Dmc Hyland

Fabrication and measurement of intrinsic Josephson junctions in misaligned films of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/

Intrinsic Josephson bridges have been patterned in films of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/. Due to our device geometry we have used films that are misaligned at 20/spl deg/ to the substrate surface. This misalignment has been confirmed using four-circle x-ray diffraction and cross-sectional transmission electron microscopy. Our films still retain a high critical temperature (108 K) despite their misalignment. The resistive anisotropy of the film is comparable to single crystal data. The product of the critical current and normal state resistance is equal to 14 mV at 4.2 K and is independent of the critical current density. These two achievements are attributed to the-lack of engineered interfaces in our intrinsic junctions. A run-to-run variation in the critical current is explained by the presence of non-Josephson shunts in the junction. Junctions with fewer non-Josephson shunts show greater hysteresis.

Josephson fluxon flow and phase diffusion in thin-film intrinsic Josephson junctions

We have measured the transport properties of intrinsic Josephson junction arrays as a function of both temperature and magnetic field, the field being applied perpendicular to the transport current. We show that they depend upon whether the junction width exceeds or is less than the Josephson penetration depth. For widths greater than the Josephson penetration depth, the transport properties are dominated by Josephson vortex flow. For widths less than the Josephson penetration depth, the zero-field dissipation is dominated by Josephson phase diffusion. For non-zero applied fields, there is a good fit to the classical “Fraunhofer” dependence of the critical current upon field in addition to a competition between Josephson phase diffusion and Josephson vortex flow.

Multiple-peak switching current distribution in Tl-Ba-Ca-Cu-O intrinsic Josephson junctions

Abstract We have fabricated series arrays of intrinsic Josephson junctions of area (2 μm) 2 in thin films of Tl 2 Ba 2 CaCu 2 O 8 grown on vicinal substrates. Measurements of the distribution of the bias current at which a voltage first appears across the array show two peaks in the temperature range between 14 and 20 K. Fitting the data to a simple thermal activation model for a small Josephson junction shows that these peaks correspond to phase-locked switching of two or three junctions respectively. Below 14 K the distribution broadens significantly which we attribute to a crossover to long-junction behaviour. Intrinsic junctions of smaller area ( 0.5×0.7 μ m 2 ) display no such fluctuations down to 4.2 K, confirming this interpretation.

Application of TBCCO based HTS devices to digital cellular communications

Linear microstrip resonators suffer from high peak current density inside the resonators which limit the power handling characteristics and hence their use in cellular transmitter applications. To realise higher power filter and combiner networks for cellular applications it is possible to use two dimensional microstrip resonators (such as disks) to equalise the internal current distribution. We have designed and tested such microstrip resonators, fabricated from TBCCO 2212 thin films deposited by RF sputtering onto 50 mm diameter LaAlO/sub 3/ substrates. The R/sub s/ of such films has been measured at 5.5 GHz using a sapphire dielectric resonator and shown to be less than 1 m/spl Omega/ scaled to 10 GHz and at 80 K. The power handling of disk resonators designed for operation in the PCS and DCS1800 cellular bands has been shown to be superior to that of linear resonators fabricated from similar material. The operation of such devices using high power levels and realistic signals encountered in 2/sup nd/ generation digital cellular systems (e.g., DQPSK and GMSK) will be presented. The application of these devices to 3/sup rd/ generation wide band CDMA systems will also be discussed.

Josephson effects in misaligned Tl-2212 films

Abstract We have measured Josephson RSJ-like static current–voltage characteristics in 2-μm-wide bridges patterned in the [cos 20° 0 sin 20°] direction in 20° misaligned thin films of Tl 2 Ba 2 CaCu 2 O 8 grown on vicinal LaAlO 3 substrates. The temperature dependence of the critical current of the bridge is well-described by Josephson SIS tunnelling between two BCS superconductors, with I c R n equal to 26 mV at 4.4 K. Conversely, the current–voltage characteristics of bridges patterned in the orthogonal [0 1 0] direction in the same films show flux–flow behaviour. This, coupled with the insensitivity of the inter-plane critical current to a small magnetic field, strongly suggests that the Josephson behaviour we have observed is due to intrinsic Josephson coupling between adjacent copper oxide planes, and not to extrinsic inter-grain coupling.

Microstructure and phase development in melt processed Tl-1223 tapes

Abstract The effect of processing conditions on microstructure and phase development has been investigated for TlBi-1223 tapes made by a melt processing route. Increasing loss of crystallinity and phase decomposition into Tl-1212 and SrCaCu-oxides is seen for melt temperatures in the range 910–925°C. The 1223 phase is reformed during a slow cool and subsequent annealing. It has been established that for samples of composition (Tl 0.78 Bi 0.22 )(Sr 0.8 Ba 0.2 ) 2 Ca 2 Cu 3 O δ a melt temperature of around 925°C and a cooling rate of 20°C/h results in a much increased grain size of up to 60–70 μm. Use of fully reacted 1223 powder was found to result in a larger increase in grain size and better phase purity than a partly reacted precursor.

Properties of Tl2Ba2CaCu2O8 thin film intrinsic Josephson junctions in an in-plane magnetic field

Abstract We have performed transport measurements on intrinsic Josephson junctions in misaligned thin films of Tl 2 Ba 2 CaCu 2 O 8 . This has been done in both zero field and in-plane aligned magnetic fields of up to 5 T. We have compared wet-etched and ion-milled devices and find that the more precise structure produced by ion-milling gives superior results. In the case of ion-milled devices we are able to individually switch on up to 50 junctions. For an n-plane field of 3 T there is complete suppression of the critical current. We have seen displaced branches for in-plane fields above 1.5 T. This branching is due to Josephson flux flow. For in-plane fields greater than 2 T the relationship between the maximum flux-flow voltage and the applied field is linear.

Development of a melt processing route for the fabrication of Tl-1223 coils

Tl-1223 tapes with engineering critical current densities, (E-J/sub c/) values of 2500 Acm/sup -2/ can be routinely fabricated but the magnetic field properties are poor due to weak link effects. We have developed a partial melt processing route which results in increased grain growth with greater potential for achieving grain alignment. We have investigated the effect of Tl-1223 phase composition on the melt process and found that Bi substitutions lead to an increased grain size in tapes made using unreacted powder but a decrease in grain growth in tapes made with reacted powder.