Pressure Effect on Superconducting NbSe2and NbS2

The pressure dependence of the superconducting transition temperature $T_c$ has been measured for Nb${\mathrm{S}}_2$ and Nb${\mathrm{Se}}_2$ in order to determine whether tunneling between laminar layers is a dominant factor controlling $T_c$. Measurements of the lattice constants by x-ray diffraction indicate that both materials are very compressible along the $c$ axis, but $T_c$ measurements do not correlate with the $c$-axis lattice constant. Nb${\mathrm{Se}}_2$ shows a rapid change in $T_c$ with pressure, whereas Nb${\mathrm{S}}_2$ shows practically no change at all. $T_c$ measurements correlate with intralaminar changes much better than they correlate with interlaminar spacings.