SHARP MAGNETOABSORPTION RESONANCES IN THE VORTEX STATE OF BI2SR2CACU2O8+DELTA

We report the observation of narrow resonances in the absorption of mm wavelength radiation (30 to 50 GHz) in the vortex state of Bi2Sr2CaCu 2O8+delta. When the applied field B (parallel-to c) is swept at fixed frequency omega, the absorption displays two distinct Lorentzian peaks at the resonance fields B0 and B1. The resonance frequency decreases with increasing field, consistent with anticyclotronic behavior. At fi xed omega, B0 and B1 also vary strongly with temperature T. B0 and B1 increase to a sharp maximum when T crosses the vortex solid-to-liquid transition line inferred from resistivity results. The coupling of cyc lotron resonance with collective modes giving rise to an anticyclotron ic mode is discussed.