Results 1 - 10 of 3594
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[en] The time evolution of structure factors (SF) in the disordering process of an initially phase-separated lattice depends crucially on the microscopic disordering mechanism, such as Kawasaki dynamics (KD) or vacancy-mediated disordering (VMD). Monte Carlo simulations show unexpected ''dips'' in the SFs. A phenomenological model is introduced to explain the dips in the odd SFs, and an analytical solution of KD is derived, in excellent agreement with simulations. The presence (absence) of dips in the even SFs for VMD (KD) marks a significant but not yet understood difference of the two dynamics
[en] In order to understand the ordered phase, if any, in a real coupled electron layers (CEL), there is a need to take into account the effect of unequal layer density. Such phase is confirmed by a strong peak in a static structure factor. With the aid of quantum/dynamical version of Singwi, Tosi, Land and Sjölander (so-called qSTLS) approximation, we have calculated the intra- and interlayer static structure factors, Sll(q) and S12(q), over a wide range of density parameter rsl and interlayer spacing d. In our present study, the sharp peak in S22(q) has been found at critical density with sufficiently lower interlayer spacing. Further, to find the resultant effect of unequal density on intra- and interlayer static structure factors, we have compared our results with that of the recent CEL system with equal layer density and isolated single electron layer
[en] Recently the application of the maximum-entropy method to direct methods has been initiated for a priori uniformly and independently distributed atoms, introducing non-uniformity in direct space by putting constraints on the expected values of the distribution. In this paper a start is made in using the maximum-entropy principle for deriving exponential joint probability distributions of structure factors for a chemically more realistic model of a priori non-uniformly and non-independently distributed atoms. The maximum-entropy equations are obtained by treating the atomic positions as well as the reciprocal vectors as random variables and applying constraints on the maximum of the distribution. (orig./WL)
[en] The static structure factor is a global and accessible observable in experimental setups like e.g. in neutron scattering. Recently it was shown how to use such structure factors as entanglement witnesses and entanglement monotones. We generalize these concepts to structure factors with arbitrary spin, and compare different types of witnesses as well as entanglement monotones.
[en] Two methods are discussed in detail. In the first method the triplet relationship is treated using the first neighborhood, and the quartet relationship using its second neighborhood. For the triplet relationship it is found that the reliability, φh+φk-φh+k≅0 is enhanced when Rh≅Rk≅Rh+k and large. This conclusion is drawn from formula giving the conditional probability of φh+φk-φh+k using an asymptotic development up to and including terms of order N-1/2. For the quartet relationship it is found that the reliability that φh+φk+φ1-φh+k+1≅π given Rh+k≅Rh+1≅Rk+1≅0 is diminished when Rh≅Rk≅R1≅Rh+k1 and large. This conclusion is drawn from formula using similar calculations for the triplet relationship. A heuristic theoretical discussion of this last result trying to explain this difference with the usual theories is given. In the second method the triplet relationship is treated using its first neighborhood. These calculations have been done using a 'normal' asymptotic development up to and including terms of order N-1/2. As a result a formula is obtained that is (at least theoretically) able to predict negative cosine values. A third method that is proposed where one uses the ideas of Patterson superposition will be discussed in detail in a forthcoming paper. (orig.)
[en] Results about the structure of liquid water under pressure and using neutron diffraction are presented. The structural data are compared with that of low density amorphous ice (LDA) and of high density amorphous ice (HDA). The low density amorphous ice which is well accounted for a continuous random network model appears as the limit of deeply supercooled water while the high density amorphous ice which is a more disordered form of ice appears as the limit of water under high pressure and at high temperature. (author). 29 refs., 6 figs
[en] Differently weighted experimental scattering data have been used to extract partial or differential structure factors or pair distribution functions in studying many materials. However, this is not done routinely partly because of the lack of user-friendly software. This paper presents MIXSCAT, a new member of the DISCUS program package. MIXSCAT allows one to combine neutron and X-ray pair distribution functions and extract their respective differential functions.
[en] Hubbard and Nozieres and Pines were some of the first to recognize the utility of studying the detailed dependence of the structure factor, for a uniform correlated electron gas, on the individual wave vectors q. Following these ideas there were several recent extensions of such calculations to nonuniform systems, with some surprising results. It was shown that any nonuniformity, superimposed on a uniform Fermion system, fundamentally changes the nature of the structure factor S/sub lambda/ (r, r') in the thermodynamic limit. The authors have extended their results for S(r,r') to small separations between r and r' and showed that the local density approximation to S(r,r') is not rigorous
[en] The authors estimate the dynamic structure factor S(q,ω) for liquid 4He in both its normal and superfluid phases. A path integral Monte Carlo simulation is performed to compute the imaginary-time polarization propagator F(q,τ), from which S(q,ω) is extracted by maximum entropy. Results for normal 4He are in good quantitative agreement with recent neutron scattering experimental data; broad agreement is found for superfluid 4He as well, though sharp features are lost, particularly at low q. The authors attribute the excessive smoothness of the results to the entropic prior probability function used in the maximum entropy reconstruction. The experimentally observed ground state excitation spectrum E(q) is accurately reproduced in the 0 ≤ q ≤ 2.5 Angstrom -1 range