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Anomalous Anderson localization behaviors in disordered pseudospin systems

Significance Wave propagation through a 1D disordered potential is always confined spatially due to the constructive interference in the backward direction. This “Anderson localization” behavior applies to all previously known 1D disordered systems in nature. Here, we show that wave propagating in 2D pseudospin-1 systems with 1D disorder has unique localization behaviors. In all conventional materials, stronger disorder always induces stronger localization. However, the localization in pseudospin-1 systems actually becomes weaker after the randomness increases beyond a critical value and a sharp transition separates the localization behavior into two regimes with different localization characteristics. Pseudospin-1 systems have been achieved in artificial crystals such as metamaterials and ultracold atom systems, which would be interesting platforms to observe the anomalous localization behaviors. We discovered unique Anderson localization behaviors of pseudospin systems in a 1D disordered potential. For a pseudospin-1 system, due to the absence of backscattering under normal incidence and the presence of a conical band structure, the wave localization behaviors are entirely different from those of conventional disordered systems. We show that there exists a critical strength of random potential (Wc), which is equal to the incident energy (E), below which the localization length ξ decreases with the random strength W for a fixed incident angle θ. But the localization length drops abruptly to a minimum at W=Wc and rises immediately afterward. The incident angle dependence of the localization length has different asymptotic behaviors in the two regions of random strength, with ξ∝sin−4θ when WWc. The existence of a sharp transition at W=Wc is due to the emergence of evanescent waves in the systems when W>Wc. Such localization behavior is unique to pseudospin-1 systems. For pseudospin-1/2 systems, there is also a minimum localization length as randomness increases, but the transition from decreasing to increasing localization length at the minimum is smooth rather than abrupt. In both decreasing and increasing regions, the θ dependence of the localization length has the same asymptotic behavior ξ∝sin−2θ.