2,628 research outputs found
Do crossover functions depend on the shape of the interaction profile?
We examine the crossover from classical to non-classical critical behaviour
in two-dimensional systems with a one-component order parameter. Since the
degree of universality of the corresponding crossover functions is still
subject to debate, we try to induce non-universal effects by adding
interactions with a second length scale. Although the crossover functions
clearly depend on the range of the interactions, they turn out to be remarkably
robust against further variation of the interaction profile. In particular, we
find that the earlier observed non-monotonic crossover of the effective
susceptibility exponent occurs for several qualitatively different shapes of
this profile.Comment: 7 pages + 4 PostScript figures. Accepted for publication in
Europhysics Letters. Also available as PDF file at
http://www.cond-mat.physik.uni-mainz.de/~luijten/erikpubs.htm
Unusual structural tuning of magnetism in cuprate perovskites
Understanding the structural underpinnings of magnetism is of great
fundamental and practical interest. Se_{1-x}Te_{x}CuO_{3} alloys are model
systems for the study of this question, as composition-induced structural
changes control their magnetic interactions. Our work reveals that this
structural tuning is associated with the position of the supposedly dummy atoms
Se and Te relative to the super-exchange (SE) Cu--O--Cu paths, and not with the
SE angles as previously thought. We use density functional theory,
tight-binding, and exact diagonalization methods to unveil the cause of this
surprising effect and hint at new ways of engineering magnetic interactions in
solids.Comment: 4 pages, with 4 postscript figures embedded. Uses REVTEX4 and
graphicx macro
Use of Natural Gas in Motor Transport as a Strategic Reference Point for Improving the Environmental Situation in Cities
The paper presents a brief analysis of the problem of environmental pollution by motor transport. Some ways of reducing the harmful effect of transport are presented. Russian legislation on the use of natural gas is analyzed. It is established that the development of the gas-engine infrastructure near major highways and cities is the most urgent issue. The criteria that must be met for the successful construction of a branched network of natural gas vehicle refueling stations are given. On the example of the Russian city of Tyumen, an experiment was conducted to account for transit traffic passing through its highways. It is concluded that improvement of the ecological situation is an important aspect of the sustainable urban development. And the actual factor that will reduce the harmful impact of transport on the environment is the use of a more environmentally friendly alternative fuel - natural gas. And to expand its use, a rational territorial and quantitative location of the natural gas vehicle refueling station network is necessary.
 
Shape of crossover between mean-field and asymptotic critical behavior in a three-dimensional Ising lattice
Recent numerical studies of the susceptibility of the three-dimensional Ising
model with various interaction ranges have been analyzed with a crossover model
based on renormalization-group matching theory. It is shown that the model
yields an accurate description of the crossover function for the
susceptibility.Comment: 4 pages RevTeX + 3 PostScript figures. Uses epsf.sty and rotate.sty.
Final version; accepted for publication in Physics Letters
SiC(0001): a surface Mott-Hubbard insulator
We present ab-initio electronic structure calculations for the Si-terminated
SiC(0001) surface. While local density approximation
(LDA) calculations predict a metallic ground state with a half-filled narrow
band, Coulomb effects, included by the spin-polarized LDA+U method, result in a
magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the
experimental value of 2.0 eV. The calculated value of the inter-site exchange
parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except
at very low temperatures. The observed Si 2p surface core level doublet can
naturally be explained as an on-site exchange splitting.Comment: RevTex, 4 pages, 4 eps-figure
Coulomb correlation and magnetic ordering in double-layered manganites: LaSrMnO
A detailed study of the electronic structure and magnetic configurations of
the 50 % hole-doped double layered manganite LaSrMnO is presented.
We demonstrate that the on-site Coulomb correlation (U) of Mn d electrons {\it
(i)} significantly modifies the electronic structure, magnetic ordering (from
FM to AFM), and interlayer exchange interactions, and {\it (ii)} promotes
strong anisotropy in electrical transport, reducing the effective hopping
parameter along the {\it c} axis for electrically active electrons. This
findng is consistent with observations of anisotropic transport -- a property
which sets this manganite apart from conventional 3D systems. A half-metallic
band structure is predicted with both the LSDA and LSDA+U methods. The
experimentally observed A-type AFM ordering in LaSrMnO is found to
be energetically more favorable with U 7 eV. A simple interpretation of
interlayer exchange coupling is given within double and super-exchange
mechanisms based on the dependencies on U of the effective exchange parameters
and state sub-band widths.Comment: 10 pages, 5 figure
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