Dissipative phenomenon in the mixed state of type II superconductors are reported, and explanations suggested.
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We have observed some striking dissipative phenomena which occur in the mixed state of type II superconductors when time-dependent magnetic fields are applied. The superconductor is probed with a small a.c. field superimposed on a swept d.c. field and its complex magnetic moment is measured for various amplitudes of the d.c. sweep rate dH/dt, the frequency ¢o and the a.c. field amplitude Hac. Earlier measurements \[1\]o n type II superconductors in swept fields were restricted to the real part, ×', of the apparent susceptibility. In our work we have studied both real and imaginary components, ×' and ×", respectively, the latter in somewhat greater detail \[2\]. Our observations bear on the nature of the dissipative mechanisms and the surface superconducting layer \[3\]i n the mixed state. In brief, we find that in the mixed state X' and ×" are functions of the single parameter 7 --~Hac/(dH/dt) and are independent of the actual value of the d.c. field, H (except near Hcl and Hc2 ). The samples used were extruded wires of In-4 at. %Bi, bothannealedandunannealed with K = 1.9. Both showed the same pattern of hehaviour. The * Now at the Boeing Company. ** Supported by the U. S. Air Force Office of Scientific Research.