Plasmons, which are collective charge oscillations, offer the potential to use optical signals in nano-scale electric circuits. Recently, plasmonics using graphene have attracted interest, particularly because of the tunable plasmon frequency through the carrier density $n$. However, the $n$ dependence of the plasmon velocity is weak ($propto n^{1/4}$) and it is difficult to tune the frequency over orders of magnitude. Here, we demonstrate that the velocity of plasmons in graphene can be changed over two orders of magnitude by applying a magnetic field $B$ and by the presence/absence of a gate; at high $B$, edge magnetoplasmons (EMPs), which are plasmons localized at the sample edge, are formed and their velocity depends on $B$ and the gate screening effect. The wide range tunability of the velocity and the observed low-loss plasmon transport encourage designing graphene nanostructures for plasmonics applications.
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