Thermodynamics of the one-dimensional SU(4) symmetric spin-orbital model

The ground state properties and the thermodynamics of the one-dimensional SU(4) symmetric spin system with orbital degeneracy are investigated using the quantum Monte Carlo loop algorithm. The spin-spin correlation functions exhibit a 4-site periodicity, and their low temperature behavior is controlled by two correlation lengths that diverge like the inverse temperature, while the entropy is linear in temperature and its slope is consistent with three gapless modes of velocity $\pi/2$. The physical implications of these results are discussed.Comment: 4 pages, 4 figures, RevTe

Theory for Phase Transitions in Insulating Vanadium Oxide

We show that the recently proposed S=2 bond model with orbital degrees of freedom for insulating V$_{2}$O$_{3}$ not only explains the anomalous magnetic ordering, but also other mysteries of the magnetic phase transition. The model contains an additional orbital degree of freedom that exhibits a zero temperature quantum phase transtion in the Ising universality class.Comment: 5 pages, 2 figure

Two-gap model for underdoped cuprate superconductors

Various properties of underdoped superconducting cuprates, including the momentum-dependent pseudogap opening, indicate a behavior which is neither BCS nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a two-gap model. This model assumes an anisotropic pairing interaction among two kinds of fermions with small and large Fermi velocities representing the quasiparticles near the M and the nodal points of the Fermi surface respectively. We find that a gap forms near the M points resulting into incoherent pairing due to strong fluctuations. Instead the pairing near the nodal points sets in with phase coherence at lower temperature. By tuning the momentum-dependent interaction, the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimally doped regime) to a mixed boson-fermion picture (in the strongly underdoped regime).Comment: 5 pages, 1 enclosed figure. For further information see http://htcs.or

X-Ray Resonant Scattering as a Direct Probe of Orbital Ordering in Transition-Metal Oxides

X-ray resonant scattering at the K-edge of transition metal oxides is shown to measure the orbital order parameter, supposed to accompany magnetic ordering in some cases. Virtual transitions to the 3d-orbitals are quadrupolar in general. In cases with no inversion symmetry, such as V$_2$O$_3$, treated in detail here, a dipole component enhances the resonance. Hence, we argue that the detailed structure of orbital order in V$_2$O$_3$ is experimentally accessible.Comment: LaTex using RevTex, 4 pages and two included postscript figure

Non-linear effects and dephasing in disordered electron systems

The calculation of the dephasing time in electron systems is presented. By means of the Keldysh formalism we discuss in a unifying way both weak localization and interaction effects in disordered systems. This allows us to show how dephasing arises both in the particle-particle channel (weak localization) and in the particle-hole channel (interaction effect). First we discuss dephasing by an external field. Besides reviewing previous work on how an external oscillating field suppresses the weak localization correction, we derive a new expression for the effect of a field on the interaction correction. We find that the latter may be suppressed by a static electric field, in contrast to weak localization. We then consider dephasing due to inelastic scattering. The ambiguities involved in the definition of the dephasing time are clarified by directly comparing the diagrammatic approach with the path-integral approach. We show that different dephasing times appear in the particle-particle and particle-hole channels. Finally we comment on recent experiments.Comment: 28 pages, 6 figures (14ps-files

Screening of Nuclear Reactions in the Sun and Solar Neutrinos

We quantitatively determine the effect and the uncertainty on solar neutrino production arising from the screening process. We present predictions for the solar neutrino fluxes and signals obtained with different screening models available in the literature and by using our stellar evolution code. We explain these numerical results in terms of simple laws relating the screening factors with the neutrino fluxes. Futhermore we explore a wider range of models for screening, obtained from the Mitler model by introducing and varying two phenomenological parameters, taking into account effects not included in the Mitler prescription. Screening implies, with respect to a no-screening case, a central temperat reduction of 0.5%, a 2% (8%) increase of Beryllium (Boron)-neutrino flux and a 2% (12%) increase of the Gallium (Chlorine) signal. We also find that uncertainties due to the screening effect ar at the level of 1% for the predicted Beryllium-neutrino flux and Gallium signal, not exceeding 3% for the Boron-neutrino flux and the Chlorine signal.Comment: postscript file 11 pages + 4 figures compressed and uuencoded we have replaced the previous paper with a uuencoded file (the text is the same) for any problem please write to [email protected]

Absence of electron dephasing at zero temperature

Dephasing of electrons due to the electron-electron interaction has recently been the subject of a controversial debate, with different calculations yielding mutually incompatible results. In this paper we prove, by means of Ward identities, that neither a Coulomb interaction nor a short-ranged model interaction can lead to phase breaking at zero temperature in spatial dimensions d>2.Comment: 7 pp., LaTeX, no figs, final version as publishe

Orbitally Degenerate Spin-1 Model for Insulating V2O3

Motivated by recent neutron, X-ray absorption and resonant scattering experiments, we revisit the electronic structure of V2O3. We propose a model in which S=1 V3+ ions are coupled in the vertical V-V pairs forming two-fold orbitally degenerate configurations with S=2. Ferro-orbital ordering of the V-V pairs gives a description which is consistent with all experiments in the antiferromagnetic insulating phase.Comment: 4 pages, including three figure