Pressure dependence of the nuclear resonance of antiferromagnetic CuO.

Abstract

The pressure dependence of the $^{63}\mathrm{Cu}$ NMR of a single crystal of CuO has been measured in the antiferromagnetic state at 77 K. The magnitude of the effective magnetic field at the nucleus (${\ensuremath{\nu}}_{\mathit{L}}$, the Larmor frequency) was found to decrease with pressure, \ensuremath{\partial} ln${\ensuremath{\nu}}_{\mathit{L}}$/\ensuremath{\partial}P=-1.39 ${\mathrm{Mbar}}^{\mathrm{\ensuremath{-}}1}$. The electric-field gradient (${\mathit{q}}_{\mathit{z}\mathit{z}}^{\mathrm{tot}}$) at the nucleus increases such that \ensuremath{\partial} ln${\mathit{q}}_{\mathit{z}\mathit{z}}^{\mathrm{tot}}$/\ensuremath{\partial}P=5.3 ${\mathrm{Mbar}}^{\mathrm{\ensuremath{-}}1}$, while the value calculated for external point charges is -2.7 ${\mathrm{Mbar}}^{\mathrm{\ensuremath{-}}1}$. The observed value of \ensuremath{\partial} ln${\mathit{q}}_{\mathit{z}\mathit{z}}^{\mathrm{tot}}$/\ensuremath{\partial}P must therefore arise from a redistribution of electrons over the copper 3p and 3d orbitals.