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تسجيل مستخدم جديدPhonons in Twisted Transition Metal Dichalcogenide Bilayers (Twistnonics): Ultra-soft Phasons, and a transition from Superlubric to Pinned Phase
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The tunability of the interlayer coupling by twisting one layer with respect to another layer of two-dimensional materials provides a unique way to manipulate the phonons and related properties. We refer to this engineering of phononic properties as Twistnonics. We study the effects of twisting on low-frequency shear (SM) and layer breathing (LBM) modes in transition metal dichalcogenide (TMD) bilayer using atomistic classical simulations. We show that these low-frequency modes are extremely sensitive to twist and can be used to infer the twist angle. We find unique ultra-soft phason modes (frequency $lesssim 1 mathrm{cm^{-1}}$, comparable to acoustic modes) for any non-zero twist, corresponding to an textit{effective} translation of the moir{e} lattice by relative displacement of the constituent layers in a non-trivial way. Unlike the acoustic modes, the velocity of the phason modes is quite sensitive to twist angle. As twist angle decreases, ($theta lesssim 3^{circ}, gtrsim 57^{circ}$) the ultra-soft modes represent the acoustic modes of the emergent soft moir{e} scale lattice. Also, new high-frequency SMs appear, identical to those in stable bilayer TMD ($theta = 0degree/60degree$), due to the overwhelming growth of stable stacking regions in relaxed twisted structures. Furthermore, we find remarkably different structural relaxation as $theta to 0^{circ}$, $to 60^{circ}$ due to sub-lattice symmetry breaking. Our study reveals the possibility of an intriguing $theta$ dependent superlubric to pinning behavior and of the existence of ultra-soft modes in textit{all} two-dimensional (2D) materials.
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