Abstract
We report a systematic study of dc electric fields produced by sinusoidal high frequency ac currents in Nb superconducting films subject to a constant magnetic field perpendicular to the film plane. At frequencies in the 100 kHz to MHz range appears a new rectification effect which has not been previously observed at lower frequencies. We have observed the dc electric field generated in this regime in films without intentionally created anisotropic pinning centres, i.e. plain films, both in strip geometry as in cross-shape geometry, and also in films with symmetric periodic pinning centres. The electric field appears in both directions along and transverse to the alternating current and is essentially different at opposite film sides. It depends strongly on the intensity of the magnetic field and may exceed by nearly an order of magnitude the rectified electric fields recently reported at lower frequencies (few kHz) in systems with artificially induced anisotropic vortex pinning. The effect has a non-monotonic dependence on the drive current frequency, being maximum around a few 100 kHz to MHz, and shows a complicated temperature dependence. It is found to be different in long strips and cross shape samples. In the case of films with symmetric periodic pinning centres the rectified voltage shows a lower magnitude than in plain films, and shows an interesting structure when the applied magnetic field crosses the matching fields. We are only able to put forward tentative ideas to explain this phenomenon, which irrespective of its explanation should be taken into account in experimental studies of rectification effects in superconductors.
Highlights
In the 1990s several groups observed rectifying effects in thin film superconductors as the appearance of ohmic resistance in a type II superconductor subject to a permanent applied magnetic field and with a dc current flowing in it, if an additional alternating magnetic field was externally applied [5, 6]
Irrespective of the possible explanations, since in experiments on rectification in superconductors this electric field adds to the one due to anisotropic vortex pinning, or to time asymmetry of the ac drive, one has to take it into account when interpreting rectification experiments in superconductors, especially if an increase of the frequencies to the MHz range is involved
We have looked for high frequency rectification effects in an artificially nanostructured Nb film with symmetric periodic pinning centres (PPCs), made up of Ni dots
Summary
The experiments were performed on plain and nanostructured Nb films with thickness (d) of 100 nm. In the case of the cross shape, the alternating current was injected through opposite arms of the cross (RF in figure 1). In our experiments a dc voltage, which we shall call rectified voltage (Udc), is detected in the superconducting sample when an ac current is injected along the sample and simultaneously a magnetic field is applied perpendicularly to the film. This voltage was measured with a Keithley 2182 nanovoltmeter, and has been detected along and transverse to the ac drive current direction.
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