1,322 research outputs found
Itinerant ferromagnetism in the multiorbital Hubbard model: a dynamical mean-field study
In order to resolve the long-standing issue of how the itinerant
ferromagnetism is affected by the lattice structure and Hund's coupling, we
have compared various three-dimensional lattice structures in the single- and
multiorbital Hubbard models with the dynamical mean-field theory with an
improved quantum Monte Carlo algorithm that preserves the spin-SU(2) symmetry.
The result indicates that {\it both} the lattice structure and the d-orbital
degeneracy are essential for the ferromagnetism in the parameter region
representing a transition metal. Specifically, (a) Hund's coupling, despite the
common belief, is important, which is here identified to come from
particle-hole scatterings, and (b) the ferromagnetism is a correlation effect
(outside the Stoner picture) as indicated from the band-filling dependence.Comment: 4 pages, 5 figure
Stochastic Liouville Equations for Femtosecond Stimulated Raman Spectroscopy
Electron and vibrational dynamics of molecules are commonly studied by
subjecting them to two interactions with a fast actinic pulse that prepares
them in a nonstationary state and after a variable delay period , probing
them with a Raman process induced by a combination of a broadband and a
narrowband pulse. This technique known as femtosecond stimulated Raman
spectroscopy (FSRS) can effectively probe time resolved vibrational resonances.
We show how FSRS signals can be modeled and interpreted using the stochastic
Liouville equations (SLE) originally developed for NMR lineshapes. The SLE
provides a convenient simulation protocol that can describe complex dynamics
due to coupling to collective coordinates at much lower cost that a full
dynamical simulation. The origin of the dispersive features which appear when
there is no separation of timescales between vibrational variations and
dephasing is clarified
Integration of fiber coupled high-Q silicon nitride microdisks with atom chips
Micron scale silicon nitride (SiN_x) microdisk optical resonators are
demonstrated with Q = 3.6 x 10^6 and an effective mode volume of 15 (\lambda /
n)^3 at near visible wavelengths. A hydrofluoric acid wet etch provides
sensitive tuning of the microdisk resonances, and robust mounting of a fiber
taper provides efficient fiber optic coupling to the microdisks while allowing
unfettered optical access for laser cooling and trapping of atoms. Measurements
indicate that cesium adsorption on the SiN_x surfaces significantly red-detunes
the microdisk resonances. A technique for parallel integration of multiple (10)
microdisks with a single fiber taper is also demonstrated.Comment: Published vesion. Minor change
Electronic structure of solid coronene: differences and commonalities to picene
We have obtained the first-principles electronic structure of solid coronene,
which has been recently discovered to exhibit superconductivity with potassium
doping. Since coronene, along with picene, the first aromatic superconductor,
now provide a class of superconductors as solids of aromatic compounds, here we
compare the two cases in examining the electronic structures. In the undoped
coronene crystal, where the molecules are arranged in a herringbone structure
with two molecules in a unit cell, the conduction band above an insulating gap
is found to comprise four bands, which basically originate from the lowest two
unoccupied molecular orbitals
(doubly-degenerate, reflecting the high symmetry of the molecular shape) in
an isolated molecule but the bands are entangled as in solid picene. The Fermi
surface for a candidate of the structure of Kcoronene with , for which
superconductivity is found, comprises multiple sheets, as in doped picene but
exhibiting a larger anisotropy with different topology.Comment: 5 pages, to be published in Phys. Rev.
Dynamical mean-filed approximation to small-world networks of spiking neurons: From local to global, and/or from regular to random couplings
By extending a dynamical mean-field approximation (DMA) previously proposed
by the author [H. Hasegawa, Phys. Rev. E {\bf 67}, 41903 (2003)], we have
developed a semianalytical theory which takes into account a wide range of
couplings in a small-world network. Our network consists of noisy -unit
FitzHugh-Nagumo (FN) neurons with couplings whose average coordination number
may change from local () to global couplings () and/or
whose concentration of random couplings is allowed to vary from regular
() to completely random (p=1). We have taken into account three kinds of
spatial correlations: the on-site correlation, the correlation for a coupled
pair and that for a pair without direct couplings. The original -dimensional {\it stochastic} differential equations are transformed to
13-dimensional {\it deterministic} differential equations expressed in terms of
means, variances and covariances of state variables. The synchronization ratio
and the firing-time precision for an applied single spike have been discussed
as functions of and . Our calculations have shown that with increasing
, the synchronization is {\it worse} because of increased heterogeneous
couplings, although the average network distance becomes shorter. Results
calculated by out theory are in good agreement with those by direct
simulations.Comment: 19 pages, 2 figures: accepted in Phys. Rev. E with minor change
Spontaneous dressed-state polarization in the strong driving regime of cavity QED
We utilize high-bandwidth phase quadrature homodyne measurement of the light
transmitted through a Fabry-Perot cavity, driven strongly and on resonance, to
detect excess phase noise induced by a single intracavity atom. We analyze the
correlation properties and driving-strength dependence of the atom-induced
phase noise to establish that it corresponds to the long-predicted phenomenon
of spontaneous dressed-state polarization. Our experiment thus provides a
demonstration of cavity quantum electrodynamics in the strong driving regime,
in which one atom interacts strongly with a many-photon cavity field to produce
novel quantum stochastic behavior.Comment: 4 pages, 4 color figure
SCIENCE TALK IN THE SECONDARY CLASSROOMS: ANALYSIS OF TEACHERS’ FEEDBACK
Feedback, the third part of Initiation-Response-Feedback (IRF) structure in typical lesson discussion, is the most crucial part of teaching and science talk. Feedback constructs cognitive scaffolding as well as dialogical pattern of discussion in the classroom. Several studies analyzing teachers’ feedback types and wait time of effect have been reported. Owing to its tremendous effect on teaching and learning, as stated by Chin (2007), a fine grained analysis has been felt. Video recorded data of fourteen science lessons in secondary level (Grade VI-X) of Bangladesh used as data source of this study. Data were analyzed with coded category. Through video analysis, the nine categories of teachers’ feedback were emerged to students’ correct and incorrect or no response. The prevalent nature of feedback was evaluative and corrective. All the generated categories were illustrated with example taken from the real lesson and tried to explain the effect of each type of feedback on lesson discussion. The results of the study are illustrated vignettes of the teachers’ varieties of feedback and the role of the feedback at secondary science lessons, and would be helpful for teachers to think and frame their practices that make a science lesson into collaborative, dialogic and facilitative one
Deterministic Dicke state preparation with continuous measurement and control
We characterize the long-time projective behavior of the stochastic master
equation describing a continuous, collective spin measurement of an atomic
ensemble both analytically and numerically. By adding state based feedback, we
show that it is possible to prepare highly entangled Dicke states
deterministically.Comment: Additional information is available at
http://minty.caltech.edu/Ensemble
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