Spin current carried by magnons

A spin current is usually carried by electrons and generated due to the imbalance of up-spin and down-spin. Here we investigate another type of spin current, which is carried by magnons. Using nonequilibrium Green's-function technique, we have derived a Landauer-Büttiker-type formula for spin current transport. The spin current satisfies conservation condition and can be expressed in terms of the magnon Green's functions of the mesoscopic ferromagnetic isolating system. As an application of this theory, we study the magnon transport properties of a two-level magnon quantum dot in the presence of the magnon-magnon scattering. By solving the self-consistent equations, we obtain the nonlinear spin current as a function of the magnetochemical potential. The spin current generated by using a parametric quantum pumping mechanism is also discussed.published_or_final_versio

Intermediate spin state stabilized by the Jahn-Teller distortion in La1/2Ba1/2CoO3

The recent high-resolution neutron and synchrotron diffraction measurements by Fauth et al. [Phys. Rev. B 65, 060401(R) (2002)] demonstrated the existence of an intermediate spin state in half-doped La1/2Ba1/2CoO3 which is accompanied by a long-range tetragonal Jahn-Teller distortion. In this paper we find that the charge uniform ferromagnetic intermediate spin state strongly couples to the Jahn-Teller effect and is stable only at moderate distortion of 1%. While the calculated magnetic moment of this state is in agreement with the experimentally measured one, the corresponding orbital ordering is the nearest-neighboring alternating d3x2-r2/d3y2-r2, instead of the observed uniform d3z2-r2 orbital ordering. A calculation is carried out using the self-consistent unrestricted Hartree-Fock approximation on a realistic multiband d-p Hubbard Hamiltonian and a generalized Jahn-Teller coupling model, in which all possible spin-, charge-, and orbital-ordered states are considered.published_or_final_versio

A spin injector

We theoretically put forward a spin injector, which consists of a three-terminal ferromagnetic-metal (FM) nonmagnetic-semiconductor (NS)-superconductor (SC) mesoscopic hybrid system. This device can inject not only the spin-up current but also the pure spin current into the NS lead. The crossed Andreev reflection plays a key role in this device. Such a spin injector may be realized within the reach of the present-day technology. © 2004 American Institute of Physics.published_or_final_versio

Does the Tail Wag the Dog?: The Effect of Credit Default Swaps on Credit Risk

We use credit default swaps (CDS) trading data to demonstrate that the credit risk of reference firms, reflected in rating downgrades and bankruptcies, increases significantly upon the inception of CDS trading, a finding that is robust after controlling for the endogeneity of CDS trading. Additionally, distressed firms are more likely to file for bankruptcy if they are linked to CDS trading. Furthermore, firms with more “no restructuring” contracts than other types of CDS contracts (i.e., contracts that include restructuring) are more adversely affected by CDS trading, and the number of creditors increases after CDS trading begins, exacerbating creditor coordination failure in the resolution of financial distress.postprin

Spin-flip effects on the current-in-plane magnetotransport in magnetic multilayers with arbitrary magnetization alignments

An extended Boltzmann equation approach, with nondiagonal components of the electron distribution, function taken into account, is proposed to study spin-flip effects on the magnetoresistance (MR) in magnetic inhomogeneous systems with arbitrary magnetization alignments. The presence of spin-flip scattering is found to reduce the MR and to decrease deviation of the MR from linear dependence on sin 2(θ/2) where θ is the angle between the magnetizations of successive magnetic films.published_or_final_versio

Mixed (s+id)-wave order parameters in the Van Hove scenario

In the Van Hove scenario including orthorhombic distortion effect, we develop a pair of coupled gap equations for the mixed (s+id)-wave order parameter. It is found that a mixed s+id symmetry state is realized in a certain range of relative strength of the s and d interactions, and there are two second-order transitions between the mixed and the pure symmetry states. Particular attention is paid to the temperature dependence of two components in the mixed order parameter as well as their evolution from a pure s to a pure d symmetry state.published_or_final_versio

Transport theory in metallic films: Crossover from the classical to the quantum regime

Using the quantum-statistical approach, we have developed a unified transport theory of metallic films. A general description for the conductivity in metallic films has been rigorously formulated in the presence of both impurity scattering and surface roughness. An explicit connection between the quasiclassical and present quantum approaches is also presented. We show that the quasiclassical theory by Fuchs [Proc. Cambridge Philos. Soc. 34, 100 (1938)] and Sondheimer [Adv. PHys. 1, 1 (1952)] can be reformed to be applicable to ultrathin metallic films by introducing a treatment of the surface via angle-dependent specularity parameters and including the quantum size effect. Moreover, to the lowest-order approximation in the theory, the previous quantum-approach results and discussions have naturally been recovered. © 1995 The American Physical Society.published_or_final_versio

The quasi-one-dimensional character of spin waves in K2Fe7Se8

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Quasiclassical approach to magnetotransport in magnetic inhomogeneous systems

The conventional quasiclassical approach to magnetotransport in magnetic multilayers and magnetic granular solids is found to be suitable only for collinear magnetization configurations. A quantum treatment of electron spin is proposed to improve the quasiclassical theory to be applicable to arbitrary magnetization alignments, in which the electron distribution functions, as well as the electric fields and currents, need to be regarded as spinor matrices which are off-diagonal in spin space of conduction electrons. An extended Boltzmann equation has been used to derive the two-point spinor conductivity in magnetic inhomogeneous systems. The result obtained is found to be identical to that obtained from the real-space Kubo formula, indicating a close link between the reformed quasiclassical approach and the quantum theory. Angular dependence of the current-in-plane magnetoresistance in magnetic superlattices is discussed.published_or_final_versio

Vortex dynamics in twinned superconductors

We numerically solve the overdamped equation of vortex motion in a twin-boundary (TB) superconductor, in which the applied Lorentz force F L, the pinning forces due to TB's and point defects, and the intervortex interacting force are taken into account. Our simulations show that TB's act as easy flow channels for the vortex motion parallel to the TB's and obstructive barriers for that normal to the TB's. Due to the barrier effect, the transverse velocity of vortices increases with F L, but if F L is strong enough, the vortices can cross through the TB's so that the transverse velocity vs F L curve exhibits peak behavior.published_or_final_versio