Experimental study of fluxon resonances in window-type long Josephson junctions

We report first systematic measurements of zero-field steps in I-V characteristics of long Nb/Al-AlO/sub x//Nb Josephson junctions made in a window of a stripline structure. We compared the data for junction series of the same dimensions but with different ratio of the junction width W to the width of the idle window part W´ surrounding them. Zero-field steps were found to be stable only in structures with W´/W<3. With increasing the ratio W´/W, the effective fluxon propagation velocity is found to increase and fine structure resonances appear on fluxon resonant steps. Experimental data are compared with numerical simulations using the modified two-dimensional sine-Gordon equation and qualitative agreement is found.<>