Yasutami Takada

Plasmon Mechanism of Superconductivity in Two-and Three-Dimensional Electron Systems

The possibility of the superconductive state with the aid of the plasmon in two- and three-dimensional single-carrier systems is investigated by numerical solutions of the gap equation in the weak-coupling theory of superconductivity, in which the effective interactions are calculated in the plasmon-pole approximation. The superconductivity does appear in rather low carrier concentrations and the highest attainable transition temperature is of the order of m * /κ 2 degrees Kelvin, where the effective mass, m * , is in the unit of the mass of a free electron and κ is the dielectric constant. Compared with a three-dimensional system, a two-dimensional one is more favorable for the plasmon mechanism of superconductivity, which stems from the difference in the dispersion relation of the plasmon.

Theory of Electronic Properties in N-Channel Inversion Layers on Narrow-Gap Semiconductors I. Subband Structure of InSb

The subband structure of n -channel inversion layers on InSb its calculated self-consistently in the Hartree approximation. A new method of the effective-mass approach for the nonparabolic band is presented. The splitting, of the spin states caused by the surface potential is calculated to be small and the resonance effect with the hole state in the bulk is found to be very small. The calculated effective masses of the occupied subbands at the Fermi level agree quite well with those measured by Daerr et al. for a wide range of the carrier concentration.

Theory of Superconductivity in Polar Semiconductors and Its Application to N-Type Semiconducting SrTiO3

By solving the gap equation numtaerically from the first principles, we have investigated the mechanism of superconductivity in n -type seumiconducting SrTiO 3 . The observed transition temperature as a function of the carrier density and the applied stress is reproduced quite well, when we take account of the plasmon and the polar optic phonon which relates to the stress-induced ferroelectric transition. In the calculation, the conduction band of SrTiO 3 is assumed to be the single-valley model proposed by Mattheiss and all the physical quantities such as the effective mass, the dielectric constant and the phonon dispersion relation are taken to be the values measured by experiments, so that there are no adjustable parameters in the theory. The effect of other phonons like the acoustic one is also evaluated and found to be small.

Subband Structures of N-Channel Inversion Layers on III–V Compounds –A Possibility of the Gate Controlled Gunn Effect–

Multi-subband structures of n-channel inversion layers on the surface of p-type III–V compounds are calculated by a variational method. Nonparabolicity is taken into account in the bulk dispersion relation of the \(\varGamma\)-valley. When the surface electron density is low, electrons occupy only the subbands in the \(\varGamma\)-valley, while when it exceeds certain critical value, most electrons occupy the ground subband in the second minimum valleys. From this behavior, the working condition of the Gunn effect in the system can be changed by applying the gate voltage in the MOS structure. The critical surface electron density depends on the charge density in the depletion layer and the surface orientation, but in case of GaAs, it is about 7×10 12 cm -2 for typical operating conditions.

Stress Effects on Electronic Properties of Silicon Inversion Layers

When a uniaxial stress is applied in the [001] direction to an n-channel inversion layer at the Si(100)/SiO 2 interface, electrons transfer from the two valleys located in the [100] direction to those in the [001] direction. The change in the subband structure and the quasi-particle properties such as the effective mass are calculated in the RPA. The Coulomb interaction is important and this transfer is discontinuous for low electron concentrations. The cyclotron resonance is also calculated by the use of the Landau Fermi-liquid theory and has a single peak throughout the transfer, if ωτ is less than three. The peak shifts from the cyclotron mass of the former valleys to that of the latter ones, which explains experiments qualitatively.

Mechanism of Superconductivity in Graphite-Alkali Metal Intercalation Compounds

The mechanism of superconductivity in the first stage graphite-alkali metal intercalation compounds C 8 M (M=K, Rb, or Cs) is investigated by the use of a method to solve the gap equation from the first principles. The observed transition temperature and anisotropy of the superconductivity can be explained well, when we consider that the three-dimensional alkali metal s -like electrons contribute to superconductivity with the aid of both the acoustic and the optic phonons in which the negatively charged carbon layers oscillate, respectively, in phase and out of phase with the positively charged alkali metal layers to give strong polar couplings with the electrons. The possibility of superconductivity in the two-dimensional graphite π-like electrons is also examined by the same method and found to be very small.

Consideration of the mechanism of superconductivity in fullerenes : beyond Migdal's theorem and the dynamic Coulomb effect

Abstract A scheme is presented to include vertex corrections up to infinite order in the calculation of the superconducting transition temperature T c . The scheme is applied to the alkali-metal-doped fullerenes. By considering the contribution from both the carrier plasmons and the intramolecular high-frequency phonon modes with λ around 0.6, we can reproduce the experimentally-observed T c vs a relation quantitatively, where a is the fcc lattice constant. For the isotope exponent α , the present calculation gives values around 0.2.

Superconductivity in the Half-Filled Hubbard-Holstein Model in the Antiadiabatic Region

We study an electron-phonon model with on-site Coulomb repulsions U e e at half filling by exact diagonalization combined with a mean-field approximation which gives accurate results in the narrow-bandwidth case. Superconductivity with off-site pairing is found to exist with an inverse isotope effect if and only if the phonon-mediated attraction is almost offset by U e e .

Exchange and correlation effects in the three-dimensional electron gas in strong magnetic fields and application to graphite

The self-consistent theory for obtaining the spin-dependent local-field correction due to Singwi, Tosi, Land and Sjolander is extended to investigate the exchange and correlation effects in the three-dimensional electron gas in strong magnetic fields. We find that barely changes with H as long as it is weak enough for the ratio of , the magnetic length, to , the average interelectron spacing, to be larger than about 0.7. With this information, we calculate the self-energy in a self-consistent way to obtain the electronic structure of graphite in strong magnetic fields. The result obtained is in agreement with experiment.

Including nonlocality in the exchange-correlation kernel from time-dependent current density functional theory: Application to the stopping power of electron liquids

We develop a scheme for building the scalar exchange-correlation (XC) kernel of time-dependent density functional theory (TDDFT) from the tensorial kernel of time-dependent current density functional theory (TDCDFT) and the Kohn-Sham current density response function. Resorting to the local approximation to the kernel of TDCDFT results in a nonlocal approximation to the kernel of TDDFT, which is free of the contradictions that plague the standard local density approximation (LDA) to TDDFT. As an application of this general scheme, we calculate the dynamical XC contribution to the stopping power of electron liquids for slow ions to find that our results are in considerably better agreement with experiment than those obtained using TDDFT in the conventional LDA.