Abstract
At the 1969 Stanford Superconductivity Conference we reported observation of anomalous dc current singularities in the I–V characteristics of Josephson tunnel junctions.1† These singularities were termed “anomalous” because their properties differ in several respects from those of the well-known Fiske steps.3 The following characteristics were observed: (1) The singularities exist in the absence of an applied magnetic field. (2) Although they appear at voltages corresponding to junction cavity resonance mode frequencies like the Fiske steps, only even-order modes occur. (3) They occur only below a temperature-dependent cutoff voltage of order 0.1/(2Δ/e). (4) The singularity amplitudes have a complex temperature dependence; in the simplest cases the amplitude first increases, then decreases as the temperature is lowered. That these singularities are nonetheless not unrelated to Fiske steps is suggested by the fact that application of a magnetic field causes a smooth decrease in the amplitudes of the even-order singularities accompanied by appearance of the missing odd-order singularities.
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