Kenji Sato

Ferroelectric Transition Induced by the Incommensurate Magnetic Ordering in LiCuVO4

A ferroelectric transition occurring simultaneously with a helical spin order has been found in both the polycrystalline and single-crystal samples of LiCuVO 4 . The system has Cu 2+ spins ( s =1/2...

Relationship between Magnetic Structure and Ferroelectricity of LiVCuO4

Neutron scattering studies and measurements of the dielectric susceptibility e and ferroelectric polarization P have been carried out under various magnetic fields H for single-crystal samples of the multiferroic system LiVCuO 4 with quasi one-dimensional spin 1/2 Cu 2+ chains formed of edge-sharing CuO 4 square planes, and the relationship between the magnetic structure and ferroelectricity has been studied. The ferroelectric polarization is significantly suppressed by the magnetic field H above 2 T applied along the a and b axes. When H = 0, a helical magnetic structure with the helical axis parallel to c has been confirmed. The magnetic structure under the field along a has been determined, where the a b -plane structure changes to the helical one with the helical axis parallel to H with increasing the field through ∼2 T. The ferroelectric polarization along a at H = 0 is found to be proportional to the neutron magnetic scattering intensity, indicating that the magnetic order is closely related to the ...

Studies of Multiferroic System of LiCu2O2: II. Magnetic Structures of Two Ordered Phases with Incommensurate Modulations

Neutron diffraction and 7 Li-NMR have been applied to determine the three dimensional magnetic structures of the multiferroic system LiCu 2 O 2 , which has four chains (ribbon chains) of edge-sharing CuO 4 square planes in a unit cell. First, we have confirmed that there are successive magnetic transitions at T N1 = 24.5 K and T N2 = 22.8 K. In the T region between T N1 and T N2 , the quasi one-dimensional spins ( S = 1/2) of Cu 2+ ions within a chain have a collinear and sinusoidally modulated structure with Cu-moments parallel to the c -axis and with the modulation vector along the b -axis. At T < T N2 , an ellipsoidal helical spin structure with the incommensurate modulation has been found. We present detailed parameters, describing the modulation amplitudes, helical axis vectors as well as the relative phases of the modulations of four ribbon chains, which can well reproduce both the NMR and neutron results, in the two magnetically ordered phases. This finding of the rather precise magnetic structures...

Surfactant effect of H atoms on the suppression of Ge segregation in Si overgrowth on Ge(n ML)/Si(100) substrates by gas source molecular beam epitaxy

Hydrogen surfactant effect on the surface segregation of Ge atoms has been studied from layer-by-layer changes in the growth rate of Si overlayers on Ge/Si(100) substrates with Ge thicknesses of 0-6 ML by gas source molecular beam epitaxy using Si 2 H 6 . The layer-by-layer growth rates were in-situ measured by reflection high-energy electron diffraction (RHEED) intensity oscillation periods. The growth rate of Si overlayers decreases exponentially with increasing Si thickness and eventually approaches that of Si/Si(100) homoepitaxial growth at substrate temperatures of 440-550°C, which corresponds to the change in surface coverages of Ge atoms. The decay length of the surface concentration of Ge atoms is suppressed by an increase in Si 2 H 6 gas pressure. The temperature dependence of the decay length has an activation energy of 1.3 eV for Si 2 H 6 gas pressures of 5×10 -6 to 5×10 -5 Torr. These facts can be explained as an effect of H atoms existing on the surface

Studies of Multiferroic System LiCu2O2: I. Sample Characterization and Relationship between Magnetic Properties and Multiferroic Nature

Single-crystal samples of LiCu2O2 with spin 1/2 Cu2+ chains of edge-sharing CuO4 square planes (ribbon chains), have been characterized by X-ray diffraction, thermogravimetric analysis, and magnetic measurements. Neither the atomic deficiency nor the mixing of Cu and Li atoms has been found, indicating that lattice defects conceived as a possible origin of the reported multiferroic behavior can be excluded. Anomalies found in the data of specific heat and neutron magnetic Bragg reflections show clear evidence that the system exhibits successive magnetic transitions at TN1=24.5 K and TN2=22.8 K. Based on the magnetic structures in the intermediate (TN2<T<TN1) and low temperature (T<TN2) phases, determined by the combined studies of neutron scattering and 7Li-NMR measurements, we can consistently understand the fact that the multiferroic properties are observed only below TN2 by considering existing theories.

Multiconductor Transmission-Line Theory with Electromagnetic Radiation

We have constructed a new multiconductor transmission-line (MTL) theory with electromagnetic radiation starting from the Maxwell equations. We express scalar and vector potentials in terms of retardation charges and currents and complete a set of MTL integro-differential equations by using the continuity equation and the ohmic-conductor equation with resistance. We naturally obtain coefficients of potential P and inductance L in the transverse electromagnetic (TEM) mode as singular terms of the retardation scalar and vector potentials. We first analyze a linear one-line antenna system and show that the remaining scalar and vector potentials after the removal of the TEM mode terms are responsible for electromagnetic radiation. We then discuss the MTL equation for a two-conductor transmission-line system. As a realistic case, we discuss a three-conductor transmission-line system to enable the inclusion of the effect of circumstances. We demonstrate that a three-conductor transmission-line system with a symm...

Three Conductor Transmission Line Theory and Origin of Electromagnetic Radiation and Noise

We construct a new multiconductor transmission-line theory with coefficients of potential and inductance. We provide coupled differential equations for potential and electric current with coefficients that are given by geometrical configurations of a transmission-line system. We provide a general solution to coupled differential equations explicitly for three-conductor transmission-lines. We derive coupled differential equations for the normal and common modes of lines 1 and 2, where common-mode current goes through line 3. The common mode acts as an antenna for radiating electromagnetic waves and any electromagnetic devices and conductors placed around the major conductor transmission-lines (lines 1 and 2) act as receivers of electromagnetic waves. This is the source of electromagnetic noise in the circumstance around the transmission-line system. The transmission-line system receives noise from the circumstance through the common mode.

Electrodynamics of Multiconductor Transmission-line Theory with Antenna Mode

In the modern life we depend completely on the electricity as the most useful form of energy. The technology on the use of electricity has been developed in all directions and also in very sophisticated manner. All the electric devices have to use electric power (energy) and they use both direct current (DC) and alternating current (AC). Today a powerful technology of manipulation of frequency and power becomes available due to the development of chopping devices as IGBT and other methods. This technology of manipulating electric current and voltage, however, unavoidably produces electromagnetic noise with high frequency. We are now filled with electromagnetic noise in our circumstance. This situation seems to be caused by the fact that we do not have a theory to describe the electromagnetic noise and to take into account the effect of the circumstance in the design of electric circuit. We have worked out such a theory in one of our papers as Three-conductor transmission-line theory and origin of electromagnetic radiation and noise (Toki & Sato (2009)). In addition to the standard two-conductor transmission-line system, we ought to introduce one more transmission object to treat the circumstance. As the most simple object, we introduce one more line to take care of the effect of the circumstance. This third transmission-line is the place where the electromagnetic noise (electromagnetic wave) goes through and influences the performance of the two major transmission-lines. If we are able to work out the three-conductor transmission-line theory by taking care of unwanted electromagnetic wave going through the third line, we understand how we produce and receive electromagnetic noise and how to avoid its influence. To this end, we had to introduce the coefficient of potential instead of the coefficient of capacity, which is used in all the standard multi-conductor transmission line theories (Paul (2008)). We are then able to introduce the normal mode voltage and current, which are usually considered in ordinary calculations, and at the same time the common mode voltage and current, which are not considered at all so far and are the sources of the electromagnetic noise (Sato & Toki (2007)). We are then able to provide the fundamental coupled differential equations for the TEM mode of the three-conductor transmission-line theory and solve the coupled equations analytically. As the most important consequence we obtain that the main two transmission-lines should have the same qualities and same geometrical shapes and their distances to the third line should be the same in order to decouple the normal mode from the common mode. The symmetrization is the key word to minimize the influence of the circumstance and hence the electromagnetic noise to the electric circuit. The symmetrization makes the normal mode decouple from the common mode and hence we are able to avoid the 9

A study on the local bonding structures of oxidized Si(111) surfaces

We have investigated the initial oxidation process at oxide thickness below 3 ML. High-resolution electron energy loss spectroscopy has been used to clarify the local bonding structures of Si-O-Si bonds on H-terminated Si( 111)-1 × I surfaces and clean Si( 111)-7 × 7 surfaces. We observed shifts of the vibrational energy of Si-O-Si symmetric bending (ν O1 ) and asymmetric stretching modes with increasing the number of adsorbed O atoms. Furthermore e, a different behavior of the energy shifts of ν O1 was clearly observed between the H-terminated I × 1 and the clean 7 × 7 surfaces. Based on a central-force-network model, it was found that the difference of the structural relaxation in the initial oxidation process between the H-terminated Si I × 1 and the clean Si 7 x 7 surfaces appears conspicuously in the bond angle of Si-O-Si. This phenomenon indicates that the Si-O-Si structure is relaxed by making the bond angle larger, which is promoted by H atoms bonding with top Si atoms.

Influences of deuterium atoms on local bonding structures of SiO2 studied by HREELS

Abstract We have investigated the initial oxidation processes on D-terminated Si(100)-2 × 1 and -1 × 1 surfaces and influences of Si-D bonding on SiO2 local bonding structures by high-resolution electron energy loss spectroscopy. It was found from experimental and simulated results that O atoms preferentially adsorb on one of two Si-Si back bonds of a surface Si atom. However, the preferential adsorption on the D-terminated Si(100)-2 × 1 surface is not as significant as the 1 × 1 surface. In addition, from the evaluation by a central-force network model, the length of Si-O bonds formed on the D-terminated 1 × 1 surface is more relaxed than on the 2 × 1 surface. This fact suggests that the existence of Si-Si dimer bonding changes the bonding states of the topmost Si atoms and then the structural change in Si-O-Si formed at the Si back bond occurs.