Resonant pairing isotope effect in polaronic systems

The intermediate coupling regime in polaronic systems, situated between the adiabatic and the anti-adiabatic limit, is characterized by resonant pairing between quasi-free electrons which is induced by an exchange interaction with localized bipolarons. The onset of this resonant pairing takes place below a characteristic temperature T* and is manifest in the opening of a pseudogap in the density of states of the electrons. The variation of T* is examined here as a function of (i) the typical frequency \omega_0 of the local lattice modes, which determines the binding energy of the bipolarons, and (ii) the doping, which amounts to a relative change of the bipolaron concentration n_B to that of the free electrons n_F. We concentrate on a doping regime, where small changes in doping give rise to a large change in T*, which is the case when n_B is small (< 0.1 per site). For finite values of n_B we find negative and practically doping independent values of the isotope coefficient \alpha^* which characterizes the formation of resonating electron pairs. Upon decreasing the total particle density such that n_B becomes exponentially small, we find a rapid change in sign of \alpha^*. This is related to the fact that the system approaches a state which is more BCS-like, where electron pairing occurs via virtual excitations into bipolaronic states and where T* coincides with the onset of superconductivity.Comment: 7 pages, 6 figures, enlarged discussion on the limits of validity of the model, to be published in Phys. Rev.

Growth and welfare: Distorting versus non-distorting taxes

In an infinitely-lived framework, taxing capital income may be growth and welfare enhancing when it allows for correcting distorting externalities in the competitive equilibrium allocation. This is the case when public capital is subject to congestion by private capital or total income [Fisher and Turnovsky (1998)] or when government expenditure exerts an external e.ect on physical capital [Corsetti and Roubini (1996)]. However, none of these features appear in simple one-sector endogenous growth models with public capital. Alternatively, we consider certain realistic fiscal policy constraints in a simple one-sector growth model with productive and unproductive public expenditures, to show that raising revenues through factor income taxes may be preferred to using lump-sum taxes.Endogenous growth, distorting taxes, public investment.

Taxing or subsidizing Factors' rents in a simple endogenous growth model with public capital

This paper tackles the fundamental issue in public finance of wether taxing or subsidizing factor rents. In a one sector endogenous growth model with private and public capital, similar to that in Barro (1990),we find that raising taxes on factors’ income as part of an optimal fiscal policy is a more pervasive result than it seems. The interaction of technological and fiscal externalities is central for this result. For instance, high enough levels of wasteful expenditures to output ratio could make positive income taxes enhance welfare. This ratio would need to be smaller, the lower the spillover externality and/or the larger the elasticities of private and public capital in the private production function.Endogenous growth, Factors’ rents subsidy, Distorting taxes, Public capital.

The Cross Sectional Dynamics of Heterogenous Trade Models

In this paper we propose a framework for studying export dynamics and market specific flows in a multicountry model of trade with heterogenous firms. Countries are asymmetric in terms of their size, the size distribution of potential entrants, properties of firms idiosyncratic shocks, and trade barriers. The model has predictions in terms of cross-sectional moments and exporters dynamics. We show that persistent productivity shocks are enough to account for, qualitatively, many features of the data. In particular, the model is consistent with observed patterns of entry and exit across markets, export sales distribution, and the life cycle of new exporters.

Reputation and Competence in Publicly Funded Science: Estimating the Effects on Research Group Productivity..

This paper estimates the "production function" for scientific research publications in the field of biotechnology. It utilizes an exceptionally rich and comprehensive data set pertaining to the universe of research groups that applied to a 1989-1993 research program in biotechnology and bio-instrumentation, sponsored by the Italian National research Council, CNR. A structural model of the resource allocation process in scientific research guides the selection of instruments in the econometric analysis, and controls for selectivity bias effects on estimates based on the performance of funded research units. The average elasticity of research output with respect to the research budget is estimated to be 0.6; but, for a small fraction of groups led by highly prestigious PIs this elasticity approaches 1. These estimates imply, conditional on the distribution of observed productivity, that a more unequal distribution of research funds would increase research output in the short-run. Past research publication performance is found to have an important effect on expect levels of grant funding, and hence on the unit's current productivity in terms of (quality adjusted) publications. The results show that the productivity of aggregate resource expenditures supporting scientific research is critically dependent on the institutional mechanisms and criteria employed in the allocation of such resources.

Bounds on Quantile Treatment Effects of Job Corps on Participants' Wages

This paper assesses the effect of the U.S. Job Corps (JC), the nation's largest and most comprehensive job training program targeting disadvantaged youths, on wages. We employ partial identification techniques and construct informative nonparametric bounds for the causal effect of interest under weaker assumptions than those conventionally used for point identification of treatment effects in the presence of sample selection. In addition, we propose and estimate bounds on quantile treatment effects of the program on participants' wages. In general, we find convincing evidence of positive impacts of JC on participants' wages. Importantly, we find that estimated impacts on lower quantiles of the distribution are higher, with the highest impact being in the 5th percentile where a positive effect on wages is bounded between 8.4 and 16.1 percent. These bounds suggest that JC results in wage compression within eligible participants.Job Corps, Nonparametric Bounds, Principal Stratification, Active Labor Market Programs., Labor and Human Capital, Public Economics, Research Methods/ Statistical Methods, J24, J68, C14, C21,