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.

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.

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.

A Theory of Entry and Exit into Exports Markets

This paper introduces persistent productivity shocks in a continuous-time mononopolistic competition model of trade with hetererogenous firms similar to Melitz (2003). In our model, the presence of sunk costs and uncertainty have three main consequences: first, firms export decisions become history-dependent. Second, the model generates firm dynamics and allows for substantial heterogeneity in export growth conditional on survival. Policy experiments modify the equilibrium along both the cross-sectional and time dimensions. Third, both the generated equilibrium firm size distribution and sales distribution of exporters into a foreign market are Pareto in the upper tail. All three consequences have been supported by empirical evidence. To solve the model we derive the stationary productivity distributions for exporters and non-exporters in general equilibrium. We point to the presence of a link between intra-industry firm heterogeneity and the degree of persistence in export status. Finally, we perform a numerical exercise to show how per-period fixed cost and up-front entry costs are differently related to persistence in export status for exporters and non-exporters.

Partial Identification of Local Average Treatment Effects with an Invalid Instrument

We derive nonparametric bounds for local average treatment effects without requiring the exclusion restriction assumption to hold or an outcome with a bounded support. Instead, we employ assumptions requiring weak monotonicity of mean potential outcomes within or across subpopulations defined by the values of the potential treatment status under each value of the instrument. We illustrate the identifying power of the bounds by analyzing the effect of attaining a GED, high school, or vocational degree on subsequent employment and weekly earnings using randomization into a training program as an invalid instrument.causal inference, instrumental variables, treatment effects, nonparametric bounds, principal stratification

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.