GIANT RADIATION LINEWIDTH OF MULTIFLUXON STATES IN LONG JOSEPHSON-JUNCTIONS

Millimeter-wave band radiation measurements of flux-flow states in lon g Josephson junctions are reported. At low fluxon density, the radiati on linewidth is found by factor of 10(4) larger than that of a lumped Josephson oscillator. With increasing the fluxon density, a sharp cros sover is observed to a much smaller linewidth approaching the lumped o scillator limit. Numerical simulations using perturbed sine-Gordon mod el show that the observed behavior is related to internal degrees of f reedom in the moving fluxon chain.