Plasmon Transport in Graphene Investigated by Time-Resolved Electrical


Norio Kumada, Shinichi Tanabe, Hiroki Hibino, Hiroshi Kamata*, Masayuki Hashisaka*,
Koji Muraki, and Toshimasa Fujisawa*
Physical Science Laboratory, *Tokyo Institute of Technology

   Plasmons, which are collective charge oscillations, could provide a means of confining electromagnetic field to nanoscale regions. Recently, plasmonics in graphene have attracted interest, particularly because of the tunable plasmon dispersion. In this work, we carried out time-resolved electrical measurements of edge magnetoplasmons (EMPs), which are plasmons localized at graphene edge in a high magnetic field, and demonstrate that the velocity of EMPs can be controlled over two orders of magnitude [1].
   Graphene used was grown on SiC substrate. We used two samples, one with and the other without a large top gate. All measurements were carried out at 1.5 K. Plasmons are injected into graphene by applying a voltage step to the injection gate and detected through the detector Ohmic contact fabricated 1.1 mm away from the injection gate [Fig. 1(a)]. From the time of flight between the injector and the detector, the plasmon velocity is determined. In the ungated sample, as the magnetic field is increased, the velocity decreases from 6000 km/s to 2000 km/s [Fig. 1(b)]. In the gated sample, on the other hand, the velocity is about 100 km/s, which is one order of magnitude smaller than that in the ungated sample [Fig. 1(c)]. The smaller velocity is due to the gate screening effect of the electric field in plasmons. At a fixed magnetic field (12 T), as the gate bias and thus the carrier density are decreased, the velocity decreases to 10 km/s with oscillations. These results indicate that the velocity of plasmons in graphene can be controlled over two orders of magnitude by applying the magnetic field, screening the plasmon electric field with a gate metal, and changing the carrier density. The wide tunability of the plasmon velocity encourages designing graphene nanostructures for plasmonic circuits.
   This work was supported by KAKENHI.

[1] N. Kumada et al., Nature Commun. 4 (2013) 1363.

Fig. 1. (a) Sample structure and the experimental setup. (b) Plasmon velocity in the ungated sample as a function of the magnetic field. (c) Plasmon velocity in the gated sample as a function of the gate bias at a magnetic field of 12 T.