3,064 research outputs found
Research on boron filaments and boron reinforced composites
Boron filaments for use as reinforcing phase in composite materials for aerospace structure
Out-of-plane nesting driven spin spiral in ultrathin Fe/Cu(001) films
Epitaxial ultrathin Fe films on fcc Cu(001) exhibit a spin spiral (SS), in
contrast to the ferromagnetism of bulk bcc Fe. We study the in-plane and
out-of-plane Fermi surfaces (FSs) of the SS in 8 monolayer Fe/Cu(001) films
using energy dependent soft x-ray momentum-resolved photoemission spectroscopy.
We show that the SS originates in nested regions confined to out-of-plane FSs,
which are drastically modified compared to in-plane FSs. From precise
reciprocal space maps in successive zones, we obtain the associated real space
compressive strain of 1.5+-0.5% along c-axis. An autocorrelation analysis
quantifies the incommensurate ordering vector q=(2pi/a)(0,0,~0.86), favoring a
SS and consistent with magneto-optic Kerr effect experiments. The results
reveal the importance of in-plane and out-of-plane FS mapping for ultrathin
films.Comment: 4 pages, 3 figure
Exchange between deep donors in semiconductors: a quantum defect approach
Exchange interactions among defects in semiconductors are commonly treated
within effective-mass theory using a scaled hydrogenic wave-function. However
such a wave-function is only applicable to shallow impurities; here we present
a simple but robust generalization to treat deep donors, in which we treat the
long-range part of the wavefunction using the well established quantum defect
theory, and include a model central-cell correction to fix the bound-state
eigenvalue at the experimentally observed value. This allows us to compute the
effect of binding energy on exchange interactions as a function of donor
distance; this is a significant quantity given recent proposals to carry out
quantum information processing using deep donors. As expected, exchange
interactions are suppressed (or increased), compared to the hydrogenic case, by
the greater localization (or delocalization) of the wavefunctions of deep
donors (or `super-shallow' donors with binding energy less then the hydrogenic
value). The calculated results are compared with a simple scaling of the
Heitler-London hydrogenic exchange; the scaled hydrogenic results give the
correct order of magnitude but fail to reproduce quantitatively our
calculations. We calculate the donor exchange in silicon including inter-valley
interference terms for donor pairs along the direction, and also show
the influence of the donor type on the distribution of nearest-neighbour
exchange constants at different concentrations. Our methods can be used to
compute the exchange interactions between two donor electrons with arbitrary
binding energy.Comment: 11 pages, 10 figures, RevTeX
Time Evolution of Spin Waves
A rigorous derivation of macroscopic spin-wave equations is demonstrated. We
introduce a macroscopic mean-field limit and derive the so-called
Landau-Lifshitz equations for spin waves. We first discuss the ferromagnetic
Heisenberg model at T=0 and finally extend our analysis to general spin
hamiltonians for the same class of ferromagnetic ground states.Comment: 4 pages, to appear in PR
Low temperature dynamics of kinks on Ising interfaces
The anisotropic motion of an interface driven by its intrinsic curvature or
by an external field is investigated in the context of the kinetic Ising model
in both two and three dimensions. We derive in two dimensions (2d) a continuum
evolution equation for the density of kinks by a time-dependent and nonlocal
mapping to the asymmetric exclusion process. Whereas kinks execute random walks
biased by the external field and pile up vertically on the physical 2d lattice,
then execute hard-core biased random walks on a transformed 1d lattice. Their
density obeys a nonlinear diffusion equation which can be transformed into the
standard expression for the interface velocity v = M[(gamma + gamma'')kappa +
H]$, where M, gamma + gamma'', and kappa are the interface mobility, stiffness,
and curvature, respectively. In 3d, we obtain the velocity of a curved
interface near the orientation from an analysis of the self-similar
evolution of 2d shrinking terraces. We show that this velocity is consistent
with the one predicted from the 3d tensorial generalization of the law for
anisotropic curvature-driven motion. In this generalization, both the interface
stiffness tensor and the curvature tensor are singular at the
orientation. However, their product, which determines the interface velocity,
is smooth. In addition, we illustrate how this kink-based kinetic description
provides a useful framework for studying more complex situations by modeling
the effect of immobile dilute impurities.Comment: 11 pages, 10 figure
Low temperature shape relaxation of 2-d islands by edge diffusion
We present a precise microscopic description of the limiting step for low
temperature shape relaxation of two dimensional islands in which activated
diffusion of particles along the boundary is the only mechanism of transport
allowed. In particular, we are able to explain why the system is driven
irreversibly towards equilibrium. Based on this description, we present a
scheme for calculating the duration of the limiting step at each stage of the
relaxation process. Finally, we calculate numerically the total relaxation time
as predicted by our results and compare it with simulations of the relaxation
process.Comment: 11 pages, 5 figures, to appear in Phys. Rev.
Self Consistent Expansion for the Molecular Beam Epitaxy Equation
Motivated by a controversy over the correct results derived from the dynamic
renormalization group (DRG) analysis of the non linear molecular beam epitaxy
(MBE) equation, a self-consistent expansion (SCE) for the non linear MBE theory
is considered. The scaling exponents are obtained for spatially correlated
noise of the general form . I find a lower critical dimension , above, which the linear MBE solution appears. Below the
lower critical dimension a r-dependent strong-coupling solution is found. These
results help to resolve the controversy over the correct exponents that
describe non linear MBE, using a reliable method that proved itself in the past
by predicting reasonable results for the Kardar-Parisi-Zhang (KPZ) system,
where DRG failed to do so.Comment: 16 page
Collective synchronization in spatially extended systems of coupled oscillators with random frequencies
We study collective behavior of locally coupled limit-cycle oscillators with
random intrinsic frequencies, spatially extended over -dimensional
hypercubic lattices. Phase synchronization as well as frequency entrainment are
explored analytically in the linear (strong-coupling) regime and numerically in
the nonlinear (weak-coupling) regime. Our analysis shows that the oscillator
phases are always desynchronized up to , which implies the lower critical
dimension for phase synchronization. On the other hand, the
oscillators behave collectively in frequency (phase velocity) even in three
dimensions (), indicating that the lower critical dimension for frequency
entrainment is . Nonlinear effects due to periodic nature of
limit-cycle oscillators are found to become significant in the weak-coupling
regime: So-called {\em runaway oscillators} destroy the synchronized (ordered)
phase and there emerges a fully random (disordered) phase. Critical behavior
near the synchronization transition into the fully random phase is unveiled via
numerical investigation. Collective behavior of globally-coupled oscillators is
also examined and compared with that of locally coupled oscillators.Comment: 18 pages, 18 figure
Surface Deformation Caused by Pressure Changes in the Fluid Core
Pressure load Love numbers are presented for the mantle deformation induced by the variation of the pressure field at the core mantle boundary (CNB). We find that the CMB geostrophic pressure fields, derived from 'frozen-flux' core surface flow estimates at epochs 1965 and 1975, produce a relative radial velocity (RRV) field in the range of 3mm/decade with uplift near the equator and subsidence near the poles. The contribution of this mechanism to the change in the length of day (l.o.d) is small --- about 2.3 x 10(exp -2) ms/decade. The contribution to the time variation of the ellipticity coefficient is more important --- -1.3 x 10(exp -11)/yr
- …