2,616 research outputs found
Starting characteristics of direct current motors powered by solar cells
Direct current motors are used in photovoltaic systems. Important characteristics of electric motors are the starting to rated current and torque ratios. These ratios are dictated by the size of the solar cell array and are different for the various dc motor types. Discussed here is the calculation of the starting to rated current ratio and starting to rated torque ratio of the permanent magnet, and series and shunt excited motors when powered by solar cells for two cases: with and without a maximum-power-point-tracker (MPPT) included in the system. Comparing these two cases, one gets a torque magnification of about 3 for the permanent magnet motor and about 7 for other motor types. The calculation of the torques may assist the PV system designer to determine whether or not to include an MPPT in the system
Calculation of the Self-energy of Open Quantum Systems
We propose an easy method of calculating the self-energy of semi-infinite
leads attached to a mesoscopic system.Comment: 6 pages, 2 figures, published in J. Phys. Soc. Jp
Towards spin injection from silicon into topological insulators: Schottky barrier between Si and Bi2Se3
A scheme is proposed to electrically measure the spin-momentum coupling in
the topological insulator surface state by injection of spin polarized
electrons from silicon. As a first approach, devices were fabricated consisting
of thin (<100nm) exfoliated crystals of Bi2Se3 on n-type silicon with
independent electrical contacts to silicon and Bi2Se3. Analysis of the
temperature dependence of thermionic emission in reverse bias indicates a
barrier height of 0.34 eV at the Si-Bi2Se3 interface. This robust Schottky
barrier opens the possibility of novel device designs based on sub-band gap
internal photoemission from Bi2Se3 into Si
Coherent spin transport through a 350-micron-thick Silicon wafer
We use all-electrical methods to inject, transport, and detect spin-polarized
electrons vertically through a 350-micron-thick undoped single-crystal silicon
wafer. Spin precession measurements in a perpendicular magnetic field at
different accelerating electric fields reveal high spin coherence with at least
13pi precession angles. The magnetic-field spacing of precession extrema are
used to determine the injector-to-detector electron transit time. These transit
time values are associated with output magnetocurrent changes (from in-plane
spin-valve measurements), which are proportional to final spin polarization.
Fitting the results to a simple exponential spin-decay model yields a
conduction electron spin lifetime (T1) lower bound in silicon of over 500ns at
60K.Comment: Accepted in PR
The MacArthur Treatment Competence Study. III: Abilities of patients to consent to psychiatric and medical treatments.
Nonequilibrium Transport through a Kondo Dot in a Magnetic Field: Perturbation Theory
Using nonequilibrium perturbation theory, we investigate the nonlinear
transport through a quantum dot in the Kondo regime in the presence of a
magnetic field. We calculate the leading logarithmic corrections to the local
magnetization and the differential conductance, which are characteristic of the
Kondo effect out of equilibrium. By solving a quantum Boltzmann equation, we
determine the nonequilibrium magnetization on the dot and show that the
application of both a finite bias voltage and a magnetic field induces a novel
structure of logarithmic corrections not present in equilibrium. These
corrections lead to more pronounced features in the conductance, and their form
calls for a modification of the perturbative renormalization group.Comment: 16 pages, 7 figure
The Kondo Effect in Non-Equilibrium Quantum Dots: Perturbative Renormalization Group
While the properties of the Kondo model in equilibrium are very well
understood, much less is known for Kondo systems out of equilibrium. We study
the properties of a quantum dot in the Kondo regime, when a large bias voltage
V and/or a large magnetic field B is applied. Using the perturbative
renormalization group generalized to stationary nonequilibrium situations, we
calculate renormalized couplings, keeping their important energy dependence. We
show that in a magnetic field the spin occupation of the quantum dot is
non-thermal, being controlled by V and B in a complex way to be calculated by
solving a quantum Boltzmann equation. We find that the well-known suppression
of the Kondo effect at finite V>>T_K (Kondo temperature) is caused by inelastic
dephasing processes induced by the current through the dot. We calculate the
corresponding decoherence rate, which serves to cut off the RG flow usually
well inside the perturbative regime (with possible exceptions). As a
consequence, the differential conductance, the local magnetization, the spin
relaxation rates and the local spectral function may be calculated for large
V,B >> T_K in a controlled way.Comment: 9 pages, invited paper for a special edition of JPSJ "Kondo Effect --
40 Years after the Discovery", some typos correcte
Suppression of Kondo effect in a quantum dot by external irradiation
We demonstrate that the external irradiation brings decoherence in the spin
states of the quantum dot. This effect cuts off the Kondo anomaly in
conductance even at zero temperature. We evaluate the dependence of the DC
conductance in the Kondo regime on the power of irradiation, this dependence
being determined by the decoherence.Comment: 4 pages, 1 figur
Conduction through a quantum dot near a singlet-triplet transition
Kondo effect in the vicinity of a singlet-triplet transition in a vertical
quantum dot is considered. This system is shown to map onto a special version
of the two-impurity Kondo model. At any value of the control parameter, the
system has a Fermi-liquid ground state. Explicit expressions for the linear
conductance as a function of the control parameter and temperature are
obtained. At T=0, the conductance reaches the unitary limit at
the triplet side of the transition, and decreases with the increasing distance
to the transition at the singlet side. At finite temperature, the conductance
exhibits a peak near the transition point
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