On the correct continuum limit of the functional-integral representation for the four-slave-boson approach to the Hubbard model: Paramagnetic phase

The Hubbard model with finite on-site repulsion U is studied via the functional-integral formulation of the four-slave-boson approach by Kotliar and Ruckenstein. It is shown that a correct treatment of the continuum imaginary time limit (which is required by the very definition of the functional integral) modifies the free energy when fluctuation (1/N) corrections beyond mean-field are considered. Our analysis requires us to suitably interpret the Kotliar and Ruckenstein choice for the bosonic hopping operator and to abandon the commonly used normal-ordering prescription, in order to obtain meaningful fluctuation corrections. In this way we recover the exact solution at U=0 not only at the mean-field level but also at the next order in 1/N. In addition, we consider alternative choices for the bosonic hopping operator and test them numerically for a simple two-site model for which the exact solution is readily available for any U. We also discuss how the 1/N expansion can be formally generalized to the four-slave-boson approach, and provide a simplified prescription to obtain the additional terms in the free energy which result at the order 1/N from the correct continuum limit.Comment: Changes: Printing problems (due to non-standard macros) have been removed, 44 page

Spin-wave spectrum of a two-dimensional itinerant electron system: Analytic results for the incommensurate spiral phase in the strong-coupling limit

We study the zero-temperature spin fluctuations of a two-dimensional itinerant-electron system with an incommensurate magnetic ground state described by a single-band Hubbard Hamiltonian. We introduce the (broken-symmetry) magnetic phase at the mean-field (Hartree-Fock) level through a \emph{spiral spin configuration} with characteristic wave vector \gmathbf{Q} different in general from the antiferromagnetic wave vector \gmathbf{Q_{AF}}, and consider spin fluctuations over and above it within the electronic random-phase (RPA) approximation. We obtain a \emph{closed} system of equations for the generalized wave vector and frequency dependent susceptibilities, which are equivalent to the ones reported recently by Brenig. We obtain, in addition, analytic results for the spin-wave dispersion relation in the strong-coupling limit of the Hubbard Hamiltonian and find that at finite doping the spin-wave dispersion relation has a \emph{hybrid form} between that associated with the (localized) Heisenberg model and that associated with the (long-range) RKKY exchange interaction. We also find an instability of the spin-wave spectrum in a finite region about the center of the Brillouin zone, which signals a physical instability toward a different spin- or, possibly, charge-ordered phase, as, for example, the stripe structures observed in the high-Tc materials. We expect, however, on physical grounds that for wave vectors external to this region the spin-wave spectrum that we have determined should survive consideration of more sophisticated mean-field solutions.Comment: 30 pages, 4 eps figure

Correlated band structure of electron-doped cuprate materials

We present a numerical study of the doping dependence of the spectral function of the n-type cuprates. Using a variational cluster-perturbation theory approach based upon the self-energy-functional theory, the spectral function of the electron-doped two-dimensional Hubbard model is calculated. The model includes the next-nearest neighbor electronic hopping amplitude $t'$ and a fixed on-site interaction $U=8t$ at half filling and doping levels ranging from $x=0.077$ to $x=0.20$. Our results support the fact that a comprehensive description of the single-particle spectrum of electron-doped cuprates requires a proper treatment of strong electronic correlations. In contrast to previous weak-coupling approaches, we obtain a consistent description of the ARPES experiments without the need to introduce a doping-dependent on-site interaction $U$.Comment: 7 pages 4 eps figure

Rare-earth impurities in Co2_2MnSi: an opportunity to improve Half-Metallicity at finite temperatures

We analyse the effects of doping Holmium impurities into the full-Heusler ferromagnetic alloy Co$_2$MnSi. Experimental results, as well as theoretical calculations within Density Functional Theory in the "Local Density Approximation plus Hubbard U" framework show that the holmium moment is aligned antiparallely to that of the transition metal atoms. According to the electronic structure calculations, substituting Ho on Co sites introduces a finite density of states in the minority spin gap, while substitution on the Mn sites preserves the half-metallic character.Comment: 22 pages, 8 figures. published in PR

Phase separation and competition of superconductivity and magnetism in the two-dimensional Hubbard model: From strong to weak coupling

Cooperation and competition between the antiferromagnetic, d-wave superconducting and Mott-insulating states are explored for the two-dimensional Hubbard model including nearest and next-nearest-neighbor hoppings at zero temperature. Using the variational cluster approach with clusters of different shapes and sizes up to 10 sites, it is found that the doping-driven transition from a phase with microscopic coexistence of antiferromagnetism and superconductivity to a purely superconducting phase is discontinuous for strong interaction and accompanied by phase separation. At half-filling the system is in an antiferromagnetic Mott-insulating state with vanishing charge compressibility. Upon decreasing the interaction strength U below a certain critical value of roughly U=4 (in units of the nearest-neighbor hopping), however, the filling-dependent magnetic transition changes its character and becomes continuous. Phase separation or, more carefully, the tendency towards the formation of inhomogeneous states disappears. This critical value is in contrast to previous studies, where a much larger value was obtained. Moreover, we find that the system at half-filling undergoes the Mott transition from an insulator to a state with a finite charge compressibility at essentially the same value. The weakly correlated state at half-filling exhibits superconductivity microscopically admixed to the antiferromagnetic order. This scenario suggests a close relation between phase separation and the Mott-insulator physics.Comment: 7 pages, 8 figures, revised version to be published in Phys. Rev.

The Supply of Female Scientists Conundrum: An International Study Exploring the Predictors of Females\u27 Intentions to Major in Science Postsecondary Education

The goal of this study was to perform an exploratory analysis of a comprehensive list of independent variables identified from literature to determine which, if any, are effective predictors in forecasting a female’s intention to study science postsecondary. This is likely to be indicative of interest to study science when pursing higher education as well as choice of major and possible career. The postulated model guiding this analysis, which was based on prior research, recognized that factors pertaining to students, parents, schools, and peers are all important. This study used logistic regression to analyze data from the 2006 Programme for International Student Assessment (PISA). The findings of this study suggest that external factors, such as those considered from the environment, are indeed important in determining a female’s intention to study science postsecondary. The findings of this study provided further refinement by demonstrating that for the 15 countries included in this analysis from the Oceania, Latin America, European, and Asian regions there were some overarching and consistent factors that are positively associated with females’ intentions to study science postsecondary. These findings essentially paint a portrait of females who intend to study science postsecondary, which are used to suggest additional research as well as interventions to help mitigate the female scientist conundrum observed worldwide

The Supply of Female Scientists Conundrum: An International Study Exploring the Predictors of Females\u27 Intentions to Major in Science Postsecondary Education

The goal of this study was to perform an exploratory analysis of a comprehensive list of independent variables identified from literature to determine which, if any, are effective predictors in forecasting a female’s intention to study science postsecondary. This is likely to be indicative of interest to study science when pursing higher education as well as choice of major and possible career. The postulated model guiding this analysis, which was based on prior research, recognized that factors pertaining to students, parents, schools, and peers are all important. This study used logistic regression to analyze data from the 2006 Programme for International Student Assessment (PISA). The findings of this study suggest that external factors, such as those considered from the environment, are indeed important in determining a female’s intention to study science postsecondary. The findings of this study provided further refinement by demonstrating that for the 15 countries included in this analysis from the Oceania, Latin America, European, and Asian regions there were some overarching and consistent factors that are positively associated with females’ intentions to study science postsecondary. These findings essentially paint a portrait of females who intend to study science postsecondary, which are used to suggest additional research as well as interventions to help mitigate the female scientist conundrum observed worldwide