of the layers is directed along the [OOl] ST0 axis. Bragg reflection measurements from YBCO lattice planes with high h, k, and I indices confirm that the film growth is epitaxial and almost single domain. For the critical current density j,(77 K) values of 2>( lo6 and 1 x lo3 A/cm2 have been found in the [OlO] and [SO]] YBCO directions, respectively.The question of the superconducting properties anisotropy in the case of the high T, superconductors (HTS) is of great importance now. Recent experiments show strong anisotropy in coherence length c,;' critical current density j, and resistivity p above the critical transition temperature T," between the directions parallel and perpendicular to the Cu02 planes in the YBazCu30, (YBCO). The origin of such behavior may be in the quasi-two-dimensional nature of high T, superconductivity.5Measurements of the anisotropy of the transport properties on YBCO single crystals are difficult because of the well pronounced layered nature that inhibits the preparation of monocrystalline structures with dimensions in the c-direction large enough for j, measurements. Requirements of YBCO thin films are also exceptionally high. One should use epitaxial films which are single domain, properly oriented and free from twins.The aim of this work is to study the possibility of the growth of coherently tilted epitaxial YBCO thin films on specially oriented (305) SrTiO, (STQ) substrates and apply these for the study of the anisotropy in the YBCO transport properties. Jia et al. showed that YBCO can cover steps on the (001) ST0 substrates. If the angle a between the normal to the (macroscopic) step surface and the (001) ST0 direction is below ~:40", the YBCO on the step is c-axis oriented.6 The surface of our (305) ST0 substrates is similar to that of a step with a = 30.96". Therefore, we expect YBCO to grow on (305) ST0 substrates with the c-axis tilted with an angle of 31" relative to the substrate surface normal, and directed along the [OOl] ST0 axis, see Fig. 1. If epitaxial and single domain films can be made, they allow us to study the anisotropy in the electrical properties of YBCO thin films for the first time, then also anisotropic behavior of many other physical properties of YBCO can be studied on these films.We have studied single and double layers of YBCO and PrBa,Cu30, (PBCO), grown on (305) ST0 (Akzo International Research SCT) using off-axis rf magnetron sputtering as a deposition technique. The deposition setup and typical sputter parameters are described elsewhere.7 X-ray diffraction analysis (XRD) and 2 MeV 4Hef Rutherford backscattering spectrometry (RBS) were used to determine the orientation of the (double) layers. The XRD experiments were performed on a Philips Materials Research Diffractometer (MRD) equipped with parallel beam x-ray optics.* The RBS data were analyzed using th...
YBaCuO nanobridges and inductively shunted dc-SQUIDS have been patterned by direct Focused Ion Beam milling. ~anobridges manifest true Josephson effect, which can be expla~ned by coherent motion of Abrikosov vortices in the bridge. An enhancement of this effect is proposed to be due to local suppression of the superconducting properties within the nanobridge area. This leads to a transition from SNS to SS'S type b e h a v i~~ of the nano ridge with decreasing temperature. TempErature d e~e n d~n c e of the critical current, dynamic resistance, and SQUID voltage-flux modulation is discussed. The flux noise level of the nanoSQUID is measured to be about 6.5 p@&€z at 62.7 K.
--Microbridges with dimensions smaller than the effective penetration depth X I, have been prepared in epitaxially grown (001) and (105) oriented YBa2Cu&7-6 thin films. The current-voltage characteristics of these bridges show specific kinks and steps which can be attributed to discrete coherent vortex motion along single or multiple paths across the microbridges. The critical current of a configuration with two very small microbridges in parallel, shows periodic SQUID modulation at 4.2 K.
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