4,994 research outputs found
Signatures of Electronic Correlations in Optical Properties of LaFeAsOF
Spectroscopic ellipsometry is used to determine the dielectric function of
the superconducting LaFeAsOF ( = 27 K) and undoped LaFeAsO
polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 350 K. The free charge carrier response in both samples is heavily
damped with the effective carrier density as low as 0.0400.005 electrons
per unit cell. The spectral weight transfer in the undoped LaFeAsO associated
with opening of the pseudogap at about 0.65 eV is restricted at energies below
2 eV. The spectra of superconducting LaFeAsOF reveal a
significant transfer of the spectral weight to a broad optical band above 4 eV
with increasing temperature. Our data may imply that the electronic states near
the Fermi surface are strongly renormalized due to electron-phonon and/or
electron-electron interactions.Comment: 4 pages, 4 figures, units in Fig.2 adde
What is the Entanglement Length in a Polymer Melt ?
We present results of molecular dynamics simulations of very long model
polymer chains analyzed by various experimentally relevant techniques. The
segment motion of the chains is found to be in very good agreement with the
repatation model. We also calculated the plateau-modulus G_N. The predicitions
of the entanglement length N_e from G_N and from the mean square displacements
of the chains segments disagree by a factor of about 2.2(2), indicating an
error in the prefactor in the standard formula for G_N. We show that recent
neutron spin echo measurements were carried out for chain lengths which are too
small for a correct determination of N_e.Comment: 5 pages, 4 figures, RevTe
Field theory of self-avoiding walks in random media
Based on the analogy with the quantum mechanics of a particle propagating in
a {\em complex} potential, we develop a field-theoretical description of the
statistical properties of a self-avoiding polymer chain in a random
environment. We show that the account of the non-Hermiticity of the quantum
Hamiltonian results in a qualitatively different structure of the effective
action, compared to previous studies. Applying the renormalisation group
analysis, we find a transition between the weak-disorder regime, where the
quenched randomness is irrelevant, and the strong-disorder regime, where the
polymer chain collapses. However, the fact that the renormalised interaction
constants and the chiral symmetry breaking regularisation parameter flow
towards strong coupling raises questions about the applicability of the
perturbative analysis.Comment: RevTeX, 9 pages; accepted for publication in J. Phys.
Interacting Growth Walk - a model for hyperquenched homopolymer glass?
We show that the compact self avoiding walk configurations, kinetically
generated by the recently introduced Interacting Growth Walk (IGW) model, can
be considered as members of a canonical ensemble if they are assigned random
values of energy. Such a mapping is necessary for studying the thermodynamic
behaviour of this system. We have presented the specific heat data for the IGW,
obtained from extensive simulations on a square lattice; we observe a broad
hump in the specific heat above the -point, contrary to expectation.Comment: 4 figures; Submitted to PR
Direct observation of substitutional Ga after ion implantation in Ge by means of extended x-ray absorption fine structure
Kinetic growth walks on complex networks
Kinetically grown self-avoiding walks on various types of generalized random
networks have been studied. Networks with short- and long-tailed degree
distributions were considered (, degree or connectivity), including
scale-free networks with . The long-range behaviour of
self-avoiding walks on random networks is found to be determined by finite-size
effects. The mean self-intersection length of non-reversal random walks, ,
scales as a power of the system size $N$: $ \sim N^{\beta}$, with an
exponent $\beta = 0.5$ for short-tailed degree distributions and $\beta < 0.5$
for scale-free networks with $\gamma < 3$. The mean attrition length of kinetic
growth walks, , scales as , with an exponent
which depends on the lowest degree in the network. Results of
approximate probabilistic calculations are supported by those derived from
simulations of various kinds of networks. The efficiency of kinetic growth
walks to explore networks is largely reduced by inhomogeneity in the degree
distribution, as happens for scale-free networks.Comment: 10 pages, 8 figure
Collapse of Randomly Self-Interacting Polymers
We use complete enumeration and Monte Carlo techniques to study
self--avoiding walks with random nearest--neighbor interactions described by
, where is a quenched sequence of ``charges'' on the
chain. For equal numbers of positive and negative charges (), the
polymer with undergoes a transition from self--avoiding behavior to a
compact state at a temperature . The collapse temperature
decreases with the asymmetry Comment: 8 pages, TeX, 4 uuencoded postscript figures, MIT-CMT-
Spinocerebellar ataxia types 1, 2, 3, and 6: disease severity and nonataxia symptoms.
OBJECTIVE: To identify factors that determine disease severity and clinical
phenotype of the most common spinocerebellar ataxias (SCAs), we studied 526
patients with SCA1, SCA2, SCA3. or SCA6.
METHODS: To measure the severity of ataxia we used the Scale for the Assessment
and Rating of Ataxia (SARA). In addition, nonataxia symptoms were assessed with
the Inventory of Non-Ataxia Symptoms (INAS). The INAS count denotes the number of
nonataxia symptoms in each patient.
RESULTS: An analysis of covariance with SARA score as dependent variable and
repeat lengths of the expanded and normal allele, age at onset, and disease
duration as independent variables led to multivariate models that explained 60.4%
of the SARA score variance in SCA1, 45.4% in SCA2, 46.8% in SCA3, and 33.7% in
SCA6. In SCA1, SCA2, and SCA3, SARA was mainly determined by repeat length of the
expanded allele, age at onset, and disease duration. The only factors determining
the SARA score in SCA6 were age at onset and disease duration. The INAS count was
5.0 +/- 2.3 in SCA1, 4.6 +/- 2.2 in SCA2, 5.2 +/- 2.5 in SCA3, and 2.0 +/- 1.7 in
SCA6. In SCA1, SCA2, and SCA3, SARA score and disease duration were the strongest
predictors of the INAS count. In SCA6, only age at onset and disease duration had
an effect on the INAS count.
CONCLUSIONS: Our study suggests that spinocerebellar ataxia (SCA) 1, SCA2, and
SCA3 share a number of common biologic properties, whereas SCA6 is distinct in
that its phenotype is more determined by age than by disease-related factors
Statistics of self-avoiding walks on randomly diluted lattice
A comprehensive numerical study of self-avoiding walks (SAW's) on randomly
diluted lattices in two and three dimensions is carried out. The critical
exponents and are calculated for various different occupation
probabilities, disorder configuration ensembles, and walk weighting schemes.
These results are analyzed and compared with those previously available.
Various subtleties in the calculation and definition of these exponents are
discussed. Precise numerical values are given for these exponents in most
cases, and many new properties are recognized for them.Comment: 34 pages (+ 12 figures), REVTEX 3.
Two-Dimensional Polymers with Random Short-Range Interactions
We use complete enumeration and Monte Carlo techniques to study
two-dimensional self-avoiding polymer chains with quenched ``charges'' .
The interaction of charges at neighboring lattice sites is described by . We find that a polymer undergoes a collapse transition at a temperature
, which decreases with increasing imbalance between charges. At the
transition point, the dependence of the radius of gyration of the polymer on
the number of monomers is characterized by an exponent , which is slightly larger than the similar exponent for homopolymers. We
find no evidence of freezing at low temperatures.Comment: 4 two-column pages, 6 eps figures, RevTex, Submitted to Phys. Rev.
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