13,986 research outputs found
Strain weakening and superplasticity in a Bi-Sn eutectic alloy processed by high-pressure torsion
High-pressure torsion (HPT) was conducted on disks of a Bi-Sn eutectic alloy under a pressure of 6.0 GPa. The microstructural evolution was studied by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Measurements of Vickers microhardness showed decreasing strength caused by strain weakening after HPT processing. Tensile testing was performed under initial strain rates from 10?4 to 10?2 s?1 at room temperature. The results demonstrate a much improved elongation to failure for the Bi-Sn alloy after HPT- processing. The Bi-Sn alloy processed through 10 turns gave an elongation to failure of more than 1200% at an initial strain rate of 10?4 s?1 at room temperature which is significantly larger than the elongation to failure of ~110% in the as-cast Bi-Sn alloy under the same tensile condition
Threshold Regression for Survival Analysis: Modeling Event Times by a Stochastic Process Reaching a Boundary
Many researchers have investigated first hitting times as models for survival
data. First hitting times arise naturally in many types of stochastic
processes, ranging from Wiener processes to Markov chains. In a survival
context, the state of the underlying process represents the strength of an item
or the health of an individual. The item fails or the individual experiences a
clinical endpoint when the process reaches an adverse threshold state for the
first time. The time scale can be calendar time or some other operational
measure of degradation or disease progression. In many applications, the
process is latent (i.e., unobservable). Threshold regression refers to
first-hitting-time models with regression structures that accommodate covariate
data. The parameters of the process, threshold state and time scale may depend
on the covariates. This paper reviews aspects of this topic and discusses
fruitful avenues for future research.Comment: Published at http://dx.doi.org/10.1214/088342306000000330 in the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Hidden Charge 2e Boson in Doped Mott Insulators: Field Theory of Mottness
We construct the low energy theory of a doped Mott insulator, such as the
high-temperature superconductors, by explicitly integrating over the degrees of
freedom far away from the chemical potential. For either hole or electron
doping, a charge 2e bosonic field emerges at low energy. The charge 2e boson
mediates dynamical spectral weight transfer across the Mott gap and creates a
new charge e excitation by binding a hole. The result is a bifurcation of the
electron dispersion below the chemical potential as observed recently in
angle-resolved photoemission on Pb-doped Bi_2Sr_2CaCu_2O_{8+\delta} (Pb2212).Comment: 4 pages, 2 figures: Correct version to appear in PRL. Revisions
include a derivation of the electron operator at low energies which reveals a
branching structure seen recently in ARPES on Pb221
Phenotypic switching of populations of cells in a stochastic environment
In biology phenotypic switching is a common bet-hedging strategy in the face
of uncertain environmental conditions. Existing mathematical models often focus
on periodically changing environments to determine the optimal phenotypic
response. We focus on the case in which the environment switches randomly
between discrete states. Starting from an individual-based model we derive
stochastic differential equations to describe the dynamics, and obtain
analytical expressions for the mean instantaneous growth rates based on the
theory of piecewise deterministic Markov processes. We show that optimal
phenotypic responses are non-trivial for slow and intermediate environmental
processes, and systematically compare the cases of periodic and random
environments. The best response to random switching is more likely to be
heterogeneity than in the case of deterministic periodic environments, net
growth rates tend to be higher under stochastic environmental dynamics. The
combined system of environment and population of cells can be interpreted as
host-pathogen interaction, in which the host tries to choose environmental
switching so as to minimise growth of the pathogen, and in which the pathogen
employs a phenotypic switching optimised to increase its growth rate. We
discuss the existence of Nash-like mutual best-response scenarios for such
host-pathogen games.Comment: 17 pages, 6 figure
Indeterminacy with No-Income-Effect Preferences and Sector-Specific Externalities
We examine a two-sector real business cycle model with sector-specific externalities in the production of distinct consumption and investment goods. In addition, the household utility is postulated to exhibit no income effect on the demand for leisure. Unlike in the one-sector counterpart, we show that equilibrium indeterminacy can result with sufficiently high returns-to-scale in the production of investment goods. We also find that the smaller the labor supply elasticity, the lower the threshold level of returns-to-scale needed for generating indeterminacy and sunspots. This finding turns out to be exactly the opposite of that in all existing RBC-based indeterminacy studies.Indeterminacy, Income Effect, Sector-Specific Externalities
Reexamining the monetarist critique of interest rate rules
Monetarist economists argued long ago that central bank interest rate rules exacerbate macroeconomic fluctuations, essentially by not allowing the interest rate to respond promptly to shifts in the supply and demand for loans. To support this critique, they pointed to the procyclicality of the money stock. Yet, when there are real shocks and a real business cycle, modern macroeconomic models imply that some procyclicality of money is desirable, to stabilize the price level. A simple interest rate rule illustrates that the monetarist critique can be valid within this model, since the rule exacerbates the response of real activity to real shocks. Other interest rate rules instead limit the macro economy's response to real shocks. But, while these interest rate rules have diverse effects on real activity, there is an important common implication: By smoothing the nominal interest rate in the short run, the rules all lead to increases in the longer-run variability in inflation and nominal interest rates.Interest rates ; Macroeconomics
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