Carrier concentration independent antiferromagnetic spin fluctuations in the electron-doped high-temperature superconducting cupratePr2−xCexCuO4

Abstract

We have performed ${}^{63}\mathrm{Cu}$ NMR measurements on the electron-doped high-temperature superconducting cuprate (HTSC) ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ $(x=0.10,$ 0.15, and 0.20), at 9 T, which is sufficient to suppress ${T}_{c}$ to zero. The Cu spin-lattice relaxation rate and the Cu spin-lattice relaxation rate anisotropy can be consistently interpreted in terms of coupling to antiferromagnetic spin fluctuations for temperatures as low as 6 K. We find that the spin-fluctuation spectrum probed by the Cu spin-lattice relaxation rate does not change with increasing electronic concentration, contrary to a recent theoretical predication. There is no evidence in the Cu spin-lattice relaxation rate data for a temperature-independent spin gap that is as large as theoretically predicted or as large as the normal-state pseudogap energy reported from infrared reflectance measurements on the electron-doped HTSC ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}.$ The Cu nuclear quadrupole resonance frequency is significantly smaller than that observed in the hole doped HTSC, which implies a nearly complete cancellation of the Cu $3d,$ O $2p,$ and nuclei contributions to the electric-field gradient at the Cu nucleus. The orbital shift anisotropy is similar to that observed in the hole doped HTSC, implying a similar relative splitting of the Cu $3d$ orbitals.