742 research outputs found
Helical Organic and Inorganic Polymers
Despite being a staple of synthetic plastics and biomolecules, helical
polymers are scarcely studied with Gaussian-basis-set {\it ab initio}
electron-correlated methods on an equal footing with molecules. This article
introduces an {\it ab initio} second-order many-body Green's-function [MBGF(2)]
method with nondiagonal, frequency-dependent Dyson self-energy for infinite
helical polymers using screw-axis-symmetry-adapted Gaussian-spherical-harmonics
basis functions. Together with the Gaussian-basis-set density-functional theory
for energies, analytical atomic forces, translational-period force, and
helical-angle force, it can compute correlated energy, quasiparticle energy
bands, structures, and vibrational frequencies of an infinite helical polymer,
which smoothly converge at the corresponding oligomer results. These methods
can handle incommensurable structures, which have an infinite translational
period and are hard to characterize by any other method, just as efficiently as
commensurable structures. We apply these methods to polyethylene ( helix),
polyacetylene (Peierls' system), and polytetrafluoroethylene ( helix) to
establish the quantitative accuracy of MBGF(2)/cc-pVDZ in simulating their
(angle-resolved) ultraviolet photoelectron spectra, and of B3LYP/cc-pVDZ or
6-31G** in reproducing their structures, infrared and Raman band positions,
phonon dispersions, and (coherent and incoherent) inelastic neutron scattering
spectra. We then predict the same properties for infinitely catenated chains of
nitrogen or oxygen and discuss their possible metastable existence under
ambient conditions. They include planar zigzag polyazene (N) (Peierls'
system), -helical isotactic polyazane (NH), -helical isotactic
polyfluoroazane (NF), and -helical polyoxane (O) as potential
high-energy-density materials
Monte Carlo study of Si(111) homoepitaxy
An attempt is made to simulate the homoepitaxial growth of a Si(111) surface
by the kinetic Monte Carlo method in which the standard Solid-on-Solid model
and the planar model of the (7x7) surface reconstruction are used in
combination.
By taking account of surface reconstructions as well as atomic deposition and
migrations, it is shown that the effect of a coorparative stacking
transformation is necessary for a layer growth.Comment: 4 pages, 5 figures. For Fig.1 of this article, please see Fig.2 of
Phys.Rev. B56, 3583 (1997). To appear in Phys.Rev.B. (June 1998
Double-Exchange Ferromagnetism and Orbital-Fluctuation-Induced Superconductivity in Cubic Uranium Compounds
A double-exchange mechanism for the emergence of ferromagnetism in cubic
uranium compounds is proposed on the basis of a - coupling scheme. The
idea is {\it orbital-dependent duality} of electrons concerning itinerant
and localized states in the cubic structure. Since
orbital degree of freedom is still active in the ferromagnetic phase,
orbital-related quantum critical phenomenon is expected to appear. In fact,
odd-parity p-wave pairing compatible with ferromagnetism is found in the
vicinity of an orbital ordered phase. Furthermore, even-parity d-wave pairing
with significant odd-frequency components is obtained. A possibility to observe
such exotic superconductivity in manganites is also discussed briefly.Comment: 4 pages, 4 figures. To appear in J. Phys. Soc. Jp
On the Puzzle of Odd-Frequency Superconductivity
Since the first theoretical proposal by Berezinskii, an odd-frequency
superconductivity has encountered the fundamental problems on its thermodynamic
stability and rigidity of a homogenous state accompanied by unphysical Meissner
effect. Recently, Solenov {\it et al}. [Phys. Rev. B {\bf 79} (2009) 132502.]
have asserted that the path-integral formulation gets rid of the difficulties
leading to a stable homogenous phase with an ordinary Meissner effect. Here, we
show that it is crucial to choose the appropriate saddle-point solution that
minimizes the effective free energy, which was assumed {\it implicitly} in the
work by Solenov and co-workers. We exhibit the path-integral framework for the
odd-frequency superconductivity with general type of pairings, including an
argument on the retarded functions via the analytic continuation to the real
axis.Comment: 6 pages, in JPSJ forma
Boundary Layer Receptivity to Localized Disturbances in Freestream Caused by a Vortex Ring Collision
The receptivity of a smooth flat plate to localized disturbances in freestream is investigated experimentally and numerically. The disturbances are generated outside a nominally-zero-pressure-gradient laminar boundary layer by a collision of two identical vortex rings with opposite signs. The vortex rings are generated by intermittent ejections of short duration jets from nozzles facing each other in the spanwise direction. A pair of rolled up vortex rings is given as the initial disturbances in the direct numerical simulation, and the growth of a boundary layer is simulated for arange of the Reynolds number based on the displacement thickness of boundary layer, 704 ≦ Re * ≦ 844. In the experimental results, high- and low-speed regions aligned in the streamwise direction are observed in the boundary layer before the external disturbances in the freestream reach the outer-edge of the boundary layer. Although velocity fluctuations inside both regions become stronger downstream, a transition to turbulence takes place only in the highspeed region at approximately Re * = 844. In the numerical results, vortical fluctuations similar to the experiment appear near the wall immediately after the vortex-ring-type disturbances are added in the freestream, but it is found that the vortical fluctuations do not directly grow into strong vortical structures. On the contrary, the development of strong vortical structures that leads to transition is triggered by the external disturbances directly intruding the boundary layer
Possible Odd-Frequency Superconductivity in Strong-Coupling Electron-Phonon Systems
A possibility of the odd-frequency pairing in the strong-coupling
electron-phonon systems is discussed. Using the Holstein-Hubbard model, we
demonstrate that the anomalously soft Einstein mode with the frequency
( is the order of the renormalized
bandwidth) mediates the s-wave odd-frequency triplet pairing against the
ordinary even-frequency singlet pairing. It is necessary for the emergence of
the odd-frequency pairing that the pairing interaction is strongly retarded as
well as the strong coupling, since the pairing interaction for the
odd-frequency pairing is effective only in the diagonal scattering channel,
with
. Namely, the odd-frequency
superconductivity is realized in the opposite limit of the original BCS theory.
The Ginzburg-Landau analysis in the strong-coupling region shows that the
specific-heat discontinuity and the slope of the temperature dependence of the
superfluid density can be quite small as compared with the BCS values,
depending on the ratio of the transition temperature and .Comment: 6 pages, 7 figures, submitted to J. Phys. Soc. Jp
A novel missense mutation of SLC7A9 frequent in Japanese cystinuria cases affecting the C-terminus of the transporter
Cystinuria is caused by the inherited defect of apical membrane transport systems for cystine and dibasic amino acids in renal proximal tubules. Mutations in either SLC7A9 or SLC3A1 gene result in cystinuria. The mutations of SLC7A9 gene have been identified mainly from Italian, Libyan Jewish, North American, and Spanish patients. In the present study, we have analyzed cystinuria cases from oriental population (mostly Japanese). Mutation analyses of SLC7A9 and SLC3A1 genes were performed on 41 cystinuria patients. The uptake of 14C-labeled cystine in COS-7 cells was measured to determine the functional properties of mutants. The protein expression and localization were examined by Western blot and confocal laser-scanning microscopy. Among 41 patients analyzed, 35 were found to possess mutations in SLC7A9. The most frequent one was a novel missense mutation P482L that affects a residue near the C-terminus end of the protein and causes severe loss of function. In MDCK II and HEK293 cells, we found that P482L protein was expressed and sorted to the plasma membrane as well as wild type. The alteration of Pro482 with amino acids with bulky side chains reduced the transport function of b0,+AT/BAT1. Interestingly, the mutations of SLC7A9 for Japanese cystinuria patients are different from those reported for European and American population. The results of the present study contribute toward understanding the distribution and frequency of cystinuria-related mutations of SLC7A9
On the Meissner Effect of the Odd-Frequency Superconductivity with Critical Spin Fluctuations: Possibility of Zero Field FFLO pairing
We investigate the influence of critical spin fluctuations on electromagnetic
responses in the odd-frequency superconductivity. It is shown that the Meissner
kernel of the odd-frequency superconductivity is strongly reduced by the
critical spin fluctuation or the massless spin wave mode in the
antiferromagnetic phase. These results imply that the superfluid density is
reduced, and the London penetration depth is lengthened for the odd-frequency
pairing. It is also shown that the zero field Flude-Ferrell-Larkin-Ovchinnikov
pairing is spontaneously realized both for even- and odd-frequency in the case
of sufficiently strong coupling with low lying spin-modes.Comment: 10 pages, 7 figure
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