In-Chain Tunneling Through Charge-Density Wave Nanoconstrictions and Break-Junctions

We have fabricated longitudinal nanoconstrictions in the charge-density wave conductor (CDW) NbSe$_{3}$ using a focused ion beam and using a mechanically controlled break-junction technique. Conductance peaks are observed below the T$_{P1}$$=145 $K and T$_{P2}$$=59 $K CDW transitions, which correspond closely with previous values of the full CDW gaps $2\Delta_{1}$ and $2\Delta_{2}$ obtained from photo-emission. These results can be explained by assuming CDW-CDW tunneling in the presence of an energy gap corrugation $\epsilon_{2}$ comparable to $\Delta_{2}$, which eliminates expected peak at $\Delta_{1}+\Delta_{2}$. The nanometer length-scales our experiments imply indicate that an alternative explanation based on tunneling through back-to-back CDW-normal junctions is unlikely.