value of the energy gap is determined by fitting the spectra using the BTK theory [18] for a large barrier parameter (tunnel limit). By fitting the positions at the maxima (solid line), the energy gap Δ = 1 . 62 meV is obtained, whereas a fit to the width of zero conductance (dashed line) results in Δ = 1 . 06 meV for the gap. These variations reflect a notable gap anisotropy in this material [14] most probably caused by the presence of charge-density wave dynamics in NbSe 2 since our measurements are relatively slow. On the other hand a dynamic broadening due to charge density waves is also possible. In an applied field B = 500 mT, line scans (’topographic mode’) through a vortex were performed (not shown) in addition to the images of the vortex lattice. The spectra recorded along the line scan clearly show a somewhat broadened BCS-like density of states in the superconducting regions and an enhancement of the differential conductance at zero bias in the normal-conducting regions of the vortex center which can be attributed to Andreev bound states [19]. This enhancement in the conductance has been reported earlier [20, 21] and is very sensitive to the position of the tip in respect to the vortex center. As representative examples, three different spectra are shown in figure 11. The data acquired FIG. 11: Differential conductance of a NbSe 2 -PtIr tunnel contact in the vortex state (left panel) at T ≈ 100 mK. The positions of the PtIr tip above the NbSe 2 sample are marked (right panel). Spectra 1 and 3 are measured in the superconducting regions outside the vortex, spectrum 2 is measured directly at the center of the vortex. Spectra are shifted along the y axis for clarity. 15