Berry-curvatures and anomalous Hall effect in Heusler compounds

Berry curvatures are computed for a set of Heusler compounds using density functional (DF) calculations and the wave functions that DF provide. The anomalous Hall conductivity is obtained from the Berry curvatures. It is compared with experimental values in the case of Co$_2$CrAl and Co$_2$MnAl. A notable trend cannot be seen but the range of values is quite enormous. The results for the anomalous Hall conductivities and their large variations can be qualitatively understood by means of the band structure and the Fermi-surface topology

Pareto improving social security reform when financial markets are incomplete!?

This paper studies an overlapping generations model with stochastic production and incomplete markets to assess whether the introduction of an unfunded social security system leads to a Pareto improvement. When returns to capital and wages are imperfectly correlated a system that endows retired households with claims to labor income enhances the sharing of aggregate risk between generations. Our quantitative analysis shows that, abstracting from the capital crowding-out effect, the introduction of social security represents a Pareto improving reform, even when the economy is dynamically effcient. However, the severity of the crowding-out effect in general equilibrium tends to overturn these gains. Klassifikation: E62, H55, H31, D91, D58 . April 2005

Collateralized Borrowing and Life-Cycle Portfolio Choice

We examine the effects of collateralized borrowing in a realistically parameterized life-cycle portfolio choice problem. We provide basic intuition in a two-period model and then solve a multi-period model computationally. Our analysis provides insights into life-cycle portfolio choice relevant for researchers in macroeconomics and finance. In particular, we show that standard models with unlimited borrowing at the riskless rate dramatically overstate the gains to holding equity when compared with collateral-constrained models. Our results do not depend on the specification of the collateralized borrowing regime: the gains to trading equity remain relatively small even with the unrealistic assumption of unlimited leverage. We argue that our results strengthen the role of borrowing constraints in explaining the portfolio participation puzzle, that is, why most investors do not own stock.

Social Security and Risk Sharing

In this paper we identify conditions under which the introduction of a pay-as-you-go social security system is ex-ante Pareto-improving in a stochastic overlapping generations economy with capital accumulation and land. We argue that these conditions are consistent with many calibrations of the model used in the literature. In our model financial markets are complete and competitive equilibria are interim Pareto efficient. Therefore, a welfare improvement can only be obtained if agents' welfare is evaluated ex ante, and arises from the possibility of inducing, through social security, an improved level of intergenerational risk sharing. We will also examine the optimal size of a given social security system as well as its optimal reform. The analysis will be carried out in a relatively simple set-up, where the various effects of social security, on the prices of long-lived assets and the stock of capital, and hence on output, wages and risky rates of returns, can be clearly identified.Intergenerational Risk Sharing, Social Security, Ex Ante Welfare Improvements, Interim Optimality, Price Effects

Recursive Contracts, Lotteries and Weakly Concave Pareto Sets

Marcet and Marimon (1994, revised 1998) developed a recursive saddle point method which can be used to solve dynamic contracting problems that include participation, enforcement and incentive constraints. Their method uses a recursive multiplier to capture implicit prior promises to the agent(s) that were made in order to satisfy earlier instances of these constraints. As a result, their method relies on the invertibility of the derivative of the Pareto frontier and cannot be applied to problems for which this frontier is not strictly concave. In this paper we show how one can extend their method to a weakly concave Pareto frontier by expanding the state space to include the realizations of an end of period lottery over the extreme points of a .at region of the Pareto frontier. With this expansion the basic insight of Marcet and Marimon goes through .one can make the problem recursive in the Lagrangian multiplier which yields significant computational advantages over the conventional approach of using utility as the state variable. The case of a weakly concave Pareto frontier arises naturally in applications where the principal’s choice set is not convex but where randomization is possible.Recursive Contracting, Recursive Multipliers, Lotteries

Pareto Improving Social Security Reform when Financial Markets are Incomplete!?

This paper studies an overlapping generations model with stochastic production and incomplete markets to assess whether the introduction of an unfunded social security system leads to a Pareto improvement. When returns to capital and wages are imperfectly correlated a system that endows retired households with claims to labor income enhances the sharing of aggregate risk between generations. Our quantitative analysis shows that, abstracting from the capital crowding-out effect, the introduction of social security represents a Pareto improving reform, even when the economy is dynamically effcient. However, the severity of the crowding-out effect in general equilibrium tends to overturn these gains.Social Security Reform, Aggregate Fluctuations, Intergenerational Risk Sharing, Incomplete Markets.

Non-vanishing Berry Phase in Chiral Insulators

The binary compounds FeSi, RuSi, and OsSi are chiral insulators crystallizing in the space group P2_13 which is cubic. By means of ab initio calculations we find for these compounds a non-vanishing electronic Berry phase, the sign of which depends on the handedness of the crystal. There is thus the possibility that the Berry phase signals the existence of a macroscopic electric polarization due to the electrons. We show that this is indeed so if a small external magnetic field is applied in the [111]-direction. The electric polarization is oscillatory in the magnetic field and possesses a signature that distinguishes the handedness of the crystal. Our findings add to the discussion of topological classifications of insulators and are significant for spintronics applications, and in particular, for a deeper understanding of skyrmions in insulators

Dollar Denominated Debt and Optimal Security Design

During a crisis, developing countries regret having issued dollar denominated debt because they have to pay more when they have less. Ex ante, however, they may be worse off issuing local currency debt because the equilibrium interest rate might rise, making it more expensive for them to borrow. Many authors have assumed that lenders and borrowers have contrary goals, and that local currency (peso) debt is better for the borrower (Bolivia), and dollar debt is better for the lender (America). We show that if each country is represented by a single consumer with quadratic utilities, in perfect competition, then both will agree ex ante on whether dollar debt or peso debt is better. (In fact all assets can be Pareto ranked). But we show that it might well be dollar debt that Pareto dominates. In particular, if the lender is sufficiently risk averse and the borrower sufficiently impatient, and the lender's endowment is sufficiently riskless, then dollar debt Pareto dominates peso debt. However, if there are persistent gains to risk sharing between the countries, then peso debt Pareto dominates dollar debt. In the special case where utilities are linear in the first period and quadratic in the second period, we can completely characterize the Pareto ranking of any asset by a formula depending only on marginal utilities at autarky. In the more general case where utilities are linear in the first period and have positive third derivative in the second period, we show that when persistent gains to risk sharing hold, America must gain from Peso debt but Bolivia might lose. Thus the presumption that peso debt is more favorable to Bolivia than to America is false. Our framework of optimal security design can be used to demonstrate one rationale for credit controls. If the purchasing power of a dollar overseas varies with the quantity of debt issued, then both America and Bolivia can gain from capital controls, because a tax that reduces the quantity of Bolivian debt might make the real dollar payoffs in Bolivia more `peso-like', and therefore under persistent gains to risk pooling, better for America and Bolivia.Dollar debt, Currency, Indexed bonds, Security design, Capital controls

Social Security and Risk Sharing

In this paper we identify conditions under which the introduction of a pay-as-you-go social security system is ex ante Pareto-improving in a stochastic overlapping generations economy with capital accumulation and land. We argue that these conditions are consistent with many calibrations of the model used in the literature. In our model financial markets are complete and competitive equilibria are interim Pareto e¢ cient. Therefore, a welfare improvement can only be obtained if agents' welfare is evaluated exante, and arises from the possibility of inducing, through social security, an improved level of intergenerational risk sharing. We will also examine the optimal size of a given social security system as well as its optimal reform. The analysis will be carried out in a relatively simple set-up, where the various effects of social security, on the prices of long-lived assets and the stock of capital, and hence on output, wages and risky rates of returns, can be clearly identified.Intergenerational Risk Sharing, Social Security, Ex Ante Welfare Improvements, Social Security Reform, Price E¤ects