5,755 research outputs found
Conductivity of a graphene strip: width and gate-voltage dependencies
We study the conductivity of a graphene strip taking into account
electrostatically-induced charge accumulation on its edges. Using a local
dependency of the conductivity on the carrier concentration we find that the
electrostatic size effect in doped graphene strip of the width of 0.5 - 3 m can result in a significant (about 40%) enhancement of the effective
conductivity in comparison to the infinitely wide samples. This effect should
be taken into account both in the device simulation as well as for verification
of scattering mechanisms in graphene.Comment: 3 pages, 4 figure
Direct observation of the effective bending moduli of a fluid membrane: Free-energy cost due to the reference-plane deformations
Effective bending moduli of a fluid membrane are investigated by means of the
transfer-matrix method developed in our preceding paper. This method allows us
to survey various statistical measures for the partition sum. The role of the
statistical measures is arousing much attention, since Pinnow and Helfrich
claimed that under a suitable statistical measure, that is, the local mean
curvature, the fluid membranes are stiffened, rather than softened, by thermal
undulations. In this paper, we propose an efficient method to observe the
effective bending moduli directly: We subjected a fluid membrane to a curved
reference plane, and from the free-energy cost due to the reference-plane
deformations, we read off the effective bending moduli. Accepting the
mean-curvature measure, we found that the effective bending rigidity gains even
in the case of very flexible membrane (small bare rigidity); it has been rather
controversial that for such non-perturbative regime, the analytical prediction
does apply. We also incorporate the Gaussian-curvature modulus, and calculated
its effective rigidity. Thereby, we found that the effective Gaussian-curvature
modulus stays almost scale-invariant. All these features are contrasted with
the results under the normal-displacement measure
d=2 transverse-field Ising model under the screw-boundary condition: An optimization of the screw pitch
A length-N spin chain with the \sqrt{N}(=v)-th neighbor interaction is
identical to a two-dimensional (d=2) model under the screw-boundary (SB)
condition. The SB condition provides a flexible scheme to construct a d\ge2
cluster from an arbitrary number of spins; the numerical diagonalization
combined with the SB condition admits a potential applicability to a class of
systems intractable with the quantum Monte Carlo method due to the
negative-sign problem. However, the simulation results suffer from
characteristic finite-size corrections inherent in SB. In order to suppress
these corrections, we adjust the screw pitch v(N) so as to minimize the
excitation gap for each N. This idea is adapted to the transverse-field Ising
model on the triangular lattice with N\le32 spins. As a demonstration, the
correlation-length critical exponent is analyzed in some detail
Driving rate effects in avalanche-mediated, first-order phase transitions
We have studied the driving rate and temperature dependence of the power-law
exponents that characterize the avalanche distribution in first-order phase
transitions. Measurements of acoustic emission in structural transitions in
Cu-Zn-Al and Cu-Al-Ni are presented. We show how the observed behaviour emerges
within a general framework of competing time scales of avalanche relaxation,
driving rate, and thermal fluctuations. We have confirmed our findings by
numerical simulations of a prototype model.Comment: 4 pages, 3 figure
Scaling Theory of Antiferromagnetic Heisenberg Ladder Models
The antiferromagnetic Heisenberg model on multi-leg ladders is
investigated. Criticality of the ground-state transition is explored by means
of finite-size scaling. The ladders with an even number of legs and those with
an odd number of legs are distinguished clearly. In the former, the energy gap
opens up as , where is the strength of the
antiferromagnetic inter-chain coupling. In the latter, the critical phase with
the central charge extends over the whole region of .Comment: 12 pages with 9 Postscript figures. To appear in J. Phys. A: Math.
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Nanostructure of cellulose microfibrils in spruce wood
The structure of cellulose microfibrils in wood is not known in detail, despite the abundance of cellulose in woody biomass and its importance for biology, energy, and engineering. The structure of the microfibrils of spruce wood cellulose was investigated using a range of spectroscopic methods coupled to small-angle neutron and wide-angle X-ray scattering. The scattering data were consistent with 24-chain microfibrils and favored a “rectangular” model with both hydrophobic and hydrophilic surfaces exposed. Disorder in chain packing and hydrogen bonding was shown to increase outwards from the microfibril center. The extent of disorder blurred the distinction between the I alpha and I beta allomorphs. Chains at the surface were distinct in conformation, with high levels of conformational disorder at C-6, less intramolecular hydrogen bonding and more outward-directed hydrogen bonding. Axial disorder could be explained in terms of twisting of the microfibrils, with implications for their biosynthesis
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