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## V. SPECTROSCOPY AT LOW TEMPERATURES 

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While running the scan head in the STM mode gives insights into the topography of the sample surface, the STS mode directly yields the electronic density of states at the surface via the differential conductance vs. voltage curves. The characteristics of the superconducting density of states in NbSe 2 are expected to occur at Δ /e = 1 . 75 · k B T$_{c}$/e = 1 . 08 mV, i. e., at low DC bias. To resolve signatures at such low energies a high energy resolution, respectively voltage resolution δV < 1 meV is needed. For that purpose a switchable voltage divider (1:1, 1:10, 1:100, 1:1000) was installed to increase the resolution of the electronics. In order to record the tunneling curves with high resolution the tunneling voltage is modulated with an AC bias ( V$_{AC}$ ≈ 20 µ V) and the current signal is measured with a lock-in amplifier. 

Figure 10 shows a typical measurement of the differential conductance of a NbSe 2 -PtIr tunnel contact (circles) at T = 50 mK in zero field. At such low temperatures the spectrum shows pronounced conductance peaks due to the BCS singularity of the density of states [17]. The 

