Direct measurement of penetration length in ultrathin and/or mesoscopic superconducting structures

Abstract

As the dimensions of thin superconducting structures become comparable with or less than the penetration length of magnetic flux into the structures, it becomes increasingly necessary to devise experimental tests of available theoretical models.One approach which we shall describe, enables penetration lengths to be derived from the measurement of the effective area of planar, thin-film structures with linear dimensions in the range 1 to 100µm. The effective area is defined by measurement of the inductive coupling of the structures to dc or low-frequency magnetic fields.The structures described consist of two parts: (1) An ultra-thin annular superconducting film with transition temperature T ca ("washer"); and (2) surrounding the washer is a superconducting ring with transition temperature T cs .Because the films are prepared in such a way that T ca < T cs , the ring-washer combination acts as a dc SQUID (Superconducting Quantum Interference Device) up to and beyond T ca , enabling the effective area of the washer to be measured over a wide temperature range. Results for the temperature dependence of the Pearl penetration length Λ(T), derived directly from measurements of the effective area, are compared both with theory and with other experimental data. Whereas alternative methods may be restricted to narrow-band, high-frequency fields and require sample dimensions of order 10 mm or greater, the method is inherently broad-band and is applicable to dimensions ≥ 1 µm.

Spatial Resolution Assessment of Nano-SQUIDs Made by Focused Ion Beam

“…inductance) of the SQUID to be controllably varied by up to a factor of 2 [1]. We report also [1] on the current-voltage characteristics of these junctions, together with the voltage-field transfer function and inductance/effective-area measurements of SQUIDs incorporating them, over a range of temperatures. The junction characteristics resemble the Dayem-bridge weaklink model and phase slip weak-link models, and when suitably shunted by an in-situ metallic layer, are free from hysteresis over a useful range of operating temperatures.…”

“…A proportion of the SQUID's effective area may consist of a reduced thickness film with suppressed , and this allows the effective area (i.e. inductance) of the SQUID to be controllably varied by up to a factor of 2 [1]. We report also [1] on the current-voltage characteristics of these junctions, together with the voltage-field transfer function and inductance/effective-area measurements of SQUIDs incorporating them, over a range of temperatures.…”

Novel Methods of Fabrication and Metrology of Superconducting NanoStructures

“…Other techniques [9] are restricted to the high-frequency (GHz) range. With a reduction of the dimensions of superconducting devices into the submicrometer regime (e.g., for quantum computing or single-particle counting applications), we have developed [10] a noninvasive in situ technique for measurement of the temperature-dependent magnetic properties of ultrathin superconducting structures. The method allows measurement of the effective area and penetration length in ultrathin-film structures of micrometer dimensions and can be extended to more general measurements of magnetic properties of nanoscale structures.…”

Focused Ion Beam NanoSQUIDs as Novel NEMS Resonator Readouts

“…Devices) developed at NPL [5], [6] as readouts. This will require operation at cryogenic temperatures but this is already necessary to satisfy inequality (1).…”

“…Here the conducting NEMS cantilever carries an ac current orthogonal to this field and vibrational excitation occurs along the third axis. We will present calculations which show that a sufficiently sensitive form of readout can be provided by our on-going development of nanoSQUIDs [5], [6]. Here a SQUID structure is fabricated, using focused ion beam etching, on the same chip as the NEMS resonator.…”