Deokjoon Cha

Development of a near-field scanning microwave microscope using a tunable resonance cavity for high resolution

We report on the operational principles and the practical implementation of a near-field scanning microwave microscope system consisting of a λ/4 coaxial resonator with a tunable resonance cavity coupled to a sharp probe tip. The changes of sensitivity and spatial resolution as a function of cavity length are clearly modulated by tuning the resonance cavity. By tuning the resonance cavity, we demonstrate improved sensitivity and spatial resolution better than 4 µm of the near-field images of a YBa2Cu3Oy thin film on a MgO substrate at an operating frequency of f = 1–1.5 GHz. We modelled the λ/4 coaxial resonator by a LCR circuit and the principles of operation can be explained by using perturbation theory, considering the radius of the probe tip, the quality factor, impedance matching, and the sample–tip distance.

Near-field scanning microwave microscope using a dielectric resonator

We describe a near-field scanning microwave microscope which uses a high-quality dielectric resonator with a tunable screw. The operating frequency is f=4.5 GHz. The probe tip is mounted in a cylindrical resonant cavity coupled to a dielectric resonator for the TE011 mode. We tuned the resonance cavity to match the impedance of 50 Ω by using a tunable screw and could improve sensitivity and spatial resolution to better than 1 μm. To demonstrate the ability of local microwave characterization, the surface resistance dependence of the metallic thin films on film thickness has been mapped.

Disorder-induced Raman scattering in MgxZn1−xTe mixed crystals

Abstract The phonon spectra of the mixed crystal Mg x Zn 1− x Te (0 ≤ X ≤ 0.48) in the zinc-blende phase are investigated by Raman scattering. The phonon frequencies exhibit a dependence on the Mg concentration. The disorder induced one-phonon and two-phonon modes are present in the Raman spectra for X ≥ 0.16. The Raman intensity ratio between first-order and second-order phonon modes was a good indicator to characterize the degree of disorder in the mixed crystals.

Spin injection and relaxation properties of anisotropic superconductors

Abstract We report the spin injection and relaxation properties of the anisotropic YBa 2 Cu 3 O y (YBCO) superconductor by injection of spin-polarized quasiparticles using a cobalt ferromagnetic injector. The observed current gain depends on the thickness of the Au interlayer and is directly related to the nonequilibrium magnetization due to spin relaxation effects.

Preparation of Bi-YIG Thick Films by Using the Metal-organic Decomposition Method

of a pre-crystallization process, which was an intermediate shorttime sintering during the fabrication of thick Bi-YIG films. The Bi-YIG thick films prepared with a pre-crystallization process showed enhanced magneto-optical properties, high crystallinity and high saturation magnetization. For the Bi-YIG thick films prepared with the pre-crystallization process, the Faraday rotation angle was measured to be 0.4 /µm, which is twice as large as the values for other Bi-YIG thick films.

Application of a sensitive near-field microwave microprobe to the nondestructive characterization of microbial rhodopsin

We study the opto-electrical properties of Natronomonas pharaonis sensory rhodopsin II (NpSRII) by using a near-field microwave microprobe (NFMM) under external light illumination. To investigate the possibility of application of NFMM to biological macromolecules, we used time dependent properties of NPSRII before/after light activation which has three distinct states - ground-state, M-state, and O-state. The diagnostic ability of NFMM is demonstrated by measuring the microwave reflection coefficient (S(11)) spectrum of NpSRII under steady-state illumination in the wavelength range of 350-650 nm. Moreover, we present microwave reflection coefficient S(11) spectra in the same wavelength range for two fast-photocycling rhodopsins: green light-absorbing proteorhodopsin (GPR) and Gloeobacter rhodopsin (GR). In addition the frequency sweep shift can be detected completely even for tiny amounts of sample (∼10(-3) OD of rhodopsin). Based on these results NFMM shows both very high sensitivity for detecting conformational changes and produces a good time-resolved spectrum.

Investigation of CdS thin films by a near-field microwave microprobe

Cadmium sulphide (CdS) thin films with different microstructures and morphologies were measured by using a near-field microwave microprobe (NFMM). The NFMM system was coupled to a dielectric resonator with a distance regulation system at an operating frequency f=4.1 GHz. The changes in dielectric permittivity of CdS thin films due to different annealing temperatures were investigated by measuring the reflection coefficient S(11). CdS thin films with different microstructures and morphology were characterized by X-ray diffraction, atomic force microscopy and NFMM.

Study of Phase Transition of Copper(II)-phthalocyanine using a Near Field Scanning Microwave Microscope

We report the changes of the microwave reflection coefficients S of copper(II)-phthalocyanine (CuPc) thin films by using a near-field microwave microscope(NSMM) in order to understand the phase transition of CuPc. For a NSMM system, a high-quality microstrip resonator coupled with a dielectric resonator was used. CuPc thin films were prepared on the pre-heated glass substrates using a thermal evaporation method. The reflection coefficients S of CuPc thin films were changed by the dependence on the substrate pre-heating temperatures. By comparing reflection coefficient S and crystal structures, we found the phase transition of CuPc thin films from -phase to -phase at the substrate heating temperature 200 ./TEX>.

Preparation of YBa/sub 2/Cu/sub 3/O/sub y/ thick films on Y/sub 2/BaCuO/sub 5/:CeO/sub 2/ substrates by a diffusion process

High density YBa/sub 2/Cu/sub 3/O/sub y/ thick films were prepared on Y/sub 2/BaCuO/sub 5/(211) substrates with CeO/sub 2/ additions by a surface diffusion process between Ba/sub 3/Cu/sub 5/O/sub y/ composite coating powder and a 211 substrate. It was found that small addition of CeO/sub 2/ from 0 to 0.5 wt% to the 211 substrate had a strong effect on the reaction between the Ba/sub 3/Cu/sub 5/O/sub y/ coating layer and the 211 substrate, which led to the crystallization of YBCO thick film grains without impurity phases such as CuO and unreacted Y/sub 2/O/sub 3/ particles along the grain boundaries. As CeO/sub 2/ additions were increased from 0 to 5 wt%, the density of 211 substrate increased up to 80-90% and the critical temperature and the critical current density of 123 thick films also increased.

Modulation of critical current and Shapiro step by current-injection based on YBCO bridge three terminal junction

Abstract Modulation of the critical current and Shapiro steps in the I-V characteristics of the granular YBCO cross-line type bridge junction by the injection current(Iinj) is presented. The modulation of Ic by Iinj is generally determined by the total current summation effect of the transport current and Iinj at the weak link. For the I-V characteristics in the presence of the external fixed microwave irradiation with Iinj, we found the flip-flop-like mode, integer steps and half integer steps were shifted by the modulation of Iinj.