9,175 research outputs found
Evaluating statistical methods used to estimate the number of postsynaptic receptors.
Calcium levels in spines play a significant role in determining the sign and magnitude of synaptic plasticity. The magnitude of calcium influx into spines is highly dependent on influx through N-methyl D-aspartate (NMDA) receptors, and therefore depends on the number of postsynaptic NMDA receptors in each spine. We have calculated previously how the number of postsynaptic NMDA receptors determines the mean and variance of calcium transients in the postsynaptic density, and how this alters the shape of plasticity curves. However, the number of postsynaptic NMDA receptors in the postsynaptic density is not well known. Anatomical methods for estimating the number of NMDA receptors produce estimates that are very different than those produced by physiological techniques. The physiological techniques are based on the statistics of synaptic transmission and it is difficult to experimentally estimate their precision. In this paper we use stochastic simulations in order to test the validity of a physiological estimation technique based on failure analysis. We find that the method is likely to underestimate the number of postsynaptic NMDA receptors, explain the source of the error, and re-derive a more precise estimation technique. We also show that the original failure analysis as well as our improved formulas are not robust to small estimation errors in key parameters
Effects of out-of-plane disorder on the nodal quasiparticle and superconducting gap in single-layer BiSrCuO ( = La, Nd, Gd)
How out-of-plane disorder affects the electronic structure has been
investigated for the single-layer cuprates
BiSrCuO ( = La, Nd, Gd) by
angle-resolved photoemission spectroscopy. We have observed that, with
increasing disorder, while the Fermi surface shape and band dispersions are not
affected, the quasi-particle width increases, the anti-nodal gap is enhanced
and the superconducting gap in the nodal region is depressed. The results
indicate that the superconductivity is significantly depressed by out-of-plane
disorder through the enhancement of the anti-nodal gap and the depression of
the superconducting gap in the nodal region
Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources
The X-ray spectral and timing properties of ultraluminous X-ray sources
(ULXs) have many similarities with the very high state of stellar-mass black
holes (power-law dominated, at accretion rates greater than the Eddington
rate). On the other hand, their cool disk components, large characteristic
inner-disk radii and low characteristic timescales have been interpreted as
evidence of black hole masses ~ 1000 Msun (intermediate-mass black holes). Here
we re-examine the physical interpretation of the cool disk model, in the
context of accretion states of stellar-mass black holes. In particular, XTE
J1550-564 can be considered the missing link between ULXs and stellar-mass
black holes, because it exhibits a high-accretion-rate, low-disk-temperature
state (ultraluminous branch). On the ultraluminous branch, the accretion rate
is positively correlated with the disk truncation radius and the bolometric
disk luminosity, while it is anti-correlated with the peak temperature and the
frequency of quasi-periodic-oscillations. Two prototypical ULXs (NGC1313 X-1
and X-2) also seem to move along that branch. We use a phenomenological model
to show how the different range of spectral and timing parameters found in the
two classes of accreting black holes depends on both their masses and accretion
rates. We suggest that ULXs are consistent with black hole masses ~ 50-100
Msun, moderately inefficiently accreting at ~20 times Eddington.Comment: 11 pages, accepted for publication in Astrophysics and Space Science.
Based on work presented at the Fifth Stromlo Symposium, Australian National
University, Dec 200
Doping evolution of the electronic structure in the single-layer cuprates BiSrLaCuO: Comparison with other single-layer cuprates
We have performed angle-resolved photoemission and core-level x-ray
photoemission studies of the single-layer cuprate
BiSrLaCuO (Bi2201) and revealed the doping
evolution of the electronic structure from the lightly-doped to optimally-doped
regions. We have observed the formation of the dispersive quasi-particle band,
evolution of the Fermi ``arc'' into the Fermi surface and the shift of the
chemical potential with hole doping as in other cuprates. The doping evolution
in Bi2201 is similar to that in CaNaCuOCl (Na-CCOC),
where a rapid chemical potential shift toward the lower Hubbard band of the
parent insulator has been observed, but is quite different from that in
LaSrCuO (LSCO), where the chemical potential does not
shift, yet the dispersive band and the Fermi arc/surface are formed around the
Fermi level already in the lightly-doped region. The (underlying) Fermi surface
shape and band dispersions are quantitatively analyzed using tight-binding fit,
and the deduced next-nearest-neighbor hopping integral also confirm the
similarity to Na-CCOC and the difference from LSCO
Effect of strong correlations on the high energy anomaly in hole- and electron-doped high-Tc superconductors
Recently, angle-resolved photoemission spectroscopy (ARPES) has been used to
highlight an anomalously large band renormalization at high binding energies in
cuprate superconductors: the high energy 'waterfall' or high energy anomaly
(HEA). This paper demonstrates, using a combination of new ARPES measurements
and quantum Monte Carlo simulations, that the HEA is not simply the by-product
of matrix element effects, but rather represents a cross-over from a
quasiparticle band at low binding energies near the Fermi level to valence
bands at higher binding energy, assumed to be of strong oxygen character, in
both hole- and electron-doped cuprates. While photoemission matrix elements
clearly play a role in changing the aesthetic appearance of the band
dispersion, i.e. the 'waterfall'-like behavior, they provide an inadequate
description for the physics that underlies the strong band renormalization
giving rise to the HEA. Model calculations of the single-band Hubbard
Hamiltonian showcase the role played by correlations in the formation of the
HEA and uncover significant differences in the HEA energy scale for hole- and
electron-doped cuprates. In addition, this approach properly captures the
transfer of spectral weight accompanying both hole and electron doping in a
correlated material and provides a unifying description of the HEA across both
sides of the cuprate phase diagram.Comment: Original: 4 pages, 4 figures; Replaced: changed and updated content,
12 pages, 6 figure
Infrared cutoff dependence of the critical flavor number in three-dimensional QED
We solve, analytically and numerically, a gap equation in parity invariant
QED_3 in the presence of an infrared cutoff \mu and derive an expression for
the critical fermion number N_c as a function of \mu. We argue that this
dependence of N_c on the infrared scale might solve the discrepancy between
continuum Schwinger-Dyson equations studies and lattice simulations of QED_3.Comment: 5 pages, 1 figure (revtex4), final versio
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