28,212 research outputs found
Effect of the Kondo correlation on Shot Noise in a Quantum Dot
The current noise in a quantum dot coupled to two leads is investigated in
the Kondo regime with and without the influence of magnetic fields by employing
a finite- slave-boson mean field theory to calculate the current-current
correlation function at zero temperature. The numerical results show that,
depending on the energy level of the QD the Coulomb interactin may reduce or
enhance the zero-frequency shot noise power spectrum in comparison with that
without Coulomb interaction. But the Fano factor is always reduced
significantly due to the Kondo-correlation effect, and the most pronounced
suppression appears at the electron-hole symmetry case . In
addition, the application of a magnetic field enhance the Fano factor of the
shot noise at small external voltage, which is attributed to the reduction of
the Kondo-enhanced density-of-state and transmission probability in a quantum
dot.Comment: accepted by J. Phys.: Condensed Matte
Pumped spin-current and shot noise spectra in a single quantum dot
We exploit the pumped spin-current and current noise spectra under
equilibrium condition in a single quantum dot connected to two normal leads, as
an electrical scheme for detection of the electron spin resonance (ESR) and
decoherence. We propose spin-resolved quantum rate equations with correlation
functions in Laplace-space for the analytical derivation of the zero-frequency
atuo- and cross-shot noise spectra of charge- and spin-current. Our results
show that in the strong Coulomb blockade regime, ESR-induced spin flip
generates a finite spin-current and the quantum partition noises in the absence
of net charge transport. Moreover, spin shot noise is closely related to the
magnetic Rabi frequency and decoherence and would be a sensitive tool to
measure them.Comment: 4 pages, 3 figures, to be published in Phys. Rev. Lette
Quantum rate equations for electron transport through an interacting system in the sequential tunneling regime
We present a set of modified quantum rate equations, with the help of the
nonequilibrium Green's function and slave-particle techniques along with the
correct quantization, for description of the quantum transport through an
interacting mesoscopic region connected with two leads, in the sequential
tunneling regime. The assumption that only leading order of ( is the
tunneling coupling between the interacting central region and the leads) has
been taken into account in deriving these equations implies that the quantum
rate equations are only valid in the case of weak coupling between the central
region and the leads. For demonstrations, we consider two special cases in the
central region, a single interacting quantum dot (SQD) with weak spin-flip
scattering and a weakly coupled double quantum dots (CQD), as examples. In the
limit of zero temperature and large bias voltage, the resulting equations are
identical to the previous results derived from the many-body Schr\"odinger
equation. The numerical simulations reveal: 1) the dependence of the spin-flip
scattering on the temperature and bias voltage in the SQD; and 2) the possible
negative differential conductance and negative tunnel magnetoresistance in the
CQD, depending on the hopping between the two quantum dots.Comment: 16 pages, 10 figures,some minor changes, to be published in Phys.
Rev.
Is monogamy of entanglement geometrical?
This work aims to understand the monogamy of quantum entanglement from a
geometrical point of view. By regarding quantum entanglement as a geometrical
structure on the state space of quantum systems and attributing all
entanglement related properties as emergent from this geometry of entanglement,
we assume there exists a genuine general monogamous relation of quantum
entanglement w.r.t. a correspondent genuine entanglement measure Q* which
possesses an underlying geometrical origin. We speculate that the monogamous
relations w.r.t. an entanglement measure Q can be understood by comparing the
different dimension dependencies of the measure Q and Q*. We gave evidences of
our conjecture by readdressing two observed properties of the monogamy
relations from this geometrical standpoint. Besides the phenomenal explanation
of the monogamy of entanglement, we also discussed a fibre bundle structure
based candidate solution for the geometry of entanglement and explained how
this idea is related to the ER=EPR conjecture and other interesting quantum
information processing problems including monogamy of entanglement,
entanglement distillation, bound entanglement and activation, and entanglement
catalyst.Comment: 23 pages, 8 figure
Spin-polarized transport through a quantum dot coupled to ferromagnetic leads: Kondo correlation effect
We investigate the linear and nonlinear transport through a single level
quantum dot connected to two ferromagnetic leads in Kondo regime, using the
slave-boson mean field approach for finite on-site Coulomb repulsion. We find
that for antiparallel alignment of the spin orientations in the leads, a single
zero-bias Kondo peak always appears in the voltage-dependent differential
conductance with peak height going down to zero as the polarization grows to
P=1. For parallel configuration, with increasing polarization from zero, the
Kondo peak descends and greatly widens by the appearance of shoulders, and
finally splits into two peaks on both sides of the bias voltage around until disappears at even larger polarization strength. At any spin
orientation angle , the linear conductance generally drops with growing
polarization strength. For a given finite polarization, the minimum linear
conductance always appears at .Comment: 7 pages, 8 figure
Full counting statistics of a single-molecular quantum dot
We investigate the full counting statistics of a single quantum dot strongly
coupled to a local phonon and weakly tunnel-connected to two metallic
electrodes. By employing the generalized nonequilibrium Green function method
and the Lang-Firsov transformation, we derive an explicit analytical formula
for the cumulant generating function, which makes one to be able to identify
distinctly the elastic and inelastic contributions to the current and
zero-frequency shot noise. We find that at zero temperature, the inelastic
effect causes upward steps in the current and downward jumps in the noise at
the bias voltages corresponding to the opening of the inelastic channels, which
are ascribed to the vibration-induced complex dependences of electronic
self-energies on the energy and bias voltage. More interestingly, the Fano
factor exhibits oscillatory behavior with increasing bias voltage and its
minimum value is observed to be smaller than one half.Comment: 12 pages, 6 figure
Inelatic cotunneling current and shot noise of an interacting quantum dot with ferromagnetic correlations
We explore inelastic cotunneling through a strongly Coulomb-blockaded quantum
dot attached to two ferromagnetic leads in the weak coupling limit using a
generic quantum Langevin equation approach. We first develop a Bloch-type
equation microscopically to describe the cotunneling-induced spin relaxation
dynamics, and then develop explicit analytical expressions for the local
magnetization, current, and its fluctuations. On this basis, we predict a novel
zero-bias anomaly of the differential conductance in the absence of a magnetic
field for the anti-parallel configuration, and asymmetric peak splitting in a
magnetic field. Also, for the same system with large polarization, we find a
negative zero-frequency differential shot noise in the low positive
bias-voltage region. All these effects are ascribed to rapid spin-reversal due
to underlying spin-flip cotunneling.Comment: 7 pages, 5 figures, submitte
Elimination of negative differential conductance in an asymmetric molecular transistor by an ac-voltage
We analyze resonant tunneling subject to a non-adiabatic time-dependent
bias-voltage through an asymmetric single molecular quantum dot with coupling
between the electronic and vibrational degrees of freedom using a {\em
Tien-Gordon-type} rate equation. Our results clearly exhibit the appearance of
photon-assisted satellites in the current-voltage characteristics and the
elimination of hot-phonon-induced negative differential conductance with
increasing ac driving amplitude for an asymmetric system. This can be ascribed
to an {\em ac-induced suppression} of unequilibrated (hot) phonons in an
asymmetric system.Comment: Accepted by Appl. Phys. Let
Optimal Lyapunov quantum control on two-level systems: convergence and extended techniques
Taking a two-level system as an example, we show that a strong control field
may enhance the efficiency of optimal Lyapunov quantum control in [Hou et al.,
Phys. Rev. A \textbf{86}, 022321 (2012)] but could decrease its control
fidelity. A relationship between the strength of the control field and the
control fidelity is established. An extended technique, which combines free
evolution and external control, is proposed to improve the control fidelity. We
analytically demonstrate that the extended technique can be used to design a
control law for steering a two-level system exactly to the target state. In
such a way, the convergence of the extended optimal Lyapunov quantum control
can be guaranteed.Comment: 9 Pages, 7 Figure
Chandra and MMT observations of low-mass black hole active galactic nuclei accreting at low rates in dwarf galaxies
We report on Chandra X-ray observations of four candidate low-mass black hole
(<10^6Msun) active galactic nuclei (AGNs) that have the estimated Eddington
ratios among the lowest (~10^(-2)) found for this class. The aims are to
validate the nature of their AGNs and to confirm the low Eddington ratios that
are derived from the broad H_alpha line, and to explore this poorly studied
regime in the AGN parameter space. Among them, two objects with the lowest
significance of the broad lines are also observed with Multi-Mirror Telescope,
and the high-quality optical spectra taken confirm them as Seyfert 1 AGNs and
as having small black hole masses. X-ray emission is detected from the nuclei
of two of the galaxies, which is variable on timescales of 10^3s, whereas no
significant (or only marginal at best) detection is found for the remaining
two. The X-ray luminosities are on the order of 10^(41) ergs/s or even lower,
on the order of 10^(40) ergs/s for non-detections, which are among the lowest
regimes ever probed for Seyfert galaxies. The low X-ray luminosities, compared
to their black hole masses derived from H_alpha, confirm their low accretion
rates assuming typical bolometric corrections. Our results hint at the
existence of a possibly large population of under-luminous low-mass black holes
in the local universe. An off-nucleus ultra-luminous X-ray source (ULX) in one
of the dwarf galaxies is detected serendipitously, with a luminosity
(6-9)x10^(39) ergs/s in 2-10 keV.Comment: ApJ in press (February 2014 issue), 6 figure
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