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 $^{63, 65}$Cu-NMR spectroscopy and Knight shift measurements on a single crystal of the electron-doped high-$T_{c}$ superconductor Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ (PCCO) 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 significant narrower below 20 K at $H$ $\parallel$ $c$ where the superconductivity is completely suppressed, indicating a significant change in the charge distribution at the Cu site, while the corresponding changes at $H$ $\perp$ $c$ 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 Cu$^{2+}$ 3$d$-orbital contributions, while its weak temperature-dependence is mainly determined by the isotropic contact hyperfine coupling to the paramagnetic Pr$^{3+}$ spins, which also gives rise to the full distribution of the internal static magnetic field at the copper for $H$ $\perp$ $c$. 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 $H$ $\parallel$ $c$ is caused by the charge ordering (CO) [E. H. da Silva Neto $et ~al.$, to be published in Science] \cite{ehdsn} or a new type of charge modulation.

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