BACKFLOW IN A FERMI-LIQUID

We calculate the backflow current around a fixed impurity in a Fermi l iquid. The leading contribution at long distances is radial and propor tional to 1/r(2). It is caused by the current induced density modulati on first discussed by Landauer [IBM J. Res. Dev. 1, 223 (1957)]. The f amiliar 1/r(3) dipolar backflow obtained in linear response is only th e next-to-leading term, whose strength is calculated here to all order s in the scattering. In the charged case the condition of perfect scre ening gives rise to a novel sum rule for the phase shifts. Similar to the behavior in a classical viscous liquid, the friction force is due only to the leading contribution in the backflow while the dipolar ter m does not contribute.