203 research outputs found
The Effect of a Magnetic Field on the Acoustoelectric current in a Narrow Channel
The effect of a perpendicular magnetic field on the quantized current induced
by a surface acoustic wave in a quasi-1D channel is studied. The channel has
been produced experimentally in a GaAs heterostructure by shallow etching
techniques and by the application of a negative gate voltage to Schottky split
gates. Commensurability oscillations of the quantized current in this
constriction have been observed in the interval of current between quantized
plateaus. The results can be understood in terms of a moving quantum dot with
the electron in the dot tunneling into the adjacent two-dimensional region. The
goal is to explain qualitatively the mechanism for the steplike nature of the
acoustoelectric current as a function of gate voltage and the oscillations when
a magnetic field is applied. A transfer Hamiltonian formalism is employed.Comment: 5 pages, 2 figure
Acoustoelectric effect in a finite-length ballistic quantum channel
The dc current induced by a coherent surface acoustic wave (SAW) of wave
vector q in a ballistic channel of length L is calculated. The current contains
two contributions, even and odd in q. The even current exists only in a
asymmetric channel, when the electron reflection coefficients r_1 and r_2 at
both channel ends are different. The direction of the even current does not
depend on the direction of the SAW propagation, but is reversed upon
interchanging r_1 and r_2. The direction of the odd current is correlated with
the direction of the SAW propagation, but is insensitive to the interchange of
r_1 and r_2. It is shown that both contributions to the current are non zero
only when the electron reflection coefficients at the channel ends are energy
dependent. The current exhibits geometric oscillations as function of qL. These
oscillations are the hallmark of the coherence of the SAW and are completely
washed out when the current is induced by a flux of non-coherent phonons. The
results are compared with those obtained previously by different methods and
under different assumptions.Comment: 7 pages, 2 figure
Giant Oscillations of Acoustoelectric Current in a Quantum Channel
A theory of d.c. electric current induced in a quantum channel by a
propagating surface acoustic wave (acoustoelectric current) is worked out. The
first observation of the acoustoelectric current in such a situation was
reported by J. M. Shilton et al., Journ. Phys. C (to be published). The authors
observed a very specific behavior of the acoustoelectric current in a
quasi-one-dimensional channel defined in a GaAs-AlGaAs heterostructure by a
split-gate depletion -- giant oscillations as a function of the gate voltage.
Such a behavior was qualitatively explained by an interplay between the
energy-momentum conservation law for the electrons in the upper transverse mode
with a finite temperature splitting of the Fermi level. In the present paper, a
more detailed theory is developed, and important limiting cases are considered.Comment: 7 pages, 2 Postscript figures, RevTeX 3.
Acoustoelectric effects in quantum constrictions
A dc current induced in a quantum constriction by a traveling acoustic wave
(or by non-equilibrium ballistic phonons) is considered. We show that in many
important situations the effect is originated from acoustically-induced
scattering between the propagating and reflecting states in the constriction.
Two particular regimes corresponding to relatively high and low acoustic
frequencies are discussed. In the first regime, the acoustoelectric effect in a
smooth constriction can be understood by semi-classical considerations based on
local conservation laws. For the low frequency regime, we show that the
acousto-conductance is closely related to the zero field conductance. The
qualitative considerations are confirmed by numerical calculations both for
smooth and abrupt channels.Comment: 10 pages, RevTeX, 9 postscript figures, submitted to Phys. Rev.
Nonlinear absorption of surface acoustic waves by composite fermions
Absorption of surface acoustic waves by a two-dimensional electron gas in a
perpendicular magnetic field is considered. The structure of such system at the
filling factor close to 1/2 can be understood as a gas of {\em composite
fermions}. It is shown that the absorption at can be strongly
nonlinear, while small deviation form 1/2 will restore the linear absorption.
Study of nonlinear absorption allows one to determine the force acting upon the
composite fermions from the acoustic wave at turning points of their
trajectories.Comment: 7 pages, 1 figure, submitted to Europhysics letter
Weiss Oscillations in Surface Acoustic Wave Propagation
The interaction of a surface acoustic wave (SAW) with a a two-dimensional
electron gas in a periodic electric potential and a classical magnetic field is
considered. We calculate the attenuation of the SAW and its velocity change and
show that these quantities exhibit Weiss oscillations.Comment: 4 pages REVTEX, 2 figures included as eps file
Nonlinear acousto-electric transport in a two-dimensional electron system
We study both theoretically and experimentally the nonlinear interaction
between an intense surface acoustic wave and a two-dimensional electron plasma
in semiconductor-piezocrystal hybrid structures. The experiments on hybrid
systems exhibit strongly nonlinear acousto-electric effects. The plasma turns
into moving electron stripes, the acousto-electric current reaches its maximum,
and the sound absorption strongly decreases. To describe the nonlinear
phenomena, we develop a coupled-amplitude method for a two-dimensional system
in the strongly nonlinear regime of interaction. At low electron densities the
absorption coefficient decreases with increasing sound intensity, whereas at
high electron density the absorption coefficient is not a monotonous function
of the sound intensity. High-harmonic generation coefficients as a function of
the sound intensity have a nontrivial behavior. Theory and experiment are found
to be in a good agreement.Comment: 27 pages, 6 figure
Surface acoustic wave attenuation by a two-dimensional electron gas in a strong magnetic field
The propagation of a surface acoustic wave (SAW) on GaAs/AlGaAs
heterostructures is studied in the case where the two-dimensional electron gas
(2DEG) is subject to a strong magnetic field and a smooth random potential with
correlation length Lambda and amplitude Delta. The electron wave functions are
described in a quasiclassical picture using results of percolation theory for
two-dimensional systems. In accordance with the experimental situation, Lambda
is assumed to be much smaller than the sound wavelength 2*pi/q. This restricts
the absorption of surface phonons at a filling factor \bar{\nu} approx 1/2 to
electrons occupying extended trajectories of fractal structure. Both
piezoelectric and deformation potential interactions of surface acoustic
phonons with electrons are considered and the corresponding interaction
vertices are derived. These vertices are found to differ from those valid for
three-dimensional bulk phonon systems with respect to the phonon wave vector
dependence. We derive the appropriate dielectric function varepsilon(omega,q)
to describe the effect of screening on the electron-phonon coupling. In the low
temperature, high frequency regime T << Delta (omega_q*Lambda
/v_D)^{alpha/2/nu}, where omega_q is the SAW frequency and v_D is the electron
drift velocity, both the attenuation coefficient Gamma and varepsilon(omega,q)
are independent of temperature. The classical percolation indices give
alpha/2/nu=3/7. The width of the region where a strong absorption of the SAW
occurs is found to be given by the scaling law |Delta \bar{\nu}| approx
(omega_q*Lambda/v_D)^{alpha/2/nu}. The dependence of the electron-phonon
coupling and the screening due to the 2DEG on the filling factor leads to a
double-peak structure for Gamma(\bar{\nu}).Comment: 17 pages, 3 Postscript figures, minor changes mad
Quantum Spin Pump in S=1/2 antiferromagnetic chains -Holonomy of phase operators in sine-Gordon theory-
In this paper, we propose the quantum spin pumping in quantum spin systems
where an applied electric field () and magnetic field () cause a finite
spin gap to its critical ground state. When these systems are subject to
alternating electromangetic fields; and travel along the {\it{loop}} which encloses
their critical ground state in this - phase diagram, the locking
potential in the sine-Gordon model slides and changes its minimum. As a result,
the phase operator acquires holonomy during one cycle along
, which means that the quantized spin current has been
transported through the bulk systems during this adiabatic process. The
relevance to real systems such as Cu-benzoate and is
also discussed.Comment: 10 pages, 5 figures, to be published in J. Phys. Soc. Jpn. 74 (2005)
no. 4. Typos corrected in the revised versio
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