Coherent modes of X-ray beams emitted by undulators in new storage rings

Synchrotron radiation emitted by electrons passing through an undulator placed in a storage ring is decomposed in coherent modes. The case of ultimate storage rings where the electron emittance is comparable to the emittance of the photon fan is analyzed by means of the cross spectral density and the coherent mode spectrum. The proposed method permits naturally the statistical analysis and propagation of the cross spectral density along the beamline optics. The coherence properties of the X-ray beam at any point of the beamline are completely given in terms of the eigenvalues and coherent modes of the cross spectral density.Comment: 5 pages, 6 Figures, 17 reference

Metastable Kinks in the Orbifold

We consider static configurations of bulk scalar fields in extra dimensional models in which the fifth dimension is an $S^1/Z_2$ orbifold. There may exist a finite number of such configurations, with total number depending on the size of the orbifold interval. We perform a detailed Sturm-Liouville stability analysis that demonstrates that all but the lowest-lying configurations - those with no nodes in the interval - are unstable. We also present a powerful general criterion with which to determine which of these nodeless solutions are stable. The detailed analysis underlying the results presented in this letter, and applications to specific models, are presented in a comprehensive companion paper.Comment: 4 pages, 4 figures, reference added, typo corrected, submitted to PR

Efficient Fiscal Policy and Amplification

We provide a rationale for the observed pro-cyclicality of tax policies in emerging markets and present a novel mechanism through which tax policy amplifies the business cycle. Our explanation relies on two features of emerging markets: limited access to financial markets and limited commitment to tax policy. We present a small open economy model with capital where a government maximizes the utility of a working population that has no access to financial markets and is subject to endowment shocks. The government's insurance motive generates pro-cyclical taxes on capital income. If the government could commit, this policy is not distortionary. However, we show that if the government lacks the ability to commit, the best fiscal policy available exacerbates the economic cycle by distorting investment during recessions. We characterize the mechanism through which limited commitment generates cycles in investment in an environment where under commitment investment would be constant. We extend our results to standard productivity shocks and to the case where the government has access to intra-period insurance markets. Lastly, we conjecture that our results would hold as well if the government could issue debt subject to borrowing constraints.

How scaling of the disturbance set affects robust positively invariant sets for linear systems

This paper presents new results on robust positively invariant (RPI) sets for linear discrete-time systems with additive disturbances. In particular, we study how RPI sets change with scaling of the disturbance set. More precisely, we show that many properties of RPI sets crucially depend on a unique scaling factor which determines the transition from nonempty to empty RPI sets. We characterize this critical scaling factor, present an efficient algorithm for its computation, and analyze it for a number of examples from the literature

Investment Cycles and Sovereign Debt Overhang

We characterize optimal taxation of foreign capital and optimal sovereign debt policy in a small open economy where the government cannot commit to policy and seeks to insure a risk averse domestic constituency. The expected tax on capital is shown to vary with the state of the economy, generating cyclicality in investment and debt in an environment where the first best capital stock is a constant. The government's lack of commitment induces a negative correlation between investment and the stock of government debt, a "debt overhang'' effect. If the government discounts the future at a rate higher than the market, then capital oscillates indefinitely at a level strictly below the first best. Debt relief is never Pareto improving and cannot affect the long-run level of investment. Further, restricting the government to a balanced budget can eliminate the cyclical distortion of investment.

Hybrid Organic−Inorganic Solids That Show Shape Selectivity

Hybrid organic−inorganic solids featuring millimolar/gram concentrations of intracrystalline organic moieties and shape-selectivity are synthesized. Pure-silica zeolite beta crystals are coated with zirconia and treated in aqueous sodium hydroxide to create defects and mesoporosity within the crystalline structure. Aminopropyl organic groups are subsequently grafted onto the generated intracrystalline silanol groups. After grafting, characterization data indicate a high organic concentration localized primarily within the intracrystalline voids. Specifically, thermogravimetric analysis shows an organic loading of 0.7 mmol of NH_2/g, ^(29)Si solid-state nuclear magnetic resonance (NMR) spectra display a quantitative decrease in Q^3 silicon atoms with a corresponding resharpening of the Q^4 resonances, and N_2 adsorption data show a decrease in micropore volume to 0.10 cm^3/g. Knoevenagel condensation reactions are catalyzed by the aminopropyl-functionalized materials using differently sized aldehydes and the results show that the zirconia-protected functionalized solid have shape selective properties