We present a systematic construction of non-Hermitian tight-binding lattices whose Bloch spectra are $n\mathrm{th}$ roots of those of Hermitian parent two-dimensional lattices, namely, graphene and the Lieb lattice. The $n$ roots of these models are constructed from connecting loop modules of unidirectional couplings whose geometrical arrangements match those of the corresponding parent system. Their energy spectrum is shown to consist of $n$-rotated and equivalent branches in the complex energy plane, each matching the real spectrum of the parent model when raised to the $n\mathrm{th}$ power, together with extra zero-energy flat bands accounted for by the generalized index theorem. We show how the low-energy Dirac cones of the parent models translate, for an appropriate choice of phase configuration for the couplings of the $n$-root lattices, as what we call an “exceptional horn” appearing at each branch, with the central Dirac point converted into zero-energy exceptional points (EPs) of order $n$ or higher at high-symmetry momenta. These exceptional horns reflect the behavior of low-lying excitations that scale with momentum as $E\sim {\left|\mathbf{q}\right|}^{\frac{1}{n}}$, with $n\ge 3$, as opposed to the linear massless modes that characterize a Dirac cone. Moreover, we derive analytic expressions for the associated Landau levels (LLs), whose energies scale with magnetic flux as $E\sim {\varphi }^{\frac{1}{2n}}$. For the case of the $n$-root Lieb lattice, the zeroth LL is shown to be exceptional. These results are analytically derived for both $n$-root models and numerically demonstrated for certain values of $n$. Finally, we propose a realistic photonic implementation based on coupled ring resonators with a split configuration of optical gain and loss. This distribution yields strongly asymmetric couplings nearing unidirectionality, and appropriate positioning of the rings allows for fine tuning of the coupling phases. We also discuss the impact of experimental imperfections on the EP signatures, modeling them through tight-binding simulations.

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• Electronic structure
• Flat bands
• Landau levels
• Non-reciprocal propagation
• Photonics
• Exceptional points
• Graphene
• Non-Hermitian systems
• Lattice models in condensed matter
• Tight-binding model