Imagine a long corridor with an uneven floor: bumps here, pits there. If you roll a toy car along it, it will get stuck in the deepest pit, not where you gave it a push. That's roughly how sound behaves in a phononic crystal — a material made of periodically arranged elements that controls the propagation of elastic waves.
In the new study, scientists built such a crystal with disorder: in different places, they broke the symmetry of the bonds between elements in different ways. It turned out that sound gets stuck where a certain quantity — the 'imaginary gauge field'—accumulates, and this spot does not depend on where the vibrations were excited. The localization peaks (main and additional) coincide exactly with the local maxima of this accumulated field, which is described by the extreme-value statistics of a random walk. This is the disordered non-Hermitian skin effect.
Moreover, in a chain of pairs of elements with different disorder strengths, one can selectively control the additional peaks. This offers a new way to manipulate waves: instead of fighting disorder, use it as a tool.
The word 'Hermitian' in physics means a system is symmetric in time: the process can be run backward. Non-Hermitian systems describe open systems with losses or gains, where time only flows forward, like sound fading in a material.