2,103 research outputs found
Gate-voltage dependence of Kondo effect in a triangular quantum dot
We study the conductance through a triangular triple quantum dot, which are
connected to two noninteracting leads, using the numerical renormalization
group (NRG). It is found that the system shows a variety of Kondo effects
depending on the filling of the triangle. The SU(4) Kondo effect occurs at
half-filling, and a sharp conductance dip due to a phase lapse appears in the
gate-voltage dependence. Furthermore, when four electrons occupy the three
sites on average, a local S=1 moment, which is caused by the Nagaoka mechanism,
is induced along the triangle. The temperature dependence of the entropy and
spin susceptibility of the triangle shows that this moment is screened by the
conduction electrons via two separate stages at different temperatures. The
two-terminal and four-terminal conductances show a clear difference at the gate
voltages, where the SU(4) or the S=1 Kondo effects occurring.Comment: 4 pages, 4 figs: typos just below (4) are corrected, results are not
affecte
Electron transport through a strongly correlated monoatomic chain
We study transport properties of a strongly correlated monoatomic chain
coupled to metallic leads. Our system is described by tight binding
Hubbard-like model in the limit of strong on-site electron-electron
interactions in the wire. The equation of motion technique in the slave boson
representation has been applied to obtain analytical and numerical results.
Calculated linear conductance of the system shows oscillatory behavior as a
function of the wire length. We have also found similar oscillations of the
electron charge in the system. Moreover our results show spontaneous spin
polarization in the wire. Finally, we compare our results with those for
non-interacting chain and discuss their modifications due to the Coulomb
interactions in the system.Comment: 7 pages, 5 figure
The Chandra view of the Largest Quasar Lens SDSS J1029+2623
We present results from Chandra observations of the cluster lens SDSS
J1029+2623 at z_l=0.58, which is a gravitationally lensed quasar with the
largest known image separation. We clearly detect X-ray emission both from the
lensing cluster and the three lensed quasar images. The cluster has an X-ray
temperature of kT = 8.1 (+2.0, -1.2) keV and bolometric luminosity of L_X =
9.6e44 erg s^-1. Its surface brightness is centered near one of the brightest
cluster galaxies, and it is elongated East-West. We identify a subpeak
North-West of the main peak, which is suggestive of an ongoing merger. Even so,
the X-ray mass inferred from the hydrostatic equilibrium assumption appears to
be consistent with the lensing mass from the Einstein radius of the system. We
find significant absorption in the soft X-ray spectrum of the faintest quasar
image, which can be caused by an intervening material at either the lens or
source redshift. The X-ray flux ratios between the quasar images (after
correcting for absorption) are in reasonable agreement with those at optical
and radio wavelengths, and all the flux ratios are inconsistent with those
predicted by simple mass models. This implies that microlensing effect is not
significant for this system and dark matter substructure is mainly responsible
for the anomalous flux ratios.Comment: 35 pages, 8 figures. Accepted for publication in Ap
Perturbation Study of the Conductance through an Interacting Region Connected to Multi-Mode Leads
We study the effects of electron correlation on transport through an
interacting region connected to multi-mode leads based on the perturbation
expansion with respect to the inter-electron interaction. At zero temperature
the conductance defined in the Kubo formalism can be written in terms of a
single-particle Green's function at the Fermi energy, and it can be mapped onto
a transmission coefficient of the free quasiparticles described by an effective
Hamiltonian. We apply this formulation to a two-dimensional Hubbard model of
finite size connected to two noninteracting leads. We calculate the conductance
in the electron-hole symmetric case using the order self-energy. The
conductance shows several maximums in the dependence in some parameter
regions of , where () is the hopping matrix element in the
- (-) directions. This is caused by the resonance occurring in some of
the subbands, and is related with the dependence of the eigenvalues of the
effective Hamiltonian.Comment: 17 pages, 12 figures, to be published in J.Phys.Soc.Jpn. 71(2002)No.
The Kondo crossover in shot noise of a single quantum dot with orbital degeneracy
We investigate out of equilibrium transport through an orbital Kondo system
realized in a single quantum dot, described by the multiorbital impurity
Anderson model. Shot noise and current are calculated up to the third order in
bias voltage in the particle-hole symmetric case, using the renormalized
perturbation theory. The derived expressions are asymptotically exact at low
energies. The resulting Fano factor of the backscattering current is
expressed in terms of the Wilson ratio and the orbital degeneracy as
at zero temperature. Then,
for small Coulomb repulsions , we calculate the Fano factor exactly up to
terms of order , and also carry out the numerical renormalization group
calculation for intermediate in the case of two- and four-fold degeneracy
(). As increases, the charge fluctuation in the dot is suppressed,
and the Fano factor varies rapidly from the noninteracting value to the
value in the Kondo limit , near the crossover region
, with the energy scale of the hybridization .Comment: 10 pages, 4 figure
Transport through a finite Hubbard chain connected to reservoirs
The dc conductance through a finite Hubbard chain of size N coupled to two
noninteracting leads is studied at T = 0 in an electron-hole symmetric case.
Assuming that the perturbation expansion in U is valid for small N (=1,2,3,...)
owing to the presence of the noninteracting leads, we obtain the self-energy at
\omega = 0 analytically in the real space within the second order in U. Then,
we calculate the inter-site Green's function which connects the two boundaries
of the chain, G_{N1}, solving the Dyson equation. The conductance can be
obtained through G_{N1}, and the result shows an oscillatory behavior as a
function of N. For odd N, a perfect transmission occurs independent of U. This
is due to the inversion and electron-hole symmetries, and is attributed to a
Kondo resonance appearing at the Fermi level. On the other hand, for even N,
the conductance is a decreasing function of N and U.Comment: 11 pages, RevTeX, 6 figures, to be published in Phys. Rev. B 59
(1999
Cosmological constant and time delay
The effect of the cosmological constant on the time delay caused by an
isolated spherical mass is calculated without using the lens equation and
compared to a recent observational bound on the time delay of the lensed quasar
SDSS J1004+4112.Comment: 8 pages, 1 figur
Determination of the phase shifts for interacting electrons connected to reservoirs
We describe a formulation to deduce the phase shifts, which determine the
ground-state properties of interacting quantum-dot systems with the inversion
symmetry, from the fixed-point eigenvalues of the numerical renormalization
group (NRG). Our approach does not assume the specific form of the Hamiltonian
nor the electron-hole symmetry, and it is applicable to a wide class of quantum
impurities connected to noninteracting leads. We apply the method to a triple
dot which is described by a three-site Hubbard chain connected to two
noninteracting leads, and calculate the dc conductance away from half-filling.
The conductance shows the typical Kondo plateaus of Unitary limit in some
regions of the gate voltages, at which the total number of electrons N_el in
the three dots is odd, i.e., N_el =1, 3 and 5. In contrast, the conductance
shows a wide minimum in the gate voltages corresponding to even number of
electrons, N_el = 2 and 4.
We also discuss the parallel conductance of the triple dot connected
transversely to four leads, and show that it can be deduced from the two phase
shifts defined in the two-lead case.Comment: 9 pages, 12 figures: Fig. 12 has been added to discuss T_
The Sloan Digital Sky Survey Quasar Lens Search. I. Candidate Selection Algorithm
We present an algorithm for selecting an uniform sample of gravitationally
lensed quasar candidates from low-redshift (0.6<z<2.2) quasars brighter than
i=19.1 that have been spectroscopically identified in the SDSS. Our algorithm
uses morphological and color selections that are intended to identify small-
and large-separation lenses, respectively. Our selection algorithm only relies
on parameters that the SDSS standard image processing pipeline generates,
allowing easy and fast selection of lens candidates. The algorithm has been
tested against simulated SDSS images, which adopt distributions of field and
quasar parameters taken from the real SDSS data as input. Furthermore, we take
differential reddening into account. We find that our selection algorithm is
almost complete down to separations of 1'' and flux ratios of 10^-0.5. The
algorithm selects both double and quadruple lenses. At a separation of 2'',
doubles and quads are selected with similar completeness, and above (below) 2''
the selection of quads is better (worse) than for doubles. Our morphological
selection identifies a non-negligible fraction of single quasars: To remove
these we fit images of candidates with a model of two point sources and reject
those with unusually small image separations and/or large magnitude differences
between the two point sources. We estimate the efficiency of our selection
algorithm to be at least 8% at image separations smaller than 2'', comparable
to that of radio surveys. The efficiency declines as the image separation
increases, because of larger contamination from stars. We also present the
magnification factor of lensed images as a function of the image separation,
which is needed for accurate computation of magnification bias.Comment: 15 pages, 17 figures, 4 tables, accepted for publication in A
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