P-wave Pairing and Colossal Magnetoresistance in Manganese Oxides

We point out that the existing experimental data of most manganese oxides show the {\sl frustrated} p-wave superconducting condensation in the ferromagnetic phase in the sense that the superconducting coherence is not long enough to cover the whole system. The superconducting state is similar to the $A_{1}$ state in superfluid He-3. The sharp drop of resistivity, the steep jump of specific heat, and the gap opening in tunneling are well understood in terms of the p-wave pairing. In addition, colossal magnetoresistance (CMR) is naturally explained by the superconducting fluctuations with increasing magnetic fields. The finite resistivity may be due to some magnetic inhomogeneities. This study leads to the possibility of room temperature superconductivity.Comment: LaTex, 14 pages, For more information, please send me an e-mail. e-mail adrress : [email protected]

Properties of short-range and long-range correlation energy density functionals from electron-electron coalescence

The combination of density functional theory with other approaches to the many-electron problem through the separation of the electron-electron interaction into a short-range and a long-range contribution is a promising method, which is raising more and more interest in recent years. In this work some properties of the corresponding correlation energy functionals are derived by studying the electron-electron coalescence condition for a modified (long-range-only) interaction. A general relation for the on-top (zero electron-electron distance) pair density is derived, and its usefulness is discussed with some examples. For the special case of the uniform electron gas, a simple parameterization of the on-top pair density for a long-range only interaction is presented and supported by calculations within the ``extended Overhauser model''. The results of this work can be used to build self-interaction corrected short-range correlation energy functionals.Comment: revised version, to appear in Phys. Rev.

System-adapted correlation energy density functionals from effective pair interactions

We present and discuss some ideas concerning an ``average-pair-density functional theory'', in which the ground-state energy of a many-electron system is rewritten as a functional of the spherically and system-averaged pair density. These ideas are further clarified with simple physical examples. We then show that the proposed formalism can be combined with density functional theory to build system-adapted correlation energy functionals. A simple approximation for the unknown effective electron-electron interaction that enters in this combined approach is described, and results for the He series and for the uniform electron gas are briefly reviewed.Comment: to appear in Phil. Mag. as part of Conference proceedings for the "Electron Correlations and Materials Properties", Kos Greece, July 5-9, 200

Broken Symmetry in Density-Functional Theory: Analysis and Cure

We present a detailed analysis of the broken-symmetry mean-field solutions using a four-electron rectangular quantum dot as a model system. Comparisons of the density-functional theory predictions with the exact ones show that the symmetry breaking results from the single-configuration wave function used in the mean-field approach. As a general cure we present a scheme that systematically incorporates several configurations into the density-functional theory and restores the symmetry. This cure is easily applicable to any density-functional approach.Comment: 4 pages, 4 figures, submitted to PR

A chiral crystal in cold QCD matter at intermediate densities?

The analogue of Overhauser (particle-hole) pairing in electronic systems (spin-density waves with non-zero total momentum $Q$) is analyzed in finite-density QCD for 3 colors and 2 flavors, and compared to the color-superconducting BCS ground state (particle-particle pairing, $Q$=0). The calculations are based on effective nonperturbative four-fermion interactions acting in both the scalar diquark as well as the scalar-isoscalar quark-hole ('$\sigma$') channel. Within the Nambu-Gorkov formalism we set up the coupled channel problem including multiple chiral density wave formation, and evaluate the resulting gaps and free energies. Employing medium-modified instanton-induced 't Hooft interactions, as applicable around $\mu_q\simeq 0.4$ GeV (or 4 times nuclear saturation density), we find the 'chiral crystal phase' to be competitive with the color superconductor.Comment: 14 pages ReVTeX, including 11 ps-/eps-figure

Evidence for an antiferromagnetic component in the magnetic structure of ZrZn2

Zero-field muon spin rotation experiments provide evidence for an antiferromagnetic component in the magnetic structure of the intermetallics ZrZn2.Comment: 5 pages, 2 figure

Effects of Backflow Correlation in the Three-Dimensional Electron Gas: Quantum Monte Carlo Study

The correlation energy of the homogeneous three-dimensional interacting electron gas is calculated using the variational and fixed-node diffusion Monte Carlo methods, with trial functions that include backflow and three-body correlations. In the high density regime the effects of backflow dominate over those due to three-body correlations, but the relative importance of the latter increases as the density decreases. Since the backflow correlations vary the nodes of the trial function, this leads to improved energies in the fixed-node diffusion Monte Carlo calculations. The effects are comparable to those found for the two-dimensional electron gas, leading to much improved variational energies and fixed-node diffusion energies equal to the release-node energies of Ceperley and Alder within statistical and systematic errors.Comment: 14 pages, 5 figures, submitted to Physical Review

Pair distribution function in a two-dimensional electron gas

We calculate the pair distribution function, $g(r)$, in a two-dimensional electron gas and derive a simple analytical expression for its value at the origin as a function of $r_s$. Our approach is based on solving the Schr\"{o}dinger equation for the two-electron wave function in an appropriate effective potential, leading to results that are in good agreement with Quantum Monte Carlo data and with the most recent numerical calculations of $g(0)$. [C. Bulutay and B. Tanatar, Phys. Rev. B {\bf 65}, 195116 (2002)] We also show that the spin-up spin-down correlation function at the origin, $g_{\uparrow \downarrow}(0)$, is mainly independent of the degree of spin polarization of the electronic system.Comment: 5 figures, pair distribution dependence with distance is calculate