Phonon Spin Selective One-Way Axial Phonon Transport in Chiral Nanohelix

Abstract

Selectively exciting and manipulating phonons at nanoscale becomes more and more important but still remains challenging in modern nano-energy control and information sensing. Here, we show that the phonon spin angular momentum provides an extra degree of freedom to achieve versatile manipulation of axial phonons in nanomaterials via coupling to spinful multi-physical fields, such as circularly polarized infrared absorption. In particular, we demonstrate the nanoscale one-way axial phonon excitation and routing in chiral nanomaterials, by converting the photon spin in circularly polarized optical fields into the collective interference phonon spin. As exemplified in the smallest chiral carbon nanotube, we show that the rectification rate can reach nearly 100\%, achieving an ideal one-way phonon router, which is verified by molecular dynamics simulations. Our results shed new light on the flexible phonon manipulation via phonon spin degree of freedom, paving the way for future spin phononics.

Figures (4)

• Figure 1: Symmetry transformation properties of unidirectional wave excitations. (a) Schematics of one-way wave excited by transverse circularly polarized sources ($\sigma_\pm$) on the surface or edge. (b) For longitudinal sources $\sigma_\pm$ through the bulk, the unidirectionality is forbidden by symmetry. (c) Unidirectional wave can be excited in chiral bulk materials with longitudinal sources $\sigma_\pm$. R and L are short for right and left handedness of materials, respectively.
• Figure 2: Collective interference phonon spin (CIPS) in chiral carbon nanotube (CNT). (a) Atomic structure of CNT (4,2) (left-handed, LH) and (2,4) (right-handed, RH), respectively. (b) Calculated phonon band structure of (4,2)- and (2,4)-CNTs (see details of calculation methods in section S5 of SI). (c) Zoom-in band structure of (b) (indicated by the white dashed box) and band-resolved $S^{\textrm{local}}_z$. (d) Same as (c) but with $S^{\textrm{local}}_z$ replaced by the CIPS $S_z$. (e) Schematics of one-way phonon routing in left- and right-haned CNT under circularly polarized light fields $\sigma_\pm$.
• Figure 3: One-way phonon routing in (4,2)-CNT under different polarization states of light fields. (a)-(c) Snapshot kinetic energy ($E_K$) distributions at $t=30$ ps as function of $z$ under right-handed, left-handed, and linear polarized light fields, respectively. The light fields are applied at the unit cell located at $z=0$ with $Q|E_0|=$ 1 eV/$\AA$ and $f=16.67$ THz.
• Figure 4: Space-time and momentum-frequency space distributions of excited phonon wave. (a) Space-time distribution of $E_K$. Two modes, A (major) and B (minor), are excited with estimated group velocities 7.7 km/s and 13.6 km/s, respectively. (b) Fourier transformation (FT) of velocity of the cell-averaged center of mass $v_{C,x}$ (see details of calculation methods in section S5 of SI) and CIPS-resolved band structure. Modes A and B coincide with $\alpha$ and $\gamma$ modes shown in Fig. \ref{['fig2']}(d). (c) $S^{\textrm{local}}_z$ and $S_z$ of $\alpha$, $\beta$, and $\gamma$ modes, respectively.