Typical dV/dI (V) spectra of Nb-Co (a-c) and Nb-Cu (d-f) contacts at 4.2 K.
Typical differential resistance versus bias voltage (thick solid curves) together with fits derived by assuming Γ = 0 (thin dashed curves) and P = 0 (thin solid curves) with the indicated fitting parameters. The curves for all the contacts are essentially indistinguishable. The deviations that occur only near the shoulder where dV/dI starts to drop from its normal-state value can be removed by introducing a “broadening parameter”. Nb-Co at T = 4.2 K with 2Δ = 2.6 meV (a); Nb-Cu at T = 4.2 K with 2Δ = 2.5 meV (b); AuIn2-Cu at T = 0.05 K with 2Δ = 65 μeV (c).
Polarization P at Γ = 0 and life-time broadening Γ at P = 0 versus Z of Nb-Co, Nb-Cu, and AuIn2-Cu contacts. The vertical solid lines represent the expected minimum Z 0 owing to Fermi momentum mismatch in the free-electron approximation. The solid curves through the data points serve as guides for the eye.
Comparison between the polarization-only (red dashed curves) and lifetime-only (blue solid curves) models for contacts with (a) small, (b) medium, and (c) large polarization. The differential resistance dV/dI is normalized to the normal contact resistance RN . The polarization-only spectra were first calculated assuming the indicated P and Z with 2Δ = 3.0 meV for niobium and T = 4.2 K. Then the lifetime-only spectra were fitted to obtain the indicated Γ and Z. For this fitting the energy gap had to be adjusted slightly.
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