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1.
The possibility of using magic Si7 clusters to form a cluster material was studied experimentally and theoretically. In experiments Si7 clusters were deposited on carbon surfaces, and the electronic structure and chemical properties of the deposited clusters were measured using X-ray photoelectron spectroscopy (XPS). A non bulk-like electronic structure of Si7 was found in the Si 2p core level spectra. Si7 is suggested to form a more stable structure than the non-magic Si8 cluster and Si atoms upon deposition on carbon surfaces. Theoretically it was possible to study the interaction between the clusters without the effect of a surface. Density functional theory (DFT) calculations of potential curves of two free Si7 clusters approaching each other in various orientations hint at the formation of cluster materials rather than the fusion of clusters forming bulk-like structures.  相似文献
2.
We present numerical calculations of the spin transfer torque resulting in current-induced domain wall motion. Rather than the conventional micromagnetic finite difference or finite element method, we use an atomistic/classical Heisenberg spin model approach, which is well suited to study geometrically confined domain walls. We compute the behaviour of domain walls in a one dimensional chain when currents are injected using adiabatic and non-adiabatic spin torque terms. Our results are compared to analytical calculations and are found to agree very well for small current densities. At larger current densities deviations are observed, which can be attributed to the approximations used in the analytical calculations.  相似文献
3.
We study via Monte Carlo simulations the influence of elastic interactions on the ordering and decomposition of a two-dimensional model binary alloy with antiferromagnetic nearest and ferromagnetic next nearest neighbor type interactions following a quench into the coexistence region. The elastic interaction leads to the development of a platelet morphology for the segregated ordered and disordered regions. A length scale characterizing the coarsening process follows a law of the type R=a+bt 1/3 with the growth b decreasing with the amount of ordered phase; this appears to be due to the presence of anti-phase boundaries between neighboring domains ordered on different sublattices which are difficult to eliminate. The application of uniaxial external stress results in rafting of the domains. Many of the simulation results are in agreement with experimentally observed effects in nickel-base superalloys.  相似文献
4.
,Static and dynamic properties of the Potts model on the simple cubic lattice with nearest neighbor -interaction are obtained from Monte Carlo simulations in a temperature range where full thermal equilibrium still can be achieved (). For a lattice size L = 16, in this range finite size effects are still negligible, but the data for the spin glass susceptibility agree with previous extrapolations based on finite size scaling of very small lattices. While the static properties are compatible with a zero temperature transition, they certainly do not prove it. Unlike the Ising spin glass, the decay of the time-dependent order parameter is compatible with a simple Kohlrausch function, , while a power law prefactor cannot be distinguished. The Kohlrausch exponent y ( T ) decreases from at [0pt] to at [0pt] however. The relaxation time is compatible with the exponential divergence postulated by McMillan for spin glasses at their lower critical dimension, but the exponent that can be extracted still differs significantly from the theoretical value, . Thus the present results support the conclusion that the Potts spin glass in d = 3 dimensions differs qualitatively from the Ising spin glass. Received: 8 October 1997 / Accepted: 27 November 1997  相似文献
5.
The finite-size scaling analysis of the density distribution function of subsystems of a system studied at constant total density is studied by a comparative investigation of two models: (i) the nearest-neighbor lattice gas model on the square lattice, choosing a total lattice size of 64×64 sites. (ii) The two-dimensional off-lattice Lennard-Jones system (truncated at a distance of 2.5 σ, σ being the range parameter of the interaction) withN=4096 particles, applying the NVT ensemble. In both models, the density distribution functionP L (ρ) is obtained forL×L subsystems for a wide range of temperaturesT, subblock linear dimensionsL and average densities <ρ>. Particular attention is paid to the question whether accurate estimates of critical temperatureT c and critical density ρ c can be obtained. In the lattice gas model these critical parameters are known exactly and the limitations of the approach can thus be definitively asserted. The final estimates for the Lennard Jones problem areT c =0.47±0.01 (in units of the Lennard Jones energy ε) and ρ c (in units of σ2), a comparison with previous estimates is made.  相似文献
6.
We study the long-time stability of oscillators driven by time-dependent forces originating from dynamical systems with varying degrees of randomness. The asymptotic energy growth is related to ergodic properties of the dynamical system: when the autocorrelation of the force decays sufficiently fast one typically obtains linear diffusive growth of the energy. For a system with good mixing properties we obtain a stronger result in the form of a central limit theorem. If the autocorrelation decays slowly or does not decay, the behavior can depend on subtle properties of the particular model. We study this dependence in detail for a family of quasiperiodic forces. The solution involves the analysis of a small-denominator problem that can be treated by fairly elementary methods. In the special case of a periodic force the quantum stability problem can be expressed in terms of spectral properties of the Floquet operator. In the presence of resonances the spectrum is absolutely continuous. We find explicitly the eigenvalues and eigenfunctions for the nonresonant case.  相似文献
7.
We investigate the behavior of a two-level quantum system in contact with a classical heat bath, e.g., a solute particle with internal degrees of freedom immersed in a solvent of massive particles. Using a combination of analytical and numerical methods, we obtain precise information about localization, time-displaced correlation functions, and the frequency-dependent susceptibility of such solute particles. We find that these quantities can have a strong dependence on the density of the solvent fluid, with the maximum changes from the behavior of the corresponding isolated quantum system occurring in many cases at very low densities. We compare the exact results with those obtained by path integral Monte Carlo. There is good agreement with the imaginary time correlations, but analytic continuation to real time proves elusive: even with the best numerical data on the former, we can only get very gross features of the latter.  相似文献
8.
Molecular solids composed of N2-molecules are studied by Monte Carlo simulations in the constant-stress ensemble utilizing Lennard-Jones and electrostatic interactions. A phase transition was found from a high-temperature orientationally-disordered cubic phase to a low-temperature phase with Pa3 structure. The transition temperature and the jump in volume are in qualitative agreement with experimental findings. An increase in the elastic constants C11 and C44 and a decrease in C12 at the fcc-Pa3 transition are predicted. An additional study was done by neglecting the electrostatic interaction in order to study the role of the intermolecular potential. In this case a transition to a low-temperature phase with trigonal structure was obtained. If, however, translationrotation coupling is omitted, the Lennard-Jones model exhibits the Pa3 phase too. In this study, phase transitions to hexagonal phases are suppressed by the choice of periodic boundary conditions. Many similarities are found with theoretical predicition of the translation-rotation coupling induced phase instabilities in molecular C60- and C70-solids.  相似文献
9.
We present molecular dynamics simulation results of quenches into the unstable region of a two-dimensional Lennard-Jones system. The evolution of the system from the non-equilibrium state into equilibrium was analyzed with a dynamical block analysis. This can lead to a new approach in the study of non-equilibrium phenomena. We show that with such an analysis one can obtain results on the dynamic evolution as the system evolves, consistent with those obtained from and analysis of the pair-distribution function, structure factor and excess energy. The simulations were carried out on the parallel computer of the condensed matter theory group at the University of Mainz.  相似文献
10.
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