Critical-current measurements in planar Josephson junctions

M. S. Rzchowski, B. M. Hinaus
1997 Applied Physics Letters  
The properties of planar high-temperature superconducting Josephson junctions are not entirely consistent with low T c models. Some of these disparities may be intrinsic, related to the unusual superconducting properties of the electrodes, while others are due to the nonstandard planar geometry. By comparing experimental measurements of a 24°bicrystal substrate Josephson junction with a simple model of nonuniform current flow in a planar geometry, we show that several unusual properties can be
more » ... properties can be explained as extrinsic. We show that nonuniform current feed in a planar geometry leads to a reduced critical current, an unchanged Meissner field H co , and a power-law dependence of the critical current on H co different from that expected in an overlap geometry. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00746-8͔ High-temperature superconducting ͑HTS͒ Josephson junction technology has advanced to the point that careful measurements are beginning to address intrinsic differences between Josephson transport in HTS and low T c materials. However, due to the unusual planar geometry of most HTS junctions, such differences cannot all be attributed to the unusual microscopic superconducting properties of the electrodes. One of the most important intrinsic junction properties is the critical current density J c . Its temperature and field dependence contain a wealth of information concerning the microstructural and electronic properties of the junction. The relation between J c and the measured critical current I c can however be complicated by several extrinsic effects. Many of these peculiar to the planar geometry have already been clarified, including field enhancement at the junction due to electrode flux-focusing, 1 critical current suppression through long-junction effects, 2 and critical-current modification by trapped flux. 3 Here we focus on an additional important planar geometry-dependent modification of the critical current: that of nonuniform bias current. The thin-film geometry of the electrodes results in a spatially nonuniform bias current distribution, where bias current is peaked near the film edge. This influences the measured critical current when it alters the supercurrent distribution in the junction, which can happen only when the junction is "long" ͑L/ J Ͼ1, with L the junction length and J the Josephson penetration depth͒. The long-junction regime is reached as the temperature is lowered and J decreases. In this regime, the physical layout of the junction is known to be important, and we find that nonuniform bias can cause the junction to behave as if it were of a different geometrical configuration. This in turn alters the measured I c even though the intrinsic J c is unchanged. For uniform bias current, junction geometry determines the relation between intrinsic critical current density and measured critical current. 4 For instance, a long junction in the overlap geometry has uniform junction current in zero field, and hence I c ϭJ c tL, where t is the film thickness. An in-line geometry junction in zero field has supercurrent flow-ing only within 2 J of the edges, giving I c ϭJ c t4 J . An applied field leads to nonuniform junction current in both geometries. The crossover in junction current distribution in the overlap case leads to a crossover in I c (H) from sublinear at low field to strictly linear near H co . The in-line I c (H) remains strictly linear at all HϽH co . The relation between I c (Hϭ0) and H co ϭ⌽ 0 /2 J L also differs, giving I c inline ϰH co and I c overlap ϰH co 2 . Although the planar HTS junction geometry is most similar to the overlap case, and hence relatively immune to long-junction effects in zero applied field, we show here that a nonuniform electrode bias modifies I c (H) as well as the value of I c (Hϭ0) and its dependence on the screening field H co throughout the long limit. In particular we show that the bias current distribution sets a new length scale that must be compared with J to determine the effect of nonuniform bias in much the same way that J is compared with the junction length L to determine the short to long transition. In the simplified model presented here, the new length scale moves I c (H co ) from the linear in-line dependence to the quadratic overlap dependence. We show that this "geometry" change is a valid interpretation in that the junction current distributions in the two limits reflect the distributions of the corresponding geometries. These effects are temperaturedependent even in the long limit due to the additional electrode bias length scale. We demonstrate these points through a comparison of experimental measurements of a 24°bicrystal-substrate YBCO Josephson junction with model calculations of nonuniform bias current in an overlap geometry. The agreement between experiment and model results in a method for determining the critical current density J c from the measured I c . The junctions were fabricated from 2000 Å YBCO films grown by pulsed laser deposition on a 24°SrTiO 3 bicrystal substrate, as described previously. 5 The substrate grain boundary is reproduced as a grain boundary in the epitaxial film, across which Josephson coupling occurs. Measurements were made in a magnetically shielded Dewar. The film was photolithographically patterned and ion-beam etched to produce a junction of length 20 m along the grain boundary. Signal and temperature-control leads were low-pass filtered to eliminate rf noise. Dynamic resistance was measured a͒ Electronic
doi:10.1063/1.120246 fatcat:vumqleycsbhgfpmrqpwz34mzse