Light Induced Electron-Phonon Scattering Mediated Resistive Switching in Nanostructured Nb Thin Film Superconductor

Abstract

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...

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