Electrical transport through constrictions in the charge-density wave conductor NbSe₃

Abstract
We have investigated the electrical transport properties of insulating and metallic constrictions of dimensions 100 nm-10 m in the charge-density wave (CDW) conductor NbSe₃. The constrictions are made in a variety of ways: focused ion beam, reactive ion etching through a resist mask, and in a mechanically-controlled break junction configuration. We find that the behaviour of the junctions is independent of the fabrication method, and, depending on the size of the constriction, that the low-temperature behaviour of the constrictions is metallic or insulating. At 4.2 K we observe peaks in the differential conductance near 100 mV and 200 mV, which vanish as the temperature is increased above the Peierls temperatures 59 K and 145 K respectively. The data is consistent with the interpretation that there are two different regimes, depending on the size of the constriction, demonstrating either tunnelling behaviour, due to CDW-insulator-CDW junctions, or metallic behaviour, due to two normal metal-CDW junctions in series.