1,465 research outputs found
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
Rare-earth impurities in CoMnSi: an opportunity to improve Half-Metallicity at finite temperatures
We analyse the effects of doping Holmium impurities into the full-Heusler
ferromagnetic alloy CoMnSi. 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
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 and
a fixed on-site interaction at half filling and doping levels ranging
from to . 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 .Comment: 7 pages 4 eps figure
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
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