M. A. N. Araújo

Phase diagram and magnetic collective excitations of the Hubbard model for graphene sheets and layers

We discuss the magnetic phases of the Hubbard model for the honeycomb lattice both in two and three spatial dimensions. A ground state phase diagram is obtained depending on the interaction strength U and electronic density n. We find a first order phase transition between ferromagnetic regions where the spin is maximally polarized (Nagaoka ferromagnetism) and regions with smaller magnetization (weak ferromagnetism). When taking into account the possibility of spiral states, we find that the lowest critical U is obtained for an ordering momentum different from zero. The evolution of the ordering momentum with doping is discussed. The magnetic excitations (spin waves) in the antiferromagnetic insulating phase are calculated from the random-phase-approximation for the spin susceptibility. We also compute the spin fluctuation correction to the mean field magnetization by virtual emission/absorpion of spin waves. In the large

Quantum waveguide theory of Andreev spectroscopy in multiband superconductors: The case of iron pnictides

U

Current-induced motion of a domain wall in a magnetic nanowire

limit, the renormalized magnetization agrees qualitatively with the Holstein-Primakoff theory of the Heisenberg antiferromagnet, although the latter approach produces a larger renormalization.

Spin-wave dispersion in La 2 CuO 4

The problem of Andreev reflection between a normal metal and a multiband superconductor is addressed. The appropriate matching conditions for the wave function at the interface are established on the basis of an extension of quantum waveguide theory to these systems. Interference effects between different bands of the superconductor manifest themselves in the conductance and the case of FeAs superconductors is specifically considered, in the framework of a recently proposed effective two-band model, in the sign-reversed s-wave pairing scenario. Resonant transmission through surface Andreev bound states is found as well as destructive interference effects that produce zeros in the conductance at normal incidence. Both these effects occur at nonzero bias voltage.

Comment on "Gapless spin-1 neutral collective mode branch for graphite".

The dynamics of current-induced motion of a magnetic domain wall in a quasi-one-dimensional ferromagnet with both easy-axis and easy-plane anisotropy, is studied. We pay a special attention to the case of a sharp domain wall, and calculate the spin torque created by the electric current. The torque has two components, one of which is acting as a driving force for the motion of the domain wall while the other distorts its shape, forcing thus the magnetic moments to deviate from the easy plane.

Superconductivity in the SU ( N ) Anderson lattice at U

We calculate the antiferromagnetic spin-wave dispersion in a half-filled Hubbard model for a two-dimensional square lattice, and find it to be in excellent agreement with recent high-resolution inelastic neutron scattering performed on La 2 CuO 4 [Phys. Rev. Lett. 86, 5377 (2001)].

Enhancement of the critical temperature in iron pnictide superconductors by finite-size effects

A Comment on the Letter by G. Baskaran and S. A. Jafari, Phys. Rev. Lett. 89, 016402 (2002)

Local-moment formation in the periodic Anderson model with superconducting correlations

We present a mean-field study of superconductivity in a generalized N-channel cubic Anderson lattice at U=\infty taking into account the effect of a nearest-neighbor attraction J. The condition U=\infty is implemented within the slave-boson formalism considering the slave bosons to be condensed. We consider the

Chern band insulators in a magnetic field

f

Chiral topological insulating phases from three-dimensional nodal loop semimetals

-level occupancy ranging from the mixed valence regime to the Kondo limit and study the dependence of the critical temperature on the various model parameters for each of three possible Cooper pairing symmetries (extended s, d-wave and p-wave pairing) and find interesting crossovers. It is found that the d- and p- wave order parameters have, in general, very similar critical temperatures. The extended s-wave pairing seems to be relatively more stable for electronic densities per channel close to one and for large values of the superconducting interaction J.