Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
松尾 まり; 岡本 敏史*; 小椎八重 航*; 森 道康; 前川 禎通
no journal, ,
The cobalt oxides, Na
CoO
have a large thermopower due to an entropy transport by the degrees of freedom of electron's spin and orbital as characteristics of strongly correlated electron systems. Wang et al. observed much larger magnetic field response of thermopower of NaCoO at low temperatures than usual metals. Motivated by their observation of the huge magnetic field response of thermopower, we investigate theoretically effects of strong correlation between electrons on the field response of thermopower at low temperatures. We study magnetic field dependence of the Seebeck coefficient of the single band Hubbard model using Dynamical Mean Field Theory and non-crossing approximation as one-site Anderson impurity solver. Our results show that the strong Coulomb interaction derives a narrow quasi-particle band structure and a large response of the Seebeck coefficient to magnetic field.
安立 裕人; 高橋 三郎; 前川 禎通; 大江 純一郎
no journal, ,
温度勾配によって強磁性絶縁体中に誘起されるスピン流に対する研究結果を発表する。揺動散逸定理から導かれるランダムトルク中の磁化のダイナミクスを数値シミュレーションによって求め、温度勾配によって駆動されるスピン波を明らかにする。また、マクロなサンプルにおけるスピン波や、逆スピンホール効果によって観測可能な電圧を議論する。
森 道康; 挽野 真一*; 高橋 三郎; 前川 禎通
no journal, ,
Nano-structure composed of superconductor (S) and magnetic materials shows various interesting phenomena. We study the supercurrent resulting from coupling of the Josephson-phase and the spin-wave excited by microwave radiation in a ferromagnetic Josephson junction, in which two S's are separated by a ferromagnet (F). To explore how the spin wave excitation affects the current-voltage curve, the resistively shunted junction model, which is an equation of motion for the Josephson-phase, is extended by considering the gauge invariance including magnetization. When the magnetization is driven by the microwave adjusted to the ferromagnetic resonance frequency, the dc supercurrent is induced in the junction, and the current-voltage curve shows step structures as a function of applied voltage. The position of each step in voltage is proportional to the microwave frequency multiplied by even number. This means that the even number of magnons is necessary for the singlet Cooper pair to go through the ferromagnetic layer. The magnitudes of step-height can be controlled by tuning the shape of interface. Our results present a new route to observe the spin-wave excitation by the Josephson effect.
前川 禎通
no journal, ,
In the Seebeck effect, the electric voltage is generated in a conductor placed in a temperature gradient. The efficiency of the effect is given by the density and the scattering of the conduction electrons in usual metals and semiconductors. Since the Seebeck effect is due to the entropy carried by the electric current, the spin and orbital degrees of freedom of conducting electrons in the oxides may enhance the Seebeck effect. In the presentation, firstly, the enhanced Seebeck effect is discussed in the transition metal oxides. Secondly, the spin-Seebeck effect is proposed, where the spin voltage is generated in a ferromagnetic metal placed in a temperature gradient. Then by utilizing the spin-detection method based on the spin-Hall effect, the spin voltage is converted to the electric voltage. The spin-Seebeck effect is induced by a pure spin current, a flow of electron spin without electric charge current, and provides a variety of spin-electronics applications.