Figure 1: (a) Example of negative refraction of metastable argon atoms Ar* (3P2) polarized in Zeeman state . Lower red curve: atomic trajectory in plane under the action of a magnetic field comoving in the direction. Blue straight line: free propagation. Parameters of the comoving field are as follows (see text): spatial period , magnitude of the magnetic field , velocity components  ms−1,  ms−1, time constant  ms, duration ms. The point-like source is on the axis at . The angle between the local velocity and the axis (see text) is shown. (b) Effective constant index (see text, (6a)) as a function of . Curve : the index is negative for . Curve : the index (multiplied by 10) is positive for any value of . (c) Index as a function of and , derived from an ensemble of trajectories similar to that shown in (a), but with different incidence angles. Positions in the ( ) plane where correspond to points where the atom velocity is parallel to the axis. In between these two values, is negative.