6,946 research outputs found
Effects of Umklapp Scattering on Electronic States in One Dimension
The effects of Umklapp scattering on electronic states are studied in one
spatial dimension at absolute zero. The model is basically the Hubbard model,
where parameters characterizing the normal () and Umklapp () scattering
are treated independently. The density of states is calculated in the t-matrix
approximation by taking only the forward and Umklapp scattering into account.
It is found that the Umklapp scattering causes the global splitting of the
density of states. In the presence of sufficiently strong Umklapp scattering, a
pole in the t-matrix appears in the upper half plane, signalling an instability
towards the 'pairing' ordered state ( is the reciprocal lattice
vector), whose consequences are studied in the mean field approximation. It
turns out that this ordered state coexists with spin-density-wave state and
also brings about Cooper-pairs. A phase diagram is determined in the plane of
and electron filling .Comment: 22 pages, LaTeX, 17 figures included, uses jpsj.st
Rotationally resolved spectroscopy of (20000) Varuna in the near-infrared
Models of the escape and retention of volatiles by minor icy objects exclude
any presence of volatile ices on the surface of TNOs smaller than ~1000km in
diameter at the typical temperature in this region of the solar system, whereas
the same models show that water ice is stable on the surface of objects over a
wide range of diameters. Collisions and cometary activity have been used to
explain the process of surface refreshing of TNOs and Centaurs. These processes
can produce surface heterogeneity that can be studied by collecting information
at different rotational phases. The aims of this work are to study the surface
composition of (20000)Varuna, a TNO with a diameter ~650km and to search for
indications of rotational variability. We observed Varuna during two
consecutive nights in January 2011 with NICS@TNG obtaining a set of spectra
covering the whole rotation period of Varuna. After studying the spectra
corresponding to different rotational phases, we did not find any indication of
surface variability. In all the spectra, we detect an absorption at 2{\mu}m,
suggesting the presence of water ice on the surface. We do not detect any other
volatiles on the surface, although the S/N is not high enough to discard their
presence. Based on scattering models, we present two possible compositions
compatible with our set of data and discuss their implications in the frame of
the collisional history of the Kuiper Belt. We find that the most probable
composition for the surface of Varuna is a mixture of amorphous silicates,
complex organics, and water ice. This composition is compatible with all the
materials being primordial. However, our data can also be fitted by models
containing up to a 10% of methane ice. For an object with the characteristics
of Varuna, this volatile could not be primordial, so an event, such as an
energetic impact, would be needed to explain its presence on the surface.Comment: 6 pages, 5 figures, to be published in A&
Study of the ionic Peierls-Hubbard model using density matrix renormalization group methods
Density matrix renormalization group methods are used to investigate the
quantum phase diagram of a one-dimensional half-filled ionic Hubbard model with
bond-charge attraction, which can be mapped from the Su-Schrieffer-Heeger-type
electron-phonon coupling at the antiadiabatic limit. A bond order wave
(dimerized) phase which separates the band insulator from the Mott insulator
always exists as long as electron-phonon coupling is present. This is
qualitatively different from that at the adiabatic limit. Our results indicate
that electron-electron interaction, ionic potential and quantum phonon
fluctuations combine in the formation of the bond-order wave phase
Fibrillar templates and soft phases in systems with short-range dipolar and long-range interactions
We analyze the thermal fluctuations of particles that have a short-range
dipolar attraction and a long-range repulsion. In an inhomogeneous particle
density region, or "soft phase," filamentary patterns appear which are
destroyed only at very high temperatures. The filaments act as a fluctuating
template for correlated percolation in which low-energy excitations can move
through the stable pattern by local rearrangements. At intermediate
temperatures, dynamically averaged checkerboard states appear. We discuss
possible implications for cuprate superconducting and related materials.Comment: 4 pages, 4 postscript figures. Discussion of implications for
experiment and theory has been expande
Variational description of the dimensional cross-over in the array of coupled one-dimensional conductors
Variational wave function is proposed to describe electronic properties of an
array of one-dimensional conductors coupled by transverse hopping and
interaction. For weak or intermediate in-chain interaction the wave function
has the following structure: Tomonaga-Luttinger bosons with momentum higher
then some variational quantity \tilde\Lambda are in their ground state while
other bosons (with |k|<\tilde\Lambda) form kinks -- fermion-like excitations of
the Tomonaga-Luttinger boson field. Nature of the ground state for this
quasiparticles can be determined by solving three dimensional effective
hamiltonian. Since the anisotropy of the effective hamiltonian is small the use
of the mean field theory is justified. For repulsive interaction possible
phases are density wave and p-wave superconductivity. Our method allows us to
calculate the low-energy part of different electronic Green's functions. In
order to do that it is enough to apply standard perturbation theory technique
to the effective hamiltonian. When the in-chain interaction is strong
\tilde\Lambda vanishes and no fermionic excitation is present in the system. In
this regime the dynamics is described by transversally coupled
Tomonaga-Luttinger bosons
Anisotropy in the helicity modulus of a 3D XY-model: application to YBCO
We present a Monte Carlo study of the helicity moduli of an anisotropic
classical three-dimensional (3D) XY-model of YBCO in superconducting state. It
is found that both the ab-plane and the c-axis helicity moduli, which are
proportional to the inverse square of the corresponding magnetic field
penetration depth, vary linearly with temperature at low temperatures. The
result for the c-axis helicity modulus is in disagreement with the experiments
on high quality samples of YBCO. Thus we conclude that purely classical phase
fluctuations of the superconducting order parameter cannot account for the
observed c-axis electrodynamics of YBCO.Comment: 7 pages, 1 figur
Local origins impart conserved bone type-related differences in human osteoblast behaviour
Osteogenic behaviour of osteoblasts from trabecular, cortical and subchondral bone were examined to determine any bone type-selective differences in samples from both osteoarthritic (OA) and osteoporotic (OP) patients. Cell growth, differentiation; alkaline phosphatase (TNAP) mRNA and activity, Runt-related transcription factor-2 (RUNX2), SP7-transcription factor (SP7), bone sialoprotein-II (BSP-II), osteocalcin/bone gamma-carboxyglutamate (BGLAP), osteoprotegerin (OPG, TNFRSF11B), receptor activator of nuclear factor-κβ ligand (RANKL, TNFSF11) mRNA levels and proangiogenic vascular endothelial growth factor-A (VEGF-A) mRNA and protein release were assessed in osteoblasts from paired humeral head samples from age-matched, human OA/OP (n = 5/4) patients. Initial outgrowth and increase in cell number were significantly faster (p < 0.01) in subchondral and cortical than trabecular osteoblasts, in OA and OP, and this bone type-related differences were conserved despite consistently faster growth in OA. RUNX2/SP7 levels and TNAP mRNA and protein activity were, however, greater in trabecular than subchondral and cortical osteoblasts in OA and OP. BSP-II levels were significantly greater in trabecular and lowest in cortical osteoblasts in both OA and OP. In contrast, BGLAP levels showed divergent bone type-selective behaviour; highest in osteoblasts from subchondral origins in OA and trabecular origins in OP. We found virtually identical bone type-related differences, however, in TNFRSF11B:TNFSF11 in OA and OP, consistent with greater potential for paracrine effects on osteoclasts in trabecular osteoblasts. Subchondral osteoblasts (OA) exhibited highest VEGF-A mRNA levels and release. Our data indicate that human osteoblasts in trabecular, subchondral and cortical bone have inherent, programmed diversity, with specific bone type-related differences in growth, differentiation and pro-angiogenic potential in vitro
Flux pinning and phase separation in oxygen rich La2-xSrxCuO4+y system
We have studied the magnetic characteristics of a series of super-oxygenated
La2-xSrxCuO4+y samples. As shown in previous work, these samples spontaneously
phase separate into an oxygen rich superconducting phase with a TC near 40 K
and an oxygen poor magnetic phase that also orders near 40 K. All samples
studied are highly magnetically reversible even to low temperatures. Although
the internal magnetic regions of these samples might be expected to act as
pinning sites, our present study shows that they do not favor flux pinning.
Flux pinning requires a matching condition between the defect and the
superconducting coherence length. Thus, our results imply that the magnetic
regions are too large to act as pinning centers. This also implies that the
much greater flux pinning in typical La2-xSrxCuO4 materials is the result of
nanoscale inhomogeneities that grow to become the large magnetic regions in the
super-oxygenated materials. The superconducting regions of the phase separated
materials are in that sense cleaner and more homogenous than in the typical
cuprate superconductor.Comment: 4 figures 8 pages Submitted to PR
Simple theory of extremely overdoped HTS
We demonstrate the existence of a simple physical picture of
superconductivity for extremely overdoped CuO2 planes. It possesses all
characteristic features of HTS, such as a high superconducting transition
temperature, the symmetry of order parameter, and the
coexistence of a single electron Fermi surface and a pseudogap in the normal
state. Values of pseudogap are calculated for different doping levels. An
orbital paramagnetism of preformed pairs is predicted.Comment: 7 pages, 1 figur
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