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

We have fabricated longitudinal nanoconstrictions in the charge-density wave conductor (CDW) NbSe₃ using a focused ion beam and using a mechanically controlled break-junction technique. Conductance peaks are observed below the TP1=145 K and TP2=59 K CDW transitions, which correspond closely with previous values of the full CDW gaps 2Δ₁ and 2Δ₂ obtained from photoemission. These results can be explained by assuming CDW-CDW tunneling in the presence of an energy gap corrugation ϵ₂ comparable to Δ₂, which eliminates expected peaks at ±|Δ₁+Δ₂|. The nanometer length scales our experiments imply indicate that an alternative explanation based on tunneling through back-to-back CDW--normal-conductor junctions is unlikely.