The elemental Nb is mainly investigated for its eminent superconducting properties. In contrary, we report of a relatively unexplored property, namely, its superior optoelectronic property in reduced dimension. We demonstrate here that nanostructured Nb thin films (NNFs), under optical illumination, behave as room temperature photo-switches and exhibit bolometric features below its superconducting critical temperature. Both photo-switch and superconducting bolometric behavior are monitored by its resistance change with light in visible and near infrared (NIR) wavelength range. Unlike the conventional photodetectors, the NNF devices switch to higher resistive states with light and the corresponding resistivity change is studied with thickness and grain size variations. At low temperature in its superconducting state, the light exposure shifts the superconducting transition towards lower temperature. The room temperature photon sensing nature of the NNF is explained by the photon assisted electron-phonon scattering mechanism while the low temperature light response is mainly related to the heat generation which essentially changes the effective temperature for the device and the device is capable of sensing a temperature difference of few tens of milli-kelvins. The observed photo-response on the transport properties of NNFs can be very important for future superconducting photon detectors, bolometers and phase slip based device applications.The refractory metal Nb is famous for its superconducting properties and it is known to provide the highest critical temperature (T c ) (bulk T c = 9.2 K) for any elemental superconductor. Being one of the most used primary superconductors, Nb is well explored material in the field of superconducting photon detectors 1, 2 and bolometric applications 3 . Besides, it exhibits various physical properties like high melting point, high thermal conductivity, high critical current density etc 4 . Since decades, dimensional effects of niobium on its physical properties like critical temperature, superconducting energy gap, critical field, coherence length, penetration depth etc. have been explored. These properties vary with the dimension of niobium particles/grains and/or films 5 . It is well known that nanomaterials can exist as various nanostructures such as quantum dots, nanowires, nanoparticles, etc. which play key role in upgrading their optoelectronic properties, mainly controlled by the quantum effect, as compared to their bulk counterpart 6,7 . For example, granular Nb thin films have been shown to act like Josephson junctions 5,8 . During 1970's the effects of light on conventional superconductors have been explored intensely 9 . The effect of light and transient photo-response on oxide based high-T c superconductors have also garnered a reasonable attention in the field of superconductivity based optoelectronic applications 10 . Here, we explore optoelectronic properties of NNFs at room temperature (RT) and at low temperature (LT), above and below the T c . To the best of ou...

PACS 74.70.-b Superconducting materials other than cuprates PACS 74.25.-q Properties of superconductors PACS 74.25.Bt Thermodynamic properties

In this paper, we present a comprehensive study of electrical transport measurements on a superconducting film of NbN (thickness, d ∼ 50 nm) and its nanostructures fabricated using Focused Ion Beam (FIB) in the form of one bridge (width, w ∼ 50 µm) and three meanders (w ∼ 500 nm, 250 nm, and 100 nm). The resistance (R) and current–voltage [V(I)] characteristics are measured as a function of temperature (2 K–16 K) and magnetic field (0 T–7 T). The photoresponse is measured under quasi-monochromatic light irradiation (wavelength of ∼800 nm). All our samples with w ≫ ξ and d > ξ are dimensionally on the borderline of the three-dimensional limit. However, the film and bridge samples show quasi-2D signatures of Brzezinski–Kosterlitz–Thouless transition in the R(T) and V(I) characteristics. On the other hand, our meander samples show two slope transitions in R(T) that seem to fit well with the thermally activated phase slip (TAPS) near the superconducting onset and quantum phase slip (QPS) at lower temperatures, expected in quasi-1D superconductors. The presence of TAPS and QPS in all the meander samples is further supported by several other observations at B = 0: (i) linear V(I) at lower excitation currents in the entire transition region; (ii) nonlinear and non-hysteretic V(I) at higher currents in the TAPS region; (iii) in the QPS region, at higher currents, the V(I) curves show a quadratic V ∝ I2 dependence before hysteretic and stepped jumps; and (iv) the switching current (IC*) reduces significantly to 5 μA–25 μA (T = 2 K) when compared to nearly ∼875 μA (T = 10.5 K) in the bridge sample. With the application and increase in the magnetic field, at fixed temperatures in the QPS region of the meander samples, the V(I) characteristics show a crossover to TAPS. This seems to be correlated with a drastic reduction in the activation barrier (Ub) extracted from the R(T,B) data. Typically, for B = 0 T–7 T, Ub varies from ∼3000 K–1200 K (film sample) to ∼1100 K–220 K (bridge sample) and ∼250 K–50 K, ∼150 K–20 K, and ∼50 K–6 K for the 500 nm, 250 nm, and 100 nm meander samples, respectively. Using the Langer, Ambegaokar, McCumber, and Halperin theory [J. S. Langer and V. Ambegaokar, Phys. Rev. 164(2), 498 (1967); D. E. McCumberand B. I. Halperin, Phys. Rev. B 1, 1054 (1970)] and considering the normal state transport properties reported earlier [Joshi et al., AIP Adv. 8, 055305 (2018)], these results are shown to be consistent with disorder induced nano-paths of ∼50 nm, ∼12 nm, ∼10 nm, and ∼7 nm width developed in the FIB fabricated bridge and 500 nm, 250 nm, and 100 nm meander samples, respectively.