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고재현,Hae-Yong Lee,Jai Young Han,Seiji Kojima,황윤회 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.2I
Brillouin spectra of Si-doped free-standing gallium-nitride single crystals of high optical quality have been measured. Five independent elastic stiffness coefficients have been obtained from the Brillouin frequency shifts, mass density, and refractive indices. The obtained coefficients are compared to those of pure bulk and epitaxial GaN samples. c33 is much larger than previously reported values, which correlates with the shorter lattice constant of the c axis and seems to be due to different levels of remaining in-plane biaxial strain and compression along the c axis caused by Si doping.
Central Peak Behaviors of Lanthanum Lead Zirconate Titanate Ferroelectric Relaxor Ceramics
고재현,Do Han Kim,Seiji Kojima 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.III
Brillouin spectra of lanthanum lead zirconate titanate (Pb1.xLax)(ZryTi1.y)O3 (PLZT- 100(x/y/1-y)) relaxor ferroelectric ceramics with the famous composition of PLZT-10/65/35 have been investigated by using high-resolution Brillouin spectroscopy. The longitudinal acoustic mode began to show a significant softening below 600 K, at which a quasi-elastic central peak of PLZT- 10/65/35 began to grow upon cooling. The central peak appeared and showed similar behaviors at both the polarized and the depolarized scattering geometries. The integrated intensity of the central peak grew on cooling and reached its maximum value at approximately 350 K, which is higher than the dielectric maximum temperatures. The relaxation time of the central peak became longer when the temperature decreased from 500 K to 300 K, which indicates slowing down of the dynamics of the polar nano-clusters.0?
고재현,전병억,황윤홰,Do Han Kim,Seiji Kojima 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.54 No.2
The acoustic properties of two potassium lithium niobate single crystals have been investigated by using Brillouin spectroscopy. The temperature dependence of the Brillouin shifts corresponding to two elastic constants, C11 and C33, was obtained over a wide temperature range including the phase transition temperature (Tc). Consistent with the uniaxial nature of the tetragonal tungsten- bronze structure and the Ising-type order parameter along the c-axis, only the C33 showed a large acoustic anomaly at Tc while C11 did not show a substantial change in both crystals. This result indicates that the longitudinal acoustic wave propagating along the polar direction couples strongly to the order parameter fluctuations, resulting in a significant softening of C33, as well as a substantial increase in the acoustic damping in the paraelectric phase. Changes in the acoustic properties above Tc have been partly ascribed to the electrostrictive coupling between the strain and the squared order parameter of polar nanoregions and fluctuation damping, which seem to be correlated with the diffuse nature of the ferroelectric phase transition observed in the present study. However, another microscopic origin seems to contribute to the softening of C33 at temperatures above Tc, which is unclear at present.
고재현,신선협,장문규,원충재,허남정 한국물리학회 2018 Current Applied Physics Vol.18 No.1
The acoustic behaviors of oxygen-reduced barium titanate (BaTiO3-d) single crystals with d~0.04 were investigated as a function of temperature by using Brillouin spectroscopy. The longitudinal acoustic mode of the moderately-reduced BaTiO3 (BTO) showed two pronounced anomalies at approximately 112 C and 11 C, which correspond to the cubic-tetragonal and tetragonal-orthorhombic phase transition temperature, respectively. These temperatures were lower by more than 10 C compared to those of the pure BaTiO3 suggesting that the disorder introduced by oxygen vacancies lowers the phase transition temperatures. The paraelectric phase of the reduced BaTiO3 were characterized by substantial softening of the longitudinal acoustic mode and the growth of central peaks centered at zero frequency. These anomalies were observed in a certain temperature range above the Curie temperature, indicating that pretransitional precursor polar clusters exist in the cubic phase and that their dynamics are responsible for the acoustic anomalies caused by electrostrictive coupling between the strain and the polarization. The relaxation time of the precursor polar clusters derived from the central peak exhibited a critical slowing-down behavior showing that their dynamics becomes more sluggish as temperature approaches the Curie point.
Elastic Anomalies Associated with the Antiferroelectric Phase Transitions of PbHfO3 Single Crystals
고재현,Krystian Roleder,Andrzej Majchrowski,Annette Bussmann-Holder 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.64 No.8
The temperature dependence of the elastic properties of antiferroelectric PbHfO3 was investigatedby using Brillouin scattering. The two structural phase transitions of antiferroelectricantiferroelectric-paraelectric phases were clearly identified from the discontinuous changes in theacoustic mode frequencies and the hypersonic damping. The substantial softening of the modefrequency, along with the remarkable increase in the acoustic damping observed in the paraelectricphase, indicated the formation of precursor noncentrosymmetric (polar) clusters and their couplingto the acoustic waves. This was corroborated by the observation of quasi-elastic central peaks,the intensity of which grew upon cooling toward the Curie point. The obtained relaxation timeexhibited a slowing-down behavior, suggesting that the dynamics of the precursor clusters becamemore sluggish on approaching the phase transition temperature.