Internal static electric and magnetic field at the copper cite in a single crystal of the electron-doped high- T c superconductor Pr 1.85 Ce 0.15 CuO 4 − y

We report ${}^{63,65}\mathrm{Cu}$-NMR spectroscopy and Knight shift measurements on a single crystal of the electron-doped high-${T}_{c}$ superconductor ${\mathrm{Pr}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4\ensuremath{-}y}$ with an applied magnetic field $(H)$ up to 26.42 T. A very small NQR frequency is obtained with the observation of the spectrum, which shows an extremely wide continuous distribution of it that becomes significantly narrower below 20 K at $H\ensuremath{\parallel}c$ where the superconductivity is completely suppressed, indicating a significant change in the charge distribution at the Cu site, while the corresponding change at $H\ensuremath{\perp}c$ is negligible when the superconductivity is present or not fully suppressed. The Knight shift and central linewidth are proportional to the applied magnetic field with a high anisotropy. We find that the magnitude of the internal static magnetic field at the copper is dominated by the anisotropic ${\mathrm{Cu}}^{2+}$ $3d$ orbital contributions, while its weak temperature dependence is mainly determined by the isotropic contact hyperfine coupling to the paramagnetic ${\mathrm{Pr}}^{3+}$ spins, which also gives rise to the full distribution of the internal static magnetic field at the copper for $H\ensuremath{\perp}c$. This internal static electric and magnetic field environment at the copper is very different from that in the hole-doped cuprates, and may provide new insight into the understanding of high-${T}_{c}$ superconductivity. Other experimental techniques are needed to verify whether the observed significant narrowing of the charge distribution at the Cu site with $H\ensuremath{\parallel}c$ is caused by the charge ordering [E. H. da Silva Neto et al., Science (to be published, 2014)] or a new type of charge modulation.