157 research outputs found
In-plane dipole coupling anisotropy of a square ferromagnetic Heisenberg monolayer
In this study we calculate the dipole-coupling-induced quartic in-plane
anisotropy of a square ferromagnetic Heisenberg monolayer. This anisotropy
increases with an increasing temperature, reaching its maximum value close to
the Curie temperature of the system. At T=0 the system is isotropic, besides a
small remaining anisotropy due to the zero-point motion of quantum mechanical
spins. The reason for the dipole-coupling-induced anisotropy is the disturbance
of the square spin lattice due to thermal fluctuations ('order-by-disorder'
effect). For usual ferromagnets its strength is small as compared to other
anisotropic contributions, and decreases by application of an external magnetic
field. The results are obtained from a Heisenberg Hamiltonian by application of
a mean field approach for a spin cluster, as well as from a many-body Green's
function theory within the Tyablikov-decoupling (RPA).Comment: 6 pages, 2 figures, accepted for publication in RP
Mesoscopic theory of the viscoelasticity of polymers
We have advanced our previous static theory of polymer entanglement involving
an extended Cahn-Hilliard functional, to include time-dependent dynamics. We go
beyond the Gaussian approximation, to the one-loop level, to compute the
frequency dependent storage and loss moduli of the system. The three parameters
in our theory are obtained by fitting to available experimental data on
polystyrene melts of various chain lengths. This provides a physical
representation of the parameters in terms of the chain length of the system. We
discuss the importance of the various terms in our energy functional with
respect to their contribution to the viscoelastic response of the polymeric
system.Comment: Submitted to Phys. Rev.
Neel Temperature for Quasi-Two-Dimensional Dipolar Antiferromagnets
We calculate the N\'eel temperature for two-dimensional isotropic
dipolar Heisenberg antiferromagnets via linear spin-wave theory and a high
temperature expansion, employing the method of Callen. The theoretical
predictions for for KMnF, RbMnF, RbMnCl and
(CHNH)MnCl are in good agreement with the measured values.Comment: 12 pages, REVTEX, TUM-CP-93-0
In-plane magnetic reorientation in coupled ferro- and antiferromagnetic thin films
By studying coupled ferro- (FM) and antiferromagnetic (AFM) thin film
systems, we obtain an in-plane magnetic reorientation as a function of
temperature and FM film thickness. The interlayer exchange coupling causes a
uniaxial anisotropy, which may compete with the intrinsic anisotropy of the FM
film. Depending on the latter the total in-plane anisotropy of the FM film is
either enhanced or reduced. Eventually a change of sign occurs, resulting in an
in-plane magnetic reorientation between a collinear and an orthogonal magnetic
arrangement of the two subsystems. A canted magnetic arrangement may occur,
mediating between these two extremes. By measuring the anisotropy below and
above the N\'eel temperature the interlayer exchange coupling can be
determined. The calculations have been performed with a Heisenberg-like
Hamiltonian by application of a two-spin mean-field theory.Comment: 4 pages, 4 figure
Disorder effects in diluted ferromagnetic semiconductors
Carrier induced ferromagnetism in diluted III-V semi-conductor is analyzed
within a two step approach. First, within a single site CPA formalism, we
calculate the element resolved averaged Green's function of the itinerant
carrier. Then using a generalized RKKY formula we evaluate the Mn-Mn long-range
exchange integrals and the Curie temperature as a function of the exchange
parameter, magnetic impurity concentration and carrier density. The effect of
the disorder (impurity scattering) appears to play a crucial role. The standard
RKKY calculation (no scattering processes), strongly underestimate the Curie
temperature and is inappropriate to describe magnetism in diluted magnetic
semi-conductors. It is also shown that an antiferromagnetic exchange favors
higher Curie temperature.Comment: tex file + 4 .eps figures are included. submited to PR
Nonlinear analysis of spacecraft thermal models
We study the differential equations of lumped-parameter models of spacecraft
thermal control. Firstly, we consider a satellite model consisting of two
isothermal parts (nodes): an outer part that absorbs heat from the environment
as radiation of various types and radiates heat as a black-body, and an inner
part that just dissipates heat at a constant rate. The resulting system of two
nonlinear ordinary differential equations for the satellite's temperatures is
analyzed with various methods, which prove that the temperatures approach a
steady state if the heat input is constant, whereas they approach a limit cycle
if it varies periodically. Secondly, we generalize those methods to study a
many-node thermal model of a spacecraft: this model also has a stable steady
state under constant heat inputs that becomes a limit cycle if the inputs vary
periodically. Finally, we propose new numerical analyses of spacecraft thermal
models based on our results, to complement the analyses normally carried out
with commercial software packages.Comment: 29 pages, 4 figure
Nonequilibrium Linear Response for Markov Dynamics, II: Inertial Dynamics
We continue our study of the linear response of a nonequilibrium system. This
Part II concentrates on models of open and driven inertial dynamics but the
structure and the interpretation of the result remain unchanged: the response
can be expressed as a sum of two temporal correlations in the unperturbed
system, one entropic, the other frenetic. The decomposition arises from the
(anti)symmetry under time-reversal on the level of the nonequilibrium action.
The response formula involves a statistical averaging over explicitly known
observables but, in contrast with the equilibrium situation, they depend on the
model dynamics in terms of an excess in dynamical activity. As an example, the
Einstein relation between mobility and diffusion constant is modified by a
correlation term between the position and the momentum of the particle
Nonmonotonic dependence of the absolute entropy on temperature in supercooled Stillinger-Weber silicon
Using a recently developed thermodynamic integration method, we compute the
precise values of the excess Gibbs free energy (G^e) of the high density liquid
(HDL) phase with respect to the crystalline phase at different temperatures (T)
in the supercooled region of the Stillinger-Weber (SW) silicon [F. H.
Stillinger and T. A. Weber, Phys. Rev. B. 32, 5262 (1985)]. Based on the slope
of G^e with respect to T, we find that the absolute entropy of the HDL phase
increases as its enthalpy changes from the equilibrium value at T \ge 1065 K to
the value corresponding to a non-equilibrium state at 1060 K. We find that the
volume distribution in the equilibrium HDL phases become progressively broader
as the temperature is reduced to 1060 K, exhibiting van-der-Waals (VDW) loop in
the pressure-volume curves. Our results provides insight into the thermodynamic
cause of the transition from the HDL phase to the low density phases in SW
silicon, observed in earlier studies near 1060 K at zero pressure.Comment: This version is accepted for publication in Journal of Statistical
Physics (11 figures, 1 table
Phase transitions in geometrothermodynamics
Using the formalism of geometrothermodynamics, we investigate the geometric
properties of the equilibrium manifold for diverse thermodynamic systems.
Starting from Legendre invariant metrics of the phase manifold, we derive
thermodynamic metrics for the equilibrium manifold whose curvature becomes
singular at those points where phase transitions of first and second order
occur. We conclude that the thermodynamic curvature of the equilibrium
manifold, as defined in geometrothermodynamics, can be used as a measure of
thermodynamic interaction in diverse systems with two and three thermodynamic
degrees of freedom
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