Masamichi Sakai

ESR Observation of the Motion of Optically Induced Paramagnetic Dimers in [Pt(en)2][Pt(en)2Cl2](ClO4)4 (en=ethylenediamine)

On the basis of the temperature dependence of the ESR spectrum, the optically induced unpaired spins in [Pt(en) 2 ][Pt(en) 2 Cl 2 ](ClO 4 ) 4 , where en is ethylenediamine, are verified to form dimeric states of Pt ions which are mobile but transport no electric charge. The dimer hops along the ···-Pt-Cl-Pt-Cl-··· chain in a manner of random walk. The hopping rate is given by ν c =ν 0 exp (-Δ/ k T ) with ν 0 =1.3×10 9 sec -1 and Δ=13±1 meV over a wide temperature range from liquid He to the ambient. This state is most likely the neutral soliton.

Excitation-Power Dependence of Free Exciton Photoluminescence of Semiconductors

We have derived an analytical formula for the excitation-power dependence of the free exciton photoluminescence (PL) intensity. It has been found that the PL intensity I depends on the power of the excitation laser L as I∝Lk, where k is the power index. We have deduced the analytical formula that describes the value of k for the free exciton PL emission under the above-band-gap excitation conditions. The results indicate that the value of k is in the region of 1<k<2 depending on both the value of L and material properties such as radiative and competitive nonradiative recombination probabilities.

Photoluminescence characterization of excitonic centers in ZnO epitaxial films

Photoluminescence properties of nominally undoped ZnO thin films grown by radical-source molecular-beam epitaxy have been investigated as a function of (i) sample growth conditions, (ii) excitation laser power density, and (iii) measurement temperatures. Altogether four excitonic emission peaks were observed at photon energy of 3.3646, 3.3606, 3.3572, and 3.3331 eV, which are tentatively denoted as emission peaks A, D, F, and G, respectively. We have classified the defect types responsible for the emission peaks into the following two groups; (i) D and F, whose responsible defect types are suggested to be residual impurities such as aluminum and indium, respectively, and (ii) A and G, whose responsible defect types are suggested to be intrinsic defects such as oxygen vacancies, interstitial zinc, and extended structural defects particularly for G.

Magnetotransport properties in near-stoichiometric hydride films of YH2+δ under weak fields

We have investigated the basic characteristics of mobile carriers in as-deposited thin films of hexagonal-close-packed Y and hydrogenated films of face-centered-cubic YH2+δ. Hall resistance and transverse magnetoresistance measurements were carried out under magnetic fields of up to 1 T at room temperature. The Hall coefficient in YH2+δ showed a minimum value of ∼5×10−12 m3/C when the δ value reached ∼−0.1, in accordance with the δ dependence of resistivity reported previously. Quantitative analysis based on the present measurements revealed that YH2+δ is a compensated metal whose carrier density and mobility were determined to be ∼1.4×1027 (∼1.9×1027) m−3 and ∼3.5×10−3 (∼3.1×10−3) m2 V−1 s−1, respectively, for δ=0.04(−0.03) for both electrons and holes. These carrier parameters were found to account concurrently for the reflection spectral line shapes observed. The compensation observed in YH2+δ is at variance with a simple criterion used previously [E. Fawcett and W. A. Reed, Phys. Rev. 131, 2463 (1963)].

Raman Scattering and X-ray Diffraction Studies on Phase Transitions in ZnCl2 under Hydrostatic Pressure

Pressure dependences of the Raman scattering and X-ray diffraction have been measured in ZnCl 2 at room temperature. As-grown crystals of the a phase transform at 2.1 GPa into a new phase, designated δ, which has the CdCl 2 type layer structure. On releasing the pressure, the δ phase transforms at 1.2 GPa into the γ phase with the red-HgI 2 type layer structure. The γ phase is stable even if the pressure is completely released. With increasing pressure on the second run and thereafter, the γ-ZnCl 2 crystal reversibly transforms into the δ phase with hysteresis between 1.2 and 2.6 GPa. This polymorphism is discussed in relation with dimensionalities of bonding networks in respective phases.

Appearance of a correlation between the Hall coefficient and electrical resistivity upon dihydrogenation of yttrium

To shed light on the correlation between the Hall coefficient (RH) and electrical resistivity (ρ), we performed simultaneous measurements of these two transport coefficients in fcc dihydride phase of yttrium (YHx), having H/Y values ranging from 1.73 to 2.04. Unlike the typical behavior of metals, an approximately linear relationship was observed between RH and ρ at room temperature after dihydrogenation of yttrium. Interpretation of this relationship, based on the Boltzmann–Bloch scheme, reveals that the transverse (cyclotron) relaxation rate (1/τc) of the carriers is relatively insensitive to the generation of hydrogen defects in the dihydride phase of yttrium, unlike the longitudinal relaxation rate (1/τ), which is affected by the presence of hydrogen defect. Low-temperature (77 K) measurements of RH and ρ on the same samples show that the approximately linear relationship observed at room temperature disappears but a certain nonlinear relationship may exist at 77 K.

Anharmonicity of Low Frequency Lattice Vibrations in Red-HgI2

The Raman scattering spectra in the layer compound red-HgI 2 have been measured in detail under hydrostatic pressure. Interatomic force constants are deduced from the data on the basis of a simple Born-von Karman model to estimate the harmonic frequency of the infrared active E u 1 (TO) mode. The frequency is found to be considerably smaller than the experimental value at low pressures. This discrepancy is attributed to the shift due to the quartic anharmonicity associated with the shear potential of a Hg–I bond. Its harmonic force constant increases rapidly with increasing pressure, so that the E u 1 (TO) mode becomes rather harmonic at high pressures. The anharmonicity due to the change in crystal volume is also discussed in relation to the pressure dependence of the interlayer interaction between iodines. Our experimental results show that the effective size of iodine ions decreases with increasing pressure.

Hall resistivity and transverse magnetoresistivity generated in simultaneous presence of spin-polarized current and external magnetic field in a nonmagnetic bipolar conductor YH2

We have carried out magnetotransport measurements of the bipolar conductor YHx () employing the ferromagnetic conductor Co as the source and drain electrodes, from which the spin-polarized currents are injected. Both the Hall resistivity (HR) and transverse magnetoresistivity (TMR) ratio are enhanced by a factor of ~3 and ~6, respectively, when Co electrodes are used instead of Au electrodes. We derived two types of closed formulae of HR and TMR, the first (second) type of which was derived by simultaneously (independently) considering the external magnetic field and spin–orbit interaction, but both types of which were derived by considering spin-dependent electrochemical potential caused by the presence of the ferromagnetic electrodes. Carrier parameter assessments based on the two types of formulae as well as the two types of electrode samples enable us to infer that coupling mechanisms between the external magnetic field and spin–orbit interaction are required for the precise interpretation of HR and TMR measured in the simultaneous presence of magnetic field and spin splitting in a diffusion force field, especially for conductors with strong spin–orbit interaction.

Magnetoresistance Generated by Combination of Spin–Orbit Interaction and Applied Magnetic Field in Bipolar Conductors

We have theoretically studied the magnetotransport properties in bipolar conductors under consideration of the simultaneous presence of an external magnetic field, left–right asymmetric carrier scattering due to spin–orbit interactions, and spin-polarized holes/electrons, predicting both positive and negative transverse magnetoresistance (TMR) terms, the mechanisms of which are completely different from the conventional mechanism in bipolar conductors. The positive TMR term is predicted only for the asymmetric carrier scattering case, the sign of which is the same (negative) between a hole and an electron. The other case of asymmetric scattering, the sign of which is opposite between a hole and an electron, was also determined to always show a negative TMR. Our proposed TMR was experimentally evidenced from the magnetotransport and magnetization measurements of a compensated metal, YH2. Also, application to logic gates is discussed on the basis of our proposed mechanism.

Generation of Spin Current in Bipolar Conductors

The ordinary Hall effect (OHE), which is caused by an external magnetic field, was studied as a mechanism for the generation of spin current. It has been theoretically elucidated that, under an open-circuit condition, the OHE can contribute to spin-current generation when spin-polarized electrons and holes are simultaneously present as mobile carriers. This OHE contribution to spin current generation is caused by the steady-state kinematics of electrons and holes whose transverse velocities have the same direction. Although anomalous Hall effects may contribute to spin-current generation, the OHE plays a principal role in the generation of spin current when electrons and holes have approximately the same transport characteristics. The experimental aspects of possible materials for the isomorphic electron and hole systems are argued on the basis of the experimental results of the magnetotransport measurement of yttrium dihydride and preliminary results of the magnetization measurement of hydrogenated films of gadolinium.