Interlayer tunnelling spectroscopy of the charge density wave state in NbSe3

Using small staked junctions of high quality we have measured interlayer tunnelling spectra of the layered quasi-one-dimensional material NbSe3. We identified a number of new features in the spectra: zero bias conductance peak (ZBCP), charge density wave (CDW) gap structure for lower and upper CDW, the sub-gap structure inside the CDW gap. The ZBCP dominates in low-temperature spectra. We discuss its origin as being related to the interlayer coherent tunnelling of the carriers uncondensed into CDW. We found that ZBCP is sensitive to magnetic field and its orientation. The field perpendicular to the layers broadens ZBCP while the parallel field narrows it. We consider the sub-gap structure as associated with self-localized states inside the gap.