Sumiaki Nagai

The Madelung Energy in Copper-Oxide-Based Ceramics

The Madelung energy in the (Y, Ca)(Ba, La) 2 Cu 3 O 6+δ system has been calculated in order to find the distribution of hole carriers among sites. We regard the compoundas a superconductor, when the lowest-energy distribution is such that holes enter sites in CuO 2 sheets, whereas we regard it as a semiconductor, when no holes enter sites in the CuO 2 sheets. Based on this criterion, we find a phase diagram, where the superconducting region is separated from the semi-conducting region by a boundary. A good agreement with the experimental finding by Tokura et al . is obtained, when account is taken of the ordering of exygen vacancies in the basal plane of Fig. 1 in the text. It is found that for some compounds with small δ(∼0.25), holes are spontaneously produced in CuO 2 sheets, so that the compounds may be a superconductor although their nominal hole content is zero.

Combined OPW-TB Method for the Band Calculation of Layer-Type Crystals. I. : General Formalism and Application to the π Band of Graphite

A new method of band calculation suitable for layer-type crystals is presented. In this method the electron motion in a monolayer is treated at first using the two-dimensional OPWs and the layer functions which are defined as to be localized near the layer. Secondly the wave function of the three-dimensional crystal is constructed by the tight binding (TB) method. As an example, the π band structure of graphite is calculated using the simple screened Coulombic model potential. The results reveal the usefulness of this new method and calculated π band structure explains fairly well the various experimental results of graphite.

Intercalation Compounds of Graphyne

Abstract We report the band structures of first stage potassium intercalated graphyne using the optimized geometries. The calculation was carried out using the full-potential linear-combination-of-atomic-orbitals method. The optimized distance between two graphyne layers sandwiching the intercalate layer is shorter than typical graphite intercalation compounds. These intercalation compounds are metallic. The character of the band around the Fermi level at ⌜ point is C2p z. The band with the character of K4s lies about 2.4 eV higher than the Fermi level at ⌜ point for the compound intercalating one potassium in a unit cell, while that lies about 0.89 eV higher than the Fermi level at ⌜ point for intercalating two potassium in a unit cell. Further we refer the amount of charge transfer between potassium and carbon atoms.

Electrons in Infinite One-Dimensional Crystals in a Uniform Electric Field

The one-dimensional Schrodinger equation with the Kroning-Penny potential plus the electric-field potential eFx has been solved numerically. All the interband effects are included in the exact numerical calculation. Although an infinite crystal was considered, localized wave functions have been obtaind at energies which make a Stark ladder. One Stark ladder is associated with one band. When the probability of the Zener tunneling to the next band is appreciable for given values of parameters, the wave function necessarily penetrates into the region of the next band and is not localized.

Resistivity of Dilute Magnetic Alloys in the Presence of External Magnetic Fields

Based on the s-d interaction model for dilute magnetic alloys, we have calculated the resistivity as a function of temperature in the presence of an external magnetic field to the second Born approximation. The logarithmic singularity of resistivity is suppressed by the external magnetic field. For 2µ H ≫ k B T the ln T term is replaced by ln 2µ H . Depending upon the values of parameters, the magnetoresistance is found to be either positive or negative, and the resistivity-versus-temperature curve may have either maximum or minimum, or both. The R - T curves are plotted for a wide variety of values of the exchange integrals, the spin-independent potentials and the external magnetic fields. It is also shown that the curves are classified into five patterns. The physical meaning of the obtained results is discussed. It is suggested that the negative and positive magnetoresistances observed in impurity conduction in semiconductors are explained from our model.

Band Structure of Layered Semiconductor α-In 2Se 3 by the Numerical-Basis-Set LCAO Method

The electronic band structure of layered semiconductor α -In 2 Se 3 is investigated by using the self-consistent numerical-basis-set LCAO method with a potential based on the local density approximation.The resulting energy band gap occurs between Γ 5 - and Γ 1 + and its value is 0.99 eV. The effective masses in conduction and valence are 0.13 m 0 and 3.2 m 0 , respectively. The self-consistent valence of each ion is as follows: In 0.95+ In 1.23+ Se 1.52- Se 0.54- Se 0.12- . Thus the compound has a considerably ionic character though the interlayer bonding is found to be partly covalent from the valence electron charge distribution.

Band Structure of Layered Semiconductor α-In2Se3

The electronic band structure of layered semiconductor α-In 2 Se 3 is calculated by using the KKR method. For the muffin-tin potential constructed from the ionized atoms In +1.30 and Se -0.86 the band gap is obtained to be about 1.45 eV which is consistent with the observed value. All of the bands corresponding to Se-4s, Se-4p, In-5s and In-5p states are separated completely one another and are comparatively flat reflecting the general characteristics of layered materials with complex structure. The calculated effective masses in conduction and valence bands are considerably large, whose values are m c =1.1 m 0 and m v =1.9 m 0 , respectively. Further it is demonstrated that the calculation of structure constants is very serious for materials with complex crystal structure such as α-In 2 Se 3 .

Inelastic Scattering of X-Rays by Light Metals

The scattering cross section of X-rays inelastically scattered by an electron gas is calculated in the random phase approximation, for various scattering angles, making use of the formula previously derived by Ohmura and Matsudaira. When the angle is small, the profile deviates from the inverse parabolla due to the pauli exclusion principle and to the electron-electron interaction. Especially when the angle is near θ c , corresponding to the plasma cut-off wave number, a pronounced peak appears, reflecting plasmon emission. It is also shown that if a scattering angle is a little larger than θ c , yet the “plasmon peak” survives. These features are in excellent agreement with the experiments of Suzuki and Tanokura, and Alexandropoulos.

Theory of Magnetoresistance in Dilute Magnetic Alloys

The effect of an external magnetic field on the resistivity of dilute magnetic alloys is investigated with the use of the Greens functions. The equation of motion for the Zubarev Greens functions with an external magnetic field included is solved correctly up to order J 3 and to all orders in V , where J is the exchange integral and V the spin-independent potential. The effect of the external magnetic field is involved in the Zeeman term of logarithmic terms and in F σ (ω) which shows the effect of freezing out of spin-flip scattering. The effect of the spin-independent potential is involved in the modified density of states \( \tilde{\rho}\) and in a phase δ v of the t matrix. The magnetoresistance obtained from numerical calculation shows a variety of patterns which are classified by the sign of \(J \tilde{\rho}\) and the value of the phase shift δ v .