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Photoinduced twist and untwist of moiré superlattices in TMDC heterobilayers

C. J. R. Duncan, A. C. Johnson, I. Maity, A. Rubio, M. Gordon, A. C. Bartnik, M. Kaemingk, W. H. Li, M. B. Andorf, C. A. Pennington, I. V. Bazarov, M. W. Tate, D. A. Muller, J. Thom-Levy, S. M. Gruner, A . M. Lindenberg, F. Liu, J. M. Maxson

TL;DR

This work demonstrates that femtosecond photoexcitation coherently twists and untwists moiré superlattices in twisted WSe$_2$/MoSe$_2$ bilayers, resolved directly by ultrafast electron diffraction. The observed enhancement and subsequent decay of moiré satellite peaks reveal a local twist-angle modulation of about $0.6^\circ$ linked to a sub-THz moiré phonon, driven by ultrafast interlayer charge transfer that transiently strengthens interlayer binding. A driven lattice model, consistent with DECP, connects the out-of-plane carrier dynamics to in-plane torsional PLD changes, distinguishing a non-thermal, coherent lattice response from simple heating. These results establish ultrafast, all-optical control of moiré potentials, with implications for excitons, polarons, and emergent correlated phases in 2D moiré materials and related heterostructures.

Abstract

Two-dimensional moiré materials are formed by artificially stacking atomically thin monolayers. A wealth of correlated and topological quantum phases can be engineered via precise choice of stacking geometry. These designer electronic properties depend crucially on interlayer coupling and atomic registry. An important open question is how atomic registry responds on ultrafast timescales to optical excitation and whether the moiré geometry can be dynamically reconfigured to tune emergent phenomena in real time. Here we show that femtosecond photoexcitation drives a coherent twist-untwist motion of the moiré superlattice in $2^\circ$ and $57^\circ$ twisted WSe$_2$/MoSe$_2$ heterobilayers, resolved directly by ultrafast electron diffraction. Upon above-band-gap photoexcitation, the moiré superlattice diffraction features are enhanced within 1 ps and subsequently suppressed several picoseconds after, deviating markedly from typical photoinduced lattice heating. Kinetic diffraction analysis, supported by simulations of the sample dynamics, indicates a peak-to-trough local twist angle modulation of $0.6^\circ$, correlated with a sub-THz frequency moiré phonon. This motion is driven by ultrafast charge transfer that transiently increases interlayer attraction. Our results could lead to ultrafast control of moiré periodic lattice distortions and, by extension, the local moiré potential that shapes excitons, polarons, and correlation-driven behaviors

Photoinduced twist and untwist of moiré superlattices in TMDC heterobilayers

TL;DR

This work demonstrates that femtosecond photoexcitation coherently twists and untwists moiré superlattices in twisted WSe/MoSe bilayers, resolved directly by ultrafast electron diffraction. The observed enhancement and subsequent decay of moiré satellite peaks reveal a local twist-angle modulation of about linked to a sub-THz moiré phonon, driven by ultrafast interlayer charge transfer that transiently strengthens interlayer binding. A driven lattice model, consistent with DECP, connects the out-of-plane carrier dynamics to in-plane torsional PLD changes, distinguishing a non-thermal, coherent lattice response from simple heating. These results establish ultrafast, all-optical control of moiré potentials, with implications for excitons, polarons, and emergent correlated phases in 2D moiré materials and related heterostructures.

Abstract

Two-dimensional moiré materials are formed by artificially stacking atomically thin monolayers. A wealth of correlated and topological quantum phases can be engineered via precise choice of stacking geometry. These designer electronic properties depend crucially on interlayer coupling and atomic registry. An important open question is how atomic registry responds on ultrafast timescales to optical excitation and whether the moiré geometry can be dynamically reconfigured to tune emergent phenomena in real time. Here we show that femtosecond photoexcitation drives a coherent twist-untwist motion of the moiré superlattice in and twisted WSe/MoSe heterobilayers, resolved directly by ultrafast electron diffraction. Upon above-band-gap photoexcitation, the moiré superlattice diffraction features are enhanced within 1 ps and subsequently suppressed several picoseconds after, deviating markedly from typical photoinduced lattice heating. Kinetic diffraction analysis, supported by simulations of the sample dynamics, indicates a peak-to-trough local twist angle modulation of , correlated with a sub-THz frequency moiré phonon. This motion is driven by ultrafast charge transfer that transiently increases interlayer attraction. Our results could lead to ultrafast control of moiré periodic lattice distortions and, by extension, the local moiré potential that shapes excitons, polarons, and correlation-driven behaviors

Paper Structure

This paper contains 19 sections, 80 equations, 19 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Results: observation of photoinduced moiré superlattice motion
  3. Discussion: Origin of the twist and untwist of moiré structures locally
  4. Outlook
  5. Methods
  6. UED
  7. Sample preparation
  8. Phonon spectra calculations
  9. Goodness of fit
  10. Acknowledgements
  11. Supplementary Information
  12. Band Structure Calculations
  13. Linearized Dynamical Model
  14. Equivalence of Driving Force with the DECP Formalism
  15. Kinematic Diffraction Analysis
  16. ...and 4 more sections

Figures (19)

  • Figure 1: Experimental scheme and representative diffraction images. (a) Schematics of the UED setup. (b) Periodic lattice distortion (PLD) in a bilayer moiré structure, consisting of alternating domains with different stacking orders. The PLD expands the size of energetically favorable stacking domains. (c) Wide-angle view of scattering patterns from $2^\circ$ WSe$_2$/MoSe$_2$ heterobilayers. (d) Zoom on the (2,-1) diffraction peak at high angular magnification. (e) Photoinduced changes of scattering intensity, highlighting the strong response of the moiré satellite peaks. The difference is obtained by subtracting the diffraction pattern 4 ps after photoexcitation from the static pattern in panel (d). The color bar is nolrmalized so that 0 represents no change and -1 the largest decrease in intensity at the delay time shown. Extended Data 1 presents diffraction snapshots at 1 ps for both the $2^\circ$ and $57^\circ$ WSe$_2$/MoSe$_2$ samples.
  • Figure 2: Transient changes in diffraction intensity. Photoexcitation is fixed at 515 nm. Error bars show Poisson uncertainties. Solid lines are fits produced by the dynamical model of Eqs. (S2)-(S4) in the Supplementary Information, and the shaded area is the 95% confidence interval. The dashed line is the prediction from lattice heating alone. The diffraction intensity we report is a sum of counts within a detector region of interest that is centered on the relevant peak. (a), (b) Normalized diffraction intensity changes of superlattice peaks and individual monolayer Bragg peaks for $2^\circ$ WSe$_2$/MoSe$_2$ moiré structure. (d), (e) Normalized intensity changes of $57^\circ$ WSe$_2$/MoSe$_2$ moiré structure. (g), (h) Normalized intensity changes of isolated WSe$_2$ (g) amd MoSe$_2$ (h) monolayers measured under an identical excitation scheme. (c), (f) Power spectral density of the model for the $2^\circ$ and $57^\circ$ WSe$_2$/MoSe$_2$ moiré structures (compare discrete Fourier transform of the experimental data, Extended Data 3). (i) Low-frequency Raman spectroscopy of $2^\circ$ WSe$_2$/MoSe$_2$ moiré structure. Further analysis of the time series data is presented in Extended Data 4, 5.
  • Figure 3: Twisting of the $2^\circ$ WSe$_2$/MoSe$_2$ lattice versus time, extracted from our dynamical model fitted to UED data. (a) At equilibrium, atoms are displaced from sites of the rigidly rotated lattice by vdW forces. Color map indicates local atomic displacement in polar coordinates: hue shows displacement direction $(\theta)$ in radians, saturation displacement magnitude $(r)$ in picometers. (b) Snapshot of the transverse displacement of atoms from equilibrium at 800 fs after photoexcitation, dominated by torsion about the $R_M^M$ stacking domain center. (c) Snapshot at 12 ps following photoexcitation, i.e., after the decay of the oscillatory transient, showing the untwisting of the equilibrium reconstruction. (d) Fitted atomic displacement as a function of radial distance from the vortex center, expressed as a twist angle, defined in Eq. \ref{['angle']}. A similar figure for the $57^\circ$ case is shown in Extended Data 8. A line-out of the displacement modulation versus radial distance is shown in Extended Data 9.
  • Figure 4: Charge transfer mechanism driving the lattice response: (a) 515 nm pump photons promote charges into the conduction band; type-II band misalignment causes electrons to move to the MoSe$_2$ layer and holes to WSe$_2$. Bands shown here are schematic: results from density functional theory calculations are presented in Extended Data 10. (b)-(c) Electrostatic forces on layer-separated charges pull the layers together. Simulated atomic displacements from equilibrium are exaggerated in the figure by a factor 100. (d)-(e) Carrier relaxation causes the lattice to heat, and thermal expansion counters the effect of electrostatic attraction. (e) The best-fit driving force that results in the solid curves shown in Fig. \ref{['fig:wiggle']}.
  • Figure S1: (a) Schematic of twisted WSe$_2$/MoSe$_2$ in reciprocal space, for general twist angle, showing all possible first-order moiré satellites. Contributions from WSe$_2$ are shown in red, MoSe$_2$ in blue. Arrows indicate the atomic-scale basis vectors ${\bf q}_0, {\bf q}_1$. Highlighted is the region of interest at $(2,-1)$, where the UED data presented in the main text is collected. (b) Definitions of the moiré scale reciprocal lattice vectors. (c) Schematic of twisted WSe$_2$/MoSe$_2$, showing only those satellites with significant intensity when the system is in equilibrium, due to strain waves caused by atomic relaxation. (d)-(e) Torsional PLDs have a significant effect on the highlighted peaks, related to the (2,-1) region of interest by rotation. (f)-(g) Radial PLDs have a significant effect on the highlighted peaks, related to the (2,-1) experimental region of interest by rotation. (h) The highlighted peaks in the (2,-1) experimental region of interest are sensitive to torsional PLDs. (i) The highlighted peaks in the (2,-1) experimental region of interest are sensitive to radial PLDs. (j) The highlighted peaks in the (2,-1) are summed in computing the satellite peak intensities shown in Figs. \ref{['2degirreps']} and \ref{['57degirreps']}.
  • ...and 14 more figures