An accurate simulation of Greens function and self-energy function of non-interacting electrons in disordered graphenes are performed. Fundamental physical quantities such as the elastic relaxation time {tau}e, the phase velocity vp, and the group velocity vg are evaluated. New features around the Dirac point are revealed, showing hints that multi-scattering induced hybridization of Bloch states plays an important role in the vicinity of the Dirac point.
The origin of ferromagnetic insulating state of La$_{7/8}$Sr$_{1/8}$MnO$_3$ is investigated. Based on the tight-binding model, it is shown that this state can be attributed to the Peierls instability arisen from the interplay of spin and orbital ordering. The importance of the hole-orbiton-phonon intercoupling in doped manganites is revealed. This picture explains well the recent experimental finding of the reentrance of ferromagnetic metal state at low temperature [Phys. Rev. Lett. 96, 097201 (2006)].
Density of states (DOS) of graphene under a high uniform magnetic field and white-noise random potential is numerically calculated. The disorder broadened zero-energy Landau band has a Gaussian shape whose width is proportional to the random potential variance and the square root of magnetic field. Wegner-type calculation is used to justify the results.
A global picture of magnetic domain wall (DW) propagation in a nanowire driven by a magnetic field is obtained: A static DW cannot exist in a homogeneous magnetic nanowire when an external magnetic field is applied. Thus, a DW must vary with time under a static magnetic field. A moving DW must dissipate energy due to the Gilbert damping. As a result, the wire has to release its Zeeman energy through the DW propagation along the field direction. The DW propagation speed is proportional to the energy dissipation rate that is determined by the DW structure. An oscillatory DW motion, either the precession around the wire axis or the breath of DW width, should lead to the speed oscillation.
This paper has been withdrawn by the author.
The propagation of a head-to-head magnetic domain-wall (DW) or a tail-to-tail DW in a magnetic nanowire under a static field along the wire axis is studied. Relationship between the DW velocity and DW structure is obtained from the energy consideration. The role of the energy dissipation in the field-driven DW motion is clarified. Namely, a field can only drive a domain-wall propagating along the field direction through the mediation of a damping. Without the damping, DW cannot propagate along the wire. Contrary to the common wisdom, DW velocity is, in general, proportional to the energy dissipation rate, and one needs to find a way to enhance the energy dissipation in order to increase the propagation speed. The theory provides also a nature explanation of the wire-width dependence of the DW velocity and velocity oscillation beyond Walker breakdown field.
Based on the modified Landau-Lifshitz-Gilbert equation for an arbitrary Stoner particle under an external magnetic field and a spin-polarized electric current, differential equations for the optimal reversal trajectory, along which the magnetization reversal is the fastest one among all possible reversal routes, are obtained. We show that this is a Euler-Lagrange problem with constrains. The Euler equation of the optimal trajectory is useful in designing a magnetic field pulse and/or a polarized electric current pulse in magnetization reversal for two reasons. 1) It is straightforward to obtain the solution of the Euler equation, at least numerically, for a given magnetic nano-structure characterized by its magnetic anisotropy energy. 2) After obtaining the optimal reversal trajectory for a given magnetic nano-structure, finding a proper field/current pulse is an algebraic problem instead of the original nonlinear differential equation.
