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브릴루앙 광산란 분광법을 이용한 응집물질의 탄성 특성 및 상전이 거동 연구
고재현 한국물리학회 2018 새물리 Vol.68 No.1
Brillouin spectroscopy is one of the inelastic laser-light scattering spectroscopies and is used to probe long-wavelength low-energy excitation processes in the characteristic frequency range of a few tens of gigahertz. Based on this technique, the velocities of acoustic waves in amorphous materials, such as polymers and liquids, and the energies and the elastic constants of acoustic phonon modes in crystals can be determined accurately. The elastic property is one of the fundamental physical properties in condensed matter, and it is sensitive to phase transitions and to changes in structure or composition. In this paper, we review the principles and the measurement methods of Brillouin spectroscopy and their main applications. Especially, we discuss property data and related physics obtainable from Brillouin spectroscopy by investigating several application examples, such as perovskite-structured oxide materials showing ferroelectricity or a relaxor state and amorphous materials including polymers. 브릴루앙 분광법은 비탄성 레이저 광산란 분광법의 하나로써 수십 GHz 정도의 특성 진동수를 가지는 장파장, 저에너지의 여기 과정을 연구하는데 활용된다. 이를 통해 고분자나 액체와 같은 비정질 물질 내 음향파의 속도와 탄성 모듈러스, 그리고 단결정 내 음향 포논의 에너지와 탄성계수를 정확히 결정할 수 있다. 응집물질의 탄성 특성은 물질의 구조나 조성 변화 혹은 상전이에 따라 민감하게 변하는 근본적인 물성 중 하나이다. 우리는 이 연구에서 브릴루앙 분광법의 원리 및 측정법, 그리고 주요 적용 분야에 대해 개괄하고자 한다. 특히 강유전 상전이 혹은 릴랙서 특성을 보이는 페로브스카이트 구조의 산화물과 폴리머를 포함한 비정질 물질에 대한 연구 사례를 검토함으로써 브릴루앙 분광법을 통해 얻을 수 있는 물성 데이터 및 이에 관련된 물리에 대해 논의한다.
Central-Peak Behaviors of Uniaxial Tungsten-Bronze Relaxor Ferroelectrics
고재현,Do Han Kim,Seiji Kojima 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.II
A Brillouin light-scattering study has been carried out on tungsten-bronze (Sr$_{0.61}$Ba$_{0.39}$)$_5$Nb$_{10}$O$_{30}$ (SBN-61), K$_{5.80}$Li$_{3.82}$Nb$_{10.12}$O$_{30}$ (KLN-1) and K$_{5.20}$Li$_{2.34}$Nb$_{10.88}$O$_{30}$ (KLN-2) single crystals. Polarized central peaks (CPs) were observed in SBN-61 and KLN-1, which exhibited diffused ferroelectric phase transitions. However, no CP was observed from KLN-2 that did not show any signature of structural phase transformation as well as relaxor behaviors. The CP in KLN-1 persisted even at temperatures above $T_c$, where dielectric dispersion was observed. In this respect, CP behaviors in uniaxial relaxor ferroelectrics were interpreted to reflect the anisotropic polarization fluctuations correlated with the fast dynamics of precursor polar nano regions (PNRs).
고재현,Anwar Hushur,Bryan C. Sih,Do Han Kim,Jong Pil Kim,Min-Su Jang,Seiji Kojima,Zuo-Guang Ye 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.42 No.IV
Elastic properties of bismuth layered perovskite compounds, LaxBi4..xTi3O12 (LBT-x) and SrBi2Ta2O9 (SBT), were examined at room temperature by Brillouin scattering. Elastic stiness coecients, c33 and c44, dened in the pseudotetragonal symmetry, were obtained. The longitudinal sound velocities propagating along the c axis, were 3985 m/s and 3118 m/s for the pure bismuth titanate (BTO) and SBT, respectively. The sound velocity of SBT is very slow and is believed to be related with the weak interlayer bonding of this layer-structure compound. A simple linear chain model showed that the dierence in the sound velocity between BTO and SBT is mainly due to the dierence in the c axis lattice constant. The sound velocity of LBT-x increased slightly with the increase of x, while c33 seems to be insensitive to x.
Angular Dispersion-type Nonscanning Fabry-Perot Interferometer Applied to Ethanol-water Mixture
고재현,Seiji Kojima 한국광학회 2009 Current Optics and Photonics Vol.13 No.2
The angular dispersion-type non-scanning Fabry-Perot was applied to an ethanol-water mixture in order to investigate its acoustic properties such as the sound velocity and the absorption coefficient. The scattered light from the mixture was analyzed by using the charge-coupled-device area detector, which made the measurement time much shorter than that obtained by using the conventional scanning tandem multi-pass Fabry-Perot interferometer. The sound velocity showed a deviation from ultrasonic sound velocities at low temperatures accompanied by the increase in the absorption coefficient, indicating acoustic dispersion due to the coupling between the acoustic waves and some relaxation process. Based on a simplified viscoelastic theory, the temperature dependence of the relaxation time was obtained. The addition of water molecules to ethanol reduced the relaxation time, consistent with dielectric measurements. The present study showed that the angular dispersion- type Fabry-Perot interferometer combined with an area detector could be a very powerful tool in the real-time monitoring of the acoustic properties of condensed matter.
고재현,Seiji Kojima 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.2
A nonscanning angular-dispersion-type Fabry-Perot interferometer (NSFPI) was built by using a high-resolution charge-coupled-device (CCD) detector and a solid etalon. It was possible to measure Brillouin spectra of condensed matter within a very short time of the order of a few seconds by using the NSFPI. Thanks to the good sensitivity and high resolution of the CCD detector, the real-timeestimation of the elastic-stiffness coefficients of a single crystal, ammonium dihydrogen phosphate, could be obtained. The results were compared with those obtained from a conventional Sandercocktype tandem Fabry-Perot interferometer, resulting in good consistency. The improvement and modification of the NSFPI will make it possible to achieve real-time monitoring of the acoustic properties of condensed matter including single crystals and thin films.
고재현,Seiji Kojima 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.42 No.IV
Brillouin spectra of a relaxor-based ferroelectric Pb[(Zn1=3Nb2=3)0:91Ti0:09]O3 (PZN-9%PT) single crystal have been examined under an electric eld oriented along the pseudocubic [001] direction. One longitudinal and two transverse modes are softened along with the increase of the Brillouin linewidth during the polarization reversal. The eld-induced changes in the frequency shift of the three modes are maximum at the coercive eld of about 2.5 kV/cm, and then they recover their initial values with further increasing the electric eld. In particular, one transverse mode, which corresponds to the (c11 .. c12)=2 elastic stiness coecient, shows the most drastic changes at two electric elds. This seems to indicate the existence of intermediate monoclinic phases during the eld-induced transformation. Our results demonstrate that Brillouin scattering is a powerful tool in investigating elastic properties of ferroelectric relaxors during the polarization reversal.