The Susceptibility in Cuprates

The unusual concentration (x) and temperature (T) dependence of the susceptibility $\chi$ in La$_{2-x}{\rm Sr}_{x}$CuO$_{4}$ is discussed in this chapter. The susceptibility $\chi$ at 400 K increases with x in the range $0.04 <x<0.25$ and decreases in the range $0.25 <x<0.33$. The maximum at $x=0.25$ is interpreted in terms of the curvature inversion of the O Fermi surface. At the inflection point the density of states is extremely high, which causes $\chi$ to have a temperature behavior $\chi=A_{0}+B_{0}/T$ ($A_{0},B_{0}=$ constant). The Cooper pair (pairon) has no net spin, and hence its spin contribution to $\chi$