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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 

<img src='content_image/352881.jpg'>

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 

