L^2-Theory for non-symmetric Ornstein-Uhlenbeck semigroups on domains

We present some new results on analytic Ornstein-Uhlenbeck semigroups and use them to extend recent work of Da Prato and Lunardi for Ornstein-Uhlenbeck semigroups on open domains O to the non-symmetric case. Denoting the generator of the semigroup by L_O, we obtain sufficient conditions in order that the domain Dom(\sqrt{-L_O}) be a first order Sobolev space.Comment: 23 pages, revised version, to appear in J. Evol. Eq. The main change is a correction in Theorem 5.5: the second assertion has been withdrawn due to a gap in the original proo

Huge metastability in high-T_c superconductors induced by parallel magnetic field

We present a study of the temperature-magnetic field phase diagram of homogeneous and inhomogeneous superconductivity in the case of a quasi-two-dimensional superconductor with an extended saddle point in the energy dispersion under a parallel magnetic field. At low temperature, a huge metastability region appears, limited above by a steep superheating critical field (H_sh) and below by a strongly reentrant supercooling field (H_sc). We show that the Pauli limit (H_p) for the upper critical magnetic field is strongly enhanced due to the presence of the Van Hove singularity in the density of states. The formation of a non-uniform superconducting state is predicted to be very unlikely.Comment: 5 pages, 2 figures; to appear in Phys. Rev.

Thermodynamics of the half-filled Kondo lattice model around the atomic limit

We present a perturbation theory for studying thermodynamic properties of the Kondo spin liquid phase of the half-filled Kondo lattice model. The grand partition function is derived to calculate chemical potential, spin and charge susceptibilities and specific heat. The treatment is applicable to the model with strong couplings in any dimensions (one, two and three dimensions). The chemical potential equals zero at any temperatures, satisfying the requirement of the particle-hole symmetry. Thermally activated behaviors of the spin(charge) susceptibility due to the spin(quasiparticle) gap can be seen and the two-peak structure of the specific heat is obtained. The same treatment to the periodic Anderson model around atomic limit is also briefly discussed.Comment: 5 pages, 3 figures, to appear in Phys. Rev.

Characterization of defects in plates using shear and Lamb waves

This work investigates the interaction of shear and Lamb waves with different kinds of defects in plates, in view of applications to defect characterization purposes. Using a finite element model, the reflection and transmission coefficients of shear and Lamb waves are determined, as a function of size parameters of the defect related to its extension and depth. Notches with elliptical and rectangular profile are examined, together with internal voids, covering both symmetric and asymmetric cases. Low and high frequency times height (2hf) regimes are considered in order to clarify how mode conversion can provide information on the shape of the defect. In this regard, also the role of symmetric and asymmetric waves is elucidated

Effect of the Three-Site Hopping Term on the t-J Model

We have used exact diagonalization and quantum Monte Carlo methods to study the one-dimensional {t-J} model including the three-site hopping term derived from the strong coupling limit of the Hubbard model. The three-site term may be important to superconducting correlations since it allows direct hopping of local singlet electron pairs. The phase diagram is determined for several values of the strength of the three-site term and compared with that of the {t-J} and Hubbard models. Phase separation, which exists in the t-J model is suppressed. In the low electron density region the formation of local singlet electron pairs is enhanced, leading to stronger superconducting correlations even for values $J/t<2$. A large spin gap region extends from low electron densities up to high densities. In the low hole density region the superconducting correlations are suppressed at $J/t>2.8$ in spite of enhanced pair formation. This is because the three-site term, while enhancing the formation of electron pairs, leads to a repulsion between holes.Comment: 9 pages including 9 figures and 1 Table. Self-unpacking postscript. Unpacking instructions are at the beginning of the file. Submitted to Physical Review

Exploring the fertility trend in Egypt

Background: The unusual fertility increase experienced by several Arab countries in the recent years is particularly visible in Egypt, where fertility declined very slowly after 2000 and started to increase again between 2008 and 2014. Objective: We first check the quality and measurement accuracy of Demographic and Health Surveys (DHS). The analysis confirms the trend since 2000. We descriptively look for possible underlying causes. Methods: We use quality criteria to check DHS data and control for tempo effect. We also perform a proximate determinants analysis to study the mechanisms affecting fertility, particularly marriage and contraceptive use patterns. Results: The trend in fertility, which has been at a level slightly below 3.5 children per woman since 2000, is due to an increase in parity one-to-three children and a steady decline in parity four-and-more children. While changes in contraception use had the largest and a growing suppressing effect before 2000, after the turn of the century there was no change in the impact of either marriage or contraception on fertility. Conclusions: We find that well-educated women between 20 and 29 years lack labour market opportunities. They may have preponed their fertility. Fertility could start declining again once the labour market situation for women has improved. On the other hand, the family model of three children is still widespread in the country. Contribution: The article studies the fertility increase in Egypt. It contributes to the literature on exceptions to the demographic transition, such as stalls in fertility decline, particularly in the context of Arab countries

Density, Social Networks and Job Search Methods: Theory and Application to Egypt

The aim of this paper is to study the impact of the size and the quality of social networks on the probability to find a job. We first develop a theoretical model in which individuals are embedded within a network of social relationships. Workers can obtain information about jobs via employed friends belonging to his/her social network. Workers can be either uneducated or educated. We show that, conditional on being employed, the probability to find a job through social networks, relative to other search methods, increases and is concave with the size of the network. The effects are stronger for the uneducated. There is however a critical size of the network above which this probability decreases. We also show that the probability to find a job through friends and relatives decreases with the local unemployment rate. We test empirically these theoretical findings for Egypt using the 1998 Labor Market Survey. The empirical evidence supports the predictions of our theoretical model.Education; Social Networks; Search Methods