4,994 research outputs found
The strong Novikov conjecture for low degree cohomology
We show that for each discrete group G, the rational assembly map
K_*(BG) \otimes Q \to K_*(C*_{max} G) \otimes \Q is injective on classes dual
to the subring generated by cohomology classes of degree at most 2 (identifying
rational K-homology and homology via the Chern character). Our result implies
homotopy invariance of higher signatures associated to these cohomology
classes. This consequence was first established by Connes-Gromov-Moscovici and
Mathai.
Our approach is based on the construction of flat twisting bundles out of
sequences of almost flat bundles as first described in our previous work. In
contrast to the argument of Mathai, our approach is independent of (and indeed
gives a new proof of) the result of Hilsum-Skandalis on the homotopy invariance
of the index of the signature operator twisted with bundles of small curvature.Comment: 11 page
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
Change of quasiparticle dispersion in crossing T_c in the underdoped cuprates
One of the most remarkable properties of the high-temperature superconductors
is a pseudogap regime appearing in the underdoped cuprates above the
superconducting transition temperature T_c. The pseudogap continously develops
out of the superconducting gap. In this paper, we demonstrate by means of a
detailed comparison between theory and experiment that the characteristic
change of quasiparticle dispersion in crossing T_c in the underdoped cuprates
can be understood as being due to phase fluctuations of the superconducting
order parameter. In particular, we show that within a phase fluctuation model
the characteristic back-turning BCS bands disappear above T_c whereas the gap
remains open. Furthermore, the pseudogap rather has a U-shape instead of the
characteristic V-shape of a d_{x^2-y^2}-wave pairing symmetry and starts
closing from the nodal k=(pi/2,pi/2) directions, whereas it rather fills in at
the anti-nodal k=(pi,0) regions, yielding further support to the phase
fluctuation scenario.Comment: 6 pages, 4 eps-figure
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
Blinking statistics of a molecular beacon triggered by end-denaturation of DNA
We use a master equation approach based on the Poland-Scheraga free energy
for DNA denaturation to investigate the (un)zipping dynamics of a denaturation
wedge in a stretch of DNA, that is clamped at one end. In particular, we
quantify the blinking dynamics of a fluorophore-quencher pair mounted within
the denaturation wedge. We also study the behavioural changes in the presence
of proteins, that selectively bind to single-stranded DNA. We show that such a
setup could be well-suited as an easy-to-implement nanodevice for sensing
environmental conditions in small volumes.Comment: 14 pages, 5 figures, LaTeX, IOP style. Accepted to J Phys Cond Mat
special issue on diffusio
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
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
Bubble dynamics in DNA
The formation of local denaturation zones (bubbles) in double-stranded DNA is
an important example for conformational changes of biological macromolecules.
We study the dynamics of bubble formation in terms of a Fokker-Planck equation
for the probability density to find a bubble of size n base pairs at time t, on
the basis of the free energy in the Poland-Scheraga model. Characteristic
bubble closing and opening times can be determined from the corresponding first
passage time problem, and are sensitive to the specific parameters entering the
model. A multistate unzipping model with constant rates recently applied to DNA
breathing dynamics [G. Altan-Bonnet et al, Phys. Rev. Lett. 90, 138101 (2003)]
emerges as a limiting case.Comment: 9 pages, 2 figure
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