Differential resistance measurements of Nb${\mathrm{Se}}_3$-insulator-lead tunnel junctions as a function of dc bias voltage are reported. The amplitude of the charge-density-wave gap, $2\Delta$, which occurs in Nb${\mathrm{Se}}_3$ at $T_2=59\mathrm{}$ K is found equal to 70 ± 5 meV. This value decreases when pressure is applied but the ratio $\frac{2\Delta }{k{T}_2}\sim 14$ is pressure independent up to 2.5 kbars. This large magnitude of the charge-density-wave gap, at variance with the BCS value, is interpreted as resulting from the strong-coupling nature of electron-phonon interactions.

• Received 25 July 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.2199