Seminar Announcement

Date： Sept. 12 (Thu.)
Room：Seminar room in prefab building
I. 15:30 - 16:45
Prof. A. Oguri （Osaka City University）“ Low and high voltage properties of theAnderson model out of equilibrium”
We will describe some properties of the Anderson impurity model forquantum dots at low and high bias voltages. At low voltages, we have calculated the low-energy asymptotic form of the nonequilibrium self-energy
using the microscopic Fermi-liquid theory of Yamada and Yosida in theKeldysh diagrammatic formalism. From this result, it has been deduced
that the nonlinear I-V characteristics has a universal form up to the thirdorder term in V, which can be written in terms of the Kondo temperatureand the Wilson ratio. In the opposite limit, the impurity Green’s functionin the high-voltage limit becomes identical to the equilibrium one in thehigh-temperature limit. This correspondence holds when the couplings ofthe dot and two leads, at left and right, are equal. These results obtainedin the two limits suggest that the Coulomb interaction can be treated adiabatically both at low and high bias voltages.

Coﬀee Break (16:45 - 17:00)

II. 17:00 Prof. J. Mathon (City University of London)“ Current and bias-induced switching of themagnetization in magnetic multilayers ”
Slonczewski showed that a bias-induced current of electrons owing between two magnetic layers separated by a nonmagnetic spacer can result inswitching of the magnetization of the second magnetic layer relative to thatof the rst layer.In his model the switching is due to the components of thenonequilibrium spin current transverse to the charge cur- rent.In this talk ageneral formulation of the current-induced switching is given.It treats boththe transverse and longitudinal components of the spin current on the samefooting. The present approach is based on the nonequilibrium Keldysh
formalism and can be im- plemented for a fully realistic band structure.It
will be shown that the nonequilibrium current-induced contribution to thelongitudinal spin current (ignored by Slonczewski) leads to an additional
torque exerted by one magnetic layer on the other.The nonequilib- rium
torque is proportional to the applied bias and has a component which remains nite for an arbitrarily thick nonmagnetic spacer layer (in the ballisticlimit).Hence this term can also result in current-induced switching.Usinga single-orbital tight-binding model,it will be shown that,for experimentally reasonable values of the bias,the current-induced torque is as strongas the equilibrium oscillatory exchange coupling.However,the current- induced torque is nonzero only for a junction in which the left-right symmetry
with respect to the center of the junction is broken.Various mechanisms of
the symmetry breaking will be discussed within the framework of the singleorbital tight-binding model.The latest results on current-induced switchingin a Co/Cu/Co trilayer,based on a fully real- istic band structure,will also
be presented.When a strong bias is applied to a magnetic tunneling junction,an additional switching e ect due to the modi cation of the potential
pro le of the insulating barrier can occur.Such a bias-induced switching
will also be discussed within the framework of the general non-equilibrium
Keldysh formalism.

連絡先：金属材料研究所
前川 禎通（内 2005）