Phononic Frequency Combs in Twisted Bilayer Transition Metal Dichalcogenide Heterostructures from First Principles
Abstract
Phononic frequency combs are equidistant spectral ladders generated when a driven vibrational mode transfers energy to an autoparametrically excited subharmonic mode through anharmonic intermode coupling. In previous studies, the coupling strength governing this energy transfer has typically been treated as a free parameter, extracted from a fabricated device, or adopted from an earlier calculation for a specific mode pair. As a result, whether a given crystal can support a phononic frequency comb, and the drive strength required to generate it, has not been predictable a priori. Here we obtain the complete parameter set from density functional theory. Twisted bilayers of \MoSSe{} and \MoSMoSe{} at the commensurate angle 21.787∘ adopt space group P3 in a 42-atom \rootseven{} moiré supercell. The second-order force constants provide the mode frequencies; the third-order constants provide the linewidths through the Boltzmann transport equation and hence the quality factors; and the same third-order tensor projected onto the phonon eigenvectors of a chosen mode pair provides the anharmonic couplings kt and kss. Both bilayers constitute a mode pair within 0.8% of the 2 : 1 resonance with ω1/ω2 = 1.9923 in \MoSSe{} and $1.9943$ in \MoSMoSe{}. By integrating the resulting two-mode equations, both modes generate combs through a discontinuous transition: at 701 fth in \MoSSe{} the envelope variance rises from 2.1×103 to 1.37×104 and the tooth count of mode 1 from 7 to 16 within a single 5 fth increment, identifying the onset as subcritical. The spectral entropy peaks \emph{before} this jump, at 341 fth. The two compounds carry comparable coupling, differing by a factor of $1.2$. However, the comb excitation threshold is set by the quality-factors of signal modes: Q2 = 672 for \MoSSe{} and Q2 = 3844 for \MoSMoSe{}.
Related
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BibTeX
@misc{arora2026phononic,
title = {{Phononic Frequency Combs in Twisted Bilayer Transition Metal Dichalcogenide Heterostructures from First Principles}},
author = {Naman Arora and Adarsh Ganesan},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.13825},
eprint = {2610.13825},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.13825},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Phononic Frequency Combs in Twisted Bilayer Transition Metal Dichalcogenide Heterostructures from First Principles
AU - Naman Arora
AU - Adarsh Ganesan
PY - 2026
DA - 2026/10/09
PB - arXiv
DO - 10.48550/arxiv.2610.13825
UR - https://arxiv.org/abs/2610.13825
AB - Phononic frequency combs are equidistant spectral ladders generated when a driven vibrational mode transfers energy to an autoparametrically excited subharmonic mode through anharmonic intermode coupling. In previous studies, the coupling strength governing this energy transfer has typically been treated as a free parameter, extracted from a fabricated device, or adopted from an earlier calculation for a specific mode pair. As a result, whether a given crystal can support a phononic frequency comb, and the drive strength required to generate it, has not been predictable a priori. Here we obtain the complete parameter set from density functional theory. Twisted bilayers of \MoSSe{} and \MoSMoSe{} at the commensurate angle 21.787^∘ adopt space group P3 in a 42-atom \rootseven{} moiré supercell. The second-order force constants provide the mode frequencies; the third-order constants provide the linewidths through the Boltzmann transport equation and hence the quality factors; and the same third-order tensor projected onto the phonon eigenvectors of a chosen mode pair provides the anharmonic couplings kt and kss. Both bilayers constitute a mode pair within 0.8% of the 2 : 1 resonance with ω_1/ω_2 = 1.9923 in \MoSSe{} and $1.9943$ in \MoSMoSe{}. By integrating the resulting two-mode equations, both modes generate combs through a discontinuous transition: at 701 fth in \MoSSe{} the envelope variance rises from 2.1×10^3 to 1.37×10^4 and the tooth count of mode 1 from 7 to 16 within a single 5 fth increment, identifying the onset as subcritical. The spectral entropy peaks \emph{before} this jump, at 341 fth. The two compounds carry comparable coupling, differing by a factor of $1.2$. However, the comb excitation threshold is set by the quality-factors of signal modes: Q_2 = 672 for \MoSSe{} and Q_2 = 3844 for \MoSMoSe{}.
ER -