Anomalous microwave absorption in quasi-one-dimensional (Sr,Ca)₁₄Cu₂₄O₄₁

Anomalously large and highly reproducible cusps in the temperature-dependent surface resistance data have been observed using contactless microwave loss measurements at 9.2 GHz in (Sr,Ca)₁₄Cu₂₄O₄₁ - a novel cuprate with quasi-one-dimensional two-legged ladder Cu₂O₃ planes and edge-sharing CuO₂ chains. The onset temperatures of the microwave cusps, which are as high as 270 K, decrease with increasing calcium content and nitrogen annealing. No indications of bulk superconductivity are evident in dc magnetic susceptibility and ac resistance measurements in the 80 to 300 K and 15 to 300 K temperature ranges respectively. However, a rapid increase in intensity of the Cu²⁺ electron paramagnetic resonance signal, which is likely to be associated with persistent currents in filamentary fractions of the samples, occurs at temperatures near the minima of the cusps. The observed cusps in the microwave data are tentatively interpreted as superconductive pairing fluctuations close to a charge density wave instability in the quasi-one-dimensionally structured ladder planes of optimally doped intergranular interfaces of the ceramic specimens.