3,637 research outputs found
Probing confined phonon modes by transport through a nanowire double quantum dot
Strong radial confinement in semiconductor nanowires leads to modified
electronic and phononic energy spectra. We analyze the current response to the
interplay between quantum confinement effects of the electron and phonon
systems in a gate-defined double quantum dot in a semiconductor nanowire. We
show that current spectroscopy of inelastic transitions between the two quantum
dots can be used as an experimental probe of the confined phonon environment.
The resulting discrete peak structure in the measurements is explained by
theoretical modeling of the confined phonon mode spectrum, where the
piezoelectric coupling is of crucial importance.Comment: 4 pages, 4 figures; final versio
Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors
We report on magnetotransport investigations of nano-scaled ferromagnetic
Co/Ni/Co single electron transistors. As a result of reduced size, the devices
exhibit single electron transistor characteristics at 4.2K. Magnetotransport
measurements carried out at 1.8K reveal tunneling magnetoresistance (TMR)
traces with negative coercive fields, which we interpret in terms of a
switching mechanism driven by the shape anisotropy of the central wire-like Ni
island. A large TMR of about 18% is observed within a finite source-drain bias
regime. The TMR decreases rapidly with increasing bias, which we tentatively
attribute to excitation of magnons in the central island.Comment: 12 pages (including 4 figures). Accepted for publishing on AP
Imaging a 1-electron InAs quantum dot in an InAs/InP nanowire
Nanowire heterostructures define high-quality few-electron quantum dots for
nanoelectronics, spintronics and quantum information processing. We use a
cooled scanning probe microscope (SPM) to image and control an InAs quantum dot
in an InAs/InP nanowire, using the tip as a movable gate. Images of dot
conductance vs. tip position at T = 4.2 K show concentric rings as electrons
are added, starting with the first electron. The SPM can locate a dot along a
nanowire and individually tune its charge, abilities that will be very useful
for the control of coupled nanowire dots
Direct Measurement of the Spin-Orbit Interaction in a Two-Electron InAs Nanowire Quantum Dot
We demonstrate control of the electron number down to the last electron in
tunable few-electron quantum dots defined in catalytically grown InAs
nanowires. Using low temperature transport spectroscopy in the Coulomb blockade
regime we propose a simple method to directly determine the magnitude of the
spin-orbit interaction in a two-electron artificial atom with strong spin-orbit
coupling. Due to a large effective g-factor |g*|=8+/-1 the transition from
singlet S to triplet T+ groundstate with increasing magnetic field is dominated
by the Zeeman energy rather than by orbital effects. We find that the
spin-orbit coupling mixes the T+ and S states and thus induces an avoided
crossing with magnitude =0.25+/-0.05 meV. This allows us to
calculate the spin-orbit length 127 nm in such systems
using a simple model.Comment: 21 pages, 7 figures, including supplementary note
Test of a Jastrow-type wavefunction for a trapped few-body system in one dimension
For a system with interacting quantum mechanical particles in a
one-dimensional harmonic oscillator, a trial wavefunction with simple structure
based on the solution of the corresponding two-particle system is suggested and
tested numerically. With the inclusion of a scaling parameter for the distance
between particles, at least for the very small systems tested here the ansatz
gives a very good estimate of the ground state energy, with the error being of
the order of ~1% of the gap to the first excited state
Signatures of Wigner Localization in Epitaxially Grown Nanowires
It was predicted by Wigner in 1934 that the electron gas will undergo a
transition to a crystallized state when its density is very low. Whereas
significant progress has been made towards the detection of electronic Wigner
states, their clear and direct experimental verification still remains a
challenge. Here we address signatures of Wigner molecule formation in the
transport properties of InSb nanowire quantum dot systems, where a few
electrons may form localized states depending on the size of the dot (i.e. the
electron density). By a configuration interaction approach combined with an
appropriate transport formalism, we are able to predict the transport
properties of these systems, in excellent agreement with experimental data. We
identify specific signatures of Wigner state formation, such as the strong
suppression of the antiferromagnetic coupling, and are able to detect the onset
of Wigner localization, both experimentally and theoretically, by studying
different dot sizes.Comment: 4 pages, 4 figure
Analyzing capacitance-voltage measurements of vertical wrapped-gated nanowires
The capacitance of arrays of vertical wrapped-gate InAs nanowires are
analyzed. With the help of a Poisson-Schr"odinger solver, information about the
doping density can be obtained directly. Further features in the measured
capacitance-voltage characteristics can be attributed to the presence of
surface states as well as the coexistence of electrons and holes in the wire.
For both scenarios, quantitative estimates are provided. It is furthermore
shown that the difference between the actual capacitance and the geometrical
limit is quite large, and depends strongly on the nanowire material.Comment: 15 pages, 6 Figures included, to appear in Nanotechnolog
Probing Spin Accumulation in Ni/Au/Ni Single-Electron Transistors with Efficient Spin Injection and Detection Electrodes
We have investigated spin accumulation in Ni/Au/Ni single-electron
transistors assembled by atomic force microscopy. The fabrication technique is
unique in that unconventional hybrid devices can be realized with unprecedented
control, including real-time tunable tunnel resistances. A grid of Au discs, 30
nm in diameter and 30 nm thick, is prepared on a SiO2 surface by conventional
e-beam writing. Subsequently, 30 nm thick ferromagnetic Ni source, drain and
side-gate electrodes are formed in similar process steps. The width and length
of the source and drain electrodes were different to exhibit different coercive
switching fields. Tunnel barriers of NiO are realized by sequential Ar and O2
plasma treatment. Using an atomic force microscope with specially designed
software, a single non-magnetic Au nanodisc is positioned into the 25 nm gap
between the source and drain electrodes. The resistance of the device is
monitored in real-time while the Au disc is manipulated step-by-step with
Angstrom-level precision. Transport measurements in magnetic field at 1.7 K
reveal no clear spin accumulation in the device, which can be attributed to
fast spin relaxation in the Au disc. From numerical simulations using the
rate-equation approach of orthodox Coulomb blockade theory, we can put an upper
bound of a few ns on the spin-relaxation time for electrons in the Au disc. To
confirm the magnetic switching characteristics and spin injection efficiency of
the Ni electrodes, we fabricated a test structure consisting of a Ni/NiO/Ni
magnetic tunnel junction with asymmetric dimensions of the electrodes similar
to those of the SETs. Magnetoresistance measurements on the test device
exhibited clear signs of magnetic reversal and a maximum TMR of 10%, from which
we deduced a spin-polarization of about 22% in the Ni electrodes.Comment: 10 pages, 5 figure
Enhanced Zeeman splitting in Ga0.25In0.75As quantum point contacts
The strength of the Zeeman splitting induced by an applied magnetic field is
an important factor for the realization of spin-resolved transport in
mesoscopic devices. We measure the Zeeman splitting for a quantum point contact
etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to
the transport direction. We observe an enhancement of the Lande g-factor from
|g*|=3.8 +/- 0.2 for the third subband to |g*|=5.8 +/- 0.6 for the first
subband, six times larger than in GaAs. We report subband spacings in excess of
10 meV, which facilitates quantum transport at higher temperatures.Comment: [Version 2] Revtex4, 11 pages, 3 figures, accepted for publication in
Applied Physics Letter
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