Charge Density Wave Gap Formation ofNbSe3Detected by Electron Tunneling

Tunneling spectra of ${\mathrm{NbSe}}_3$ between 77 K and room temperature have been measured carefully. A charge density wave (CDW) pseudogap exists up to a temperature higher than $\sim 260\mathrm{K}$. While the gap parameter undergoes an accelerated change between $\sim 130$ and $\sim 160\mathrm{K}$, the junction conductance at zero bias does not show any anomaly around $T_1=145\mathrm{}\mathrm{K}$, the upper CDW transition temperature. Below the mean field transition temperature there are four regimes: a regime where a true CDW state exists, a regime where pinned and mobile CDW coexist, a regime of a pseudogap, and a regime where the CDW fluctuation can barely be detected by current measurements.