Tomohiko Kato

Theory of the intersurfacedband plasmon

Abstract The dispersion relation of the intersurfacedband plasmon, i.e., collective excitation of the surface electrons of the clean or adsorbed surfaces, is investigated by the self-consistent field approach. The dispersion is predicted to start with the linear term in the wave number parallel to the surface, which will be helpful to distinguish the plasmon from the individual excitations. The sign of the dispersion and the depolarization shift depend on the polarization of the interband transition. The contribution of the plasmon to the electron energy loss cross section is given.

Coverage dependence of density of states and work function of a random adsorbate-surface system: Application to alkali-metal/Si(001)2 × 1 surface

Abstract A cell-CPA theory is developed for a random adsorbate-substrate system based on the LCAO model. Effects of the adsorbate-substrate and adsorbate-adsorbate interactions and the depolarization shifts of the adsorbate level are treated self-consistently. Short-range correlation in the adsorbate distribution is taken into account. The theory is applied to alkali metals adsorbed on the silicon (001) surface. Local density of states, charge transfer, and the change of the work function are calculated and illustrated as a function of the coverage. The calculated coverage dependence of the work function of the Cs/Si(001) system agrees fairly well with the experimental results. The nature of the adsorbate-substrate bonding is analyzed. The bonding is almost ionic at low coverage, while the covalent character increases with coverage, as the adsorbate levels come down due to the depolarization shift and the lateral interaction among the adsorbates. The transition is sensitive to the inter-adsorbate correlation. An effect of the nonorthogonality between the adsorbate and substrate orbitals is briefly discussed.

Theory on spectroscopy and work function of alkali chains adsorbed on Si(001)2×1

Abstract The coverage dependence of the work function in the alkali-adsorbed Si(001) system is investigated by a model which includes a hopping term between adsorbed atoms as well as a mixing term between the states of the adsorbate and the substrate. Local densities of states of the adsorbate and the substrate are calculated in the coherent potential approximation. For the Cs/Si(001) system, the calculated result reproduces the experimental data of the work function by reasonable values of parameters. By use of the same model with the same parameters, the energy loss function of this system at full coverage due to the interband transition is calculated. The calculated value of the excitation energy of the intersurface-band plasmon agrees well with the experimental one.

Cell model of Si(111)7 × 7 structure

Abstract The phase transition of the Si(111)7×7 structure and its growth on a quenched surface are investigated theoretically. The surface structure is described in terms of states of faulted and unfaulted triangular halves of the 7×7 unit cell. A model Hamiltonian which satisfies peculiar features of the structure is derived. The Monte-Carlo simulation for this model shows that the 7×7 structure undergoes the first-order phase transition. Growth patterns of the 7×7 structure on the quenched surface are calculated with the same parameters and they reproduce characteristic features of experimental results. The calculation also qualitatively reproduces the temperature dependence of the critical nucleus size for the 7×7 domain growth on the quenched terrace.

Existence of slowly fluctuating spin clusters at Néel temperature in the mixture Co1-xMnxCl2.2H2O

Proton NMR measurements are carried out on the mixture Co 1- x Mn x Cl 2 ·2H 2 O with various Mn concentrations in the region from 1 to 4.2 K. For x =0.54 proton NMR lines are observed near 17 MHz at just Neel temperature in zero applied field. This result shows the existence of slowly fluctuating spin clusters freezing for more than 10 -4 sec with the spin cluster of the diameter of more than 14 A near Neel temperature. The temperature dependence of a Bragg reflection by neutron diffraction is reinterpreted in relation to the amount of such spin clusters.

Effect of steps on structural phase transitions of Si(111) 7 × 7 surfaces

The effect of steps on the structural phase transition for the Si(111) 7×7 structure dimer-adatom-stacking fault (DAS) structure is investigated using a cell model. This model describes the state of the Si(111) surface in units of 7×7 triangular halves. In the previous work, the subject of which was an infinite terrace, the result calculated for the temperature dependence of the DAS order parameter exhibited much larger hysteresis than in the experiments, although the main features of the phase transition were consistent with experiments. The presence of steps is expected to suppress hysteresis. Calculations for the temperature dependence of the order parameter are performed using a Monte Carlo simulation in the presence of steps. By assuming reasonable values of the formation energy of the DAS cell at the step edges and the intervals between the steps, the calculated result agrees with the experimental results in terms of hysteresis.

Monte-Carlo simulation on growth patterns of DAS structure on quenched Si(111) surface

Abstract The growth patterns of the Si(111)-(7×7) structure (DAS structure) on the quenched surface are calculated using Monte-Carlo simulation. The model Hamiltonian, which was constructed in terms of the states of each 7×7 triangular half, based on an assumption of one-by-one formation of faulted halves in the previous paper, is employed. In the simulation the out-of-phase effect for growth is taken into account. Calculated patterns of the DAS domain growth reproduce the characteristic features of the observed patterns for growths on the terrace, from the F- and U-steps, respectively.

Temperature Dependence of ESR Lines Related to Phosphorus in Silicon

Temperature dependence of ESR in a silicon crystal containing 1.3×1017 phosphorus atoms/cm3 has been investigated at 4–40 K. The ESR signals depend strongly on the specimen temperature. Typical hyperfine structures of the phosphorus ground state were found in the low-temperature region. The hyperfine lines vanished at 20 K and a single line corresponding to the first excited state of phosphorus was observed in the high-temperature region. The amplitudes of the hyperfine lines and the single line show sharp maxima at 11 K and 30 K, respectively. The change of the signal amplitudes is due to the temperature dependence of spin relaxation time. The change of the linewidth for the hyperfine lines can be explained by the temperature dependence of spin relaxation time, and that for the single line is considered to be caused by motional narrowing.

The ferromagnetic?spin glass transition in PdMn alloys: symmetry breaking of ferromagnetism and spin glass studied by a multicanonical method

The magnetism of Pd(1-x)Mn(x) is investigated theoretically. A localized spin model for Mn spins that interact with short-range antiferromagnetic interactions and long-range ferromagnetic interactions via itinerant d electrons is set up, with no adjustable parameters. A multicanonical Monte Carlo simulation, combined with a procedure of symmetry breaking, is employed to discriminate between the ferromagnetic and spin glass orders. The transition temperature and the low-temperature phase are determined from the temperature variation of the specific heat and the probability distributions of the ferromagnetic order parameter and the spin glass order parameter at different concentrations. The calculation results reveal that only the ferromagnetic phase exists at x < 0.02, that only the spin glass phase exists at x > 0.04, and that the two phases coexist at intermediate concentrations. This result agrees semi-quantitatively with experimental results.

Simulation of non-equilibrium phase transition in a random magnetic system

The time-dependent phase transition of a random magnetic system is investigated using a dynamic Monte Carlo simulation. The present random system is a simple dilution of magnetic atoms with Ising spins in a square lattice. The time-dependent order parameters are calculated as a function of temperature for several concentrations of magnetic atoms. The results indicate that as the averaging time becomes shorter, the phase transition becomes more gradual. This tendency is most pronounced around the percolation concentration. These results are compared to results of a recent NMR spin echo experiment on MnxCd1-x(HCOO)2?2(NH2)2CO reported by Kubo et al. (2007). Although the present simulation includes essentially no adjustable parameter, the results are in satisfactory agreement.