Seminars

The Intrinsic Josephson Effect in anisotropic oxide superconductor films

Wednesday, 19 Oct 2005, 14.00, W0.03

With both scientific and technological goals in mind we have therefore fabricated intrinsic Josephson junctions in thin films of the anisotropic superconductor Tl₂Ba₂CaCu₂O₈. The junctions have a range of cross-sectional areas from ~ 4 mm² down to 0.25 mm². Junctions larger than ~ 1 mm² show strongly fluctuating low-temperature critical currents with a peak J_c (4.2 K) equal to 1.7 ± 0.4 10⁴ Acm^-2. Such junction arrays show coexistence of linear and non-linear dissipative branches in the current-voltage characteristics. We demonstrate that this coexistence is consistent with a Josephson “washboard” model in which dissipation is not constant for each junction in the array.

For junctions with area less than ~ 1 mm² we observe strongly hysteretic non-linear current-voltage characteristics which are comparable with those seen in single crystal Bi-Sr-Ca-Cu-O. The collapse of plots of the current at low bias versus V/N (where V is the voltage across the array and N is the number of junctions in the dissipative state) onto a single curve is taken by most observers as an indication that the non-linearity reflects the quasiparticle density of states and is not an artifact of heating. At higher bias heating becomes significant which precludes measurement of the superconducting gap.

Sub-micron junctions are also characterized by the absence of critical current fluctuations and a suppression of the switching current density, which falls to zero at an area of 0.25 mm². The suppression is due to thermally-activated phase diffusion, and the temperature-dependence of the switching current for junctions with E_J > kT can be qualitatively accounted for by a simple washboard model. The magnetic field dependence of the critical current (a) confirms the sinusoidal current-phase relationship and (b) establishes the size of the junction to be given by the separation of adjacent copper-oxide double-planes.

Adjacent copper-oxide double planes in anisotropic layered high-temperature superconductors are Josephson coupled. Hence a patterned “stack” of copper-oxide layers behaves like a one-dimensional array of underdamped “SIS” Josephson junctions when the transport current is normal to the layers. These so-called intrinsic Josephson junctions (IJJs) have been widely studied in the bismuth- and thallium-based cuprate families, and have been proposed for a variety of applications including tuneable THz oscillators and sensors. From a scientific point of view it is important to establish that (a) the current – phase relationship is sinusoidal and (b) the dissipative current – voltage characteristics reflect the quasiparticle density of states, as would be expect for a classical “SIS” junction.