3,646 research outputs found
Dispersive photoluminescence decay by geminate recombination in amorphous semiconductors
The photoluminescence decay in amorphous semiconductors is described by power
law at long times. The power-law decay of photoluminescence at
long times is commonly observed but recent experiments have revealed that the
exponent, , is smaller than the value 1.5 predicted from a
geminate recombination model assuming normal diffusion. Transient currents
observed in the time-of-flight experiments are highly dispersive characterized
by the disorder parameter smaller than 1. Geminate recombination rate
should be influenced by the dispersive transport of charge carriers. In this
paper we derive the simple relation, . Not only the
exponent but also the amplitude of the decay calculated in this study is
consistent with measured photoluminescence in a-Si:H.Comment: 18pages. Submitted for the publication in Phys. Rev.
Detection of Iron Emission in the z = 5.74 QSO SDSSp J104433.04-012502.2
We obtained near-infrared spectroscopy of the z=5.74 QSO, SDSSp
J104433.04-012 502.2 with the Infrared Camera and Spectrograph of the Subaru
telescope. The redshift of 5.74 corresponds to a cosmological age of 1.0 Gyr
for the current Lambda-dominated cosmology. We found a similar strength of the
Fe II (3000-3500 A) emission lines in SDSSp J104433.04-012502.2 as in low
redshift QSOs. This is the highest redshift detection of iron. We subtracted a
power-law continuum from the spectrum and fitted model Fe II emission and
Balmer continuum. The rest equivalent width of Fe II (3000-3500 A) is ~30 A
which is similar to those of low redshift QSOs measured by the same manner. The
chemical enrichment models that assume the life time of the progenitor of SNe
Ia is longer than 1 Gyr predict that weaker Fe II emission than low red shift.
However, none of the observed high redshift (z > 3) QSOs show a systematic dec
rease of Fe II emission compared with low redshift QSOs. This may due to a
shorter lifetime of SNe Ia in QSO nuclei than in the solar neighborhood.
Another reason of strong Fe II emission at z=5.74 may be longer cosmological
age due to smaller Omega_M.Comment: 5 pages, 3 figure
Universal Behavior in Large-scale Aggregation of Independent Noisy Observations
Aggregation of noisy observations involves a difficult tradeoff between
observation quality, which can be increased by increasing the number of
observations, and aggregation quality which decreases if the number of
observations is too large. We clarify this behavior for a protypical system in
which arbitrarily large numbers of observations exceeding the system capacity
can be aggregated using lossy data compression. We show the existence of a
scaling relation between the collective error and the system capacity, and show
that large scale lossy aggregation can outperform lossless aggregation above a
critical level of observation noise. Further, we show that universal results
for scaling and critical value of noise which are independent of system
capacity can be obtained by considering asymptotic behavior when the system
capacity increases toward infinity.Comment: 10 pages, 3 figure
Stress concentration in the vicinity of a hole defect under conditions of Hertzian contact
Two dimensional photoelastic stress analyses were conducted for epoxy resin models containing a hole defect under the conditions of Hertzian contact. Stress concentrations around the defect were determined as a function of several parameters. The effect of tangential traction on the stress concentration was also determined. Sharp stress concentrations occur in the vicinity of both the left and the right side of the hole. The stress concentration becomes more distinct the larger the hole diameter and the smaller distance between the hole and the contact surface. The stress concentration is greatest when the disk imposing a normal load is located at the contact surface directly over the hole. The magnitude and the location of stress concentration varies with the distance between the Hertzian contact area and the hole. The area involved in a process of rolling contact fatigue is confined to a shallow region at both sides of the hole. It was found that the effect of tangential traction is comparatively small on the stress concentration around the hole
Impurity effects at finite temperature in the two-dimensional S=1/2 Heisenberg antiferromagnet
We discuss effects of various impurities on the magnetic susceptibility and
the specific heat of the quantum S=1/2 Heisenberg antiferromagnet on a
two-dimensional square lattice. For impurities with spin S_i > 0 (here S_i=1/2
in the case of a vacancy or an added spin, and S_i=1 for a spin coupled
ferromagnetically to its neighbors), our quantum Monte Carlo simulations
confirm a classical-like Curie susceptibility contribution S_i^2/4T, which
originates from an alignment of the impurity spin with the local N\'eel order.
In addition, we find a logarithmically divergent contribution, which we
attribute to fluctuations transverse to the local N\'eel vector. We also study
frustrated and nonfrustrated bond impurities with S_i=0. For a simple intuitive
picture of the impurity problem, we discuss an effective few-spin model that
can distinguish between the different impurities and reproduces the
leading-order simulation data over a wide temperature range.Comment: 15 pages, 14 figures, submitted to PRB. v2, published version with
cosmetic change
Simple Scheme for Gauge Mediation
We present a simple scheme for constructing models that achieve successful
gauge mediation of supersymmetry breaking. In addition to our previous work [1]
that proposed drastically simplified models using metastable vacua of
supersymmetry breaking in vector-like theories, we show there are many other
successful models using various types of supersymmetry breaking mechanisms that
rely on enhanced low-energy U(1)_R symmetries. In models where supersymmetry is
broken by elementary singlets, one needs to assume U(1)_R violating effects are
accidentally small, while in models where composite fields break supersymmetry,
emergence of approximate low-energy U(1)_R symmetries can be understood simply
on dimensional grounds. Even though the scheme still requires somewhat small
parameters to sufficiently suppress gravity mediation, we discuss their
possible origins due to dimensional transmutation. The scheme accommodates a
wide range of the gravitino mass to avoid cosmological problems.Comment: 13 page
Cascade events at IceCube + DeepCore as a definitive constraint on the dark matter interpretation of the PAMELA and Fermi anomalies
Dark matter decaying or annihilating into μ^+μ^- or τ^+τ^- has been proposed as an explanation for the e^± anomalies reported by PAMELA and Fermi. Recent analyses show that IceCube, supplemented by DeepCore, will be able to significantly constrain the parameter space of decays to μ^+μ^-, and rule out decays to τ^+τ^- and annihilations to μ^+μ^- in less than five years of running. These analyses rely on measuring tracklike events in IceCube + DeepCore from down-going ν_μ. In this paper we show that by instead measuring cascade events, which are induced by all neutrino flavors, IceCube + DeepCore can rule out decays to μ^+μ^- in only three years of running, and rule out decays to τ^+τ^- and annihilation to μ^+μ^- in only one year of running. These constraints are highly robust to the choice of dark matter halo profile and independent of dark matter-nucleon crosssection
Renormalization Group Invariance of Exact Results in Supersymmetric Gauge Theories
We clarify the notion of Wilsonian renormalization group (RG) invariance in
supersymmetric gauge theories, which states that the low-energy physics can be
kept fixed when one changes the ultraviolet cutoff, provided appropriate
changes are made to the bare coupling constants in the Lagrangian. We first
pose a puzzle on how a quantum modified constraint (such as Pf(Q^i Q^j) =
\Lambda^{2(N+1)} in SP(N) theories with N+1 flavors) can be RG invariant, since
the bare fields Q^i receive wave function renormalization when one changes the
ultraviolet cutoff, while we naively regard the scale \Lambda as RG invariant.
The resolution is that \Lambda is not RG invariant if one sticks to canonical
normalization for the bare fields as is conventionally done in field theory. We
derive a formula for how \Lambda must be changed when one changes the
ultraviolet cutoff. We then compare our formula to known exact results and show
that their consistency requires the change in \Lambda we have found. Finally,
we apply our result to models of supersymmetry breaking due to quantum modified
constraints. The RG invariance helps us to determine the effective potential
along the classical flat directions found in these theories. In particular, the
inverted hierarchy mechanism does not occur in the original version of these
models.Comment: LaTeX, 26 page
Linear Complexity Lossy Compressor for Binary Redundant Memoryless Sources
A lossy compression algorithm for binary redundant memoryless sources is
presented. The proposed scheme is based on sparse graph codes. By introducing a
nonlinear function, redundant memoryless sequences can be compressed. We
propose a linear complexity compressor based on the extended belief
propagation, into which an inertia term is heuristically introduced, and show
that it has near-optimal performance for moderate block lengths.Comment: 4 pages, 1 figur
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