Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

esistance oscillation in low magnetic fields and to coherent states in fractional quantum Hall effect which
occur due to an interlayer interaction between neighbored quantum wells. First, we focus on a bilayer system
which is exposed to microwave irradiation and exhibits photo-resistance oscillations, caused by the
interference of magneto-intersubband (MIS) oscillations and microwave induced resistance oscillations
(MIROs, observed systems with one occupied subband). The investigated samples are GaAs double quantum
wells (DQWs) with AlGaAs barriers and high total sheet electron density ns = 10 12 cm -2 and a mobility of μ =
10 6 cm 2 /Vs, coupled by tunneling. Transport measurements in a perpendicular magnetic field reveal MIS
oscillations owing to the alignment of Landau levels which pass consecutively the Fermi level with
increasing magnetic field. The MIS oscillation picture is strongly frequency dependent and shows enhanced
MIS peaks for high frequency, damped features and inversed MIS peaks for low frequencies. The inelastic
mechanism, generalized to the two-subband case explains behavior taking into account heating of electrons
due to microwave irradiation and the characteristic scattering times τq (quantum lifetime), τtr (transport time)
and τq (quantum lifetime) (Wie8).
Triple quantum wells (TQWs) consist of three quantum wells in a close proximity and coupled by
tunneling. In contrast to single wells, new fractional quantum Hall states are predicted in these systems if the
interlayer electron-electron interaction is comparable to the ordinary intralayer interaction, determined by the
ratio of the layer separation d to the magnetic length lb (ratio in the order of unity). The samples have an
electron density of ns = 10 12 cm -2 and a mobility of μ ≈ 0.5·10 5 cm 2 /Vs and are separated by thin barriers. In
this type of samples, density of the central well is 30% smaller than in the lateral wells. We have observed a
collapse of integer quantum Hall effect as well as two phenomena in fractional quantum Hall effect: (i)
emergence of fractional states and (ii) re-entrance of on the right side of filling factor ν = 5/2. Physics of
FQHE for n layers (n = 3: triple quantum well) with n > 2 opens interesting perspectives to study such
coherent states by varying several parameters. New theoretical works about multilayer fractional quantum
Hall states, e.g., the part on construction may lead to further studies of such systems.
- Quantum interferences of edge channels in magnetically confined quantum wires
We report on the observation of quantum interferences in a multichannel quantum wire coupled by a
microwave field at two pinning sites where electrons experience forward scattering. We use a magnetic field
to tune the phase difference between charge density waves propagating in each channel. Their interference
gives a magnetic field dependent transmission through the second pinning site which we detect through
oscillations in the magneto-resistance. Magnetically confined quantum wires (MCQW) were obtained by
fabricating Dysprosium microstrips at the centre of narrow Hall bars made of a GaAs/AlGaAs quantum well.
The depth of this MCQW was tuned between 0 and 25meV by applying a magnetic field in the plane to
magnetize the strip. At low field, the energy separation between MCQW subbands is sufficiently small for
microwave absorption through inter-channel transitions. At high magnetic field (B>9T), the energy gaps
between 1D subbands become wider than the photon energy. As a result, there is no oscillatory structure in
the magnetoresistance above 9T.
We have analyzed the oscillatory structure below 9T and conclude to the formation of 1D magnetic
subbands in the gradient of magnetic field. Microwaves excite transport in 1D subbands above the Fermi
level which results in 1D charge density waves. The magnetic field tunes the phase difference of the
charge density waves circulating in each branch by changing the Fermi wavevector. Charge density
excitations therefore interfere at two points (magnetic defects) in the manner of a Mach-Zehnder
interferometer. The phase difference between the two arms of the Mach-Zehnder interferometer gives the
magnetoresistance oscillations seen at low field. We have fitted the resistance peaks with the theoretical
maxima of power absorption by the charge density waves and find as sole adjustment parameter the distance
between the magnetic defects to be d=11μm. We thus conclude to a mean free path of charge density waves
at least equal to 3 times the electron mean free path. We have demonstrated microwave assisted
interferences of edge channels.
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