82,787 research outputs found
Size dependence of second-order hyperpolarizability of finite periodic chain under Su-Schrieffer-Heeger model
The second hyperpolarizability of
double-bond finite chain of trans-polyactylene is analyzed using the
Su-Schrieffer-Heeger model to explain qualitative features of the
size-dependence behavior of . Our study shows that is
{\it nonmonotonic} with and that the nonmonotonicity is caused by the
dominant contribution of the intraband transition to in polyenes.
Several important physical effects are discussed to reduce quantitative
discrepancies between experimental and our resultsComment: 3 figures, 1 tabl
Cell wall microstructure, pore size distribution and absolute density of hemp shiv
This paper, for the first time, fully characterizes the intrinsic physical parameters of hemp shiv including cell wall microstructure, pore size distribution and absolute density. Scanning electron microscopy revealed microstructural features similar to hardwoods. Confocal microscopy revealed three major layers in the cell wall: middle lamella, primary cell wall and secondary cell wall. Computed tomography improved the visualization of pore shape and pore connectivity in three dimensions. Mercury intrusion porosimetry (MIP) showed that the average accessible porosity was 76.67 ± 2.03% and pore size classes could be distinguished into micropores (3–10 nm) and macropores (0.1–1 µm and 20–80 µm). The absolute density was evaluated by helium pycnometry, MIP and Archimedes’ methods. The results show that these methods can lead to misinterpretation of absolute density. The MIP method showed a realistic absolute density (1.45 g cm−3) consistent with the density of the known constituents, including lignin, cellulose and hemi-cellulose. However, helium pycnometry and Archimedes’ methods gave falsely low values owing to 10% of the volume being inaccessible pores, which require sample pretreatment in order to be filled by liquid or gas. This indicates that the determination of the cell wall density is strongly dependent on sample geometry and preparation
Reversibility Checking for Markov Chains
In this paper, we present reversibility preserving operations on Markov chain
transition matrices. Simple row and column operations allow us to create new
reversible transition matrices and yield an easy method for checking a Markov
chain for reversibility
Strong magnetic coupling between an electronic spin qubit and a mechanical resonator
We describe a technique that enables a strong, coherent coupling between a
single electronic spin qubit associated with a nitrogen-vacancy impurity in
diamond and the quantized motion of a magnetized nano-mechanical resonator tip.
This coupling is achieved via careful preparation of dressed spin states which
are highly sensitive to the motion of the resonator but insensitive to
perturbations from the nuclear spin bath. In combination with optical pumping
techniques, the coherent exchange between spin and motional excitations enables
ground state cooling and the controlled generation of arbitrary quantum
superpositions of resonator states. Optical spin readout techniques provide a
general measurement toolbox for the resonator with quantum limited precision
Interface Between Topological and Superconducting Qubits
We propose and analyze an interface between a topological qubit and a
superconducting flux qubit. In our scheme, the interaction between Majorana
fermions in a topological insulator is coherently controlled by a
superconducting phase that depends on the quantum state of the flux qubit. A
controlled phase gate, achieved by pulsing this interaction on and off, can
transfer quantum information between the topological qubit and the
superconducting qubit.Comment: 12 pages, 7 figures. V2: Final version as published in Phys. Rev.
Lett, with detailed clarifications in the Appendi
Modeling urban street patterns
Urban streets patterns form planar networks whose empirical properties cannot
be accounted for by simple models such as regular grids or Voronoi
tesselations. Striking statistical regularities across different cities have
been recently empirically found, suggesting that a general and
details-independent mechanism may be in action. We propose a simple model based
on a local optimization process combined with ideas previously proposed in
studies of leaf pattern formation. The statistical properties of this model are
in good agreement with the observed empirical patterns. Our results thus
suggests that in the absence of a global design strategy, the evolution of many
different transportation networks indeed follow a simple universal mechanism.Comment: 4 pages, 5 figures, final version published in PR
Time dependent diffusion in a disordered medium with partially absorbing walls: A perturbative approach
We present an analytical study of the time dependent diffusion coefficient in
a dilute suspension of spheres with partially absorbing boundary condition.
Following Kirkpatrick (J. Chem. Phys. 76, 4255) we obtain a perturbative
expansion for the time dependent particle density using volume fraction of
spheres as an expansion parameter. The exact single particle -operator for
partially absorbing boundary condition is used to obtain a closed form
time-dependent diffusion coefficient accurate to first order in the
volume fraction . Short and long time limits of are checked against
the known short-time results for partially or fully absorbing boundary
conditions and long-time results for reflecting boundary conditions. For fully
absorbing boundary condition the long time diffusion coefficient is found to be
, to the first order of
perturbation theory. Here is small but non-zero, the diffusion
coefficient in the absence of spheres, and the radius of the spheres. The
validity of this perturbative result is discussed
Finite-Volume Energy Spectrum, Fractionalized Strings, and Low-Energy Effective Field Theory for the Quantum Dimer Model on the Square Lattice
We present detailed analytic calculations of finite-volume energy spectra,
mean field theory, as well as a systematic low-energy effective field theory
for the square lattice quantum dimer model. The analytic considerations explain
why a string connecting two external static charges in the confining columnar
phase fractionalizes into eight distinct strands with electric flux
. An emergent approximate spontaneously broken symmetry
gives rise to a pseudo-Goldstone boson. Remarkably, this soft phonon-like
excitation, which is massless at the Rokhsar-Kivelson (RK) point, exists far
beyond this point. The Goldstone physics is captured by a systematic low-energy
effective field theory. We determine its low-energy parameters by matching the
analytic effective field theory with exact diagonalization results and Monte
Carlo data. This confirms that the model exists in the columnar (and not in a
plaquette or mixed) phase all the way to the RK point.Comment: 35 pages, 16 figure
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