Knight Shifts and Band Structure in the Lead-Salt Semiconductors

The temperature and carrier-concentration dependences of the ${\mathrm{Pb}}^{207}$ and ${\mathrm{Te}}^{125}$ Knight shifts have been measured in both $n$- and $p$-type PbTe. Qualitatively similar data have been reported by others for ${\mathrm{Pb}}^{207}$ in PbSe. The very large shifts of ${\mathrm{Pb}}^{207}$ in $p$-type material and the relatively small shifts of ${\mathrm{Te}}^{125}$ in $n$-type material are attributed to an $s$-character contact interaction with the carriers of the valence and conduction bands, respectively. The sign of the isotropic $g$ factor $g_{\mathrm{eff}}$ is negative in the $L$-point valence band and positive in the $L$-point conduction band, in agreement with the results of k·p perturbation theory. The temperature dependence of the Knight shift is ascribed to the motion of the Fermi level, the temperature dependence of the energy gap and related band parameters, and to the shift in carriers from the $L$-point valence band to a second valence band at temperatures above about 200°K. The localized orbital hyperfine interaction is suggested as the source of the Knight shifts of ${\mathrm{Pb}}^{207}$ in $n$-type material and ${\mathrm{Te}}^{125}$ in $p$-type material.