Microwave Absorption Peaks : Signatures of Spin Dynamics in Cuprates

We show that a common feature of temperature-dependent microwave absorption is the presence of absorption peaks. ac loss peaks can arise when the internal T -dependent magnetic relaxation time crosses the measurement frequency. These features are observed in the insulating (SrxCa14−xCu24O41, La5/3Sr1/3NiO4 and Y Ba2Cu3O6.0), pseudo-gap (T > Tc in underdoped Y Ba2Cu3O7−δ , Hg : 1223 and Hg : 1201) and superconducting (T < Tc) states of the oxides. The commonality of these features suggests a magnetic (spin) mechanism, rather than a quasiparticle origin, for the so-called “conductivity” peaks observed in the cuprate superconductors. During the last few years, there have been extensive and careful experimental [1,2] as well as theoretical [3] studies of the microwave properties of the cuprate superconductors. In parallel, several experiments have been shown to be consistent with a d-wave order parameter (OP) [4]. The linear behavior of the penetration depth λ(T ) at low temperature T is frequently cited as evidence of d-wave symmetry. However a consensus is emerging that the totality of the microwave data is not explainable in terms of a pure d-wave gap quasiparticle scenario, and that the (dynamic) microwave response may be decoupled from the (static) OP symmetry. The principal issues are summarized below : 1. Y BCO and Hg : 1223 are definitely not pure d-wave superconductors as deduced from the microwave measurements. This is signaled by the presence of multiple “conductivity” peaks, as shown in Fig. 1(c). Within a conventional gap-quasiparticle scenario, the data are consistent with mixed symmetry, e.g. d + s. While mixed symmetry is allowed in orthorhombic Y Ba2Cu3O7−δ , it is not allowed in tetragonal Hg : 1223. This suggests that the OP symmetry may be decoupled from the crystal symmetry. 2. The measured microwave absorption is significantly higher than estimates based upon d-wave calculations, using acceptable estimates of the scattering times. The discrepancies are very large (orders of magnitude) for all superconductors such as Bi : 2212, with the possible exception of Y Ba2Cu3O7−δ. 3. Anomalous state above Tc: In materials such as Hg : 1223, Hg : 1201 and underdoped Y Ba2Cu3O7−δ we find that the surface resistance Rs is not equal to the surface reactance Xs (Rs 6= Xs). This indicates that the pseudogap state above Tc is not a normal metal with ordinary Ohmic conductivity, and may signify the importance of magnetic contributions to the microwave impedance. Thus the transition to the superconducting state takes place from an unconventional state, and that suggests that sum rules may not hold [5]. 4. 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 [6]. 5. Internal Josephson effect: A unique feature of the cuprate superconductors is the strong microwave response at very low field levels well below Hc1. In ultraclean Y Ba2Cu3O6.95 samples, a nonlinear response which can be described as a single Josephson junction in the ab-plane, is observed [7]. This can arise from