Evidence for a conduction-electron transition from pressure dependence of NMR in CeP and CeAs

Nuclear-magnetic-resonance (NMR) measurements as a function of temperature and pressure have been made of the Knight shift $K$ of ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ and ${}_{}{}^{75}{}_{}{}^{}\mathrm{As}$ contained in CeP and CeAs, respectively. The pressure derivative of $K$ ($\frac{d\ln K}{\mathrm{dP}}$) for both compounds shows a large increase near the temperature where $K$ is known to become nonlinearly related to the susceptibility. Detailed analyses of the NMR data show that behavior expected from either nonlinear hyperfine effects or valence changes of the Ce qualitatively fail to account for the pressure results. It is shown, however, that the effects of a sharp feature in the conduction-electron density of states, located very near the Fermi energy, can produce the observed NMR. The pressure dependence of $K$ for ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ in YbP was also studied. Very small pressure-induced changes in $K$ occurred at low temperature with no anomalies in the pressure derivative such as observed in the Ce compounds.