Optically induced persistent current in carbon nanotubes

We demonstrate theoretically that an off-resonant circularly polarized electromagnetic field can induce a persistent current in carbon nanotubes, which corresponds to electron rotation about the nanotube axis. As a consequence, the nanotubes acquire magnetic moment along the axis, which depends on their crystal structure and can be detected in state-of-the-art measurements. This effect and related phenomena are analyzed within the developed Floquet theory describing the electronic properties of the nanotubes irradiated by the field.

Figure 1

Sketch of the system under consideration. (a) The cross section of the CNT (the yellow ring strip) with radius $R$ irradiated by a circularly polarized electromagnetic wave with electric field amplitude $E_0$, which propagates along the CNT axis corresponding to the $z$ coordinate of the Cartesian coordinate system $x,y,z$ associated with the CNT. The red line marks the irradiation-induced persistent current $J$. (b) The graphene sheet as a basis of the (4,4) armchair CNT with radius $R=L_1/2\pi$ and length $L_2$. The blue and red circles mark the carbon atoms from the $A$ and $B$ graphene sublattices, respectively. The axes ${\xi }_{1,2}$ mark the coordinate system associated with the graphene sheet. The CNT is fabricated by the rolling up of the sheet along the ${\xi }_1$ axis.

Figure 2

(a) The electron energy spectrum of the (4,4) armchair CNT. The Fermi energy of the electron system is ${\varepsilon }_F=0$, the threshold of interband optical absorption is $\mathrm{\Delta }$, and the Dirac point wave vector $k_D=2\pi /3a$ corresponds to the band edge. (b) The electron energy spectrum of the CNT near the band edge in the absence of the dressing field (dashed lines) and in the presence of the dressing field (solid lines), where ${\varepsilon }_g$ is the field-induced band gap.

Figure 3

The dependences of the optically induced band gap ${\varepsilon }_g$ and persistent current $J$ on the field amplitude $E_0$ and the photon energy $\hslash {\omega }_0$ for the $(n,n)$ armchair CNT with $n=75$ (the CNT radius is $R\approx 5.1$ nm).