21,613 research outputs found
Spherical to deformed shape transition in the nucleon-pair shell model
A study of the shape transition from spherical to axially deformed nuclei in
the even Ce isotopes using the nucleon-pair approximation of the shell model is
reported. As long as the structure of the dominant collective pairs is
determined using a microscopic framework appropriate to deformed nuclei, the
model is able to produce a shape transition. However, the resulting transition
is too rapid, with nuclei that should be transitional being fairly well
deformed, perhaps reflecting the need to maintain several pairs with each
angular momentum.Comment: 7 pages, 5 figure
A Herschel Study of 24 micron-Selected AGNs and Their Host Galaxies
We present a sample of 290 24-micron-selected active galactic nuclei (AGNs)
mostly at z ~ 0.3 -- 2.5, within 5.2 square degrees distributed as 25' X 25'
fields around each of 30 galaxy clusters in the Local Cluster Substructure
Survey (LoCuSS). The sample is nearly complete to 1 mJy at 24 microns, and has
a rich multi-wavelength set of ancillary data; 162 are detected by Herschel. We
use spectral templates for AGNs, stellar populations, and infrared emission by
star forming galaxies to decompose the spectral energy distributions (SEDs) of
these AGNs and their host galaxies, and estimate their star formation rates
(SFRs), AGN luminosities, and host galaxy stellar masses. The set of templates
is relatively simple: a standard Type-1 quasar template; another for the
photospheric output of the stellar population; and a far infrared star-forming
template. For the Type-2 AGN SEDs, we substitute templates including internal
obscuration, and some Type-1 objects require a warm component (T > 50 K). The
individually Herschel- detected Type-1 AGNs and a subset of 17 Type-2 ones
typically have luminosities > 10^{45} ergs/s, and supermassive black holes of ~
3 X 10^8 Msun emitting at ~ 10% of the Eddington rate. We find them in about
twice the numbers of AGN identified in SDSS data in the same fields, i.e., they
represent typical high luminosity AGN, not an infrared-selected minority. These
AGNs and their host galaxies are studied further in an accompanying paper
Counting statistics of tunneling through a single molecule: effect of distortion and displacement of vibrational potential surface
We analyze the effects of a distortion of the nuclear potential of a
molecular quantum dot (QD), as well as a shift of its equilibrium position, on
nonequilibrium-vibration-assisted tunneling through the QD with a single level
() coupled to the vibrational mode. For this purpose, we derive an
explicit analytical expression for the Franck-Condon (FC) factor for a
displaced-distorted oscillator surface of the molecule and establish rate
equations in the joint electron-phonon representation to examine the
current-voltage characteristics and zero-frequency shot noise, and skewness as
well. Our numerical analyses shows that the distortion has two important
effects. The first one is that it breaks the symmetry between the excitation
spectra of the charge states, leading to asymmetric tunneling properties with
respect to and . Secondly, distortion (frequency
change of the oscillator) significantly changes the voltage-activated cascaded
transition mechanism, and consequently gives rise to a different nonequilibrium
vibrational distribution from that of the case without distortion. Taken in
conjunction with strongly modified FC factors due to distortion, this results
in some new transport features: the appearance of strong NDC even for a
single-level QD with symmetric tunnel couplings; a giant Fano factor even for a
molecule with an extremely weak electron-phonon interaction; and enhanced
skewness that can have a large negative value under certain conditions.Comment: 29 pages, 11 figures, published versio
The ratchet effect and the transporting islands in the chaotic sea
We study directed transport in a classical deterministic dissipative system.
We consider the generic case of mixed phase space and show that large ratchet
currents can be generated thanks to the presence, in the Hamiltonian limit, of
transporting stability islands embedded in the chaotic sea. Due to the
simultaneous presence of chaos and dissipation the stationary value of the
current is independent of initial conditions, except for initial states with
very small measure.Comment: 5 pages, 6 figure
Mesoscopic Kondo effect of a quantum dot embedded in an Aharonov-Bohm ring with intradot spin-flip scattering
We study the Kondo effect in a quantum dot embedded in a mesoscopic ring
taking into account intradot spin-flip scattering . Based on the finite-
slave-boson mean-field approach, we find that the Kondo peak in the density of
states is split into two peaks by this coherent spin-flip transition, which is
responsible for some interesting features of the Kondo-assisted persistent
current circulating the ring: (1) strong suppression and crossover to a sine
function form with increasing ; (2) appearance of a "hump" in the
-dependent behavior for odd parity. -induced reverse of the persistent
current direction is also observed for odd parity.Comment: 7 pages,6 figures, to be published by Europhys. Let
Self-induced and induced transparencies of two-dimensional and three- dimensional superlattices
The phenomenon of transparency in two-dimensional and three-dimensional
superlattices is analyzed on the basis of the Boltzmann equation with a
collision term encompassing three distinct scattering mechanisms (elastic,
inelastic and electron-electron) in terms of three corresponding distinct
relaxation times. On this basis, we show that electron heating in the plane
perpendicular to the current direction drastically changes the conditions for
the occurrence of self-induced transparency in the superlattice. In particular,
it leads to an additional modulation of the current amplitudes excited by an
applied biharmonic electric field with harmonic components polarized in
orthogonal directions. Furthermore, we show that self-induced transparency and
dynamic localization are different phenomena with different physical origins,
displaced in time from each other, and, in general, they arise at different
electric fields.Comment: to appear in Physical Review
Fully Gapped Superconducting State Based on a High Normal State Quasiparticle Density of States in BaKFeAs Single Crystals
We report the specific heat (SH) measurements on single crystals of hole
doped -based superconductor . It is found that
the electronic SH coefficient is not temperature dependent and
increases almost linearly with the magnetic field in low temperature region.
These point to a fully gapped superconducting state. Surprisingly the sharp SH
anomaly reaches a value of 98 suggesting a
very high normal state quasiparticle density of states (). A detailed analysis reveals that the cannot be
fitted with a single gap of s-wave symmetry due to the presence of a hump in
the middle temperature region. However, our data indicate that the dominant
part of the superconducting condensate is induced by an s-wave gap with the
magnitude of about 6 meV.Comment: 5 pages, 5 figure
Role of semicore states in the electronic structure of group-III nitrides: An exact exchange study
The bandstructure of the zinc-blende phase of AlN, GaN, InN is calculated
employing the exact-exchange (EXX) Kohn-Sham density-functional theory and a
pseudopotential plane-wave approach. The cation semicore d electrons are
treated both as valence and as core states. The EXX bandgaps of AlN and GaN
(obtained with the Ga 3d electrons included as core states) are in excellent
agreement with previous EXX results, GW calculations and experiment. Inclusion
of the semicore d electrons as valence states leads to a large reduction in the
EXX bandgaps of GaN and InN. Contrary to common belief, the removal of the
self-interaction, by the EXX approach, does not account for the large
disagreement for the position of the semicore d electrons between the LDA
results and experiment.Comment: 10 pages including 3 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
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Light-Induced Currents at Domain Walls in Multiferroic BiFeO3.
Multiferroic BiFeO3 (BFO) films with spontaneously formed periodic stripe domains can generate above-gap open circuit voltages under visible light illumination; nevertheless the underlying mechanism behind this intriguing optoelectronic response has not been understood to date. Here, we make contact-free measurements of light-induced currents in epitaxial BFO films via detecting terahertz radiation emanated by these currents, enabling a direct probe of the intrinsic charge separation mechanisms along with quantitative measurements of the current amplitudes and their directions. In the periodic stripe samples, we find that the net photocurrent is dominated by the charge separation across the domain walls, whereas in the monodomain samples the photovoltaic response arises from a bulk shift current associated with the non-centrosymmetry of the crystal. The peak current amplitude driven by the charge separation at the domain walls is found to be 2 orders of magnitude higher than the bulk shift current response, indicating the prominent role of domain walls acting as nanoscale junctions to efficiently separate photogenerated charges in the stripe domain BFO films. These findings show that domain-wall-engineered BFO thin films offer exciting prospects for ferroelectric-based optoelectronics, as well as bias-free strong terahertz emitters
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