Ko Sugihara

Magnon drag effect in magnetic semiconductors

Abstract Magnon drag effect in antiferromagnetic- and ferromagnetic semiconductors is calculated by solving the coupled Boltzmann equation for the magnon and carrier systems, where the carriers are specified by the mobile carriers in a broad band with an isotropic effective mass m*. Discussions are limited to the temperature range T « TN or Tc. Magnons and carriers are coupled via the sdinteraction. If the Rayleigh scattering due to the point defects or impurities, plays a dominant role in limiting the relaxation rate of magnon except the sd-scattering, the magnon drag thermopower Download : Download full-size image has the following temperature dependence: Download : Download full-size image The first two qualitatively explain the observed behavior of the thermoelectric power in MnTe.

Concentration dependent spin-lattice relaxation in n-type silicon

Abstract Spin-lattice relaxation mechanisms for the donor electrons in uncompensated silicon are presented. Major isolated spins transfer their excitation energies via the spin-diffusion process to the fast-relaxing centers. For lightly doped samples. N d ≲ 10 16 cm 3 , exchange-coupled donor pairs act as the fast-relaxing centers. Theory provides the correct order of magnitude for the relaxation rate 1 T s . However, the calculated relaxation rate 1 T s , for this process is field independent, while the observed rate shows a weak field dependence. For more heavily doped samples, N d > 10 16 /cm 3 , the relaxation rate can be explained by assuming the presence of a small concentration of neutral-ionized donor pairs. The relaxation process for these pairs is the resultant of two different mechanisms, a field dependent mechanism and a field independent one. The former depends strongly on the donor concentration and the latter shows relatively weak dependence on N d .

Anomalous Galvanomagnetic Properties of Graphite in Strong Magnetic Fields

Our experiments on the galvanomagnetic effects of graphite in strong magnetic fields revealed that 1) σ x y B is not a constant but depends on the field strength, 2) the resistivity at low temperatures has a field dependence of (rho{cong}B/(p{+}qB^{n})), (n{cong}1), and 3) in an applied magnetic field the ρ vs T curve has a maximum at T =20 K∼25 K. These results can not be explained by simple theory. However, if the transitions: D + +(-e)→D 0 and/or A - +(+e)→A 0 are induced in strong fields, where D corresponds to donor and A represents acceptor, then co-existence of ionized impurity scattering and neutral impurity scattering can explain the qualitative feature of the ( B , T )-dependence of the resistivity at low temperatures. At high temperatures it is necessary to consider phonon scattering and carrier-carrier scattering. Without the carrier-carrier scattering the ( B , T )-dependence of ρ for T >25 K can not be explained.

Thermoelectric power of antiferromagnetic semiconductors near the néel temperature

Abstract Thermoelectric power in antiferromagnetic semiconductors is calculated in the vicinity of the Neel temperature by assuming the presence of the well-defined propagating modes (magnon) and a temperature-dependent magnon velocity. Carriers are specified by the mobile carriers in a broad band with an isotropic effective mass. Magnons and carriers are coupled via the sd-interaction. Anomaly near TN does not result from the diffusion term Sdiff. and it is ascribed to the magnon-drag contribution Sdrag which is obtained from S drag = − ∑ q R ( q ) c m ( q ) / 3 e , where R(q) is the momentum transfer ratio between the magnon system and carriers, cm(q) denotesthe specific heat of the magnon q. The assumed temperature dependence of the magnon velocity makesSdru divergent at TN with TN − T → +0 and it might be removed in consideration of the magnon frequency spreading. Except this divergence the calculation provides a qualitative explanation for the anomalous feature in MnTe.

Anomalous galvanomagnetic properties of graphite in the quantum limit

Abstract Recent experiments on the galvanomagnetic effects of graphite in the quantum limit revealed that (1), σ xy H is not a constant but depends on the field strength; (2), the resistivity ϱ ⋟ σ xx −1 at low temperatures has a field dependence ofϱ = ∣(A + BH n ) , n ⋟ 1; and (3), the ϱ vs T curve has a maximum at about T = 25K. These results cannot be explained by a simple theory. However, in consideration of the appearance of the bound states D 0 and A 0 in the quantum limit we can provide a qualitative explanation of the above observed results, where D correspond to donor and A represents acceptor. In the quantum limit the possibility of the transitions D + + (−e) → D 0 and A − + (+e) → A 0 is pointed out. Co-existence of the ionized impurity scattering and the neutral impurity scattering explains the qualitative features of the ( H,T )-dependence of the resistivity at low temperatures. At high temperatures it is necessary to consider the phonon scattering and the carrier-carrier scattering. Without the carrier-carrier scattering the ( H,T )-dependence of ϱ at T > 25K cannot be explained.

Acoustomagnetoelectric effect in graphite

The AME effect has been observed in graphite at 4.2 K by applying a magnetic field parallel to the c-axis. The frequency of the ultrasonic wave is 16 MHz. The AME voltage is proportional to H0.91 for H 3 kOe. dHsB-oscillation has been observed for H ⪆ 7 kOe. The observed field dependence of the AME voltage is different from that of bismuth. Calculation based on the theory of Cohen, Harrison and Harrison has been presented and compared with experimental results.