Surface plasmonics of Weyl semimetals

Smooth interfaces of topological systems are known to host massive surface states in addition to the topologically protected chiral one. We show that in Weyl semimetals these massive states, along with the chiral Fermi arc, strongly alter the form of the Fermi-arc plasmon. Most saliently, they yield further collective plasmonic modes that are absent in a conventional interface. The plasmon modes are completely anisotropic as a consequence of the underlying anisotropy in the surface model and expected to have a clear-cut experimental signature, e.g., in electron-energy loss spectroscopy.

Figure 1

Various particle-hole excitations involving the surface bands for a fixed transverse momentum. The chiral FA is denoted by the red line, whereas the massive VP bands are shown in green. As discussed in the main text, for $q_z=0$, only $n\to \pm n$ excitations are allowed across the Fermi level (dashed line). Inset: Global view on surface bands connecting two Weyl nodes. Along the transverse momentum, the quasi-1D surface bands do not disperse.

Figure 2

Profile of the imaginary part of the noninteracting dynamical polarization $-\text{Im}\left({\chi }^{\left(0\right)}\right)$ in the $(q_y,\omega )$ phase space at $\mu =0.01e_0,0.90e_0,1.01e_0$, and $1.30e_0$ for $q_z=0$, $v=1$ eV.Å, $\ell =10$ Å, $\mathrm{\Delta }=1$ eV, and $k_0=10$ Å${}^{-1}$. The zeros of the real part of ${\epsilon }^{\text{RPA}}$ (red dashed lines) indicate the plasmon modes.

Figure 3

Same as Fig. 2 except for $q_z{\ell }_S=0.2$. The zeros of the real part of ${\epsilon }^{\text{RPA}}$ (red dashed lines) indicate the plasmon modes

Figure 4

Electron loss function at $\mu =1.01e_0$ for $q_y{\ell }_S\in [-2.0,2.0]$: Left for $q_z=0$ and right for $q_z{\ell }_S=0.2$. Yellow dashed lines show the symmetric VP intraband plasmon in the absence of the FA state. Three plasmon modes are all nonreciprocal and visible.