Theory of terahertz electric oscillations by supercooled superconductors

Abstract

We predict that below Tc a regime of negative differential conductivity (NDC) can be reached. The superconductor should be supercooledto T<Tc in the normal phase under DC voltage. In such a nonequilibrium situation the NDC of thesuperconductor is created by the excess conductivity of the fluctuation Cooperpairs. We propose NDC of supercooled superconductors to be used as an activemedium for generation of electric oscillations. Such generators can be used in thesuperconducting electronics as a new type THz source of radiation. Oscillations can bemodulated by the change of the bias voltage, electrostatic doping by a gate electrodewhen the superconductor is the channel of a field effect transistor, or by light.When small amplitude oscillations are stabilized near the critical temperatureTc the generator can be used as a bolometer. NDC, which is essential for the applications, ispredicted on the basis of analysis of known results for fluctuation conductivity, obtained inprevious papers by solving the Boltzmann kinetic equation for the Cooper pairs metastablein the normal phase. The Boltzmann equation for fluctuation Cooper pairs is a result ofstate-of-the-art application of the microscopic theory of superconductivity. Our theoreticalconclusions are based on some approximations like time dependent Ginzburg–Landautheory initially derived for gapless superconductors, but nevertheless can reliably predictthe appearance of NDC. NDC is the main ingredient of the proposed technical applications.The maximal frequency at which superconductors can operate as generators is determinedby the critical temperature . For high-Tc superconductors this maximal frequency falls well inside the terahertz range. Technical conditions toavoid nucleation of the superconducting phase are briefly discussed. We suggest that nanostructuredhigh-Tc superconductors patterned in a single chip can give the best technical performance of theproposed oscillator.