Gap anisotropy, spin fluctuations, and normal-state properties of the electron-doped superconductorSr0.9La0.1CuO2

Abstract

We report the results from a thermopower and a Cu nuclear magnetic resonance (NMR) study of the infinite-${\mathrm{CuO}}_{2}$-layer electron-doped high-temperature superconducting cuprate (HTSC) ${\mathrm{Sr}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{CuO}}_{2}.$ We find that the temperature dependence of the thermopower, $S(T),$ is different from that observed in the hole-doped HTSC. In particular, we find that $\mathrm{dS}(T)/dT$ is positive above \ensuremath{\sim}120 K. However, we show that $S(T)$ can still be described by the same model developed for the hole-doped HTSC and hence $S(T)$ is not anomalous and does not imply phonon-mediated pairing as has previously been suggested. The Cu NMR data reveal a Knight shift and spin lattice relaxation rate below ${T}_{c}$ that are inconsistent with isotropic s-wave pairing. The Cu spin lattice relaxation rate in the normal state, however, is Curie-Weiss like and is comparable to that of the optimally and overdoped hole-doped HTSC ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}.$ The magnitude of the Knight shift indicates that the density of states at the Fermi level is anomalously small when compared with the hole-doped HTSC with the same ${T}_{c},$ indicating that the size of ${N(E}_{f})$ is of little importance in the HTSC. We find no evidence of the normal state pseudogap that is observed in the hole-doped HTSC and which was recently reported to exist in the electron-doped HTSC ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4}$ from infrared reflectance measurements.