Photoconductivity with anomalies in insulators correlated with high-T_c superconductivity

Abstract

We have studied anomalies in the temperature dependence of transient photoconductivity in host oxide insulators or semiconductors La-Cu-O, Y-Cu-O, Ba-Bi-O, Bi-(La, Y)-Cu-O etc. with various levels of Ba-, Sr-, Pb-, (Sr, Ca)- doping or oxygen deficiency in the photoexcitation range Θ=450-950 nm and T=4.2-300K, including compositions close to the Cu-O-based or Bi-O-based high-T_c superconductors. As temperature decreased, we observed several unexpected emergences of photoconductivity for these host oxide insulators, instead of the submergence of photoconductivity found in most normal photoconductors, below the step temperatures T_pS, corresponding with the critical temperatures T_sC of the superconductors. The Cu₂O- or Bi₂O₃-like part in these insulators or semiconductors at the photoexcitation exhibits behaviour similar to that in doped Cu- or Bi-O based superconductors in the dark, irrespective of various crystal symmetry, dimensionality, and huge differences in carrier concentration. Surprisingly, we have observed quite similar phenomena even in single crystals of the host insulators Cu₂O and Bi₂O₃ themselves. Moreover, we have succeeded in observing nonlinear optical growth of photodiamagnetism in optical-microwave double cyclotron resonance absorption lines of photocarriers at 35GHz and at 735 nm in Cu₂O, most likely caused by two-body effects, such as bipolarons via induced mid-gap states. Thus, we speculate that Cu₂O or Bi₂O₃ may be the basic substance at photoexcitation equivalent to the Cu-O or Bi-O based oxide superconductors in the dark. We make the hypothesis that all the values of T_pS and T_sC are ruled by the quantum numbers, [n, l] of virtual intraband excitons CuO in the dark or in Cu₂O at photoexcitation. We attribute the variety of T_sC in the Cu-O-based superconductors to “excitonic isomer shifts” corresponding with “isotope shifts” in the phonon mechanism. Furthermore, we propose that, by combining these photoconductors for the gate materials and relevant superconductors for the source and drain materials, both become effective simultaneously below T_sC, one can fabricate novel devices such as optically controllable Josephson devices or FET-transistors and accumulate them into high density. Therefore, we believe that our discovery of such phenomena may lead to a new field of science, that we may call “Superconductive Optoelectronics”.

Key Words:

• Photoconductivity
• phase transition
• and high-T_c superconductivity