The reorientation process in a magnetic field in two-dimensional isotropic and XY quantum Heisenberg antiferromagnets is shown to occur through the intermediate phase with unbroken continuous symmetry and constant magnetization equal to one third of the saturation value. The same reorientation process is also found in the more complicated classical models.

We construct the 1/S spin wave expansion for double exchange ferromagnets at T = 0. It is assumed that the value of Hund's rule coupling, J(H), is sufficiently large, resulting in a fully saturated, ferromagnetic half-metallic ground state. We evaluate corrections to the magnon dispersion law, and we also find that, in contrast to earlier statements in the literature, magnon-electron scattering does give rise to spin wave damping. We analyze the momentum dependence of these quantities and discuss the experimental implications for colossal magnetoresistance compounds.

Measurements of the transmission phase in transport through a quantum dot embedded in an Aharonov-Bohm interferometer show systematic sequences of phase lapses separated by Coulomb peaks. Using a two-level quantum dot as an example we show that this phenomenon can be accounted for by the combined effect of asymmetric dot-lead coupling and interaction-induced "population switching" of the levels, rendering this behavior generic. In addition, we use the notion of spectral shift function to analyze the relationship between transmission phase lapses and the Friedel sum rule.In a series of experiments by the Weizmann group, the transmission phase, Θ tr , characterizing transport through a quantum dot (QD) has been systematically studied 1,2,3 , embedding the QD in an Aharonov-Bohm interferometer 4,5 . Arguably the most intriguing finding of these experiments has been the correlated behavior of Θ tr as function of the leads' chemical potential µ (or the gate voltage): it appears to undergo a lapse (phase lapse, PL), seemingly of −π, between any two consecutive Coulomb peaks. It is clear that this effect cannot be explained within a single-particle framework 6 . Moreover, in spite of a substantial body of theoretical work (see, e.g., Refs. 7,8,9), some of which gained important insight on the underlying physics, no clear cut theoryexperiment connection has been established as yet.In the present article we revisit this problem. We do this by studying a (spinless) two-level QD, attached to two leads. We account for the difference in the couplings of level 1 and level 2 to the leads ("1 − 2 asymmetry") and, for the first time, probe the effect of the (generically expected) asymmetric coupling to the left and the right leads ("L-R asymmetry"). We find unexpectedly that these two asymmetries give rise to a qualitatively new behavior of Θ tr (µ), and render the appearance of PL between consecutive Coulomb peaks generic. This conclusion is in line with recent renormalisation group results for a QD with degenerate levels 10 .Throughout the discussion of transmission PLs in the literature, much attention was paid to the Friedel sum rule, which, in one dimension, relates the transmission phase to the change of carrier population in the system (see, e.g., Refs. 11,12). Since the latter varies monotonously with the chemical potential (or gate voltage), one may perceive a contradiction between this sum rule and the occurrence of PLs. We revisit this issue in Appendix A and show, in particular, that the correct formulation of Friedel sum rule in one dimension allows for transmission phase lapses.The minimal model for studying the phase lapse mechanism includes a two-level QD,Here, the operatorsd i with i = 1, 2 annihilate electrons on the two dot sites (with bare energies E. The QD is coupled to the two leads by the tunnelling termThe operatorsĉ j (with half-integer j) are defined on the tight-binding sites of the left and right lead (cf. Fig. 1). We begin with summarizing the results of Ref.13 (see also Refs. 12,14,15,16) in the cas...

We investigate the zero-temperature phase diagram and spin-wave properties of a double exchange magnet with on-site Hubbard repulsion. It is shown that even within a simple Hartree -Fock approach this interaction (which is often omitted in theoretical treatments) leads to qualitatively important effects which are highly relevant in the context of experimental data for the colossal magnetoresistance compounds. These include the asymmetry of the doping dependence of spin stiffness, and the zone-boundary "softening" of spin wave dispersion. Effects of Hubbard repulsion on phase separation are analyzed as well. We also show that in the ferromagnetic phase, an unusual temperature-dependent effective electron-electron interaction arises at finite T . The mean-field scheme, however, does not yield the experimentally observed density of states depletion near the Fermi level. We speculate that proper treatment of electron-electron interactions may be necessary for understanding both this important feature and more generally the physics of colossal magnetoresistance phenomenon.