Research Group for Condensed Matter Theory
Group Leader : Michiyasu Mori
Members : Hiroaki Onishi, Katsunori Kubo, Jun'ichi Ieda, Hiroto Adachi, Bo Gu, Mari Matsuo,Mamoru Matsuo, Yuta Yamane, Yuichi Ohnuma
Group Activities

We study new functional materials emerging from theelectrons internal degrees of freedom (spin, charge and orbital)
and the electrons correlation. New principle and new function ofdevices are also our important targets. So far, we have found (1)a new method of spin injection into all types of materials and (2)a spin current generation by a mechanical vibration (phonon).
Details of these two results are summarized in ResearchHighlights. Studies on spin Seebeck effect, spin current
generation by mechanical rotation, ferromagnetic Josephson
junction with magnetic domain wall, and enhanced spin Hall
effect are also progressing soundly.
Theory of spinmotive force and its continuous generation byhighly asymmetric geometry
A spin-originated motive force, i.e., “spinmotive force”, isthe conversion of the magnetic energy in a ferromagnet into
the electrical energy of conduction electrons via the magnetic
exchange interaction. The spinmotive force is given by the spinelectric ﬁeld,,
where m is the local magnetization, P is the spin polarization ofconduction electrons. The Planck constant and the elementarycharge are denoted by ħ and e, respectively. Note that both thetime and the spatial variations of m are necessary to obtainthe spinmotive force. So far, a motion of magnetization or
ferromagnetic nano-particles has been used to generate thespinmotive force. Hence, the obtained results are limited to apulse or ac-type generations.
We proposed a new method to generate a dc spinmotive force[1]. By exciting a ferromagnetic resonance in a comb-shaped
ferromagnetic thin ﬁlm as shown in Fig. 1, it is found that thecontinuous spinmotive force can be obtained. The spin electricﬁeld in the comb-shaped ferromagnet is calculated by supposingthat the local magnetization dynamics is determined by theLandau-Lifshitz-Gilbert equation. Experimental results arequantitatively well reproduced by our theoretical calculations.
Our theory can provide a microscopic understanding of thespinmotive force.

Non-monotonic temperature dependence of thermopower incorrelated electron systems
Since the discovery of unusually large thermopowerin Na xCoO 2, transition-metal oxides with strong electron
correlation have attracted much attention as promising
candidates for high-performance thermoelectric materials.
Thermopower is none other than the amount of entropy flow
along with the electric current. The consideration on the entropy
in thermodynamics tells us the low and high temperature(T) limits of the thermopower: In the metallic systems, thethermopower goes to zero as T goes to 0 K. On the other
hand, the high-temperature limit of thermopower is given by
the entropy consideration in the atomic limit. In the strongly
correlated systems, the spin and orbital degrees of freedom canenhance the high-temperature thermopower in principle.
We studied the role of strong Coulomb interaction on
thermopower, whose temperature dependence is particularly
examined in detail. For this purpose, the single- and multiband Hubbard models are adopted as a minimum model andthe strong Coulomb interaction is treated in the dynamicalmean ﬁeld theory (DMFT), which can capture the coherent-toincoherent crossover due to the strong Coulomb interaction,
U. This method based on the local picture is useful tounderstand the overall behavior of thermopower as a functionof temperature. It is found that the Coulomb interaction gives
rise to a non-monotonic temperature-dependence, which is welldescribed by the entropy consideration at high temperatures. In
the light of our theoretical results, the thermoelectric response in
the transition metal oxides, La1-xSrxVO3 is well explained.

Fig. 2 (a) Thermopower in the high temperature limit, Q 1(T → ∞ with kBT<U) and Q2 (T → ∞ with U<kBT), vs.
electron density, n in the single band Hubbard model. (b)Temperature dependence of the thermopower calculated
by DMFT with the non-crossing approximation as
impurity solver, for various parameter sets of U and n.

Fig. 1 Schematic of the comb-shaped ferromagnetic thin ﬁlm.
The ferromagnetic resonance frequency in the pad region(left) is different from that in the wire region (right).
9 ASRC

References[1] Y. Yamane et al., Phys. Rev. Lett. 107, 236602 (2011).
[2] M. Uchida et al., Phys. Rev. B 83, 165127 (2011).
[3] M. Matsuo et al., Phys. Rev. B 84, 153107 (2011).