d_x²-y²-STATE OF HIGH TEMPERATURE SUPERCONDUCTORS WITH A SMALL ADMIXTURE OF d_xy-STATE

Abstract

It is well known that most of the high temperature superconductors (at least hole-type) are in d-wave state. But it is still an unsolved problem whether it is a pure d-wave state or one has some kind of mixed state. Among the candidates for an admixture, there are s- and d-wave states. Existing experiments could not resolve this issue. New possibilities for experimental resolution of this problem are opened via recent observation of the collective modes in UBe 13 (heavy fermion superconductor) by microwave impedance technique experiments and in Sr 2 RuO 4 (high temperature superconductor) by ultrasound attenuation experiments. Some theoretical treatments show that the most likely state is a mixture of two d-wave states: dx2-y2 and dxy with a small admixture of former state. To create the theoretical basis for investigation of possible mixed superconducting state in unconventional superconductors by sound attenuation and microwave absorption experiments, I derive for the first time a full set of equations for collective modes spectrum in dx2-y2-state with small admixture of dxy state. These equations allow to calculate the whole collective mode spectrum in mixed dx2-y2+iεdxy state and distinguish this state from pure d-wave states (whose collective mode spectrum has been calculated earlier) by ultrasound attenuation and microwave absorption experiments.