Non - quasiparticle microwave absorption in Bi 2 Sr 2 CaCu 2 O 8 + δ 1 S . Sridhar

We show that a non-quasiparticle charge collective mode, in parallel and coincident with the d-wave pair conductivity, leads to a quantitative understanding of microwave surface impedance measurements on superconducting Bi2Sr2CaCu2O8+δ. The analysis suggests an inhomogeneous charge ground state in Bi2Sr2CaCu2O8+δ and other HTS. A complete understanding of the microwave response of high Tc superconductors has proved to be as elusive as the mechanism of superconductivity itself. Two principal outstanding issues are: (i) The basic mechanism of linear microwave absorption is still not understood. While one would have thought that an identification of the order parameter symmetry as dx2−y2 would have provided quantitative description of the linear microwave properties, this has not turned out to be the case. Calculations of the absorption based upon d-wave (or even mixed s+ d symmetry) are actually lower by orders of magnitude (as we show below), even though they can account for the penetration depth measurements [1,2]. A unified picture of the microwave loss of single crystals, thin films and ceramics has not yet been achieved. (ii) The HTS display a surprisingly high level of nonlinear response which is not properly understood, despite several attempts at modelling it. The measured nonlinear response (which is a major limitation of the use of the cuprate superconductors in microwave applications) is significantly higher than estimates based upon d-wave calculations [3]. Particularly intriguing manifestations of nonlinearity are the Josephson-like response in single crystals [4], the so-called magnetic recovery effect [5](a) and 2nd harmonic generation [5](b). In this paper we discuss a new analysis of the microwave response in HTS. In the superconducting state, electromagnetic relaxation is shown to occur predominantly via a charge mode even at frequencies in theGHz ranges. This model quantitatively 1) Work supported by NSF-ECS, ONR and AFOSR. describes the available microwave data for Bi2Sr2CaCu2O8+δ (Bi : 2212) and also for several other cuprate HTS. 0 20 40 60 80 100 120 10 −2 10 −1 10 0