22,877 research outputs found
Monte Carlo Simulations of Doped, Diluted Magnetic Semiconductors - a System with Two Length Scales
We describe a Monte Carlo simulation study of the magnetic phase diagram of
diluted magnetic semiconductors doped with shallow impurities in the low
concentration regime. We show that because of a wide distribution of
interaction strengths, the system exhibits strong quantum effects in the
magnetically ordered phase. A discrete spin model, found to closely approximate
the quantum system, shows long relaxation times, and the need for specialized
cluster algorithms for updating spin configurations. Results for a
representative system are presented.Comment: 12 pages, latex, 7 figures; submitted to International Journal of
Modern Physics C, Proceedings of the U.S.-Japan Bilateral Seminar:
Understanding and Conquering Long Time Scales in Computer Simulation
Nonlinear ER effects in an ac applied field
The electric field used in most electrorheological (ER) experiments is
usually quite high, and nonlinear ER effects have been theoretically predicted
and experimentally measured recently. A direct method of measuring the
nonlinear ER effects is to examine the frequency dependence of the same
effects. For a sinusoidal applied field, we calculate the ac response which
generally includes higher harmonics. In is work, we develop a multiple image
formula, and calculate the total dipole moments of a pair of dielectric
spheres, embedded in a nonlinear host. The higher harmonics due to the
nonlinearity are calculated systematically.Comment: Presented at Conference on Computational Physics (CCP2000), held at
Gold Coast, Australia from 3-8, December 200
Electronic Structure Calculations of Magnetic Exchange Interactions in Europium Monochalcogenides
Using a combination of local spin density and Hubbard 1 approximations we
study the mechansim of exchange interacion in EuX (X=O, S, Se and Te). We
reproduce known experimental results about bulk modulus, critical pressure for
structural phase transition, magnetic ordering temperature, spin--wave
dispersions as well as momentum-- and tempearuture--dependent band shift. Our
numerical results show pressure induced competition between the hybirization
enhanced exchange interaction and Kondo--like coupling in EuO. Possible ways to
enhance T_{c} are discussed
Sticker systems over monoids
Molecular computing has gained many interests among researchers since Head introduced the first theoretical model for DNA based computation using the splicing operation in 1987. Another model for DNA computing was proposed by using the sticker operation which Adlemanused in his successful experiment for the computation of Hamiltonian paths in a graph: a double stranded DNA sequence is composed by prolonging to the left and to the right a sequence of (single or double) symbols by using given single stranded strings or even more complex dominoes with sticky ends, gluing these ends together with the sticky ends of the current sequence according to a complementarity relation. According to this sticker operation, a language generative mechanism, called a sticker system, can be defined: a set of (incomplete) double-stranded sequences (axioms) and a set of pairs of single or double-stranded complementary sequences are given. The initial sequences are prolonged to the left and to the right by using sequences from the latter set, respectively. The iterations of these prolongations produce “computations” of possibly arbitrary length. These processes stop when a complete double stranded sequence is obtained. Sticker systems will generate only regular languages without restrictions. Additional restrictions can be imposed on the matching pairs of strands to obtain more powerful languages. Several types of sticker systems are shown to have the same power as regular grammars; one type is found to represent all linear languages whereas another one is proved to be able to represent any recursively enumerable language. The main aim of this research is to introduce and study sticker systems over monoids in which with each sticker operation, an element of a monoid is associated and a complete double stranded sequence is considered to be valid if the computation of the associated elements of the monoid produces the neutral element. Moreover, the sticker system over monoids is defined in this study
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