Theory of Inelastic Electron Tunneling from a Localized Spin in the Impulsive Approximation

A simple expression for the conductance steps in the inelastic electron tunneling from spin excitations in a single magnetic atom adsorbed on a non-magnetic metal surfaces is derived. The inelastic coupling between the tunneling electron and the spin is via the exchange coupling and is treated in an impulsive approximation using the Tersoff-Hamann approximation for the tunneling between the tip and the sample. Our results for conductance steps justify the analysis carried out by Hirjebedin et al. [Science 317, 1199 (2007)] of observed step-like conductances by inelastic electron tunneling from spin excitations in a single magnetic adatom using a simple spin matrix element. In addition, our result gives a simple expression for the magnitudes of conductance steps and their lateral spatial variation with respect to the tip position, which can be calculated directly from spin-polarized wave functions at the Fermi level of the sample

A simplified density functional theory method for charged adsorbates on an ultrathin, insulating film supported by a metal substrate

A simplified density functional theory (DFT) method for charged adsorbates on an ultrathin, insulating film supported by a metal substrate is developed and presented. This new method is based on a previous DFT development that uses a perfect conductor (PC) model to approximate the electrostatic response of the metal substrate, while the film and the adsorbate are both treated fully within DFT [I. Scivetti and M. Persson, Journal of Physics: Condensed Matter 25, 355006 (2013)]. The missing interactions between the metal substrate and the insulating film in the PC approximation are modelled by a simple force field (FF). The parameters of the PC model and the force field are obtained from DFT calculations of the film and the substrate, here shown explicitly for a NaCl bilayer supported by a Cu(100) surface. In order to obtain some of these parameters and the polarisability of the force field, we have to include an external, uniformly charged plane in the DFT calculations, which has required the development of a periodic DFT formalism to include such a charged plane in the presence of a metal substrate. This extension and implementation should be of more general interest and applicable to other challenging problems, for instance, in electrochemistry. As illustrated for the gold atom on the NaCl bilayer supported by a Cu(100) surface, our new DFT-PC-FF method allows us to handle different charge states of adsorbates in a controlled and accurate manner with a considerable reduction of the computational time. In addition, it is now possible to calculate vertical transition and reorganisation energies for charging and discharging of adsorbates that cannot be obtained by current DFT methodologies that include the metal substrate. We find that the computed vertical transition energy for charging of the gold adatom is in good agreement with experiments

Theoretical study of electron states in Au chains on NiAl(110)

We have carried out a density functional study of unoccupied, resonance states in a single Au atom, dimers, a trimer and infinite Au chains on the NiAl(110) surface. Two inequivalent orientations of the ad-chains with substantially different interatomic distances were considered. From the study of the evolution of the electron states in an Au chain from being isolated to adsorbed, we find that the resonance states derive from the 6$s$ states of the Au atoms, which hybridize strongly with the substrate states and develop a $p$-like polarization. The calculated resonance states and LDOS images were analyzed in a simple tight-binding, resonance model. This model clarifies (1) the physics of direct and substrate-mediated adatom-adatom interactions and (2) the physics behind the enhancements of the LDOS at the ends of the adatom chains, and (3) the physical meaning of the "particle-in-box" model used in the analysis of observed resonance states. The calculated effective mass and band bottom energy are in good agreement with experimental data obtained from scanning tunnelling spectroscopy

Time Consistency of Fiscal and Monetary Policy: A Solution

This paper demonstrates how time consistency of the Ramsey policy–the optimal fiscal and monetary policy under commitment–can be achieved. Each government should leave its successor with a unique maturity structure for the nominal and indexed debt, such that the marginal benefit of a surprise inflation exactly balances the marginal cost. Unlike in earlier papers on the topic, the result holds for quite a general Ramsey policy, including time varying polices with positive inflation and positive nominal interest rates. We compare our results with those in Persson, Persson, and Svensson (1987), Calvo and Obstfeld (1990), and Alvarez, Kehoe, and Neumeyer (2004).time consistency; Ramsey policy; surprise inflation

The Effects on Sick Leave of Changes in the Sickness Insurance System

In order to get a more complete picture of how labor supply is affected by economic incentives, the effects on absenteeism and not just on contracted hours should be taken into account. In particular, absenteeism due to sick leave can be considerable. In this paper we examine whether the level of sick leave compensation affects sick leave behavior. Using time-series data for Sweden spanning a long period (1955–99) with numerous changes of the compensation level, we generally find strong effects of the expected sign. Reforms implying more generous compensation for sick leave tend to be associated with permanent increases in total sick leave per person employed and vice versa. These findings are reinforced in a panel study covering the 1983–91 period.Absenteeism; Labor supply; Sick leave; Sickness insurance; Social security

A Model of Income Insurance and Social Norms

A large literature on ex ante moral hazard in income insurance emphasizes that the individual can affect the probability of an income loss by choice of lifestyle and hence, the degree of risk-taking. The much smaller literature on moral hazard ex post mainly analyzes how a “moral hazard constraint” can make the individual abstain from fraud (“mimicking”). The present paper instead presents a model of moral hazard ex post without a moral hazard constraint; the individual's ability and willing­ness to work is represented by a continuous stochastic variable in the utility function, and the extent of moral hazard depends on the generosity of the insurance system. Our model is also well suited for analyzing social norms concerning work and benefit dependency.Moral Hazard; Sick Pay Insurance; Labor Supply; Asymmetric Information  

The Gains from Pension Reform

We characterize pension systems along three dimensions: 1) actuarial vs. non-actuarial, 2) funded vs. pay-as-you-go, 3) defined-contribution vs. defined-benefit. Increasing the degree of actuarial fairness, by strengthening the linkage between contributions and benefits, reduces labor market distortions and may increase welfare in a Pareto-efficiency sense. Increasing the degree of funding implies mainly a redistribution of income among generations, although a partial shift to funding also provides better risk-return combinations for individuals. Shifting from defined-benefit to defined-contribution schemes (with fixed contribution rates) shifts the income risk from workers and taxpayers to pensioners.Social security; Funding