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61.
G. Uimin Y. Kudasov P. Fulde A. Ovchinnikov 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(2):241-244
We discuss the effects of an applied magnetic field on the low-energy excitations in the low temperature phase of Yb4As3. We show also why the magnetic interaction of the Yb3+ ions is nearly of an isotropic Heisenberg spin-1/2 type. A small anisotropy due to an intrachain dipolar interaction leads
to the opening of a gap when a magnetic field is applied. The model agrees with available experimental data. Simple experiments
are suggested in order to further test the present theory.
Received 3 February 1999 相似文献
62.
P. Fröbrich P.J. Jensen P.J. Kuntz A. Ecker 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):579-594
The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Green's
function theory by considering all components of the magnetization. We present a new method for the calculation of expectation
values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green's functions. This
formulation allows a straightforward extension of the monolayer case to thin films with many layers and for arbitrary spin
and moreover provides a practicable procedure for numerical computation. The model Hamiltonian includes a Heisenberg term,
an external magnetic field, a second-order uniaxial single-ion anisotropy, and the magnetic dipole-dipole coupling. We utilize
the Tyablikov (RPA) decoupling for the exchange interaction terms and the Anderson-Callen decoupling for the anisotropy terms.
The dipole coupling is treated in the mean-field approximation, a procedure which we demonstrate to be a sufficiently good
approximation for realistic coupling strengths. We apply the new method to monolayers with spin and to multilayer systems with S=1. We compare some of our results to those where mean-field theory (MFT) is applied to all interactions, pointing out some
significant differences.
Received 19 June 2000 and Received in final form 2 August 2000 相似文献
63.
We develop a phenomenological model of the double-phase-conjugate mirror grating formation based on geometrical considerations.
We show that the grating develops from one lateral side of the interaction zone towards the other side. This model is confirmed
by the behaviour of different configurations of a 2-zone double-phase-conjugate mirror. Finally, we obtained stable operation
in InP:Fe, this made it possible to implement a 1×8 switch between single-mode fibres at telecom wavelength with 12% global
efficiency and 200 ms reconfiguration time.
Received: 6 November 1998 / Revised version: 18 February 1999 / Published online: 7 April 1999 相似文献
64.
J.M. Lee K.G. Watkins W.M. Steen 《Applied Physics A: Materials Science & Processing》2000,71(6):671-674
Laser removal of small particles from a metal surface is carried out by changing the incident angle of the laser beam. It
has been found that a dramatic improvement of cleaning efficiency in terms of area and energy is observed when using the laser
at glancing angle of incidence as compared to perpendicular. Furthermore substrate damage is greatly reduced and probably
eliminated at glancing angles. The process mechanism is discussed by considering the adhesion and the laser-induced cleaning
forces for different incident angles. It is shown that there are different laser–matter interactions operating.
Received: 25 April 2000 / Accepted: 9 May 2000 / Published online: 5 October 2000 相似文献
65.
In this paper we will report the results obtained by femtosecond Pulsed Laser Deposition on vanadium carbide. These results, compared with those obtained for another group 5 carbide, tantalum carbide, evidence large analogies between the two systems. Optical emission spectroscopy shows in both cases the presence of particles in the secondary plume and in both cases the films are formed by nanoparticles and present a stoichiometry corresponding to the hemicarbide. 相似文献
66.
In an effort to establish the optimal parameters for the cleaning of complex layers of polymers (mainly based on acrylics, vinyls, epoxys known as Elvacite, Laropal, Paraloid B72, among others) applied during past conservation treatments on the surface of wall paintings, laser cleaning tests were performed with particular emphasis on the plausible morphological modifications induced in the remaining polymeric material. Pulse duration effects were studied using laser systems of different pulse durations (ns and fs) at 248 nm. Prior to tests on real fragments from the Monumental Cemetery in Pisa (Italy) which were coated with different polymers, attention was focused on the study of model samples consisting of analogous polymer films cast on quartz disks. Ultraviolet irradiation is strongly absorbed by the studied materials both in ns and fs irradiation regimes. However, it is demonstrated that ultrashort laser pulses result in reduced morphological alterations in comparison to ns irradiation. In addition, the dependence of the observed alterations on the chemical composition of the consolidation materials in both regimes was examined. Most importantly, it was shown that in this specific conservation problem, an optimum cleaning process may rely not only on the minimization of laser-induced morphological changes but also on the exploitation of the conditions that favour the disruption of the adhesion between the synthetic material and the painting. 相似文献
67.
Yanhua Han 《Applied Surface Science》2009,255(17):7524-7528
Two-dimensional ‘hat-scratch’ structures are fabricated on silica glass by the interference of three non-coplanar beams originating from a single femtosecond laser pulse. The scanning electron microscope (SEM) characterizations show that the as-formed structures are composed of hat holes and scratch marks. The experimental results indicate that the structures are dependent on the intensity of laser beam. The formation of the two-dimensional ‘hat-scratch’ structures is mainly due to the combined laser ablation effects including ionization, shock wave, plasma expansion, and phase explosion. 相似文献
68.
Sharon R. George 《Applied Surface Science》2009,255(24):9558-9561
Two important fluoropolymers, polytetrafluoroethylene [PTFE—(C2F4)N] and polyvinylidene fluoride [PVDF—(C2H2F2)N], respond to 157-nm laser radiation in dramatically different ways. At fluences sufficient to produce rapid etching, the volatile emissions from PTFE are dominated by (CF2)N fragments. The velocities of the fastest (CF2)N molecules at each mass are consistent with kinetic energies on the order of an electron volt—and change little with fluence. This fluence independence suggests that the velocities are not affected by collisions after emission. To account for the high kinetic energies and the unusual, half-monomer mass distribution, we propose that these fragments are produced by photochemical scission of the polymer backbone, and that a fraction of the excitation energy is delivered to each fragment as kinetic energy. In contrast, the principle neutral species from PVDF is HF. HF is produced by the scission of C-F bonds, followed by chemical reactions with nearby hydrogen. This process is accompanied by the conjugation of backbone C-C bonds. The photochemical cleavage of C-C bonds in PTFE and C-F bonds in PVDF is consistent with the lower C-C bond energy of PTFE. 相似文献
69.
For increasing the packing density of electronic devices and enabling 3D wiring, new concepts of interconnection for flexible circuit boards are required. The backside wiring is one innovative concept which, however, requires interconnections from the back to the front side by means of vias.Results on backside opening of polymer foils for exposing a thin metal film deposited at the front side are presented. For the experiments, a thin polyimide foil covered with a thin molybdenum metal film was used. By using mask projection of a pulsed UV-laser beam (248 nm, 20 ns) polymer foil was ablated. The laser ablation process must be adjusted in the manner to avoid damage of the thin metal film, to prevent cones formation at laser ablation, but still enabling the clean ablation of the polymer. The influence of process parameters on the backside opening is discussed and compared with theoretical estimations of the laser-induced temperatures. Using a two-step ablation process applying first high fluences to ablate the main part of the foil and finishing with low laser fluence turns out to be advantageous. This backside opening (BSO) can be used to perform an electrical contact from the backside. 相似文献
70.
We have observed several kinds of hydrocarbon cations after the nanosecond and the femtosecond laser ablation (nsLA and fsLA) of solid C60. The observation indicates that the carbon fragments produced just after laser ablation of the C60 molecule react with the hydrogen atoms and ions coexisting in the ablation plume. In the case of fsLA, clear dependence of the product hydrocarbon species on the ablation laser power has been observed although the dependence is not clearly observed in nsLA. The production of CnH5+ (n = 8, 10, and 12) is only observed in fsLA suggesting the unique nature of the transient carbon fragments produced by fsLA. 相似文献