Electronic screening in graphite*

Nonlocal screening in highly-oriented pyrolitic graphite was investigated with inelastic x-ray scattering. Measurements were performed over a sufficiently broad range of momentum and energy to permit complete inversion of the loss function, $-Im [1/\epsilon(k,\omega)]$, allowing real-time, microscopic imaging of the induced electron density around a charged impurity. In addition, we found evidence for a sign change in the zero-frequency dielectric function, $\epsilon(k,0)$, over a sizeable range of momentum. This ``antiscreening" should cause the Coulomb interaction to be attractive, perhaps assisting superconductivity in this system.