1,310 research outputs found
Analysis of band-gap formation in squashed arm-chair CNT
The electronic properties of squashed arm-chair carbon nanotubes are modeled
using constraint free density functional tight binding molecular dynamics
simulations. Independent from CNT diameter, squashing path can be divided into
{\it three} regimes. In the first regime, the nanotube deforms with negligible
force. In the second one, there is significantly more resistance to squashing
with the force being nN/per CNT unit cell. In the last regime,
the CNT looses its hexagonal structure resulting in force drop-off followed by
substantial force enhancement upon squashing. We compute the change in band-gap
as a function of squashing and our main results are: (i) A band-gap initially
opens due to interaction between atoms at the top and bottom sides of CNT. The
orbital approximation is successful in modeling the band-gap opening at
this stage. (ii) In the second regime of squashing, large
interaction at the edges becomes important, which can lead to band-gap
oscillation. (iii) Contrary to a common perception, nanotubes with broken
mirror symmetry can have {\it zero} band-gap. (iv) All armchair nanotubes
become metallic in the third regime of squashing. Finally, we discuss both
differences and similarities obtained from the tight binding and density
functional approaches.Comment: 16 pages and 6 figures, To appear in PR
Break-down of the density-of-states description of scanning tunneling spectroscopy in supported metal clusters
Low-temperature scanning tunneling spectroscopy allows to probe the
electronic properties of clusters at surfaces with unprecedented accuracy. By
means of quantum transport theory, using realistic tunneling tips, we obtain
conductance curves which considerably deviate from the cluster's density of
states. Our study explains the remarkably small number of peaks in the
conductance spectra observed in recent experiments. We demonstrate that the
unambiguous characterization of the states on the supported clusters can be
achieved with energy-resolved images, obtained from a theoretical analysis
which mimics the experimental imaging procedure.Comment: 5 pages, 3 figure
Hybrid Quantum Mechanical/ Molecular Mechanical Methods for Studying Energy Transduction in Biomolecular Machines
Hybrid quantum mechanical/molecular mechanical (QM/MM) methods have become indispensable tools for the study of biomolecules. In this article, we briefly review the basic methodological details of QM/MM approaches and discuss their applications to various energy transduction problems in biomolecular machines, such as long-range proton transports, fast electron transfers, and mechanochemical coupling. We highlight the particular importance for these applications of balancing computational efficiency and accuracy. Using several recent examples, we illustrate the value and limitations of QM/MM methodologies for both ground and excited states, as well as strategies for calibrating them in specific applications. We conclude with brief comments on several areas that can benefit from further efforts to make QM/MM analyses more quantitative and applicable to increasingly complex biological problems
Isotropic Spin Wave Theory of Short-Range Magnetic Order
We present an isotropic spin wave (ISW) theory of short-range order in
Heisenberg magnets, and apply it to square lattice S=1/2 and S=1
antiferromagnets. Our theory has three identical (isotropic) spin wave modes,
whereas the conventional spin wave theory has two transverse and one
longitudinal mode. We calculate temperature dependences of various
thermodynamic observables analytically and find good (several per cent)
agreement with independently obtained numerical results in a broad temperature
range.Comment: 4 pages, REVTeX v3 with 3 embedded PostScript figure
Various series expansions for a Heisenberg antiferromagnet model for SrCu(BO)
We use a variety of series expansion methods at both zero and finite
temperature to study an antiferromagnetic Heisenberg spin model proposed
recently by Miyahara and Ueda for the quasi two-dimensional material
SrCu(BO). We confirm that this model exhibits a first-order quantum
phase transition at T=0 between a gapped dimer phase and a gapless N\'eel phase
when the ratio of nearest and next-nearest neighbour interactions is
varied, and locate the transition at . Using longer series we are
able to give more accurate estimates of the model parameters by fitting to the
high temperature susceptibility data.Comment: RevTeX, 13 figure
Quantum phase transitions in the Triangular-lattice Bilayer Heisenberg Model
We study the triangular lattice bilayer Heisenberg model with
antiferromagnetic interplane coupling and nearest neighbour
intraplane coupling , which can be ferro- or
antiferromagnetic, by expansions in . For negative a phase
transition is found to an ordered phase at a critical which is in the 3D classical Heisenberg universality class. For
, we find a transition at a rather large . The
universality class of the transition is consistent with that of Kawamura's 3D
antiferromagnetic stacked triangular lattice. The spectral weight for the
triplet excitations, at the ordering wavevector, remains finite at the
transition, suggesting that a phase with free spinons does not exist in this
model.Comment: revtex, 4 pages, 3 figure
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