Josephson nanoconstrictions made by AFM plowing of YBa2Cu3O7-x films

Using atomic force microscope (AFM) as a lithography tool, we ploughed (a) constant thickness nanoconstriction (similar to 492 nm) and (b) variable thickness bridges (similar to 25 to 50 nm) on YBa2Cu3O7-x thin films. We measured I-V characteristics and observed Shapiro steps, giving positive evidence of Josephson effect in these nanobridges. Further the linear temperature dependence of the critical current (I-c) and the well defined Fraunhofer-type pattern of I-c versus the applied magnetic field, rules out any flux flow behavior and firmly establishes Josephson effect in the constant thickness nanoconstriction type junction. The temperature dependence of I-c is again linear in the variable thickness bridge. However the magnetic field dependence of I-c did not show Fraunhofer-type pattern, possibly due to averaging effects. We believe that the observed low field dependence of I-c in these nanobridges allows one to tune the Josephson energy by small applied fields, making these nanojunctions useful for quantum applications like qubits. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3481425]