Magnetoconductance of Carbon Nanotube $p-n$ Junctions

The magnetoconductance of $p\mathrm{\text{−}}n$ junctions formed in clean single wall carbon nanotubes is studied in the geometry where a magnetic field is along the tube axis. For long junctions the low temperature magnetoconductance is anomalously large; the relative change in the conductance becomes of order unity even when the flux through the tube is much smaller than the flux quantum. The magnetoconductance is negative for metallic tubes. For semiconducting and small gap tubes the magnetoconductance is nonmonotonic: positive at small and negative at large fields. The identified magnetoconductance mechanism is relevant to magnetotransport in undoped metallic and small gap tubes in the presence of a long range disorder potential.

Figure 1

(a) Device sketch: a nanotube rests on an insulating substrate. The $n$ and $p$ regions are created by biasing the top gates separated by a distance $2L$. (b) Band diagram of the device. Tilted solid lines represent the bottom of the conduction and the top of the valence bands. The width of the classically forbidden region is $2x_o=2\Delta /eE$. Inset: one-dimensional spectrum is obtained by dissecting the graphene Brillouin zone by the $k_y=(\Phi /{\Phi }_0+m)/R$ line (dashed line). The low energy states lie in the vicinity of $K$ and $K^{\prime }$ points.

Figure 2

(a) Dimensionless electron density as a function of $\xi$ for $q=0.3$, $q=0.6$, and $q=0.9$. (b) The ratio of the correction to the transmission coefficient to the interaction constant $\lambda$, Eq. (14), as a function of $q$.