4,955 research outputs found
Modified critical correlations close to modulated and rough surfaces
Correlation functions are sensitive to the presence of a boundary. Surface
modulations give rise to modified near surface correlations, which can be
measured by scattering probes. To determine these correlations, we develop a
perturbative calculation in deformations in height from a flat surface. The
results, combined with a renormalization group around four dimensions, are also
used to predict critical behavior near a self-affinely rough surface. We find
that a large enough roughness exponent can modify surface critical behavior.Comment: 4 pages, 1 figure. Revised version as published in Phys. Rev. Lett.
86, 4596 (2001
Correlation functions near Modulated and Rough Surfaces
In a system with long-ranged correlations, the behavior of correlation
functions is sensitive to the presence of a boundary. We show that surface
deformations strongly modify this behavior as compared to a flat surface. The
modified near surface correlations can be measured by scattering probes. To
determine these correlations, we develop a perturbative calculation in the
deformations in height from a flat surface. Detailed results are given for a
regularly patterned surface, as well as for a self-affinely rough surface with
roughness exponent . By combining this perturbative calculation in
height deformations with the field-theoretic renormalization group approach, we
also estimate the values of critical exponents governing the behavior of the
decay of correlation functions near a self-affinely rough surface. We find that
for the interacting theory, a large enough can lead to novel surface
critical behavior. We also provide scaling relations between roughness induced
critical exponents for thermodynamic surface quantities.Comment: 31 pages, 2 figure
Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy
Hexagonal boron nitride (h-BN) is a layered two-dimensional material with
properties that make it promising as a dielectric in various applications. We
report the growth of h-BN films on Ni foils from elemental B and N using
molecular beam epitaxy. The presence of crystalline h-BN over the entire
substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used
to examine the morphology and continuity of the synthesized films. A scanning
electron microscopy study of films obtained using shorter depositions offers
insight into the nucleation and growth behavior of h-BN on the Ni substrate.
The morphology of h-BN was found to evolve from dendritic, star-shaped islands
to larger, smooth triangular ones with increasing growth temperature
Dynamical Properties of Two Coupled Hubbard Chains at Half-filling
Using grand canonical Quantum Monte Carlo (QMC) simulations combined with
Maximum Entropy analytic continuation, as well as analytical methods, we
examine the one- and two-particle dynamical properties of the Hubbard model on
two coupled chains at half-filling. The one-particle spectral weight function,
, undergoes a qualitative change with interchain hopping
associated with a transition from a four-band insulator to a two-band
insulator. A simple analytical model based on the propagation of exact rung
singlet states gives a good description of the features at large . For
smaller , is similar to that of the
one-dimensional model, with a coherent band of width the effective
antiferromagnetic exchange reasonably well-described by renormalized
spin-wave theory. The coherent band rides on a broad background of width
several times the parallel hopping integral , an incoherent structure
similar to that found in calculations on both the one- and two-dimensional
models. We also present QMC results for the two-particle spin and charge
excitation spectra, and relate their behavior to the rung singlet picture for
large and to the results of spin-wave theory for small .Comment: 9 pages + 10 postscript figures, submitted to Phys.Rev.B, revised
version with isotropic t_perp=t data include
Long-Range Ordering of Vibrated Polar Disks
Vibrated polar disks have been used experimentally to investigate collective
motion of driven particles, where fully-ordered asymptotic regimes could not be
reached. Here we present a model reproducing quantitatively the single, binary
and collective properties of this granular system. Using system sizes not
accessible in the laboratory, we show in silico that true long-range order is
possible in the experimental system. Exploring the model's parameter space, we
find a phase diagram qualitatively different from that of dilute or point-like
particle systems.Comment: 5 pages, 4 figure
Long term variability of Cygnus X-1: VII. Orbital variability of the focussed wind in Cyg X-1 / HDE 226868 system
Binary systems with an accreting compact object are a unique chance to
investigate the strong, clumpy, line-driven winds of early type supergiants by
using the compact object's X-rays to probe the wind structure. We analyze the
two-component wind of HDE 226868, the O9.7Iab giant companion of the black hole
Cyg X-1 using 4.77 Ms of RXTE observations of the system taken over the course
of 16 years. Absorption changes strongly over the 5.6 d binary orbit, but also
shows a large scatter at a given orbital phase, especially at superior
conjunction. The orbital variability is most prominent when the black hole is
in the hard X-ray state. Our data are poorer for the intermediate and soft
state, but show signs for orbital variability of the absorption column in the
intermediate state. We quantitatively compare the data in the hard state to a
toy model of a focussed Castor-Abbott-Klein-wind: as it does not incorporate
clumping, the model does not describe the observations well. A qualitative
comparison to a simplified simulation of clumpy winds with spherical clumps
shows good agreement in the distribution of the equivalent hydrogen column
density for models with a porosity length on the order of the stellar radius at
inferior conjunction; we conjecture that the deviations between data and model
at superior conjunction could be either due to lack of a focussed wind
component in the model or a more complicated clump structure.Comment: proposed for acceptance in A&A, 11 pages, 11 figures (two in
appendix
Phase-fluctuation induced reduction of the kinetic energy at the superconducting transition
Recent reflectivity measurements provide evidence for a "violation" of the
in-plane optical integral in the underdoped high-T_c compound
Bi_2Sr_2CaCu_2O_{8+\delta} up to frequencies much higher than expected by
standard BCS theory. The sum rule violation may be related to a loss of
in-plane kinetic energy at the superconducting transition. Here, we show that a
model based on phase fluctuations of the superconducting order parameter can
account for this change of in-plane kinetic energy at T_c. The change is due to
a transition from a phase-incoherent Cooper-pair motion in the pseudogap regime
above T_c to a phase-coherent motion at T_c.Comment: 5 pages, 3 eps-figure
Distribution of transmitted charge through a double-barrier junction
The distribution function of transmitted charge through a double-barrier
junction is studied at zero temperature and at small applied voltage. Both a
semiclassical model, in which the transport is described by jump rates, and a
quantum mechanical model, which averages over resonant and non-resonant states,
are used to determine the characteristic function of the transmitted electrons.
It is demonstrated that for large times the logarithm of the characteristic
function is equal within the two approaches. The charge distribution is in
between a Gaussian and a Poissonian distribution if both barriers have equal
height and reduces to a Poissonian if one barrier is much higher than the
other.Comment: 6 pages, revtex, 2 figures include
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