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 2Δ. The tunneling always demonstrates a threshold V_t≈0.2Δ, followed, for both CDWs in NbSe₃, by a staircase fine structure. T dependencies of V_t(T) and Δ(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/Δ. 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.