1,529 research outputs found
Absorption spectrum in the wings of the potassium second resonance doublet broadened by helium
We have measured the reduced absorption coefficients occurring in the wings
of the potassium 4S-5P doublet lines at 404.414 nm and at 404.720 nm broadened
by helium gas at pressures of several hundred Torr. At the experimental
temperature of 900 K, we have detected a shoulder-like broadening feature on
the blue wing of the doublet which is relatively flat between 401.8 nm and
402.8 nm and which drops off rapidly for shorter wavelengths, corresponding to
absorption from the X doublet Sigma+ state to the C doublet Sigma+ state of the
K-He quasimolecule. The accurate measurements of the line profiles in the
present work will sharply constrain future calculations of potential energy
surfaces and transition dipole moments correlating to the asymptotes He-K(5p),
He-K(5s), and He-K(3d).Comment: 2 figure
Experimental and Theoretical Studies of Pressure Broadened Alkali-Metal Atom Resonance Lines
We are carrying out a joint theoretical and experimental research program to study the broadening of alkali atom resonance lines due to collisions with helium and molecular hydrogen for applications to spectroscopic studies of brown dwarfs and extrasolar giant planets
Valley Splitting Theory of SiGe/Si/SiGe Quantum Wells
We present an effective mass theory for SiGe/Si/SiGe quantum wells, with an
emphasis on calculating the valley splitting. The theory introduces a valley
coupling parameter, , which encapsulates the physics of the quantum well
interface. The new effective mass parameter is computed by means of a tight
binding theory. The resulting formalism provides rather simple analytical
results for several geometries of interest, including a finite square well, a
quantum well in an electric field, and a modulation doped two-dimensional
electron gas. Of particular importance is the problem of a quantum well in a
magnetic field, grown on a miscut substrate. The latter may pose a numerical
challenge for atomistic techniques like tight-binding, because of its
two-dimensional nature. In the effective mass theory, however, the results are
straightforward and analytical. We compare our effective mass results with
those of the tight binding theory, obtaining excellent agreement.Comment: 13 pages, 7 figures. Version submitted to PR
Probing microwave fields and enabling in-situ experiments in a transmission electron microscope.
A technique is presented whereby the performance of a microwave device is evaluated by mapping local field distributions using Lorentz transmission electron microscopy (L-TEM). We demonstrate the method by measuring the polarisation state of the electromagnetic fields produced by a microstrip waveguide as a function of its gigahertz operating frequency. The forward and backward propagating electromagnetic fields produced by the waveguide, in a specimen-free experiment, exert Lorentz forces on the propagating electron beam. Importantly, in addition to the mapping of dynamic fields, this novel method allows detection of effects of microwave fields on specimens, such as observing ferromagnetic materials at resonance
Efficient approach to solve the Bethe-Salpeter equation for excitonic bound states
Excitonic effects in optical spectra and electron-hole pair excitations are
described by solutions of the Bethe-Salpeter equation (BSE) that accounts for
the Coulomb interaction of excited electron-hole pairs. Although for the
computation of excitonic optical spectra in an extended frequency range
efficient methods are available, the determination and analysis of individual
exciton states still requires the diagonalization of the electron-hole
Hamiltonian . We present a numerically efficient approach for the
calculation of exciton states with quadratically scaling complexity, which
significantly diminishes the computational costs compared to the commonly used
cubically scaling direct-diagonalization schemes. The accuracy and performance
of this approach is demonstrated by solving the BSE numerically for the
Wannier-Mott two-band model in {\bf k} space and the semiconductors MgO and
InN. For the convergence with respect to the \vk-point sampling a general
trend is identified, which can be used to extrapolate converged results for the
binding energies of the lowest bound states.Comment: 13 pages, 12 figures, 1 table, submitted to PR
H-NMR Study of the Random Bond Effect in the Quantum Spin System (CH)CHNHCu(ClBr)
Spin-lattice relaxation rate of H-NMR has been measured in
(CH)CHNHCu(ClBr) with , which has been
reported to be gapped system with singlet ground state from the previous
macroscopic magnetization and specific heat measurements, in order to
investigate the bond randomness effect microscopically in the gapped composite
Haldane system (CH)CHNHCuCl. It was found that the spin-lattice
relaxation rate in the present system includes both fast and slow
relaxation parts indicative of the gapless magnetic ground state and the gapped
singlet ground state, respectively. We discuss the obtained results with the
previous macroscopic magnetization and specific heat measurements together with
the microscopic SR experiments.Comment: 4 pages, 2 figures, to be published in J. Phys. Soc. Jpn. vol.76
(2007) No.
Dilute gas of ultracold two-level atoms inside a cavity; generalized Dicke model
We consider a gas of ultracold two-level atoms confined in a cavity, taking
into account for atomic center-of-mass motion and cavity mode variations. We
use the generalized Dicke model, and analyze separately the cases of a
Gaussian, and a standing wave mode shape. Owing to the interplay between
external motional energies of the atoms and internal atomic and field energies,
the phase-diagrams exhibit novel features not encountered in the standard Dicke
model, such as the existence of first and second order phase transitions
between normal and superradiant phases. Due to the quantum description of
atomic motion, internal and external atomic degrees of freedom are highly
correlated leading to modified normal and superradiant phases.Comment: 10 pages, 7 figure
Development of a New DNA Marker for Fusarium Yellows Resistance in Brassica rapa Vegetables
In vegetables of Brassica rapa L., Fusarium oxysporum f. sp. rapae (For) or F. oxysporum f. sp. conglutinans (Foc) cause Fusarium yellows. A resistance gene against Foc (FocBr1) has been identified, and deletion of this gene results in susceptibility (focbr1-1). In contrast, a resistance gene against For has not been identified. Inoculation tests showed that lines resistant to Foc were also resistant to For, and lines susceptible to Foc were susceptible to For. However, prediction of disease resistance by a dominant DNA marker on FocBr1 (Bra012688m) was not associated with disease resistance of For in some komatsuna lines using an inoculation test. QTL-seq using four F2 populations derived from For susceptible and resistant lines showed one causative locus on chromosome A03, which covers FocBr1. Comparison of the amino acid sequence of FocBr1 between susceptible and resistant alleles (FocBr1 and FocBo1) showed that six amino acid differences were specific to susceptible lines. The presence and absence of FocBr1 is consistent with For resistance in F2 populations. These results indicate that FocBr1 is essential for For resistance, and changed amino acid sequences result in susceptibility to For. This susceptible allele is termed focbr1-2, and a new DNA marker (focbr1-2m) for detection of the focbr1-2 allele was develope
Theory of optical spectra of polar quantum wells: Temperature effects
Theoretical and numerical calculations of the optical absorption spectra of
excitons interacting with longitudinal-optical phonons in quasi-2D polar
semiconductors are presented. In II-VI semiconductor quantum wells, exciton
binding energy can be tuned on- and off-resonance with the longitudinal-optical
phonon energy by varying the quantum well width. A comprehensive picture of
this tunning effect on the temperature-dependent exciton absorption spectrum is
derived, using the exciton Green's function formalism at finite temperature.
The effective exciton-phonon interaction is included in the Bethe-Salpeter
equation. Numerical results are illustrated for ZnSe-based quantum wells. At
low temperatures, both a single exciton peak as well as a continuum resonance
state are found in the optical absorption spectra. By contrast, at high enough
temperatures, a splitting of the exciton line due to the real phonon absorption
processes is predicted. Possible previous experimental observations of this
splitting are discussed.Comment: 10 pages, 9 figures, to appear in Phys. Rev. B. Permanent address:
[email protected]
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