The anapole form factor of the nucleon

The anapole form factor of the nucleon is calculated in chiral perturbation theory in leading order. To this order, the form factor originates from the pion cloud, and is proportional to the non-derivative parity-violating pion-nucleon coupling. The momentum dependence of the form factor - and in particular, its radius - is completely determined by the pion mass.Comment: 9 pages, 2 eps figures included by epsf.sty, minor changes in note adde

Double Chargino Production in e−e−e^{-}e^{-} scattering

We point out the production of the charginos and neutralinos in electron-electron process in several supersymmetric models, in order to show that the International Linear Collider can discover double charged charginos if these particles really exist in nature.Comment: 9 pages, 9 figures, Talk given at CTP symposium on Supersymmetry at LHC: Theoretical and Experimental Perspectives, The British University in Egypt, Cairo, Egypt, 11-14 March 200

Gauge Coupling Unification in GUT with Anomalous U(1) Symmetry

We show that in the framework of grand unified theory (GUT) with anomalous $U(1)_A$ gauge symmetry, the success of the gauge coupling unification in the minimal SU(5) GUT is naturally explained, even if the mass spectrum of superheavy fields does not respect SU(5) symmetry. Because the unification scale for most realizations of the theory becomes smaller than the usual GUT scale, it suggests that the present level of experiments is close to that sufficient to observe proton decay via dimension 6 operators, $p\to e+\pi$.Comment: 4 pages, RevTeX, to appear in Phys.Rev.Let

Calculation of model Hamiltonian parameters for LaMnO_3 using maximally localized Wannier functions

Maximally localized Wannier functions (MLWFs) based on Kohn-Sham band-structures provide a systematic way to construct realistic, materials specific tight-binding models for further theoretical analysis. Here, we construct MLWFs for the Mn e_g bands in LaMnO_3, and we monitor changes in the MLWF matrix elements induced by different magnetic configurations and structural distortions. From this we obtain values for the local Jahn-Teller and Hund's rule coupling strength, the hopping amplitudes between all nearest and further neighbors, and the corresponding reduction due to the GdFeO_3-type distortion. By comparing our results with commonly used model Hamiltonians for manganites, where electrons can hop between two "e_g-like" orbitals located on each Mn site, we find that the most crucial limitation of such models stems from neglecting changes in the underlying Mn(d)-O(p) hybridization.Comment: 15 pages, 11 figures, 3 table

Mesoscopic magnetoelectric effect in chaotic quantum dots

The magnitude of the inverse Faraday effect (IFE), a static magnetization due to an ac electric field, can be strongly increased in a mesoscopic sample, sensitive to time-reversal symmetry (TRS) breaking. Random rectification of ac voltages leads to a magnetization flux, which can be detected by an asymmetry of Hall resistances in a multi-terminal setup. In the absence of applied magnetic field through a chaotic quantum dot the IFE scale, quadratic in voltage, is found as an analytic function of the ac frequency, screening, and coupling to the contacts and floating probes, and numerically it does not show any effect of spin-orbit interaction. Our results qualitatively agree with a recent experiment on TRS-breaking in a six-terminal Hall cross.Comment: 4+ pages, 2 figures; v2-published version, small change

Anderson impurity in the one-dimensional Hubbard model on finite size systems

An Anderson impurity in a Hubbard model on chains with finite length is studied using the density-matrix renormalization group (DMRG) technique. In the first place, we analyzed how the reduction of electron density from half-filling to quarter-filling affects the Kondo resonance in the limit of Hubbard repulsion U=0. In general, a weak dependence with the electron density was found for the local density of states (LDOS) at the impurity except when the impurity, at half-filling, is close to a mixed valence regime. Next, in the central part of this paper, we studied the effects of finite Hubbard interaction on the chain at quarter-filling. Our main result is that this interaction drives the impurity into a more defined Kondo regime although accompanied in most cases by a reduction of the spectral weight of the impurity LDOS. Again, for the impurity in the mixed valence regime, we observed an interesting nonmonotonic behavior. We also concluded that the conductance, computed for a small finite bias applied to the leads, follows the behavior of the impurity LDOS, as in the case of non-interacting chains. Finally, we analyzed how the Hubbard interaction and the finite chain length affect the spin compensation cloud both at zero and at finite temperature, in this case using quantum Monte Carlo techniques.Comment: 9 pages, 9 figures, final version to be published in Phys. Rev.

Extrinsic Spin Hall Effect Induced by Iridium Impurities in Copper

We study the extrinsic spin Hall effect induced by Ir impurities in Cu by injecting a pure spin current into a CuIr wire from a lateral spin valve structure. While no spin Hall effect is observed without Ir impurity, the spin Hall resistivity of CuIr increases linearly with the impurity concentration. The spin Hall angle of CuIr, $(2.1 \pm 0.6)$% throughout the concentration range between 1% and 12%, is practically independent of temperature. These results represent a clear example of predominant skew scattering extrinsic contribution to the spin Hall effect in a nonmagnetic alloy.Comment: 5 pages, 4 figure

Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study

We systematically study the influence of ferromagnetic leads on the Kondo resonance in a quantum dot tuned to the local moment regime. We employ Wilson's numerical renormalization group method, extended to handle leads with a spin asymmetric density of states, to identify the effects of (i) a finite spin polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in the bands (governed by the band edges) and (iii) an arbitrary shape of the leads density of states. For a generic lead density of states the quantum dot favors being occupied by a particular spin-species due to exchange interaction with ferromagnetic leads leading to a suppression and splitting of the Kondo resonance. The application of a magnetic field can compensate this asymmetry restoring the Kondo effect. We study both the gate-voltage dependence (for a fixed band structure in the leads) and the spin polarization dependence (for fixed gate voltage) of this compensation field for various types of bands. Interestingly, we find that the full recovery of the Kondo resonance of a quantum dot in presence of leads with an energy dependent density of states is not only possible by an appropriately tuned external magnetic field but also via an appropriately tuned gate voltage. For flat bands simple formulas for the splitting of the local level as a function of the spin polarization and gate voltage are given.Comment: 18 pages, 18 figures, accepted for publication in PR

Quantum dot with ferromagnetic leads: a density-matrix renormalization group study

A quantum dot coupled to ferromagnetically polarized one-dimensional leads is studied numerically using the density matrix renormalization group method. Several real space properties and the local density of states at the dot are computed. It is shown that this local density of states is suppressed by the parallel polarization of the leads. In this case we are able to estimate the length of the Kondo cloud, and to relate its behavior to that suppression. Another important result of our study is that the tunnel magnetoresistance as a function of the quantum dot on-site energy is minimum and negative at the symmetric point.Comment: 4 pages including 5 figures. To be published as a Brief Report in Phys. Rev.