283 research outputs found
Excitons in Mott insulators
Motivated by recent Raman and resonant inelastic X-ray scattering experiments
performed for Mott insulators, which suggest formation of excitons in these
systems, we present a theory of exciton formation in the upper Hubbard band.
The analysis based on the spin polaron approach is performed in the framework
of an effective t-J model for the subspace of states with one doubly occupied
site. Our results confirm the existence of excitons and bear qualitative
resemblance to experimental data despite some simplifications in our approach.
They prove that the basic underlying mechanismof exciton formation is the same
as that which gives rise to binding of holes in weakly doped antiferromagnets.Comment: 4 pages, 1 figur
The size of selected human skull foramina in relation to skull capacity
An anatomical study was undertaken in order to investigate whether the sizes of
selected human skull foramina with significant venous compartments correlated
significantly with skull capacity. A total of 100 macerated human skulls were
examined to determine the diameter of the foramina and the skull capacity.
Measurements of the surface area of the foramina were made using a computerised
digital analysis system.
Only the size of the hypoglossal canal and jugular foramen were found to correlate
significantly with the capacity of the skull. This correlation, together with
the considerable size of the hypoglossal canal, indicated its important role in the
venous drainage of the brain.
There was considerable centralisation of venous outflow from the brain, with
60% of the area of all venous foramina of the skull occupied by jugular foramina.
Asymmetry between the right and left jugular foramina was identified, with an
average ratio of 1.6 (ranging between 1 and 3.47). In the case of right-sided
domination the correlation between the skull capacity and the size of both jugular
foramina was negative (the larger the skull cavity, the less the asymmetry),
while in the case of left-sided domination the correlation was positive. Perhaps
the left-sided domination is less advantageous for the haemodynamics of blood
outflow, as the left brachiocephalic vein is longer and is often compressed by
the sternum and aortic arch
Preservation and sterilization methods of the meniscal allografts : literature review
Nowadays, there are four types of meniscal
allografts known: fresh, cryopreserved, deep-frozen
and lyophilized ones but only two of them are widely
used in clinical practice. Use of different types of
meniscal allografts still remains controversial due to
preparation method, their biomechanical properties as
well as cost which is connected with processing and
storage. The main aim of this review is to present the
current status of knowledge concerning meniscal
allograft preservation and sterilization, especially the
advantages and disadvantages of each method.
Authors wanted to show a broad spectrum of methods
used and conceptions presented by other authors. The
second aim is to gather available information about
meniscal preservation and sterilization methods in one
paper. Deep-frozen and cryopreserved meniscal allografts
are the most frequently used ones in the clinical
practice. The use of fresh grafts stays controversial but
also has many followers. Lyophilized grafts in turn are
not applied at present due to some serious drawbacks
including reduction of tensile strength, poor rehydration,
graft shrinkage and post-transplantation joint
effusion as well as increased risk of meniscal size
reduction. An application of sterilizing agents make
the meniscal allograft free from the bacteria and
viruses, but also it may cause serious structure changes.
Therefore, choosing just one ideal method of meniscal
allograft preservation and sterilization is complicated
and should be based on broad knowledge and experience
of surgeon performing the transplantation
Relation between flux formation and pairing in doped antiferromagnets
We demonstrate that patterns formed by the current-current correlation
function are landmarks which indicate that spin bipolarons form in doped
antiferromagnets. Holes which constitute a spin bipolaron reside at opposite
ends of a line (string) formed by the defects in the antiferromagnetic spin
background. The string is relatively highly mobile, because the motion of a
hole at its end does not raise extensively the number of defects, provided that
the hole at the other end of the line follows along the same track. Appropriate
coherent combinations of string states realize some irreducible representations
of the point group C_4v. Creep of strings favors d- and p-wave states. Some
more subtle processes decide the symmetry of pairing. The pattern of the
current correlation function, that defines the structure of flux, emerges from
motion of holes at string ends and coherence factors with which string states
appear in the wave function of the bound state. Condensation of bipolarons and
phase coherence between them puts to infinity the correlation length of the
current correlation function and establishes the flux in the system.Comment: 5 pages, 6 figure
Universal Behavior of the Resistance Noise across the Metal-Insulator Transition in Silicon Inversion Layers
Studies of low-frequency resistance noise show that the glassy freezing of
the two-dimensional (2D) electron system in the vicinity of the metal-insulator
transition occurs in all Si inversion layers. The size of the metallic glass
phase, which separates the 2D metal and the (glassy) insulator, depends
strongly on disorder, becoming extremely small in high-mobility samples. The
behavior of the second spectrum, an important fourth-order noise statistic,
indicates the presence of long-range correlations between fluctuators in the
glassy phase, consistent with the hierarchical picture of glassy dynamics.Comment: revtex4; 4+ pages, 5 figure
Stripes in Doped Antiferromagnets: Single-Particle Spectral Weight
Recent photoemission (ARPES) experiments on cuprate superconductors provide
important guidelines for a theory of electronic excitations in the stripe
phase. Using a cluster perturbation theory, where short-distance effects are
accounted for by exact cluster diagonalization and long-distance effects by
perturbation (in the hopping), we calculate the single-particle Green's
function for a striped t-J model. The data obtained quantitatively reproduce
salient (ARPES-) features and may serve to rule out "bond-centered" in favor of
"site-centered" stripes.Comment: final version as appeared in PRL; (c) 2000 The American Physical
Society; 4 pages, 4 figure
Spectral density for a hole in an antiferromagnetic stripe phase
Using variational trial wave function based on the string picture we study
the motion of a single mobile hole in the stripe phase of the doped
antiferromagnet. The holes within the stripes are taken to be static, the
undoped antiferromagnetic domains in between the hole stripes are assumed to
have alternating staggered magnetization, as is suggested by neutron scattering
experiments. The system is described by the t-t'-t''-J model with realistic
parameters and we compute the single particle spectral density.Comment: RevTex-file, 9 PRB pages with 15 .eps and .gif files. To appear in
PRB. Hardcopies of figures (or the entire manuscript) can be obtained by
e-mail request to: [email protected]
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