Voltage-Controlled Group Velocity of Edge Magnetoplasmon in the Quantum Hall Regime


Hiroshi Kamata1, 2, Takeshi Ota1, Koji Muraki1, and Toshimasa Fujisawa2
1Physical Science Laboratory, 2Tokyo Institute of Technology

 When a two-dimensional electron system is subjected to a strong perpendicular magnetic field, the Lorentz force makes electrons to propagate along the edge of the sample. In the quantum Hall regime, as the Fermi level in the sample interior lies in the gap between the energy levels discretized by the magnetic field, back scattering between channels at opposite edges is prohibited, which makes these edge channels ideal coherent one dimensional channels without dissipation. Electronic analogues of quantum optics experiments using various interferometers defined with edge channels have been demonstrated, allowing one to study coherent transport properties and quantum statistics of electrons. These experiments suggest the possibility of using edge channels as quantum channels, through which quantum states may be transmitted over a macroscopic distance comparable to the device size. To this end, the group velocity of electrons is one important physical parameters to be controlled.
 We investigate the group velocity of edge magnetoplasmons (EMPs) in the quantum Hall regime by means of time-of-flight measurement [1]. EMPs generated at t = 0 by applying a voltage pulse to the source electrode propagate along the sample edge toward the quantum point contact (QPC). By temporarily opening the QPC at a delay time td with another voltage pulse, we are able to selectively detect EMPs arriving at a given time delay td as a drain current IDS (Fig. 1). We find that the group velocity of EMPs traveling along the edges defined by a metallic gate strongly depends on the voltage VG on the gate (Fig. 2). The observed variation of the velocity with VG can be understood as reflecting the degree of screening provided by the metallic gate, which damps the in-plane electric field and hence reduces the velocity. The degree of screening varies as VG changes the distance between the gate and the edge channel.

[1] H. Kamata, T. Ota, K. Muraki, and T. Fujisawa, Phys. Rev. B 81 (2010) 085329.

Fig. 1. (a) Schematic illustration of device structure and
experimental setup for time-of-flight measurement.
A short voltage pulse VPS(t) is applied to the source
contact to inject a pulse of EMPs. Another voltage
pulse VPG(t) is applied to the QPC to probe the local
potential. The time interval between the two voltage
pulses is varied using the mechanical delay line.
Four delay gates between the source contact and
the QPC are used to add extra path length.
Fig. 2. (a) IDS vs td for various VG. Data are offset for clarity.
(b) Group velocity νg as a function of VG.
(c) Schematic illustrating the cross-sectional view of
the sample structure.

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