Takuro Koike

Thermal Expansion Coefficient of Synthetic Diamond Single Crystal at Low Temperatures

The lattice parameter of synthetic diamond single crystals has been measured in the range 4.2-320 K by the X-ray diffraction method. The lattice parameter is found to be nearly constant between 4.2 and 90 K. The thermal expansion coefficient α calculated from the experimental results is very small (of the order of 10-7 or less) and no definite evidence of the negative thermal expansion is found within our relative experimental accuracy of the order of ±1×10-6.

The thermal expansion coefficient and Gruneisen parameter of InP crystal at low temperatures

The thermal expansion of the lattice constant of InP crystal has been measured in the range 4.2-300 K by the Bond method and the results are shown graphically. The thermal expansion coefficient alpha calculated from the experimental results is negative between 15 and 80 K and positive below 15 K. The corresponding Gruneisen parameter gamma closely follows the behaviour of alpha and the values of alpha and gamma are tabulated and shown graphically. The results do not agree with the phenomenological theory in which only a negative alpha is predicted for InP at low temperatures.

General equivalent circuit models for magnetostatic wave devices

A general equivalent circuit approach to magnetostatic wave (MSW) devices is extended and improved. Various possibilities of general equivalent circuit models for MSW devices are analyzed. The appropriate general equivalent circuit models for MSW devices have been developed. The insertion losses of the MSW devices based on the equivalent circuit models were calculated, and excellent agreement with experimental results was obtained.<<ETX>>

The negative thermal expansion coefficient of GaP crystal at low temperatures

The thermal expansion of the lattice constant of a GaP crystal was measured in the range 4.2-300K using the Bond method. The authors found a negative thermal expansion coefficient for GaP at low temperatures. They also calculated the Gruneisen parameter and this was negative below 50K. These experimental results do not agree with the phenomenological theory in which a positive thermal expansion coefficient for GaP is predicted at low temperatures.

Phase-Shift Control of Resonant Frequencies of Magnetostatic Wave Resonators

We discuss a possible technique to control the resonant frequencies of a straight-edge magnetostatic wave (MSW) resonator without changing the external applied magnetic field and the circuit parameters of a feedback load circuit. The method is to use two additional microstrip electrodes at the edges of the resonator and two varactor diodes connected in series. Upon varying the bias voltages to the varactor diodes, the input admittance at the center electrode can be changed. Theoretical investigation reveals that very large resonant frequency shifts can be obtained by changing only the bias voltage change to the varactor diodes, which may be useful in mobile telephone applications in the gigaherz frequency range.

Improved Equivalent Circuit Analysis for Magnetostatic Wave Devices

We extended and improved the general equivalent circuit approach to the magnetostatic wave (MSW) devices we proposed and discussed earlier. In the theory we analyzed various possibilities of general equivalent circuit models for the MSW devices (MSSW, MSFVW and MSBVW). Appropriate general equivalent circuit models for all the MSW devices have been developed. The insertion losses of these devices based on the equivalent circuit models were calculated and excellent agreement with experimental results was obtained.

Analysis of MSW Transducer Electrode Design by Use of Weighting Functions

We theoretically analyzed MSW transducer electrode design by use of weighting functions. The method is based on the general equivalent circuit model for the MSW devices we developed and extended earlier. The position of each electrode and the geometrical factor are weighted spatially using a function whose Fourier transform is a square pulse in the k domain. This technique is very effective in the design of an MSW filter with a flat frequency response. The method may be extended to various signal processing applications in the future.

Experimental investigation of magnetostatic wave soliton propagation and losses in a long YIG sample

Recently nonlinear magnetostatic wave (MSW) propagation in thin YIG films has been attracting great attention for its potential use in microwave and millimeter wave signal processing devices. In the last few years, we have been investigating the soliton propagation in a very long YIG sample (more than 10 mm) from the viewpoint of equivalent circuit design analysis for developing possible devices. In this report, we compared the linear magnetostatic wave propagation characteristics with the soliton waveform at various input power levels and carefully examined the relation between the linear and the nonlinear waves. We also find that the insertion loss of the linear wave propagation characteristics (filter curve) is closely related to the resulting soliton formation. The details of the experimentally observed results are discussed. These results will be very useful for the development of nonlinear MSW devices in the near future.

Simulation of Magnetostatic Wave Envelope Soliton Propagation in Yttrium Iron Garnet Films

Although formation and propagation of magnetostatic soliton waves in YIG (yttrium iron garnet) films are attracting great attention, there has been no discussion about simulating soliton formation and propagation in YIG films from the viewpoint of circuit analysis and simulation. In this report, we propose a new algorithm to simulate the magnetostatic soliton wave output. We first describe the linear magnetostatic wave equivalent circuit model we have developed and then discuss the simulation algorithm, which combines the linear equivalent circuit model and the soliton wave solution of the nonlinear Schrodinger equation. The simulated results are in excellent agreement with reported experimental results. We also find that the proposed simulation is very effective to determine various physical parameters in the YIG film for given experimental conditions.

Effects of Directional Coupling on the Magnetostatic Surface Wave Propagation

In this report we describe the experimental confirmation of the directional coupling of the magnetostatic surface wave between two parallel YIG films. We also describe a theory to calculate the coupling coefficient and the over-all passband characteristics of the devices. With this technique, a bandstop filter with the minimum insertion loss of 18 dB and the rejection of at least 25 dB from the top of the passband at 7.08 GHz was demonstrated. The stopband width at 10 dEi below the top of the passband was 15 MHz. A narrow bandpass filter was also built. We observed the reduction of up to 75 of the 30 dB passband with the increase of the insertion loss of less than several decibels. This method is useful at higher frequencies because we can choose a material with the lowest intrinsic loss and control the bandwidth by the directional coupling. The experimental results agreed qualitatively well with the theoretical predictions. This will provide a new possibility to the design of magnetostatic signal processing devices.