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11.
Amit Sircar Rajesh K. Dwivedi R. K. Thareja 《Applied physics. B, Lasers and optics》1996,63(6):623-627
An experimental investigation of laser-induced breakdown using Nd:YAG laser harmonics for argon, nitrogen and oxygen gases is reported. Pressure dependence as well as wavelength dependence of the breakdown threshold irradianceI
th is investigated. The experimental observations for 1.064 and 0.532 m laser wavelengths are in agreement with theoretical calculations which include the effects of multiphoton ionization and cascade ionization. 相似文献
12.
S. Pflalzner 《Applied physics. B, Lasers and optics》1992,55(4):368-372
A new model is described for the absorption of laser light by a plasma. Two issues important for high laser intensity are considered: changes in the electron velocity distribution and the absorption non-linearity. The effects of anisotropy and non-Maxwellian distribution function are analysed with a hard sphere model. It is shown that anisotropy alters the electron-ion collision frequency by an amount which depends on the ratio of the temperatures in direction of and perpendicular to the laser field.The effect of high laser intensity is considered assuming Coulomb collisions, and a new expression obtained for the collision frequency as a function of quiver velocity. It will be shown that the result is in agreement with the Spitzer result in the case of small fields and with the result of Catto and Speziale in the case of very strong fields, but shows an unexpected maximum of the collision frequency in an intermediate regime. 相似文献
13.
The one-dimensional spin 1 bilinear-biquadratic model is re-expressed in a symmetrically frustrated SU(3) model, which facilitates us to introduce a fermionic representation and related bond-operator mean-field theory. By analyzing the gap and the static spin susceptibility, we shows that this treatment can easily capture the commensurate and incommensurate Haldane gap phases. 相似文献
14.
S. Mahdavifar 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(3):285-289
We have studied the behavior of the energy gap of the 1D AF
spin-
XXZ model in a transverse magnetic field (h)
using the exact diagonalization technique. The ground state phase
diagram consists of two spin-flop and paramagnetic phases. Using a
modified finite-size scaling approach, we have computed the
critical exponent of the energy gap in the vicinity of the
critical transverse field hc(Δ). Our numerical results
confirm that the continuous phase transition from the spin-flop
phase to the paramagnetic one is in the universality class of the
Ising model in the transverse field (ITF). By applying conformal
estimates of a small perturbation (h≪1), we have also
justified our numerical results. 相似文献
15.
P. F. Yu J. G. Cai J. M. Liu G. T. Shen 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,44(1):151-158
We investigate the phase decoherence effects on
the entanglement of a two-qubit anisotropic Heisenberg
model
with a nonuniform magnetic field in the x–z-plane. As a
measure of the entanglement, the concurrence of the system is
calculated. It is shown that when the magnetic field is along the
z-axis, the nonuniform and uniform components of the field have
no influence on the entanglement for the cases of
and
, respectively. But when
the magnetic field is not along the z-axis, both the uniform and
the nonuniform components of the field will introduce the
decoherence effects. It is found that the effects of the
Heisenberg chain's anisotropy in the Z-direction on the
entanglement are dependent on the direction of the field.
Moreover, the larger the initial concurrence is, the higher value
it will exhibit during the time evolution of the system for a
proper set of the parameters ν, Δ, θ, γ
, B and b. 相似文献
16.
We construct integrable generalized models in a systematic way exploring different representations of the gl(N) algebra. The models are then interpreted in the context of atomic and molecular physics, most of them related to different types of Bose–Einstein condensates. The spectrum of the models is given through the analytical Bethe ansatz method. We further extend these results to the case of the superalgebra gl(M|N), providing in this way models which also include fermions. 相似文献
17.
R. Zinke J. Schulenburg J. Richter 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(2):147-152
We investigate the ground-state magnetic long-range order of
quasi-one-dimensional quantum Heisenberg antiferromagnets for spin
quantum numbers s = 1/2 and s = 1. We use the coupled
cluster method to calculate the sublattice magnetization and
its dependence on the inter-chain coupling J⊥.
We find that for the unfrustrated spin-1/2 system,
an infinitesimal inter-chain coupling is sufficient to stabilize
magnetic long-range order, in agreement with results obtained
by other methods.
For s = 1, we find that a finite inter-chain coupling
is necessary to stabilize magnetic long-range order.
Furthermore, we consider a quasi one-dimensional spin-1/2 system,
where a frustrating next-nearest neighbor in-chain coupling is included.
We find that for stronger frustration as well, a finite inter-chain coupling
is necessary to have magnetic long-range order in the ground state, and that the strength of the inter-chain coupling necessary
to establish magnetic long-range order is related to the size of the spin gap of the isolated chain. 相似文献
18.
G. Cristoforetti S. Legnaioli V. Palleschi A. Salvetti E. Tognoni 《Applied physics. B, Lasers and optics》2005,80(4-5):559-568
A collinear double pulse laser-induced plasma was characterised by means of a spectrally and time resolved imaging technique. The beams of two Q-switched Nd:YAG lasers were focused on a brass target in a vacuum chamber to form the plasma. The plume emission intensity and spatial distribution were recorded with temporal resolution using an intensified CCD. Using a set of interference filters, we collected images of the emission from the major target components as well as from oxygen. Both the laser inter-pulse separation (in the range between 0 and 10 s) and the ambient air pressure value (in the range between 105 and 10 Pa) were varied during the experiment. At atmospheric pressure, an enhancement of the line emission from the target elements was observed for delayed laser pulses compared to coincident pulses. However, this enhancement effect tends to fall at low pressure values, and a decrease of the signal is observed for pressures under about 104 Pa. Moreover, it was observed that the evaluation of the enhancement factor strongly depends on the detector field of view. The propagation of the emitting plume was also studied at several pressures and inter-pulse delays. 相似文献
19.
Vanja Margetic Ticijana Ban Ota Samek Franz Leis Kay Niemax Roland Hergenröder 《Czechoslovak Journal of Physics》2004,54(4):423-429
Fluorescence measurements have been used to characterize the velocity of atoms in a femtosecond-laser-produced plasma. Nanogram
amounts of a copper sample were ablated by the focused radiation (λ=775 nm) of an all-solid-state laser. The laser was operated
at a pulse rate of 10 Hz with an energy of 200μJ per pulse. The microplasma expanded into a defined argon atmosphere of pressures between 0.02 and 850 mbar. Atomic fluorescence
was excited in the laser plume by a dye-laser pulse with the wavelength set to the line Cu I 282.4 nm. The narrowed beam of
the dye-laser was directed into the plasma at different heights above the sample surface. The fluorescence radiation was measured
with an échelle-spectrometer, equipped with an intensified-charge-coupled device as the detector. The velocity depends strongly
on the pressure of the ambient atmosphere and the distance from the sample surface. The highest velocity found at an argon
pressure of 0.02 mbar was 1.0×106 cm s−1. 相似文献
20.
Düsterer S. Schwoerer H. Ziegler W. Salzmann D. Sauerbrey R. 《Applied physics. B, Lasers and optics》2003,76(1):17-21
The influence of prepulses on the conversion efficiency (CE) of laser radiation into 13 nm extreme ultraviolet (EUV) radiation,
resulting from lithium-like oxygen ions from O20 μm water droplets, was investigated. The laser pulse durations in the experiment
ranged from 200 fs to 120 ps. Applying prepulses preceding the main pulse at various delays of up to 11 ns, it was shown that
the CE increases differently for each measured pulse duration. The strongest dependence on the introduction of a prepulse
was observed for 2 ps laser pulses with a 20 mJ laser pulse energy. The EUV CE was improved by a factor of 15 by the introduction
of a prepulse. Calculations on the atomic physics of oxygen ions and simulations of the laser–plasma interaction revealed
the influence of the prepulse on the EUV yield.
Received: 25 October 2002 / Published online: 22 January 2003
RID="*"
ID="*"Corresponding author. Fax: +49-3641/947-202, E-mail: duesterer@ioq.uni-jena.de 相似文献