Point-contact spectroscopy of the charge-density-wave gap along the chains in NbSe3

We have measured the differential current-voltage (IV) characteristics of Au-NbSe3 point contacts formed in the direction along the conducting chains. We clearly observed charge density wave (CDW) gap features below the upper Peierls transition temperature in the variation of the differential resistance as a function of bias voltage for a number of contacts with very different zero-bias contact resistances. The temperature dependence of the upper CDW gap, Delta(p1), extracted from the data follows the BCS dependence with Delta(p1)(0) = 100 meV, similarly to previous measurements with the current applied perpendicularly to the chains. For contacts with direct conductivity, we found an excess resistance when the applied voltage is less than +/-Delta/e. This is attributed to an analog of Andreev reflection, in which the incident electron returns along its original path with its charge unchanged. [S0163-1829(99)11331-6].