Nonlinear phononic slidetronics

TL;DR

The work demonstrates that conventional coherent phonon excitation pathways cannot efficiently switch ferroelectricity in bilayer h-BN due to weak light–matter coupling and the interlayer energy barrier, but nonlinear excitation of high-frequency intralayer phonons can tilt the interlayer potential and enable ultrafast, all-optical control of stacking order. Using first-principles calculations and a semiclassical model, the authors show that a targeted 40 THz intralayer mode dynamically biases the double-well landscape such that AB to BA switching occurs at experimentally accessible pulse energies (e.g., E0≈20 MV/cm, τ≈0.5 ps). This nonlinear phononic slidetronics mechanism provides a robust route to ultrafast, energy-efficient manipulation of stacking in van der Waals materials, with potential extensions to other electronic orders. The findings open avenues for ultrafast, optically driven memory and device applications in low-dimensional systems.

Abstract

Van der Waals ferroelectrics are conventionally switched by sliding the different layers between stacking orders with opposing electric polarizations. Ultrashort laser pulses have been proposed to launch shear modes and induce switching, with often unfeasible large pulse energies however. Here, we demonstrate switching of ferroelectricity in bilayer hexagonal boron nitride through nonlinearly excited phonons. We show that the efficiencies of conventional coherent phonon excitation mechanisms, including infrared absorption and Raman scattering techniques, are too low to overcome the energy barrier separating the two ferroelectric states. We demonstrate instead that excitation of high-frequency intralayer modes leads to a tilting of the interlayer potential-energy landscape that enables changing the stacking order. Our results provide an avenue towards efficient phononic slidetronics, enabling ultrafast control of the stacking order in van der Waals materials.

Figures (11)

• Figure 1: Phononic sliding in hexagonal boron nitride. (a) Potential energy landscape as a function of shear-mode amplitude. The six-fold rotationally symmetric minima correspond to AB and BA stacking orders with opposing ferroelectric polarizations, $P_\text{FE}$. The white dashed line corresponds to the double-well potential shown in (b). Double-well potential of the ferroelectric AB and BA stacking orders. In AB stacking, nitrogen atoms from the upper layer are centered over the hexagonal ring, whereas boron atoms from the upper layer are located over nitrogen atoms from the lower layer---vice versa for BA stacking. The red dots are calculated data and the blue line is a fit. Grey arrows denote motion of the layers along the shear mode.
• Figure 2: Shear-mode excitation mechanisms for circular polarization. Energy diagrams (top row) depict the scattering processes, where red pulses indicate incoming and outgoing photons and dashed lines mark intermediate electronic $|\text{e}\rangle$ and phononic $|\text{ph}\rangle$ states in the Raman-type processes. We plot the time evolutions of the amplitudes of the two orthogonal shear-mode components following the excitation for a time window up to 6 ps. (a) Infrared absorption (IRA), (b) ionic Raman scattering (IRS), (c) impulsive stimulated Raman scattering (ISRS), (d) terahertz sum-frequency excitation (THz-SFE), and (e) infrared resonant Raman scattering (IRRS).
• Figure 3: Ferroelectric switching through nonlinear phononic slidetronics. (a) Tilting of the double-well potential upon displacement of the high-frequency intralayer $E$ mode at 40.5 THz. The dotted black curve is the average of $\tilde{V}(\tilde{Q})$ obtained from tiltings due to the maximum and minimum amplitudes of the 40.5 THz mode. (b) Time evolution of shear-mode amplitudes following the excitation of the 40.5 THz mode. The red curve shows rectified coherent oscillations of the shear mode for a pulse energy below the switching threshold ($E_0$ = 17.5 MV/cm), whereas the green curve shows a changing of the stacking order from AB to BA for a pulse energy above the switching threshold ($E_0$ = 20 MV/cm).
• Figure S1: Visualization of eigenvectors. (a) Shear modes showing relative motion of the layers. (b) High-frequency $E$ modes showing the intralayer motion of the atoms, where the lengths of the arrows indicate the relative atomic displacements.
• Figure S2: Time evolution of the amplitudes of the high-frequency intralayer modes following the excitation by an ultrashort pulse with a peak electric field of $E_0= 40.2$ MV/cm and a pulse duration of $\tau=0.05$ ps. Blue graphs represent the amplitudes for $Q_{n,x}$, and red graphs represent the amplitudes for $Q_{n,y}$. We show plots for the (a) 40.45 THz mode and (b) 40.5 THz mode.