2,018 research outputs found
High Kondo temperature (TK ~ 80 K) in self-assembled InAs quantum dots laterally coupled to nanogap electrodes
We have fabricated single electron tunneling structures by forming nanogap
metallic electrodes directly upon single self-assembled InAs quantum dots
(QDs). The fabricated samples exhibited clear Coulomb blockade effects.
Furthermore, a clear Kondo effect was observed when strong coupling between the
electrodes and the QDs was realized using a large QD with a diameter of ~ 100
nm. From the temperature dependence of the linear conductance at the Kondo
valley, the Kondo temperature TK was determined to be ~ 81 K. This is the
highest TK ever reported for artificial quantum nanostructures.Comment: 3 pages, 3 figure
Fabrication of graphene nanoribbon by local anodic oxidation lithography using atomic force microscope
We conducted local anodic oxidation (LAO) lithography in single-layer,
bilayer, and multilayer graphene using tapping-mode atomic force microscope.
The width of insulating oxidized area depends systematically on the number of
graphene layers. An 800-nm-wide bar-shaped device fabricated in single-layer
graphene exhibits the half-integer quantum Hall effect. We also fabricated a
55-nm-wide graphene nanoribbon (GNR). The conductance of the GNR at the charge
neutrality point was suppressed at low temperature, which suggests the opening
of an energy gap due to lateral confinement of charge carriers. These results
show that LAO lithography is an effective technique for the fabrication of
graphene nanodevices.Comment: 4 pages, 4 figure
Transport properties of diluted magnetic semiconductors: Dynamical mean field theory and Boltzmann theory
The transport properties of diluted magnetic semiconductors (DMS) are
calculated using dynamical mean field theory (DMFT) and Boltzmann transport
theory. Within DMFT we study the density of states and the dc-resistivity,
which are strongly parameter dependent such as temperature, doping, density of
the carriers, and the strength of the carrier-local impurity spin exchange
coupling. Characteristic qualitative features are found distinguishing weak,
intermediate, and strong carrier-spin coupling and allowing quantitative
determination of important parameters defining the underlying ferromagnetic
mechanism. We find that spin-disorder scattering, formation of bound state, and
the population of the minority spin band are all operational in DMFT in
different parameter range. We also develop a complementary Boltzmann transport
theory for scattering by screened ionized impurities. The difference in the
screening properties between paramagnetic () and ferromagnetic ()
states gives rise to the temperature dependence (increase or decrease) of
resistivity, depending on the carrier density, as the system goes from the
paramagnetic phase to the ferromagnetic phase. The metallic behavior below
for optimally doped DMS samples can be explained in the Boltzmann theory
by temperature dependent screening and thermal change of carrier spin
polarization.Comment: 15 pages, 15 figure
Electrical control of Kondo effect and superconducting transport in a side-gated InAs quantum dot Josephson junction
We measure the non-dissipative supercurrent in a single InAs self-assembled
quantum dot (QD) coupled to superconducting leads. The QD occupation is both
tuned by a back-gate electrode and lateral side-gate. The geometry of the
side-gate allows tuning of the QD-lead tunnel coupling in a region of constant
electron number with appropriate orbital state. Using the side-gate effect we
study the competition between Kondo correlations and superconducting pairing on
the QD, observing a decrease in the supercurrent when the Kondo temperature is
reduced below the superconducting energy gap in qualitative agreement with
theoretical predictions
Spin transport through a single self-assembled InAs quantum dot with ferromagnetic leads
We have fabricated a lateral double barrier magnetic tunnel junction (MTJ)
which consists of a single self-assembled InAs quantum dot (QD) with
ferromagnetic Co leads. The MTJ shows clear hysteretic tunnel magnetoresistance
(TMR) effect, which is evidence for spin transport through a single
semiconductor QD. The TMR ratio and the curve shapes are varied by changing the
gate voltage.Comment: 4 pages, 3 figure
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