We investigate intrinsic and extrinsic decay of edge magnetoplasmons (EMPs) in graphene quantum Hall (QH) systems by high-frequency electronic measurements. From EMP resonances in disk shaped graphene, we show that the dispersion relation of EMPs is
nonlinear due to interactions, giving rise to intrinsic decay of EMP wavepacket. We also identify extrinsic dissipation mechanisms due to interaction with localized states in bulk graphene from the decay time of EMP wavepackets. We indicate that, owing to the unique linear and gapless band structure, EMP dissipation in graphene can be lower than that in GaAs systems.
Nuclear electric resonance (NER) is based on nuclear magnetic resonance mediated by spatial oscillations of electron spin domains excited by a radio frequency (RF) electric field, and it allows us to investigate the spatial distribution of the nuclea
r spin polarization around domain walls (DWs). Here, NER measurements were made of the dynamic nuclear spin polarization (DNP) at the spin phase transition of the fractional quantum Hall state at a Landau level filling factor of $ u=2/3$. From the RF pulse power and pulse duration dependence of the NER spectrum, we show that the DNP occurs only within $sim 100$ nm around DWs, and that it does not occur in DWs. We also show that DWs are pinned by the hyperfine field from polarized nuclear spins.
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.
We investigate the quasiparticle excitation of the bilayer quantum Hall (QH) system at total filling factor $ u_{mathrm{T}} = 1$ in the limit of negligible interlayer tunneling under tilted magnetic field. We show that the intrinsic quasiparticle exc
itation is of purely pseudospin origin and solely governed by the inter- and intra-layer electron interactions. A model based on exciton formation successfully explains the quantitative behavior of the quasiparticle excitation gap, demonstrating the existence of a link between the excitonic QH state and the composite fermion liquid. Our results provide a new insight into the nature of the phase transition between the two states.
We study decoherence of nuclear spins in a GaAs quantum well structure using resistively detected nuclear magnetic resonance. The transverse decoherence time T2 of 75As nuclei is estimated from Rabi-type coherent oscillations as well as by using spin
-echo techniques. By analyzing T2 obtained by decoupling techniques, we extract the role of dipole-dipole interactions as sources of decoherence in GaAs. Under the condition that the device is tilted in an external magnetic field, we exhibit enhanced decoherence induced by the change in strength of the direct dipole-dipole interactions between first nearest-neighbor nuclei. The results agree well with simple numerical calculations.
