Electrical Transport Measurement of Niobium Thin Superconducting Film Above An Array of Magnetic Quantum Dots
The dynamic properties of a Square Nb/Fe magnetic quantum dot array have been studied with and without magnetic field in the presence of high frequency signals. We map this sample to a typical proximity-coupled Superconductor-Normal-Metal-Superconductor (SNS) Josephson junction arrays. The sample consists of a square array of Fe dots covered by a thin Nb film sputtered on top.
When the array is biased with both dc and ac current, their time-averaged dc voltage characteristics shows the some constant voltage steps. These steps are called Shapiro steps. In this dissertation, we biased the array with a high frequency signal and some dip structures corresponding to the Shapiro steps equidistant in voltage were found in the dynamical resistance curve.
For a SNS Josephson array, the width of the steps shows the Bessel-function-like behavior and it is dependent on the amplitude of the rf current. Since the Shapiro steps were rounded up in our sample, instead the "strength" was studied and it showed the similar properties as the rf power-dependent step width. The rf frequency dependence of the integer Shapiro step also was present in this dissertation.
If a transverse magnetic field is applied to the array, the vortex sublattice is then commensurate with the underlying junction lattice. As a result, the array exhibits some half-integer Shapiro steps (Fractional step). Even in the absence of magnetic field, the half-integer step (known as subharmonic step) also can occur. Both half-integer steps were studied in the influence of rf power and rf frequency.
Some factors, like thermal fluctuation and site disorder, that affected the measurement was discussed in the last chapter. The site disorder can disrupt the phase coherence between the superconducting islands and thus the appearance of Shapiro steps, especially the half-integer steps, are strongly affected by the site disorder.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:quantum dots josephson junction array shapiro step
Date of Publication:01/01/2008