Spectral response of point-contact Josephson junctions

Abstract

The spectral response of niobium point‐contact Josephson junctions with resistance values R between 1 kΩ and 10 Ω has been measured. For R≳100 Ω the response peaks at maxima of d²V̄/dĪ², whereas the spectral response peaks at frequencies ν_R≲200 GHz. Decreasing the resistance by burning‐in of the contact, it was found that for underdamped contacts ν_R∝R^−1/3, almost independent of the junction geometry. Tapping the contact, i.e., changing the microscopic properties of the flat contact region (diameter, 5–10 μm), changed the absolute value of ν_R. Maximum resonant response occurred approximately for critical damping at R=80–100 Ω. The observed behavior is interpreted as resonant detection at the plasma frequency. At low resistance values ν_R disappears, but additional resonances at ν≳150 GHz appear, some of which could be identified as antenna resonances.