PACS. 74.30G -Response to electromagnetic fields, nuclear magnetic resonance, ultrasonic attenuation.PACS. 74.60G -Flux pinning, fluxon-defect interactions. PACS. 74.70 -Superconducting materials.Abstract. -Series of microwave-absorption lines spaced uniformly in the magnetic field have been observed in single crystals of YBa2Cu307-6. As the crystals are rotated, the regularly periodic lines move in field so that the component of the magnetic field along a [1101 direction remains constant. Line series appear at successive threshold microwave powers and temperatures, and the lines widen dramatically as either the microwave power or the temperature is increased. The absorptions are believed to arise from the mixing of macroscopic flux states, stabilized by the microwave excitation.

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