Subgap collective tunneling and its staircase structure in charge density waves

Tunneling spectra of chain materials NbSe3 and TaS3 were studied in nanoscale mesa devices. Current-voltage I-V characteristics related to all charge density waves (CDWs) reveal universal spectra within the normally forbidden region of low V, below the electronic CDW gap 2 Delta. The tunneling always demonstrates a threshold V-t approximate to 0.2 Delta, followed, for both CDWs in NbSe3, by a staircase fine structure. T dependencies of V-t(T) and Delta(T) scale together for each CDW, while the low T values V-t(0) correlate with the CDWs' transition temperatures T-p. Fine structures of CDWs perfectly coincide when scaled along V/Delta. The results evidence the sequential entering of CDW vortices (dislocations) in the junction area with the tunneling current concentrated in their cores. The subgap tunneling proceeds via the phase channel: coherent phase slips at neighboring chains.