13,205 research outputs found
Resonant tunneling and Fano resonance in quantum dots with electron-phonon interaction
We theoretically study the resonant tunneling and Fano resonance in quantum
dots with electron-phonon (e-ph) interaction. We examine the bias-voltage ()
dependence of the decoherence, using Keldysh Green function method and
perturbation with respect to the e-ph interaction. With optical phonons of
energy , only the elastic process takes place when , in
which electrons emit and absorb phonons virtually. The process suppresses the
resonant amplitude. When , the inelastic process is possible which
is accompanied by real emission of phonons. It results in the dephasing and
broadens the resonant width. The bias-voltage dependence of the decoherence
cannot be obtained by the canonical transformation method to consider the e-ph
interaction if its effect on the tunnel coupling is neglected. With acoustic
phonons, the asymmetric shape of the Fano resonance grows like a symmetric one
as the bias voltage increases, in qualitative accordance with experimental
results.Comment: 28 pages, 11 figure
Dilute-Bose-Gas Approach to ground state phases of 3D quantum helimagnets under high magnetic field
We study high-field phase diagram and low-energy excitations of
three-dimensional quantum helimagnets. Slightly below the saturation field, the
emergence of magnetic order may be mathematically viewed as Bose-Einstein
condensation (BEC) of magnons. The method of dilute Bose gas enables an
unbiased quantitative analysis of quantum effects in three-dimensional
helimagnets and thereby three phases are found: cone, coplanar fan and an
attraction-dominant one. To investigate the last phase, we extend the usual BEC
approach so that we can handle 2-magnon bound states. In the case of 2-magnon
BEC, the transverse magnetization vanishes and long-range order occurs in the
quadrupolar channel (spin-nematic phase). As an application, we map out the
phase diagram of a 3D helimagnet which consists of frustrated J1-J2 chains
coupled by an interchain interaction J3.Comment: 4pages, 3figures, International Conference on Magnetism (ICM) 2009
(Karlsruhe, Germany, July 26-31, 2009)
Crossover trimers connecting continuous and discrete scaling regimes
For a system of two identical fermions and one distinguishable particle
interacting via a short-range potential with a large s-wave scattering length,
the Efimov trimers and Kartavtsev-Malykh trimers exist in different regimes of
the mass ratio. The Efimov trimers are known to exhibit a discrete scaling
invariance, while the Kartavtsev-Malykh trimers feature a continuous scaling
invariance. We point out that a third type of trimers, "crossover trimers",
exist universally regardless of short-range details of the potential. These
crossover trimers have neither the discrete nor continuous scaling invariance.
We show that the crossover trimers continuously connect the discrete and
continuous scaling regimes as the mass ratio and the scattering length are
varied. We identify the regions for the Kartavtsev-Malykh trimers, Efimov
trimers, crossover trimers, and non-universal trimers as a function of the mass
ratio and the s-wave scattering length by investigating the scaling property
and model-independence of the trimers.Comment: 14 pages, 9 figure
Phonon Spectroscopy by Electric Measurements of Coupled Quantum Dots
We propose phonon spectroscopy by electric measurements of the
low-temperature conductance of coupled-quantum dots, specifically employing
dephasing of the quantum electronic transport by the phonons. The setup we
consider consists of a T-shaped double-quantum-dot (DQD) system in which only
one of the dots (dot 1) is connected to external leads and the other (dot 2) is
coupled solely to the first one. For noninteracting electrons, the differential
conductance of such a system vanishes at a voltage located in-between the
energies of the bonding and the anti-bonding states, due to destructive
interference. When electron-phonon (e-ph) on the DQD is invoked, we find that,
at low temperatures, phonon emission taking place on dot 1 does not affect the
interference, while phonon emission from dot 2 suppresses it. The amount of
this suppression, as a function of the bias voltage, follows the effective e-ph
coupling reflecting the phonon density of states and can be used for phonon
spectroscopy.Comment: 9 pages, 6 figure
Comoving Space Density and Obscured Fraction of High-Redshift Active Galactic Nuclei in the Subaru/{\it XMM-Newton} Deep Survey
We study the comoving space density of X-ray-selected luminous active
galactic nuclei (AGNs) and the obscured AGN fraction at high redshifts () in the Subaru/{\it XMM-Newton} Deep Survey (SXDS) field. From an X-ray
source catalog with high completeness of optical identification thanks to deep
optical images, we select a sample of 30 AGNs at with intrinsic
(de-absorbed and rest-frame 2--10 keV) luminosities of
erg s detected in the 0.5--2 keV band, consisting of 20 and 10 objects
with spectroscopic and photometric redshifts, respectively. Utilizing the
method, we confirm that the comoving space density of luminous
AGNs decreases with redshift above . When combined with the {\it
Chandra}-COSMOS result of Civano et al.\ (2011), the density decline of AGNs
with erg s is well represented by a power law
of . We also determine the fraction of X-ray obscured
AGNs with cm in the Compton-thin population to be
0.54, by carefully taking into account observational biases
including the effects of photon statistics for each source. This result is
consistent with an independent determination of the type-2 AGN fraction based
on optical properties, for which the fraction is found to be 0.590.09.
Comparing our result with that obtained in the local Universe, we conclude that
the obscured fraction of luminous AGNs increases significantly from to
by a factor of 2.51.1.Comment: 12 pages, 12 figures, 1 table. Accepted for publication in Ap
Suzaku Confirms NGC~3660 is an Unabsorbed Seyfert 2
An enigmatic group of objects, unabsorbed Seyfert 2s may have intrinsically
weak broad line regions, obscuration in the line of sight to the BLR but not to
the X-ray corona, or so much obscuration that the X-ray continuum is completely
suppressed and the observed spectrum is actually scattered into the line of
sight from nearby material. NGC 3660 has been shown to have weak broad
optical/near infrared lines, no obscuration in the soft X-ray band, and no
indication of "changing look" behavior. The only previous hard X-ray detection
of this source by Beppo-SAX seemed to indicate that the source might harbor a
heavily obscured nucleus. However, our analysis of a long-look Suzaku
observation of this source shows that this is not the case, and that this
source has a typical power law X-ray continuum with normal reflection and no
obscuration. We conclude that NGC 3660 is confirmed to have no unidentified
obscuration and that the anomolously high Beppo-SAX measurement must be due to
source confusion or similar, being inconsistent with our Suzaku measurements as
well as non-detections from Swift-BAT and RXTE.Comment: Accepted to PAS
Many body effects in finite metallic carbon nanotubes
The non homogeneity of the charge distribution in a carbon nanotube leads to
the formation of an excitonic resonance, in a similar way to the one observed
in X-ray absorption in metals. As a result, a positive anomaly at low bias
appears in the tunnelling density of states. This effect depends on the
screening of the electron--electron interactions by metallic gates, and it
modifies the coupling of the nanotube to normal and superconducting electrodes.Comment: 5 page
Precision Measurements Using Squeezed Spin States via Two-axis Counter-twisting Interactions
We show that the two-axis counter twisting interaction squeezes a coherent
spin state into three states of interest in quantum information, namely, the
twin-Fock state, the equally-weighted superposition state, and the state that
achieves the Heisenberg limit of optimal sensitivity defined by the Cramer-Rao
inequality in addition to the well-known Heisenberg-limited state of spin
fluctuations.Comment: 5 pages, 3 figure
Population of individual Ne 1s<sup>-1</sup>np Rydberg states measured across the 1s ionization threshold and the evolution of shake-up into shake-down
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