Bloch oscillations are among the most symmetric phenomena in solid-state physics. A particle in a periodic potential under a constant force oscillates back and forth with textbook regularity — equal amplitude, equal speed, mirror symmetry in each direction. The oscillation is a direct consequence of the lattice periodicity and should be robust against perturbation.
Li, Shang, and Malomed (arXiv:2603.19049) break this symmetry using non-Abelian gauge fields. Spinor wavepackets in a honeycomb lattice, subjected to gauge fields created by tuning the ratio of Rashba and Dresselhaus spin-orbit coupling, exhibit Bloch oscillations that freeze during half the cycle. The particle oscillates normally in one direction, then stops entirely for the return stroke. Half the oscillation simply vanishes.
The asymmetry is continuously tunable. Adjusting the spin-orbit coupling parameters controls both the degree of freezing and which half of the cycle is suppressed. The mechanism is topological: the non-Abelian gauge structure introduces a geometric phase that selectively damps one direction of motion without affecting the other. No asymmetric potential is needed. No directional force is applied. The spatial asymmetry emerges entirely from the internal spin structure coupling to the lattice geometry.
This is motion that becomes directionally selective without any directional bias in the system's energy landscape. The particle doesn't “prefer” one direction because it's energetically easier — it prefers one direction because the gauge field makes the other direction geometrically invisible. The freezing isn't a barrier. It's an absence — the return path simply ceases to exist in the relevant gauge sector.
Li, Shang, and Malomed, "Anomalous Topological Bloch Oscillations under Non-Abelian Gauge Fields," arXiv:2603.19049 (2026).