Kenji Makoshi

Effect of Spin Fluctuations on the Specific Heat of Weakly and Nearly Ferromagnetic Metals

The specific heat due to the effect of spin fluctuations in itinerant ferromagnets is calculated by using the renormalized spin fluctuation theory of Moriya and Kawabata. The calculation continuously covers all the interesting temperature ranges, both below and above the Curie temperature. The specific heat is shown to be generally enhanced in weakly and nearly ferromagnetic metals. This is particularly so at low temperatures and the effect decreases as temperature goes up far more rapidly than the previous results based on the random phase approximation which generally over-estimates the spin fluctuation effect. An anomaly around the Curie temperature turns out to be rather small.

Contribution to a theory of vibrational scanning tunneling spectroscopy of adsorbates: Nonequilibrium Green's function approach

Abstract Formal theory of a phonon-assisted tunneling between a metal surface with an adsorbate and a metal scanning tip through an adsorbate (i.e., vibrational scanning tunneling spectroscopy (STS)) has been presented on the basis of the self-consistent nonequilibrium Keldyshs diagram technique. Within this approach, it is possible to take into account a finite lifetime of vibrational excitation, and the balance between elastic and opening inelastic tunneling channels. The relaxation of the vibrational degrees of freedom is included into the phonon Keldysh–Green function and plays a very important role both in the elastic and the inelastic tunneling through the adsorbate. The tunnel current (the total as well as the inelastic one) is calculated in terms of the one-particle nondiagonal substrate–adsorbate Keldysh–Greens function supplemented with the vibrational lifetime and filling factors of adsorbate orbital and phonons. The general expression of the inelastic tunnel current and conductance via the excitation of phonons is also presented in terms of the phonon occupation number and the phonon damping function in the stationary condition.

Theory of Helical Spin Structure in Itinerant Electron Systems. II

The self-consistent renormalization theory of spin fluctuations as developed previously for ferro- and antiferromagnetic metals is extended to the case of a helical spin structure with the use of a model for the dynamical susceptibility χ 0 ( q ,ω) of the noninteracting system. The magnetic properties of MnSi having a helical spin structure of a long period below T C =29 K are discussed on the basis of this theory.

Two-Copper-Atom Units Induce a Pseudo Jahn–Teller Polaron in Hole-Doped Cuprate Superconductors

Multiconfiguration molecular orbital cluster calculation results combined with experimental observations suggest that the local electronic state unit for a doped hole contains at least two Cu atoms in contrast to the Zhang–Rice singlet state that includes only one Cu atom. The cluster calculation shows that a doped hole in a two copper containing cluster induces a pseudo Jahn–Teller instability with stabilization energy that agrees with a peak in the photoinduced conductivity measurement, and the Cu–O bond length fluctuations that explain the EXAFS experiment; the charge-transfer energy gap observed in the optical conductivity measurement, and an excitation energy that corresponds to a peak in the energy loss function are also obtained from the calculation including two Cu atoms.

Charge Transfer in Hartree-Fock Approximation of Time-Dependent Anderson Model

The effect of the intraatomic Coulomb interaction on the charge transfer is treated in the Hartree-Fock approximation with use of the time-dependent Anderson model which has been widely employed in gas-surface collision without the intraatomic interaction so far. The charge state of the scattered ion from the metal surface is calculated numerically for a case of simple time-dependence in a surface-adatom interaction within the wide band approximation. The total number of electrons transferred to the adatom-orbital is found to saturate in a rather short time interval. The spin polarisation grows exponentially, dependent on the spin splitting of the adatom level. Also shown is an oscillatory behaviour of spin polarisation near its equilibrium value after the exponential growth.

Theoretical Study of Structures of Solid Oxygen under High Pressure

The structure of solid oxygen under high pressure is studied up to 6 GPa with no empirical parameters. The potential of the crystal is described by superposition of O 2 pair potentials calculated by the ab initio method. Structural optimization is achieved with this potential energy surface. The monoclinic structure is stable up to 6 GPa and a monoclinic-orthorhombic structural transition is not found. The pressure dependence of lattice constants agrees with the result of recent X-ray diffraction experiments.

Oscillation of the Spin Polarization in the Time-Dependent Newns-Anderson Model

The oscillatory approach in time to the equilibrium value of the spin polarization of moving atoms near metal surfaces is analyzed in detail, which appears in the Hartree-Fock approximation for the time-dependent Newns-Anderson model. The approximate asymptotic solution is obtained for the long time behavior of the spin polarization and also the detailed analytic property of the exact solution for long time is discussed.

Effect of Spin Fluctuations on the Ultrasonic Attenuation in Itinerant Electron Systems with Helical Spin Structures

The effect of spin fluctuations on the ultrasonic attenuation coefficient is studied on the basis of the previously developed theory of the helical spin structure in the itinerant electron systems. The observed double peak structure in the attenuation coefficient as a function of the temperature under a rather high magnetic field is explained as due to the existence of the Stoner boundary and the mixing between up and down spin electrons due to helical ordering, which reflect themselves significantly on the low frequency part of the dynamical susceptibilities.

Theory of the relation between inelastic scanning tunneling spectroscopy of adsorbates and their vibrational deexcitation

Abstract We show how the rate for controlled excitation of adsorbate vibrational modes by scanning tunneling microscope can be quantitatively related to the lifetimes of these modes when deexciting by electron–hole pair mechanism. To this end we present a Green function formalism that accounts for the two processes in a unified way. A model calculation for an adsorbate like CO is studied and compared with existing experimental results.

Theory of photon-assisted tunneling through quantum dot

Abstract Based on the two-channel Anderson Hamiltonian with the non-equilibrium electron distribution under an external electric field, we develop the theory of stationary photon-assisted tunneling current via strongly correlated energy levels in a quantum dot. The non-equilibrium (Mahanthappa–Baksi–Keldysh) Green function formalism is employed to study the Kondo effect and Coulomb blockade phenomenon in the presence of an external field. The Coulomb interaction is treated within the second order perturbation in the self-energy. The change of electron distribution function in the quantum dot due to emission and absorption of photons is taken into account and is found to play a very important role for the suppression of the Kondo effect. We show that an opening of new tunneling channel through photon side band accounts for a Coulomb staircase in the electric current.