147 research outputs found
Enhanced quantized current driven by surface acoustic waves
We present the experimental realization of different approaches to increase
the amount of quantized current which is driven by surface acoustic waves
through split gate structures in a two dimensional electron gas. Samples with
driving frequencies of up to 4.7 GHz have been fabricated without a
deterioration of the precision of the current steps, and a parallelization of
two channels with correspondingly doubled current values have been achieved. We
discuss theoretical and technological limitations of these approaches for
metrological applications as well as for quantum logics.Comment: 3pages, 4eps-figure
Acoustoelectric current transport through single-walled carbon nanotubes
We have contacted single-walled carbon nanotubes after aligning the tubes by
the use of surface acoustic waves. The acoustoelectric current has been
measured at 4.2 K and a probing of the low-dimensional electronic states by the
surface acoustic wave has been detected. By decreasing the acoustic wavelength
resulting in an adjustment to the length of the defined carbon nanotube
constriction a quantization of the acoustoelectric current has been observed.Comment: submitted to PR
Quantized charge transport through a static quantum dot using a surface acoustic wave
We present a detailed study of the surface acoustic wave mediated quantized
transport of electrons through a split gate device containing an impurity
potential defined quantum dot within the split gate channel. A new regime of
quantized transport is observed at low RF powers where the surface acoustic
wave amplitude is comparable to the quantum dot charging energy. In this regime
resonant transport through the single-electron dot state occurs which we
interpret as turnstile-like operation in which the traveling wave amplitude
modulates the entrance and exit barriers of the quantum dot in a cyclic fashion
at GHz frequencies. For high RF powers, where the amplitude of the surface
acoustic wave is much larger than the quantum dot energies, the quantized
acoustoelectric current transport shows behavior consistent with previously
reported results. However, in this regime, the number of quantized current
plateaus observed and the plateau widths are determined by the properties of
the quantum dot, demonstrating that the microscopic detail of the potential
landscape in the split gate channel has a profound influence on the quantized
acoustoelectric current transport.Comment: 9 page
Quantized charge pumping through a quantum dot by surface acoustic waves
We present a realization of quantized charge pumping. A lateral quantum dot
is defined by metallic split gates in a GaAs/AlGaAs heterostructure. A surface
acoustic wave whose wavelength is twice the dot length is used to pump single
electrons through the dot at a frequency f=3GHz. The pumped current shows a
regular pattern of quantization at values I=nef over a range of gate voltage
and wave amplitude settings. The observed values of n, the number of electrons
transported per wave cycle, are determined by the number of electronic states
in the quantum dot brought into resonance with the fermi level of the electron
reservoirs during the pumping cycle.Comment: 8 page
Single-electron transport driven by surface acoustic waves: moving quantum dots versus short barriers
We have investigated the response of the acoustoelectric current driven by a
surface-acoustic wave through a quantum point contact in the closed-channel
regime. Under proper conditions, the current develops plateaus at integer
multiples of ef when the frequency f of the surface-acoustic wave or the gate
voltage Vg of the point contact is varied. A pronounced 1.1 MHz beat period of
the current indicates that the interference of the surface-acoustic wave with
reflected waves matters. This is supported by the results obtained after a
second independent beam of surface-acoustic wave was added, traveling in
opposite direction. We have found that two sub-intervals can be distinguished
within the 1.1 MHz modulation period, where two different sets of plateaus
dominate the acoustoelectric-current versus gate-voltage characteristics. In
some cases, both types of quantized steps appeared simultaneously, though at
different current values, as if they were superposed on each other. Their
presence could result from two independent quantization mechanisms for the
acoustoelectric current. We point out that short potential barriers determining
the properties of our nominally long constrictions could lead to an additional
quantization mechanism, independent from those described in the standard model
of 'moving quantum dots'.Comment: 25 pages, 12 figures, to be published in a special issue of J. Low
Temp. Phys. in honour of Prof. F. Pobel
Experimental investigation towards a periodically pumped single-photon source
Experiments towards a periodically pumped single-photon source are presented. The lateral piezoelectric field of a surface acoustic wave dissociates laser-generated two-dimensional excitons into electrons and holes. These carriers are separated by the wave potential and are transported over macroscopic length scales without recombining. When reaching a stress-induced quantum dot in the quantum well they periodically populate the zero-dimensional states and recombine, emitting single photons periodically in time according to the surface acoustic-wave frequency. We have successfully reduced the number of pumped quantum dots down to 100 and have detected a strong blinking photoluminescence signal. By further reducing the number of quantum dots down to 1 a periodically pumped single photon source could be realized.Peer reviewe
A numerical investigation of a piezoelectric surface acoustic wave interaction with a one-dimensional channel
We investigate the propagation of a piezoelectric surface acoustic wave (SAW)
across a GaAs/AlGaAs heterostructure surface, on which there is
fixed a metallic split-gate. Our method is based on a finite element
formulation of the underlying equations of motion, and is performed in
three-dimensions fully incorporating the geometry and material composition of
the substrate and gates. We demonstrate attenuation of the SAW amplitude as a
result of the presence of both mechanical and electrical gates on the surface.
We show that the incorporation of a simple model for the screening by the
two-dimensional electron gas (2DEG), results in a total electric potential
modulation that suggests a mechanism for the capture and release of electrons
by the SAW. Our simulations suggest the absence of any significant turbulence
in the SAW motion which could hamper the operation of SAW based quantum devices
of a more complex geometry.Comment: 8 pages, 8 figure
Non-invasive probing of random local potential fluctuations in ZnCdSe/ZnSe quantum wells
Temperature dependence and recombination behavior of trapped charge carriers
in ZnCdSe/ZnSe multiple quantum wells are investigated employing surface
acoustic waves. These weakly perturb the carrier system, but remain highly
sensitive even at small conductivities. Using this non-invasive probe we are
able to detect persistent photoconductivity minutes after optical excitation.
Measurement of exciting photon energies, the temperature dependence and ability
to quench the conductivity with energies lower than the bandgap, support the
notion of spatial separation of electrons and holes in the wells, due to random
local potential fluctuations possibly induced by compositional fluctuations
Charge transport through single molecules, quantum dots, and quantum wires
We review recent progresses in the theoretical description of correlation and
quantum fluctuation phenomena in charge transport through single molecules,
quantum dots, and quantum wires. A variety of physical phenomena is addressed,
relating to co-tunneling, pair-tunneling, adiabatic quantum pumping, charge and
spin fluctuations, and inhomogeneous Luttinger liquids. We review theoretical
many-body methods to treat correlation effects, quantum fluctuations,
nonequilibrium physics, and the time evolution into the stationary state of
complex nanoelectronic systems.Comment: 48 pages, 14 figures, Topical Review for Nanotechnolog
A Josephson Quantum Electron Pump
A macroscopic fluid pump works according to the law of Newtonian mechanics
and transfers a large number of molecules per cycle (of the order of 10^23). By
contrast, a nano-scale charge pump can be thought as the ultimate
miniaturization of a pump, with its operation being subject to quantum
mechanics and with only few electrons or even fractions of electrons transfered
per cycle. It generates a direct current in the absence of an applied voltage
exploiting the time-dependence of some properties of a nano-scale conductor.
The idea of pumping in nanostructures was discussed theoretically a few decades
ago [1-4]. So far, nano-scale pumps have been realised only in system
exhibiting strong Coulombic effects [5-12], whereas evidence for pumping in the
absence of Coulomb-blockade has been elusive. A pioneering experiment by
Switkes et al. [13] evidenced the difficulty of modulating in time the
properties of an open mesoscopic conductor at cryogenic temperatures without
generating undesired bias voltages due to stray capacitances [14,15]. One
possible solution to this problem is to use the ac Josephson effect to induce
periodically time-dependent Andreev-reflection amplitudes in a hybrid
normal-superconducting system [16]. Here we report the experimental detection
of charge flow in an unbiased InAs nanowire (NW) embedded in a superconducting
quantum interference device (SQUID). In this system, pumping may occur via the
cyclic modulation of the phase of the order parameter of different
superconducting electrodes. The symmetry of the current with respect to the
enclosed magnetic flux [17,18] and bias SQUID current is a discriminating
signature of pumping. Currents exceeding 20 pA are measured at 250 mK, and
exhibit symmetries compatible with a pumping mechanism in this setup which
realizes a Josephson quantum electron pump (JQEP).Comment: 7+ pages, 6 color figure
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