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 2� 1 and 2� 2 obtained from photoemission. These results can be explained by assuming CDW-CDW tunneling in the presence of an energy gap corrugation � 2 comparable to � 2, which eliminates expected peaks atj � 1 � � 2j. 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. focused ion beam (FIB), shows conductance peaks at 105 mV and 190 mV corresponding to 2� 1 and 2� 2 ,a s illustrated in Fig. 1. We reproduce the data at 4.2 K using a mechanically controlled break-junction (MCBJ) tech- nique, demonstrating that the FIB results are not dominated by Ga ion damage. Our results can be explained by CDW- CDW tunneling in the presence of a large transverse gap corrugation, although tunneling through back-to-back CDW-normal-conductor junctions cannot be conclusively ruled out. CDWs form in metals with quasi-one-dimensional Fermi surfaces. Electron-hole pairs near the Fermi level kF form a macroscopic condensate and associated periodic modulations of the electron density and atomic positions. The condensate arises from the electron-phonon interac- tion, as described by the mean-field Hamiltonian H P � P