1,141 research outputs found
Ising Dipoles on the Triangular Lattice
A cluster Monte Carlo method for systems of classical spins with purely
dipolar couplings is presented. It is tested and applied for finite arrays of
perpendicular Ising dipoles on the triangular lattice. This model is a
modification with long-range interactions of the geometrically frustrated Ising
antiferromagnet. From measurements of integrated autocorrelation times for
energy, magnetization and staggered magnetizations, a high efficiency of the
cluster Monte Carlo (MC) algorithm compared to a single-spin-flip algorithm is
found. For the investigated model, a finite temperature transition is found
which is characterized by a peak in the specific heat and in the staggered
susceptibilities.Comment: 4 pages, 3 figures. Presented at the 8th Joint MMM-Intermag
Conference, San Antonio 2001. To appear in J.Appl.Physic
Modification of primordial ices by cosmic rays as simulated by cyclotron irradiation
Frozen CH4 and CH4/Ar mixtures closed into metal cuvettes and open to the vacuum were irradiated at 15 and 77 K with 10 - 20 MeV p and He-3(2+) ions in order to simulate the effect of cosmic rays on solid organic matter in space. Ices exposed to vacuum represent surfaces of icy systems whereas closed systems stand for bulk ices. The products were analyzed by MS, SEM, RBS, ERDA, H-1-NMR, HPLC, GC-MS, NEXAFS, and FT-IR. Volatile products consisted of a mixture of low molecular species, e.g., C2H2, C2H4, C2H6, and long linear aliphatic and olefinic compounds. The formation of polycyclic aromatic hydrocarbons (PAH's) and related species in solid CH4 is due to a multi-center reaction within one collision cascade and is governed by energy density effects with critical linear energy transfer values L(sub T) between 2 and 10 keV/micron. Open ices exhibit preferential hydrogen release resulting in an increased carbonization as compared to more hydrogen rich molecules protected inside large icy bodies
- …