Imaging of μm wavelength, collective cavity resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junction stacks under microwave irradiation

We have used low-temperature scanning electron microscopy to image the electromagnetic field distribution in microstructured Bi2Sr2CaCu2O8+x intrinsic Josephson junction stacks under micro. wave irradiation. Even for microwave frequencies. below 10 GHz, standing waves along the stack with wavelengths of order. 10 mum can be observed. The data, together with numerical simulations based on coupled sine-Gordon equations, strongly indicate that the standing waves are due to collective Josephson plasma cavity resonances, with a large number of junctions oscillating in phase. The resonance frequencies are estimated to be of order 100-500 GHz, that is, much higher than the drive frequency. We suggest that, under the low-frequency microwave irradiation, fluxon/ antifluxon pairs form; in turn exciting the high-frequency collective cavity resonances. (C) 2004 American Institute of Physics.