260 research outputs found
LS Penrose’s limit theorem: Tests by simulation
LS Penrose’s limit theorem (PLT) – which is implicit in Penrose [5, p. 72] and for which he gave no rigorous proof – says that, in simple weighted voting games, if the number of voters increases indefinitely while existing voters retain their weights and the relative quota is pegged, then – under certain conditions – the ratio between the voting powers of any two voters converges to the ratio between their weights. Lindner and Machover [3] prove some special cases of PLT; and conjecture that the theorem holds, under rather general conditions, for large classes of weighted voting games, various values of the quota, and with respect to several measures of voting power. We use simulation to test this conjecture. It is corroborated w.r.t. the Penrose–Banzhaf index for a quota of 50% but not for other values; w.r.t. the Shapley–Shubik index the conjecture is corroborated for all values of the quota (short of 100%).limit theorems, majority games, simulation, weighted voting games
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Evolution of superconductivity in K2-xFe4+ySe5: Spectroscopic studies of X-ray absorption and emission.
This study investigates the evolution of superconductivity in K2-xFe4+ySe5 using temperature-dependent X-ray absorption and resonant inelastic X-ray scattering techniques. Magnetization measurements show that polycrystalline superconducting (SC) K1.9Fe4.2Se5 has a critical temperature (T c) of ∼31 K with a varying superconducting volume fraction, which strongly depends on its synthesis temperature. An increase in Fe-structural/vacancy disorder in SC samples with more Fe atoms occupying vacant 4d sites is found to be closely related to the decrease in the spin magnetic moment of Fe. Moreover, the nearest-neighbor Fe-Se bond length in SC samples exceeds that in the non-SC (NS) sample, K2Fe4Se5, which indicates a weaker hybridization between the Fe 3d and Se 4p states in SC samples. These results clearly demonstrate the correlations among the local electronic and atomic structures and the magnetic properties of K2-xFe4+ySe5 superconductors, providing deeper insight into the electron pairing mechanisms of superconductivity
Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors
We calculate the c-axis infrared conductivity in
underdoped cuprate superconductors for spinfluctuation exchange scattering
within the CuO-planes including a phenomenological d-wave pseudogap of
amplitude . For temperatures decreasing below a temperature , a gap for develops in in the
incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting'
behavior, i.e. it increases for low temperatures above the constant behavior
for . We find that the pseudogap structure in the in-plane optical
conductivity is about twice as big as in the interplane conductivity
, in qualitative agreement with experiment. This is a
consequence of the fact that the spinfluctuation exchange interaction is
suppressed at low frequencies as a result of the opening of the pseudogap.
While the c-axis conductivity in the underdoped regime is described best by
incoherent transmission, in the overdoped regime coherent conductance gives a
better description.Comment: to be published in Phys. Rev. B (November 1, 1999
Collective Modes and the Superconducting State Spectral Function of Bi2212
Photoemission spectra of the high temperature superconductor Bi2212 near
(pi,0) show a dramatic change when cooling below Tc: the broad peak in the
normal state turns into a sharp low energy peak followed by a higher binding
energy hump. Recent experiments find that this low energy peak persists over a
significant range in momentum space. We show in this paper that these data are
well described by a simple model of electrons interacting with a collective
mode which appears only below Tc.Comment: 4 pages, revtex, 4 encapsulated postscript figure
Bosonic Mode Mixing in the Superconducting State Spectral Function of Bi_2Sr_2CaCu_2O_{8+delta}
Photoemission spectra of Bi_2Sr_2CaCu_2O_{8+delta} below T_c show two
features near the (pi,0) point of the zone: a sharp peak at low energy and a
higher binding energy hump. We find that the sharp peak persists at low energy
even as one moves towards (0,0), while the broad hump shows significant
dispersion which correlates well with the normal state dispersion. We argue
that these features are naturally explained by the mixing of electrons with a
bosonic mode which appears only below T_c, and speculate that the latter may be
related to the resonance seen in recent neutron data.Comment: 4 pages, revtex, 4 postscript figure
Increased HIV Incidence in Men Who Have Sex with Men Despite High Levels of ART-Induced Viral Suppression: Analysis of an Extensively Documented Epidemic
Background: There is interest in expanding ART to prevent HIV transmission, but in the group with the highest levels of ART use, men-who-have-sex-with-men (MSM), numbers of new infections diagnosed each year have not decreased as ART coverage has increased for reasons which remain unclear.
Methods: We analysed data on the HIV-epidemic in MSM in the UK from a range of sources using an individual-based simulation model. Model runs using parameter sets found to result in good model fit were used to infer changes in HIV-incidence and risk behaviour.
Results: HIV-incidence has increased (estimated mean incidence 0.30/100 person-years 1990–1997, 0.45/100 py 1998–2010), associated with a modest (26%) rise in condomless sex. We also explored counter-factual scenarios: had ART not been introduced, but the rise in condomless sex had still occurred, then incidence 2006–2010 was 68% higher; a policy of ART initiation in all diagnosed with HIV from 2001 resulted in 32% lower incidence; had levels of HIV testing been higher (68% tested/year instead of 25%) incidence was 25% lower; a combination of higher testing and ART at diagnosis resulted in 62% lower incidence; cessation of all condom use in 2000 resulted in a 424% increase in incidence. In 2010, we estimate that undiagnosed men, the majority in primary infection, accounted for 82% of new infections.
Conclusion: A rise in HIV-incidence has occurred in MSM in the UK despite an only modest increase in levels of condomless sex and high coverage of ART. ART has almost certainly exerted a limiting effect on incidence. Much higher rates of HIV testing combined with initiation of ART at diagnosis would be likely to lead to substantial reductions in HIV incidence. Increased condom use should be promoted to avoid the erosion of the benefits of ART and to prevent other serious sexually transmitted infections
Which biomarker predicts benefit from EGFR-TKI treatment for patients with lung cancer?
Subsets of patients with non-small cell lung cancer respond remarkably well to small molecule tyrosine kinase inhibitors (TKI) specific for epidermal growth factor receptor (EGFR) such as gefitinib or erlotinib. In 2004, it was found that EGFR mutations occurring in the kinase domain are strongly associated with EGFR-TKI sensitivity. However, subsequent studies revealed that this relationship was not perfect and various predictive markers have been reported. These include EGFR gene copy numbers, status of ligands for EGFR, changes in other HER family genes or molecules downstream to EGFR including KRAS or AKT. In this review, we would like to review current knowledge of predictive factors for EGFR-TKI. As all but one phase III trials failed to show a survival advantage of the treatment arm involving EGFR-TKIs, it is necessary to select patients by these biomarkers in future clinical trials. Through these efforts, it would be possible to individualise EGFR-TKI treatment for patients suffering from lung cancer
Theory of Electric Transport in the Pseudogap State of High-Tc Cuprates
We theoretically investigate the electric transport in the pseudogap state of
High-Tc cuprates. Starting from the repulsive Hubbard model, we perform the
microscopic calculation to describe the pseudogap phenomena which are induced
by the superconducting fluctuations. The single particle Green function, spin
susceptibility and superconducting fluctuations are self-consistently
determined by the SC-FLEX+T-matrix approximation. The longitudinal and
transverse conductivities are calculated by using the Eliashberg and
Kohno-Yamada formalism. The effects of the spin fluctuations and
superconducting fluctuations are estimated, respectively. The vertex
corrections arising from the two fluctuations are also calculated. The
additional contribution from the Aslamazov-Larkin term is also estimated beyond
the Eliashberg formalism.
It is shown that the main effect of the superconducting fluctuations is the
feedback effect through the spin fluctuations. The correct results are obtained
by considering the superconducting fluctuations and the spin fluctuations
simultaneously. The temperature and doping dependences of the resistivity and
the Hall coefficient are well explained. We point out that the characteristic
momentum dependence of the systems plays an essential role in this explanation.Comment: To appear in J. Phys. Soc. Jpn. Vol.71 No.1 (2002
Renormalized mean-field theory of the neutron scattering in cuprate superconductors
The magnetic excitation spectrum of the t-t'-J-model is studied in mean-field
theory and compared to inelastic neutron-scattering (INS) experiments on YBCO
and BSCCO superconductors. Within the slave-particle formulation the dynamical
spin response is calculated from a renormalized Fermi liquid with an effective
interaction ~J in the magnetic particle--hole channel. We obtain the so-called
41meV resonance at wave vector (pi,pi) as a collective spin-1 excitation in the
d-wave superconducting state. It appears sharp (undamped), if the underlying
Fermi surface is hole-like with a sufficient next-nearest-neighbor hopping
t'<0. The double-layer structure of YBCO or BSCCO is not important for the
resonance to form. The resonance energy \omega_{res} and spectral weight at
optimal doping come out comparable to experiment. The observed qualitative
behavior of \omega_{res} with hole filling is reproduced in the underdoped as
well as overdoped regime. A second, much broader peak becomes visible in the
magnetic excitation spectrum if the 2D wave-vector is integrated over. It is
caused by excitations across the maximum gap, and in contrast to the resonance
its energy is almost independent of doping. At energies above or below
\omega_{res} the commensurate resonance splits into incommensurate peaks,
located off (pi,pi). Below \omega_{res} the intensity pattern is of `parallel'
type and the dispersion relation of incommensurate peaks has a negative
curvature. This is in accordance with recent INS experiments on YBCO.Comment: 17pp including 14 figure
Two-Particle-Self-Consistent Approach for the Hubbard Model
Even at weak to intermediate coupling, the Hubbard model poses a formidable
challenge. In two dimensions in particular, standard methods such as the Random
Phase Approximation are no longer valid since they predict a finite temperature
antiferromagnetic phase transition prohibited by the Mermin-Wagner theorem. The
Two-Particle-Self-Consistent (TPSC) approach satisfies that theorem as well as
particle conservation, the Pauli principle, the local moment and local charge
sum rules. The self-energy formula does not assume a Migdal theorem. There is
consistency between one- and two-particle quantities. Internal accuracy checks
allow one to test the limits of validity of TPSC. Here I present a pedagogical
review of TPSC along with a short summary of existing results and two case
studies: a) the opening of a pseudogap in two dimensions when the correlation
length is larger than the thermal de Broglie wavelength, and b) the conditions
for the appearance of d-wave superconductivity in the two-dimensional Hubbard
model.Comment: Chapter in "Theoretical methods for Strongly Correlated Systems",
Edited by A. Avella and F. Mancini, Springer Verlag, (2011) 55 pages.
Misprint in Eq.(23) corrected (thanks D. Bergeron
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