Rectification of Vibrational Energy Transfer in Driven Chiral Molecules

TL;DR

The paper addresses directional vibrational energy transfer in chiral molecules under driven conditions. It employs two complementary approaches: analytical harmonic modeling of a single helical chain and numerical MD simulations of a polyethylene double-helix, with a phase-coherent driving force and end baths. The key finding is that combining chirality with phase-controlled driving yields a rectification of vibrational energy flow, with the direction and magnitude governed by the driving frequency $ω$ and phase $φ$, and the effect persisting at room temperature. The results suggest a feasible route to molecular energy transport diodes, potentially realizable with circularly polarized light.

Abstract

We show that the combination of molecular chirality and phase-controlled driving can lead to rectification of vibrational energy transfer. We demonstrate this effect using classical models of (1) a single helical chain and (2) a more realistic model of polyethylene double helix. We examine the effect of the driving frequency, polarization, and temperature on this phenomenon. Notably, we find that the direction and magnitude of the observed directionality preference depend on the driving frequency and phase, and that the effect persists at room temperature.

Figures (6)

• Figure 1: Schematic of the construction used for this study: A chiral molecular chain is suspended in the $z$-direction by keeping the terminal atoms at either end of the chain fixed. Atoms adjacent to these terminal atoms are subject to Langevin thermostats set to $T_\text{cold}$. Periodic driving forces of the form $\vec{F}(t) = F_0(\text{cos}\omega t,\text{cos}(\omega t+\phi),0)$ are applied to atoms at the center of the chain, and the steady-state heat flux $J_{L}$ ($J_{R}$) that develops in the minus (plus) $z$-direction is measured.
• Figure 2: Spatial profiles of temperature and nuclear angular momentum under phase-shifted driving: (a–b) Harmonic chain (model 1, $N=201$) driven at $\omega=530.9~\mathrm{cm^{-1}}$ ($15.9$ THz). (c–d) Polyethylene double helix (model 2, $N=98$; 15 left-handed twists) driven at $\omega=955~\mathrm{cm^{-1}}$ (28.6 THz). All simulations use a driving force with amplitude $F_{0}=0.5~\mathrm{kcal}/ \mathrm{mol}^{-1}$Å$^{-1}$ and phase $\phi=0$ (orange circles) or $\phi=\pi/2$ (blue triangles). $Z$ denotes position along the chain axis. (a,c) The time-averaged local kinetic temperature $T(Z)$. (b,d) Site-resolved nuclear angular momentum $\langle L_{z,i}\rangle/\hbar$. Error bars indicate variability within a block of atoms.
• Figure 3: Analytical and numerical results for the rectification ratio $J_{L}/J_{R}$ plotted against driving frequency $\omega$ for the harmonic chain (Model 1, $N=201$). Here the $y$-axis ratio is $J_L/J_R$ rather than $J_R/J_L$ in order to show the magnitude of the peaks located at $200-600~\mathrm{cm}^{-1}$.
• Figure 4: Numerical results for $J_{R}/J_{L}$ plotted against driving frequency $\omega$, for the polyethylene double-helix (Model 2, $N=98$) containing 15 twists (black circles) and 3 twists (blue diamonds). Due to large variations in the range of $J_{R}/J_{L}$ for Model 2 across the frequency spectrum, results for the low-frequency range (a) and high-frequency range (b) are displayed separately.
• Figure 5: The rectification ratio $J_{R}/J_{L}$ as a function of phase $\phi$ for (a) the harmonic chain (Model 1, $N=201$) driven at $\omega=530.9~\mathrm{cm^{-1}}$ ($15.9$ THz), and (b) the polyethylene double-helix (Model 2, $N=98$) driven at $\omega=955~\mathrm{cm^{-1}}$ ($28.6$ THz). The blue line shows the non-result ($J_{R}/J_{L}=1$) for driving without phase coherence.