Spin Tunneling, Berry phases and Doped Antiferromagnets

Interference effects between Berry phase factors in spin tunneling systems have been discussed in recent Letters by Loss, DiVincenzo and Grinstein and von Delft and Henley. This Comment points out that Berry phases in spin tunneling are important in another interesting case: the two dimensional doped antiferromagnet. I show that the dispersion of a single hole in the t-J model changes sign as $e^{2\pi s}$ where $s$ is the size of the spins. This provides an interpretation of the numerical results for the s=\half model, and a prediction for other spin sizes.Comment: 5 pages, LaTe

Optimal Long-Run Fiscal Policy: Constraints, Preferences and the Resolution of Uncertainty

We construct a computational dynamic stochastic overlapping generations general equilibrium model with uncertain lifetimes and explore the impact of policy stickiness (specifically, a major reform will preclude future reforms for a generation) on optimal long-run fiscal policy. Under such circumstances, entitlement reforms exhaust a valuable option to move in the future. We explore the conditions under which the gain to waiting is large enough to induce optimizing policymakers to delay reforming a suboptimal system. We also allow for the uncertainty to have ARCH characteristics and explore the impact of time-varying uncertainty on the optimality of delayed policy action.

Corporate Financial Policy, Taxes, and Uncertainty: An Integration

In this paper, we present a simple general equilibrium model of the portfolio behavior of households and institutions, paying particular attention to the influence of differences in tax rates and attitudes toward risk. Under the plausible assumptions that households are more risk averse than institutions and possess a greater relative "tax preference" for equity versus debt, we are able to characterize the equilibria which may result when debt is subject to bankruptcy risk. Among the issues which we study are the effects of tax rate changes, changes in risk, and changes in firm leverage on the relative asset holdings of the two types of investor and the rates of return earned on equity and debt. Numerical simulations provide additional understanding of the model's characteristics.

Taxation, Portfolio Choice, and Debt-Equity Ratios: A General Equilibrium Model

This paper explores the portfolio behavior of investors differing with respect to both tax rates and risk-aversion, emphasizing the role of constraints on individual and firm behavior in ensuring the existence of and characterizing portfolio equilibrium. Under certain conditions on the securities available in the market, which also are required for shareholders to be unanimous in supporting firm value maximization, investors will be segmented by tax rate into two groups, one specialized in equity and the other in debt. Though the relative wealths of the two groups determines the aggregate debt-equity ratio, each firm will be indifferent to its financial policy.

Uncertainty and the Design of Long-Run Fiscal Policy

This paper explores optimal fiscal policy in an overlapping-generations general-equilibrium model under uncertainty and the impact on optimal policy of the introduction of a type of policy stickiness intended to account for the stylized fact that major reforms happen infrequently. In general, our analysis suggests not only that action should not be delayed, but further that action should actually be accelerated. The added realism of restrictions on the frequency of policy changes alters this result in two ways. The prospect of being unable to set policy in the future occasions even more precautionary saving today, if the government acts. However, the government may also choose not to set policy, and its inaction range is very asymmetric. Because the impact of its policies on the current elderly cannot be reversed in the future, the government is much more likely to choose inaction when fiscal tightening is called for. Thus, the optimal policy response over time might best be characterized by great caution in general, but punctuated by occasional periods of apparent irresponsibility.

Dividend Taxes and Firm Valuation: New Evidence

This paper extends our previous analysis (Auerbach and Hassett 2005) of the effects of the "Jobs and Growth Tax Relief Act of 2003" on firm valuation. That paper found that firms with higher dividend yields benefited more than other dividend paying firms, a result that, in itself, is consistent with both new and traditional views of dividend taxation. But further evidence favored the new view. We also found that non-dividend-paying "immature" firms experienced larger abnormal returns than other firms and that a similar bonus accrued to firms likely to issue new shares, two results that are consistent with an anticipated transition to higher dividend payments. Here, we extend our earlier analysis in two ways. First, we consider the impact of the 2004 Presidential election on option prices, to gain further insight into and confirmation of the mechanism through which the 2003 legislation affected firm values. Second, we explore in more detail the determinants of the "immaturity premium" noted above. In contrast to claims in a recent paper by Amromin et al. (2005), we find that the premium is associated with the likelihood of new share issuance, as inferred but not demonstrated in our original analysis.

The 2003 Dividend Tax Cuts and the Value of the Firm: An Event Study

The "Jobs and Growth Tax Relief Act of 2003" (JGTRA03) contained a number of significant tax provisions, but the most noteworthy may have been the reduction in dividend tax rates. The political debate over the dividend tax reductions of 2003 took a number of surprising twists and turns. Accordingly, it is likely that the views of market participants concerning the probability of significant dividend tax reduction fluctuated significantly during 2003. In this paper, we use this fact to estimate the effects of dividend tax policy on firm value. We find that firms with higher dividend yields benefited more than other dividend paying firms, a result that, in itself, is consistent with both new and traditional views of dividend taxation. But further evidence points toward the new view and away from the traditional view. We also find that non-dividend-paying firms experienced larger abnormal returns than other firms as the result of the dividend tax cut, and that a similar bonus accrued to firms likely to issue new shares, two results that may appear surprising at first but are consistent with the theory developed in the paper.

Oscillating Superfluidity of Bosons in Optical Lattices

We follow up on a recent suggestion by C. Orzel et. al., Science, 291, 2386 (2001), whereby bosons in an optical lattice would be subjected to a sudden parameter change from the Mott to the superfluid phase. We analyze the Bose Hubbard model with a modified coherent states path integral which can escribe - both - phases. The saddle point theory yields collective oscillations of the uniform superfluid order parameter. These would be seen in time resolved interference patterns made by the released gas. We calculate the collective oscillation's damping rate by phason pair emission. In two dimensions the overdamped region largely overlaps with the quantum critical region. Measurements of critical dynamics on the Mott side are proposed.Comment: 4 pages 1 eps figures; Final version as appears in PRL. Added discussion on spontaneous generation of vortice

Domain Patterns in the Microwave-Induced Zero-Resistance State

It has been proposed that the microwave-induced ``zero-resistance'' phenomenon, observed in a GaAs two-dimensional electron system at low temperatures in moderate magnetic fields, results from a state with multiple domains, in which a large local electric field \bE(\br) is oriented in different directions. We explore here the questions of what may determine the domain arrangement in a given sample, what do the domains look like in representative cases, and what may be the consequences of domain-wall localization on the macroscopic dc conductance. We consider both effects of sample boundaries and effects of disorder, in a simple model, which has a constant Hall conductivity, and is characterized by a Lyapunov functional.Comment: 19 pages, 5 figures; submitted to a special issue of Journal of Statistical Physics, in honor of P. C. Hohenberg and J. S. Lange

A Path Intergal Approach to Current

Discontinuous initial wave functions or wave functions with discontintuous derivative and with bounded support arise in a natural way in various situations in physics, in particular in measurement theory. The propagation of such initial wave functions is not well described by the Schr\"odinger current which vanishes on the boundary of the support of the wave function. This propagation gives rise to a uni-directional current at the boundary of the support. We use path integrals to define current and uni-directional current and give a direct derivation of the expression for current from the path integral formulation for both diffusion and quantum mechanics. Furthermore, we give an explicit asymptotic expression for the short time propagation of initial wave function with compact support for both the cases of discontinuous derivative and discontinuous wave function. We show that in the former case the probability propagated across the boundary of the support in time $\Delta t$ is $O(\Delta t^{3/2})$ and the initial uni-directional current is $O(\Delta t^{1/2})$. This recovers the Zeno effect for continuous detection of a particle in a given domain. For the latter case the probability propagated across the boundary of the support in time $\Delta t$ is $O(\Delta t^{1/2})$ and the initial uni-directional current is $O(\Delta t^{-1/2})$. This is an anti-Zeno effect. However, the probability propagated across a point located at a finite distance from the boundary of the support is $O(\Delta t)$. This gives a decay law.Comment: 17 pages, Late