  Counterpropagating Alfv\'en waves are ubiquitously observed in many
astrophysical environments, such as a star surface and a planetary foreshock.
We discuss an efficient particle acceleration mechanism in two
counterpropagating circularly polarized Alfv\'en waves. Phase transitions of
particle behavior occur when wave amplitudes exceed two critical values. Above
the critical amplitudes, the numerical simulation shows that any particles
irreversibly gain relativistic energy within a short time regardless of their
initial position and energy once the coherent waveform is formed. The
accelerated particles have spatial coherence. Higher wave phase velocity
requires smaller critical amplitudes, while the maximum attainable energy
increases as the wavenumber and the frequency decrease. The results may be
applicable in some astrophysical phenomena, as well as a future laboratory
experiment using high-power lasers.
