We describe an experiment in superconductivity suitable for an advanced undergraduate laboratory. Point-contact spectroscopy is performed by measuring the differential conductance between an electrochemically etched gold tip and a 100-nm thick superconducting niobium film with a transition temperature Tc ≈ 7 K. By fitting the results to Blonder–Tinkham–Klapwijk theory using a finite lifetime of quasiparticles, we obtain a superconducting gap energy Δ ≈ 1.53 meV, a lower bound to the Fermi velocity vF ≥ 3.1 × 107 cm/s, and a BCS coherence length ξ ≈ 43 nm for niobium. These results are in good agreement with previous measurements.
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