Abstract
The transport properties of superconducting NbN nanostructures in the form of thin film, bridge of width (w) = 50 μm and three meanders of w = 500, 250 and 100 nm have been investigated by resistance (R) measurements in temperature (T) range = 2 -300 K and magnetic field (B) range = 0 - 7 Tesla. The nanostructuring was carried out using Focused Ion Beam (FIB) milling. Reduction of sample width results in significant changes in the normal and superconducting state properties. For instance, the observed metallic behavior in the thin film sample is lost and the normal state resistance increases drastically from 2.4 Ω to 418 kΩ for the 100 nm meander. In the superconducting state, the value of critical temperature Tc (upper critical field Bc2 at T = 0 K) reduces gradually with width reduction, it changes from 13.15 K (42.8 Tesla) in the case of thin film sample to 5.7 K (12.7 Tesla) for the 100 nm meander sample. The superconducting transitions are found to get broader for the bridge sample and the meanders additionally show low-temperature resistive tails. In case of all the samples with reduced width, the transition onsets are found to be rounded at surprisingly high values of T ∼ 25 K >> Tc. These results are discussed in terms of the possible effects of FIB processing and weak localization in our samples.
Highlights
Nb-based superconductors have been well known for many applications in bulk form and in nanostructured form as well.[1,2] In the latter case, various characteristics have been exploited to build superconducting electronic devices.[3]
Thin films of superconductors have been found to be ideal for devices like Transition Edge Sensor (TES), which utilises the property of high dR/dT of superconductors in transition regime;[4] superconducting loop with one or two Josephson junctions forms ultra-sensitive superconducting quantum interference devices which use quantum tunneling of electrons from weakly linked superconductors;[5] and the superconducting nanowire functioning as single photon detector has proven to be very promising for communication applications.[6,7]
The effect of width reduction on the transport properties of superconducting NbN nanostructures has been studied via focused ion beam (FIB) milled thin film in the form of bridge and meander samples
Summary
Nb-based superconductors have been well known for many applications in bulk form and in nanostructured form as well.[1,2] In the latter case, various characteristics have been exploited to build superconducting electronic devices.[3]. In addition to the size effects, the nanofabrication process itself may induce microstructural/chemical changes in the nanostructures.[12,13] it becomes important to study and differentiate the two effects for the realization of devices with tailored properties. We have attempted to address these issues through transport studies of NbN nanostructures, where a host NbN film of 50 nm thickness is gradually reduced by FIB milling to varying sample widths of 50 μm, 500 nm, 250 nm and 100 nm. The highlight of the present work is the observed scaling of both the normal and superconducting state properties of our nanostructures with width, which can be understood by the combined effect of width reduction and FIB induced microstructural changes
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