O17NMR Evidence for a Pseudogap in the MonolayerHgBa2CuO4+δ

Measurements of the spin shift ${}^{17}{\mathrm{K}}_s$ have been performed for the ${}^{17}$O NMR line in the monolayer cuprate ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$, and yield the spin susceptibility ${\chi }_s\left(T\right)$ of the ${\mathrm{CuO}}_2$ planes. In the underdoped regime, ${\chi }_s$ is found to extrapolate to zero at $T0\mathrm{}$. This indicates that a pseudogap opens at $T^{*}>T_c$, as in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$. Therefore, the pseudogap features cannot be explained in terms of a magnetic coupling between ${\mathrm{CuO}}_2$ planes in bilayer systems. In the overdoped regime, ${\chi }_s\left(T\right)$ obeys a Pauli-like law, as found in the overdoped ${\mathrm{Ti}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_6$. Comparing the different monolayer systems, we suggest some correlation between $T_c$ and distortion effects in the planes, as monitored by the ${}^{17}$O NMR linewidth.