Collision of $\phi^4$ kinks free of the Peierls-Nabarro barrier in the regime of strong discreteness

TL;DR

This paper investigates how strong discreteness in a modified $^4$ field model affects kink-antikink collisions, revealing new phenomena such as repulsive solitary waves and the formation of discrete breathers due to narrow and inverted phonon spectra.

Contribution

It introduces an exceptional discretization where the Peierls-Nabarro potential is zero, enabling radiationless kink propagation and uncovering novel collision behaviors in strongly discrete lattices.

Findings

Kinks can propagate with negligible radiation in the exceptional discretization.

Strong discreteness leads to a narrow phonon band and spectrum inversion.

Collision outcomes include repulsive solitary waves and discrete breather formation.

Abstract

The two major effects observed in collisions of the continuum $\phi^4$ kinks are (i) the existence of critical collision velocity above which the kinks always emerge from the collision and (ii) the existence of the escape windows for multi-bounce collisions with the velocity below the critical one, associated with the energy exchange between the kink's internal and translational modes. The potential merger (for sufficiently low collision speeds) of the kink and antikink produces a bion with oscillation frequency $\omega_B$, which constantly radiates energy, since its higher harmonics are always within the phonon spectrum. Similar effects have been observed in the discrete $\phi^4$ kink-antikink collisions for relatively weak discreteness. Here we analyze kinks colliding with their mirror image antikinks in the regime of strong discreteness considering an exceptional discretization of the $\phi^4$ field equation where the static Peierls-Nabarro potential is precisely zero and the not-too-fast kinks can propagate practically radiating no energy. Several new effects are observed in this case, originating from the fact that the phonon band width is small for strongly discrete lattices and for even higher discreteness an inversion of the phonon spectrum takes place with the short waves becoming low-frequency waves. When the phonon band is narrow, not a bion but a discrete breather with frequency $\omega_{DB}$ and all higher harmonics outside the phonon band is formed. When the phonon spectrum is inverted, the kink and antikink become mutually repulsive solitary waves with oscillatory tails, and their collision is possible only for velocities above a threshold value sufficient to overcome their repulsion.