832 research outputs found
Constraints on the composition of Jupiter's stratospheric aerosols from ultraviolet photometry
The absolute reflectivity of Jupiter has been obtained in 50 A-wide regions centering on 0.221, 0.233, 0.252, and 0.330 microns from three series of IUE satellite spectra taken in November 1979. The data indicate a strong decrease in reflectivity for latitudes greater than about 30 deg, in keeping with Voyager measurements. An additional 24 spectra were also obtained in a west-east series along the equator, as well as near 40 deg N latitude. These data favor models in which the haze particles have effective radii within a factor of 2 of 0.2 microns. Near the equator, the haze aerosols produce much less absorption than near 40 deg N; the aerosol distributions and optical properties derived are noted to be more dependent on the assumed location and reflectivity of the top of the tropospheric cloud
Spectrophotometry of planets, asteroids and satellites from the international ultraviolet explorer satellite
A total of 14 8 hour I.U.E. observing sessions resulted in 39 spectra of 11 asteroids and 9 solar type stars as well as 57 spectra at various locations on the disk of Jupiter. The Jupiter observations include a total of 5 center to limb series of spectra at various latitudes and a North South series along the central meridian. In the range from 2000-3000 A, the planet shows a striking decrease in brightness at latitudes greater than about 30 degrees, and exhibits limb brightening at low latitudes and limb darkening at high latitudes. Preliminary results indicate that about 6 km-amagats of clean hydrogen are required above a haze of absorbing aerosols to reproduce the limb brightening observed at 2500 A in the equatorial regions. At higher latitudes, the aerosols extend to even higher levels of the atmosphere. Comparison of the Jovian data with detailed model calculations and the analyses of the asteroid spectra are still in progress with other support
Kondo effect in quantum dots coupled to ferromagnetic leads
We study the Kondo effect in a quantum dot which is coupled to ferromagnetic
leads and analyse its properties as a function of the spin polarization of the
leads. Based on a scaling approach we predict that for parallel alignment of
the magnetizations in the leads the strong-coupling limit of the Kondo effect
is reached at a finite value of the magnetic field. Using an equation-of-motion
technique we study nonlinear transport through the dot. For parallel alignment
the zero-bias anomaly may be split even in the absence of an external magnetic
field. For antiparallel spin alignment and symmetric coupling, the peak is
split only in the presence of a magnetic field, but shows a characteristic
asymmetry in amplitude and position.Comment: 5 pages, 2 figure
Kondo effect near the Van Hove singularity in biased bilayer graphene
Magnetic impurity adsorbed on one of the carbon planes of a bilayer graphene
is studied. The formation of the many-body SU(2) and SU(4) resonances close to
the bandgap is analyzed within the mean field Kotliar-Ruckenstein slave boson
approach. Impact of enhanced hybridization and magnetic instability of bilayer
doped near the Van Hove singularity on the screening of magnetic moment is
discussed.Comment: 10 pages, 8 figure
Frequency-Dependent Current Noise through Quantum-Dot Spin Valves
We study frequency-dependent current noise through a single-level quantum dot
connected to ferromagnetic leads with non-collinear magnetization. We propose
to use the frequency-dependent Fano factor as a tool to detect single-spin
dynamics in the quantum dot. Spin precession due to an external magnetic and/or
a many-body exchange field affects the Fano factor of the system in two ways.
First, the tendency towards spin-selective bunching of the transmitted
electrons is suppressed, which gives rise to a reduction of the low-frequency
noise. Second, the noise spectrum displays a resonance at the Larmor frequency,
whose lineshape depends on the relative angle of the leads' magnetizations.Comment: 12 pages, 15 figure
Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study
We systematically study the influence of ferromagnetic leads on the Kondo
resonance in a quantum dot tuned to the local moment regime. We employ Wilson's
numerical renormalization group method, extended to handle leads with a spin
asymmetric density of states, to identify the effects of (i) a finite spin
polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in
the bands (governed by the band edges) and (iii) an arbitrary shape of the
leads density of states. For a generic lead density of states the quantum dot
favors being occupied by a particular spin-species due to exchange interaction
with ferromagnetic leads leading to a suppression and splitting of the Kondo
resonance. The application of a magnetic field can compensate this asymmetry
restoring the Kondo effect. We study both the gate-voltage dependence (for a
fixed band structure in the leads) and the spin polarization dependence (for
fixed gate voltage) of this compensation field for various types of bands.
Interestingly, we find that the full recovery of the Kondo resonance of a
quantum dot in presence of leads with an energy dependent density of states is
not only possible by an appropriately tuned external magnetic field but also
via an appropriately tuned gate voltage. For flat bands simple formulas for the
splitting of the local level as a function of the spin polarization and gate
voltage are given.Comment: 18 pages, 18 figures, accepted for publication in PR
Crossover from Kondo assisted suppression to co-tunneling enhancement of tunneling magnetoresistance via ferromagnetic nanodots in MgO tunnel barriers
Recently, it has been shown that magnetic tunnel junctions with thin MgO
tunnel barriers exhibit extraordinarily high tunneling magnetoresistance (TMR)
values at room temperature1, 2. However, the physics of spin dependent
tunneling through MgO barriers is only beginning to be unravelled. Using planar
magnetic tunnel junctions in which ultra-thin layers of magnetic metals are
deposited in the middle of a MgO tunnel barrier here we demonstrate that the
TMR is strongly modified when these layers are discontinuous and composed of
small pancake shaped nanodots. At low temperatures, in the Coulomb blockade
regime, for layers less than ~1 nm thick, the conductance of the junction is
increased at low bias consistent with Kondo assisted tunneling. In the same
regime we observe a suppression of the TMR. For slightly thicker layers, and
correspondingly larger nanodots, the TMR is enhanced at low bias, consistent
with co-tunneling.Comment: Nano Letters (in press
Interference effects in electronic transport through metallic single-wall carbon nanotubes
In a recent paper Liang {\it et al.} [Nature {\bf 411}, 665 (2001)] showed
experimentally, that metallic nanotubes, strongly coupled to external
electrodes, may act as coherent molecular waveguides for electronic transport.
The experimental results were supported by theoretical analysis based on the
scattering matrix approach. In this paper we analyze theoretically this problem
using a real-space approach, which makes it possible to control quality of
interface contacts. Electronic structure of the nanotube is taken into account
within the tight-binding model. External electrodes and the central part
(sample) are assumed to be made of carbon nanotubes, while the contacts between
electrodes and the sample are modeled by appropriate on-site (diagonal) and
hopping (off-diagonal) parameters. Conductance is calculated by the Green
function technique combined with the Landauer formalism. In the plots
displaying conductance {\it vs.} bias and gate voltages, we have found typical
diamond structure patterns, similar to those observed experimentally. In
certain cases, however, we have found new features in the patterns, like a
double-diamond sub-structure.Comment: 15 pages, 4 figures. To apear in Phys. Rev.
Electric-field controlled spin reversal in a quantum dot with ferromagnetic contacts
Manipulation of the spin-states of a quantum dot by purely electrical means
is a highly desirable property of fundamental importance for the development of
spintronic devices such as spin-filters, spin-transistors and single-spin
memory as well as for solid-state qubits. An electrically gated quantum dot in
the Coulomb blockade regime can be tuned to hold a single unpaired spin-1/2,
which is routinely spin-polarized by an applied magnetic field. Using
ferromagnetic electrodes, however, the properties of the quantum dot become
directly spin-dependent and it has been demonstrated that the ferromagnetic
electrodes induce a local exchange-field which polarizes the localized spin in
the absence of any external fields. Here we report on the experimental
realization of this tunneling-induced spin-splitting in a carbon nanotube
quantum dot coupled to ferromagnetic nickel-electrodes. We study the
intermediate coupling regime in which single-electron states remain well
defined, but with sufficiently good tunnel-contacts to give rise to a sizable
exchange-field. Since charge transport in this regime is dominated by the
Kondo-effect, we can utilize this sharp many-body resonance to read off the
local spin-polarization from the measured bias-spectroscopy. We show that the
exchange-field can be compensated by an external magnetic field, thus restoring
a zero-bias Kondo-resonance, and we demonstrate that the exchange-field itself,
and hence the local spin-polarization, can be tuned and reversed merely by
tuning the gate-voltage. This demonstrates a very direct electrical control
over the spin-state of a quantum dot which, in contrast to an applied magnetic
field, allows for rapid spin-reversal with a very localized addressing.Comment: 19 pages, 11 figure
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