Accurate Measurements of the Chemical Potential of Polymeric Systems by Monte-Carlo Simulation

We present a new Monte-Carlo method for estimating the chemical potential of model polymer systems. The method is based upon the gradual insertion of a penetrable `ghost' polymer into the system and is effective for large chain lengths and at high densities. Insertion of the ghost chain is facilitated by use of an expanded ensemble in which weighted transitions are permitted between states characterising the strength of the excluded volume and thermal interactions experienced by the ghost chain. We discuss the implementation and optimisation of the method within the framework of the bond fluctuation model, and demonstrate its precision by a calculation of the finite-size corrections to the chemical potential.Comment: 12 pages Latex, Report Number #IP-94.12

Liquid-vapour asymmetry in pure fluids: A Monte Carlo simulation study

Monte Carlo simulations within the grand canonical ensemble are used to obtain the joint distribution of density and energy fluctuations $p_L(\rho,u)$ for two model fluids: a decorated lattice gas and a polymer system. In the near critical region the form of $p_L(\rho,u)$ is analysed using a mixed field finite-size-scaling theory that takes account of liquid-vapour asymmetry. Field mixing transformations are performed that map $p_L(\rho,u)$ onto the joint distribution of critical scaling operators \ptMEstar\ appropriate to the Ising fixed point. Carrying out this procedure permits a very accurate determination of the critical point parameters. By forming various projections of \ptMEstar , the full universal finite-size spectrum of the critical density and energy distributions of fluids is also obtained. In the sub-critical coexistence region, an examination is made of the influence of field mixing on the asymmetry of the density distribution.Comment: 19 pages Latex, 15 Figures available on request. Report Number #IP-94.15

The Lot of the Unemployed: A Time Use Perspective

This paper provides new evidence on time use and subjective well-being of employed and unemployed individuals in 14 countries. We devote particular attention to characterizing and modeling job search intensity, measured by the amount of time devoted to searching for a new job. Job search intensity varies considerably across countries, and is higher in countries that have higher wage dispersion. We also examine the relationship between unemployment benefits and job search.unemployment, job search, time use, unemployment benefits, inequality

New Two-Dimensional Models of Supernova Explosions by the Neutrino-Heating Mechanism: Evidence for Different Instability Regimes in Collapsing Stellar Cores

The neutrino-driven explosion mechanism for core-collapse supernovae in its modern flavor relies on the additional support of hydrodynamical instabilities in achieving shock revival. Two possible candidates, convection and the so-called standing accretion shock instability (SASI), have been proposed for this role. In this paper, we discuss new successful simulations of supernova explosions that shed light on the relative importance of these two instabilities. While convection has so far been observed to grow first in self-consistent hydrodynamical models with multi-group neutrino transport, we here present the first such simulation in which the SASI grows faster while the development of convection is initially inhibited. We illustrate the features of this SASI-dominated regime using an explosion model of a 27 solar mass progenitor, which is contrasted with a convectively-dominated model of an 8.1 solar mass progenitor with subsolar metallicity, whose early post-bounce behavior is more in line with previous 11.2 and 15 solar mass explosion models. We analyze the conditions discriminating between the two different regimes, showing that a high mass-accretion rate and a short advection time-scale are conducive for strong SASI activity. We also briefly discuss some important factors for capturing the SASI-driven regime, such as general relativity, the progenitor structure, a nuclear equation of state leading to a compact proto-neutron star, and the neutrino treatment. Finally, we evaluate possible implications of our findings for 2D and 3D supernova simulations. Our results show that a better understanding of the SASI and convection in the non-linear regime is required.Comment: 12 pages, 13 figures; revised version accepted for publication in Ap

Job Search and Unemployment Insurance: New Evidence from Time Use Data

This paper provides new evidence on job search intensity of the unemployed in the U.S., modeling job search intensity as time allocated to job search activities. The main findings are: 1) the average unemployed worker in the U.S. devotes about 41 minutes to job search on weekdays, which is substantially more than his or her European counterpart; 2) workers who expect to be recalled by their previous employer search substantially less than the average unemployed worker; 3) across the 50 states and D.C., job search is inversely related to the generosity of unemployment benefits, with an elasticity between -1.6 and -2.2; 4) the predicted wage is a strong predictor of time devoted to job search, with an elasticity in excess of 2.5; 5) job search intensity for those eligible for Unemployment Insurance (UI) increases prior to benefit exhaustion; 6) time devoted to job search is fairly constant regardless of unemployment duration for those who are ineligible for UI. A nonparametric Monte Carlo technique suggests that the relationship between job search effort and the duration of unemployment for a cross-section of job seekers is only slightly biased by length-based sampling.unemployment, unemployment insurance, job search, time use, unemployment benefits, inequality

Resonant photon absorption in the low spin molecule V15

We report the first study of the micro-SQUID response of a molecular system to electromagnetic radiation. The advantages of our micro-SQUID technique in respect to pulsed electron paramagnetic resonance (EPR) techniques consist in the possibility to perform time-resolved experiments (below 1 ns) on submicrometer sizes samples (about 1000 spins) at low temperature (below 100 mK). Resonant photon absorption in the GHz range was observed via low temperature micro-SQUID magnetization measurements of the spin ground state S = 1/2 of the molecular complex V15. The line-width essentially results from intra-molecular hyperfine interaction. The results point out that observing Rabi oscillations in molecular nanomagnets requires well isolated low spin systems and high radiation power. Our first results open the way for time-resolved observations of quantum superposition of spin-up and spin-down states in SMMs.Comment: 7 pages, 5 figure