Theory of terahertz electric oscillations by supercooled superconductors

We predict that below Tc a regime of negative differential conductivity (NDC) can be reached. The superconductor should be supercooled to T<Tc in the normal phase under DC voltage. In such a nonequilibrium situation the NDC of the superconductor is created by the excess conductivity of the fluctuation Cooper pairs. We propose NDC of supercooled superconductors to be used as an active medium for generation of electric oscillations. Such generators can be used in the superconducting electronics as a new type THz source of radiation. Oscillations can be modulated by the change of the bias voltage, electrostatic doping by a gate electrode when the superconductor is the channel of a field effect transistor, or by light. When small amplitude oscillations are stabilized near the critical temperature Tc the generator can be used as a bolometer. NDC, which is essential for the applications, is predicted on the basis of analysis of known results for fluctuation conductivity, obtained in previous papers by solving the Boltzmann kinetic equation for the Cooper pairs metastable in the normal phase. The Boltzmann equation for fluctuation Cooper pairs is a result of state-of-the-art application of the microscopic theory of superconductivity. Our theoretical conclusions are based on some approximations like time dependent Ginzburg-Landau theory initially derived for gapless superconductors, but nevertheless can reliably predict the appearance of NDC. NDC is the main ingredient of the proposed technical applications. The maximal frequency at which superconductors can operate as generators is determined by the critical temperature ℎ/2πωmax ∼ kBTc. For high-Tc superconductors this maximal frequency falls well inside the terahertz range. Technical conditions to avoid nucleation of the superconducting phase are briefly discussed. We suggest that nanostructured high-Tc superconductors patterned in a single chip can give the best technical performance of the proposed oscillator