15,032 research outputs found
Design of Copolymeric Materials
We devise a method for designing materials that will have some desired
structural characteristics. We apply it to multiblock copolymers that have two
different types of monomers, A and B. We show how to determine what sequence of
A's and B's should be synthesised in order to give a particular structure and
morphology. %For example in a melt of such %polymers, one may wish to engineer
a body-centered %cubic structure. Using this method in conjunction with the
theory of microphase separation developed by Leibler, we show it is possible to
efficiently search for a desired morphology. The method is quite general and
can be extended to design isolated heteropolymers, such as proteins, with
desired structural characteristics. We show that by making certain
approximations to the exact algorithm, a method recently proposed by
Shakhnovich and Gutin is obtained. The problems with this method are discussed
and we propose an improved approximate algorithm that is computationally
efficient.Comment: 15 pages latex 2.09 and psfig, 1 postscript figure
A gapless charge mode induced by the boundary states in the half-filled Hubbard open-chain
We discuss the ground state and some excited states of the half-filled
Hubbard model defined on an open chain with L sites, where only one of the
boundary sites has a different value of chemical potential. We consider the
case when the boundary site has a negative chemical potential -p and the
Hubbard coupling U is positive. By an analytic method we show that when p is
larger than the transfer integral some of the ground-state solutions of the
Bethe ansatz equations become complex-valued. It follows that there is a
``surface phase transition'' at some critical value p_c; when p<p_c all the
charge excitations have the gap for the half-filled band, while there exists a
massless charge mode when p>p_c.Comment: Revtex, 25 pages, 3 eps figures; Full revision with Appendixes adde
Multi-Stability of Electromagnetically Induced Transparency in Atom-Assisted Optomechanical Cavities
We study how an oscillating mirror affects the electromagnetically induced
transparency (EIT) of an atomic ensemble, which is confined in a gas cell
placed inside a micro-cavity with an oscillating mirror in one end. The
oscillating mirror is modeled as a quantum mechanical harmonic oscillator. The
cavity field acts as a probe light of the EIT system and also produces a light
pressure on the oscillating mirror. The back-action from the mirror to the
cavity field results in several (from one to five) steady-states for this
atom-assisted optomechanical cavity, producing a complex structure in its EIT.
We calculate the susceptibility with respect to the few (from one to three)
stable solutions found here for the equilibrium positions of the oscillating
mirror. We find that the EIT of the atomic ensemble can be significantly
changed by the oscillating mirror, and also that the various steady states of
the mirror have different effects on the EIT.Comment: 10 pages, 9 figure
Precision determination of absolute neutron flux
A technique for establishing the total neutron rate of a highly-collimated
monochromatic cold neutron beam was demonstrated using a method of an
alpha-gamma counter. The method involves only the counting of measured rates
and is independent of neutron cross sections, decay chain branching ratios, and
neutron beam energy. For the measurement, a target of 10B-enriched boron
carbide totally absorbed the neutrons in a monochromatic beam, and the rate of
absorbed neutrons was determined by counting 478keV gamma rays from neutron
capture on 10B with calibrated high-purity germanium detectors. A second
measurement based on Bragg diffraction from a perfect silicon crystal was
performed to determine the mean de Broglie wavelength of the beam to a
precision of 0.024 %. With these measurements, the detection efficiency of a
neutron monitor based on neutron absorption on 6Li was determined to an overall
uncertainty of 0.058 %. We discuss the principle of the alpha-gamma method and
present details of how the measurement was performed including the systematic
effects. We also describe how this method may be used for applications in
neutron dosimetry and metrology, fundamental neutron physics, and neutron cross
section measurements.Comment: 44 page
First Limits on Left-Right Symmetry Scale from LHC Data
We use the early Large Hadron Collider data to set the lower limit on the
scale of Left-Right symmetry, by searching for the right-handed charged gauge
boson via the final state with two leptons and two jets, for 33/pb
integrated luminosity and 7 TeV center-of-mass energy. In the absence of a
signal beyond the Standard Model background, we set the bound M_WR > 1.4 TeV at
95% C.L.. This result is obtained for a range of right-handed neutrino masses
of the order of few 100 GeV, assuming no accidental cancelation in right-handed
lepton mixings.Comment: 4 pages, added reference
Parametrical optimization of laser surface alloyed NiTi shape memory alloy with Co and Nb by the Taguchi method
Different high-purity metal powders were successfully alloyed on to a nickel titanium (NiTi) shape memory alloy (SMA) with a 3 kW carbon dioxide (CO2) laser system. In order to produce an alloyed layer with complete penetration and acceptable composition profile, the Taguchi approach was used as a statistical technique for optimizing selected laser processing parameters. A systematic study of laser power, scanning velocity, and pre-paste powder thickness was conducted. The signal-to-noise ratios (S/N) for each control factor were calculated in order to assess the deviation from the average response. Analysis of variance (ANOVA) was carried out to understand the significance of process variables affecting the process effects. The Taguchi method was able to determine the laser process parameters for the laser surface alloying technique with high statistical accuracy and yield a laser surface alloying technique capable of achieving a desirable dilution ratio. Energy dispersive spectrometry consistently showed that the per cent by weight of Ni was reduced by 45 per cent as compared with untreated NiTi SMA when the Taguchi-determined laser processing parameters were employed, thus verifying the laser's processing parameters as optimum
Flavor changing t -> c l_1^- l_2^+ decay in the general two Higgs doublet model
We study the flavor changing t-> c l_1^- l_2^+ decay in the framework of the
general two Higgs doublet model, the so called model III. We predict the
branching ratio for l_1=\tau, l_2=\mu at the order of magnitude of BR \sim
10^{-8}.Comment: 12 Pages, 5 Figure
Gapped spin liquid states in a one-dimensional Hubbard model with antiferromagnetic exchange interaction
We study the phase diagram of a one-dimensional extended Hubbard model with
antiferromagnetic exchange interaction analytically and numerically. The
bosonization and transfer-matrix renormalization group methods are used in the
corresponding coupling regimes. At half-filling, the system is a Mott insulator
with a finite spin excitation gap if the on-site Coulomb repulsion is fairly
smaller than the antiferromagnetic exchange J. This Mott-insulator is
characterized by the bond-charge-density-wave order or spontaneously
dimerization. In the weak-coupling regime where the spin-charge separation
holds approximately, the critical point separating the gapless and gapped spin
liquid phases is U_c\sim J/2. However, as J increases, the spin-charge
couplings become important and the critical point U_c is significantly
suppressed and eventually tends to zero as J\to \infty. Away from half-filling,
the charge gap completely collapses but the spin gap persists.Comment: 5 pages, 2 figures, to appear in PR
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