Comment on 'Lagrange-multiplier method in correlated-electron systems: Exact diagonalization study'

We discuss the Lagrange-multiplier method in a many-body system, and how to apply the Lagrange multiplier correctly to realize quantum constraint in a Hamiltonian for a quantum system. It is pointed out that the discussion in Yanagisawa’s paper [Phys. Rev. B 57, 6208 (1998)] lacks a basic knowledge of mathematics, and that his conclusion is generally wrong.published_or_final_versio

Antiferromagnetism in the S=1/2 antiferromagnetic Heisenberg model on a two-dimensional square lattice

We present a rigorous result and strong evidence to support existence of antiferromagnetic long-range order on a square lattice for the S = 1/2 antiferromagnetic Heisenberg model.published_or_final_versio

Spin transverse force on spin current in an electric field

As a relativistic quantum mechanical effect, it is shown that the electron field exerts a transverse force on an electron spin 1/2 only if the electron is moving. The spin force, analogue to the Lorentz for an electron charge in a magnetic field, is perpendicular to the electric field and the spin current whose spin polarization is projected along the electric field. This spin-dependent force can be used to understand the Zitterbewegung of the electron wave packet with spin-orbit coupling and is relevant to the generation of the charge Hall effect driven by the spin current in semiconductors. © 2005 The American Physical Society.published_or_final_versio

Exact ground state in an orbitally degenerate Hubbard model

We present an exact solution on the ground state of a one-dimensional orbitally degenerate Hubbard model in the case of strong coupling, and the phase diagram in terms of electron filling and relevant parameters. The ground state is nondegenerate apart for spin SU(2) symmetry and a fully saturated and metallic ferromagnet except the case of the quarter filling, at which a ferromagnetic Mott insulator transition occurs. The phase diagram shows coexistence of ferromagnetism and spin-triplet pairing superconductivity when the Hund coupling is sufficiently larger. A mechanism for spin-triplet pairing superconductivity and its possible relevance to the quasi-one-dimensional organic superconductor are proposed.published_or_final_versio

Phase separation and charge ordering in doped manganite perovskites: Projection perturbation and mean-field approaches

A theory is developed to explain various types of electronic collective behaviors in doped manganites R 1-xX xMnO 3 (R = La, Pr, Nd, etc. and X=Ca, Sr, Ba, etc.). Starting from a realistic electronic model, we derive an effective Hamiltonian by utilizing the projection perturbation techniques and develop a spin-charge-orbital coherent-state theory, in which the Jahn-Teller effect and the orbital degeneracy of e g electrons in Mn ions are taken into account. Physically, the experimentally observed charge-ordering state and electronic phase separation are two macroscopic quantum phenomena with opposite physical mechanisms, and their physical origins are elucidated in this theory. The interplay of the Jahn-Teller effect, the lattice distortion, as well as the double-exchange mechanism leads to different magnetic structures and to different charge-ordering patterns and phase separation. ©2000 The American Physical Society.published_or_final_versio

Spin current in the Kondo lattice model

By using the projection operator technique it is observed that the strong Hund's rule coupling and s-d interaction in transition metal elements may lead to an effective coupling between the spin current and spin chirality. As a result, the spin chirality can be regarded as a driving force to produce a spin current. The spin current may give rise to a novel type of field acting on the spins. A spin battery is designed based on the interactions between the spin current and spin chirality.published_or_final_versio

Nonexponential relaxation and quantum tunnel splitting in the molecular magnet Fe8

Magnetic relaxation in molecular magnets under a sweeping field is studied by taking into account local stray fields. It is found that the randomness of local stray field leads to a distribution of the relaxation rate which subsequently makes the relaxation deviate from the exponential law as predicted by the Landau-Zener model such that the Landau-Zener method needs to be revised to deduce an exact tunneling splitting. The tunneling splitting and distribution width of local stray fields are derived from the experimental data for molecular magnets Fe8.published_or_final_versio

Coloumb interaction and instability of CE-structure in half doped manganites

In their Letter (Phys. Rev. Lett. 83, 5118 (1999)), den Brink, Khaliullin, and Khomskii proposed theoretically that the one-dimensional ferromagnetic zigzag chains in CE phase in half-doped manganites play an essential role in forming the orbital ordering, and, more surprisingly, the on-site Coulomb interaction U between electrons with different orbitals leads to experimentally observed charge ordering. In this Comment, I point out that the strong U will destroy the stability of CE-type phase, which is stable in a very narrow regime in the parameter space for electronic model.To solve this issue finally, we have to take into account other interactions, such as the long-range Coulomb interaction, Jahn-Teller distortion, and physics of topological berry phase. For example, the effect of finite large J$_{H}$ leads to an attractive particle-hole interaction, which favors to stabilize the charge ordering.Comment: 1 page, 1 figure, To appear in Phys. Rev. Let

Spintronic Faraday rotation spectroscopy and geometrical modulation of spin current in an Aharonov-Casher ring

We study the ballistic spin transmission through an AC ring between two ferromagnetic filter-electrodes in the presence of spin-orbit interaction, theoretically. It is shown that the spin precession of polarized electrons can be controlled via a cylindrically symmetric electric field, and the sequence of the polarized conductance depends on the relative orientation of magnetization between the left and right filters. The injected spin-polarized electrons can be blocked completely for the special values of electric field and angle of texture, which makes the magnetoresistance to be drastically enhanced. We also propose an interference scheme to detect the geometric phase.published_or_final_versio

Quantum spin Hall effect induced by nonmagnetic and magnetic staggered potentials

We conducted a comparative study of the quantum spin Hall (QSH) effects induced by nonmagnetic and magnetic staggered potentials, respectively, and show that they have the same effect in driving the topological phase transition. The result implies that both time-reversal (T) preserving and breaking systems can host a QSH effect. We also investigate the stability of the resulting QSH effect for disorder and find that, for T invariant systems, the edge states are always robust while those of the T breaking system are also robust if there is additional symmetry in the system. © 2011 American Physical Society.published_or_final_versio