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.