8,684 research outputs found
Surface critical behavior of driven diffusive systems with open boundaries
Using field theoretic renormalization group methods we study the critical
behavior of a driven diffusive system near a boundary perpendicular to the
driving force. The boundary acts as a particle reservoir which is necessary to
maintain the critical particle density in the bulk. The scaling behavior of
correlation and response functions is governed by a new exponent eta_1 which is
related to the anomalous scaling dimension of the chemical potential of the
boundary. The new exponent and a universal amplitude ratio for the density
profile are calculated at first order in epsilon = 5-d. Some of our results are
checked by computer simulations.Comment: 10 pages ReVTeX, 6 figures include
On the surface critical behaviour in Ising strips: density-matrix renormalization-group study
Using the density-matrix renormalization-group method we study the surface
critical behaviour of the magnetization in Ising strips in the subcritical
region. Our results support the prediction that the surface magnetization in
the two phases along the pseudo-coexistence curve also behaves as for the
ordinary transition below the wetting temperature for the finite value of the
surface field.Comment: 15 pages, 9 figure
Boundary critical behavior at m-axial Lifshitz points for a boundary plane parallel to the modulation axes
The critical behavior of semi-infinite -dimensional systems with
-component order parameter and short-range interactions is
investigated at an -axial bulk Lifshitz point whose wave-vector instability
is isotropic in an -dimensional subspace of . The associated
modulation axes are presumed to be parallel to the surface, where . An appropriate semi-infinite model representing the
corresponding universality classes of surface critical behavior is introduced.
It is shown that the usual O(n) symmetric boundary term
of the Hamiltonian must be supplemented by one of the form involving a
dimensionless (renormalized) coupling constant . The implied boundary
conditions are given, and the general form of the field-theoretic
renormalization of the model below the upper critical dimension
is clarified. Fixed points describing the ordinary, special,
and extraordinary transitions are identified and shown to be located at a
nontrivial value if . The surface
critical exponents of the ordinary transition are determined to second order in
. Extrapolations of these expansions yield values of these
exponents for in good agreement with recent Monte Carlo results for the
case of a uniaxial () Lifshitz point. The scaling dimension of the surface
energy density is shown to be given exactly by , where
is the anisotropy exponent.Comment: revtex4, 31 pages with eps-files for figures, uses texdraw to
generate some graphs; to appear in PRB; v2: some references and additional
remarks added, labeling in figure 1 and some typos correcte
Boundary critical behaviour at -axial Lifshitz points: the special transition for the case of a surface plane parallel to the modulation axes
The critical behaviour of -dimensional semi-infinite systems with
-component order parameter is studied at an -axial bulk
Lifshitz point whose wave-vector instability is isotropic in an -dimensional
subspace of . Field-theoretic renormalization group methods are
utilised to examine the special surface transition in the case where the
potential modulation axes, with , are parallel to the surface.
The resulting scaling laws for the surface critical indices are given. The
surface critical exponent , the surface crossover exponent
and related ones are determined to first order in
\epsilon=4+\case{m}{2}-d. Unlike the bulk critical exponents and the surface
critical exponents of the ordinary transition, is -dependent already
at first order in . The \Or(\epsilon) term of is
found to vanish, which implies that the difference of and
the bulk exponent is of order .Comment: 21 pages, one figure included as eps file, uses IOP style file
Thermodynamic Casimir effects involving interacting field theories with zero modes
Systems with an O(n) symmetrical Hamiltonian are considered in a
-dimensional slab geometry of macroscopic lateral extension and finite
thickness that undergo a continuous bulk phase transition in the limit
. The effective forces induced by thermal fluctuations at and above
the bulk critical temperature (thermodynamic Casimir effect) are
investigated below the upper critical dimension by means of
field-theoretic renormalization group methods for the case of periodic and
special-special boundary conditions, where the latter correspond to the
critical enhancement of the surface interactions on both boundary planes. As
shown previously [\textit{Europhys. Lett.} \textbf{75}, 241 (2006)], the zero
modes that are present in Landau theory at make conventional
RG-improved perturbation theory in dimensions ill-defined. The
revised expansion introduced there is utilized to compute the scaling functions
of the excess free energy and the Casimir force for temperatures
T\geqT_{c,\infty} as functions of , where
is the bulk correlation length. Scaling functions of the
-dependent residual free energy per area are obtained whose
limits are in conformity with previous results for the Casimir amplitudes
to and display a more reasonable
small- behavior inasmuch as they approach the critical value
monotonically as .Comment: 23 pages, 10 figure
Spin transport in magnetic multilayers
We study by extensive Monte Carlo simulations the transport of itinerant
spins travelling inside a multilayer composed of three ferromagnetic films
antiferromagnetically coupled to each other in a sandwich structure. The two
exterior films interact with the middle one through non magnetic spacers. The
spin model is the Ising one and the in-plane transport is considered. Various
interactions are taken into account. We show that the current of the itinerant
spins going through this system depends strongly on the magnetic ordering of
the multilayer: at temperatures below (above) the transition temperature
, a strong (weak) current is observed. This results in a strong jump of
the resistance across . Moreover, we observe an anomalous variation,
namely a peak, of the spin current in the critical region just above . We
show that this peak is due to the formation of domains in the temperature
region between the low- ordered phase and the true paramagnetic disordered
phase. The existence of such domains is known in the theory of critical
phenomena. The behavior of the resistance obtained here is compared to a recent
experiment. An excellent agreement with our physical interpretation is
observed. We also show and discuss effects of various physical parameters
entering our model such as interaction range, strength of electric and magnetic
fields and magnetic film and non magnetic spacer thicknesses.Comment: 8 pages, 17 figures, submitted to J. Phys.: Cond Matte
Distribution of ions near a charged selective surface in critical binary solvents
Near-critical binary mixtures containing ionic solutes near a charged wall
preferentially adsorbing one component of the solvent are studied. Within the
Landau-Ginzburg approach extended to include electrostatic interactions and the
chemical preference of ions for one component of the solvent, we obtain a
simple form for the leading-order correction to the Debye-Huckel theory result
for the charge density profile. Our result shows that critical adsorption
influences significantly distribution of ions near the wall. This effect may
have important implications for the screening of electrostatic interactions
between charged surfaces immersed in binary near-critical solvents.Comment: 24 pages, 3 figure
Unusual transport properties of ferromagnetic Heusler alloy CoTiSn
We report results of magnetization, zero field resistivity and
magnetoresistance measurements in ferromagnetic Heusler alloy CoTiSn. There
is a striking change in the character of electron transport as the system
undergoes the paramagnetic to ferromagnetic transition. In the paramagnetic
state the nature of the electron transport is like that of a semiconductor and
this changes abruptly to metallic behaviour at the onset of ferromagnetic
ordering. Application of external magnetic field tends to suppress this
semiconducting like transport leading to a negative magnetoresistance which
reaches a peak in the vicinity of Curie temperature. Comparison is made with
the similar unusual behaviour observed in other systems including UNiSn and
manganites.Comment: 9 pages of text including 5 figures. Submitted to Physical Review
Surface critical behavior of random systems at the ordinary transition
We calculate the surface critical exponents of the ordinary transition
occuring in semi-infinite, quenched dilute Ising-like systems. This is done by
applying the field theoretic approach directly in d=3 dimensions up to the
two-loop approximation, as well as in dimensions. At
we extend, up to the next-to-leading order, the previous
first-order results of the expansion by Ohno and Okabe
[Phys.Rev.B 46, 5917 (1992)]. In both cases the numerical estimates for surface
exponents are computed using Pade approximants extrapolating the perturbation
theory expansions. The obtained results indicate that the critical behavior of
semi-infinite systems with quenched bulk disorder is characterized by the new
set of surface critical exponents.Comment: 11 pages, 11 figure
Surface critical exponents at a uniaxial Lifshitz point
Using Monte Carlo techniques, the surface critical behaviour of
three-dimensional semi-infinite ANNNI models with different surface
orientations with respect to the axis of competing interactions is
investigated. Special attention is thereby paid to the surface criticality at
the bulk uniaxial Lifshitz point encountered in this model. The presented Monte
Carlo results show that the mean-field description of semi-infinite ANNNI
models is qualitatively correct. Lifshitz point surface critical exponents at
the ordinary transition are found to depend on the surface orientation. At the
special transition point, however, no clear dependency of the critical
exponents on the surface orientation is revealed. The values of the surface
critical exponents presented in this study are the first estimates available
beyond mean-field theory.Comment: 10 pages, 7 figures include
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