Liu Yu-Long

Structural properties of Bi1?xLaxFeO3 studied by micro-Raman scattering

This paper reports that La-doped BiFeO3 (Bi1?xLaxFeO3, x = 0, 0.1, 0.2, 0.3, 0.6, 0.8 and 1.0) were studied by using micro-Raman spectroscopy and x-ray diffraction (XRD). The XRD patterns indicate that the structure of Bi1?xLaxFeO3 changes from rhombohedral BiFeO3 to orthorhombic LaFeO3. The results of Raman spectroscopy show good agreement with the XRD results. Strikingly, the phonon peak at around 610 cm?1 and the two-phonon peaks in the high frequency range exist in all compounds and enhance with increasing La substitution. The increasing intensity of the 610 cm?1 peak is attributed to the changes in the FeO6 octahedron during the rhombohedral?orthorhombic phase transition. The enhancements of the two-phonon peaks are associated with the breakdown of the cycloid spin configuration with the appearance of the orthorhombic structure. These results indicate the existence of strong spin?phonon coupling in Bi1?xLaxFeO3, which may provide useful information for understanding the effects of La content on the structural and magnetic properties of Bi1?xLaxFeO3.

Surface-enhanced Raman spectroscopy of monolayer C60 Langmuir-Blodgett films

The Langmuir-Blodgett (LB) films of C60 mixed with stearic acid (SA) on a water subphase are deposited on optical glass substrates and their UV-Vis absorption spectra are measured. The surface-enhanced Raman spectra (SERS) of C60 LB films has been observed for the first time. Silver-island film is used to obtain SERS of a well-ordered single-monolayer C60 LB film. The SERS contain all fundamental Raman modes, as well as several new weak bands attributed to higher-order Raman modes as determined from factor group analysis. The enhancement of higher-order Raman scattering is probably related to C60 molecular structure. The present results illustrate the potential of SRES in detecting monolayer C60 LB films and in obtaining complete C60 Raman spectra.

Polarized Micro-Raman Study of the Temperature-Induced Phase Transition In 0.67 Pb(Mg1/3Nb2/3)O3-0.33PbTiO3

Polarized Raman spectra of ferroelectric relaxor 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (0.67PMN-0.33PT) single crystal are systematically investigated in a wide temperature range from -196 to 600°C by micro-Raman scattering technique. The results clearly reveal that there are two structural phase transitions in such composite ferroelectric relaxor: the rhombohedral-tetragonal (R - T) phase transition and the tetragonal-cubic (T - C) phase transition. The former occurs at about TR-T = 34°C, corresponding to the vanishing of the soft A1 mode at 106 cm−1 recorded in the parallel polarization. The latter appears at about TT-C = 144°C, which can be verified with the vanishing of mode at 780 cm−1 measured in the crossed polarization.

Enhanced Raman scattering from nano-SnO2 grains

We present the Raman spectra of nano-SnO2 grains with sizes from 4nm to 80nm excited by 532nm and 1.06μm lines. The enhanced Raman scattering of the nanograins is observed for both exciting lines when the grain size is less than 8nm. The less the grain size is, the more intensely the Raman scattering is enhanced. According to our results, the enhancements of the Raman intensity are a few tenfolds and different for different exciting lines when the grain size is 4nm. It can be attributed to enhanced Raman scattering by electron–hole pair excitations in the nanograins that originate from sub-microscopic (10nm) size and other defect- and surface-related features. A critical size that divides respective predominance of bulk properties and the defect-, surface- and size-related features can be determined to be about 8nm.

Elastic, dielectric, and piezoelectric characterization of 0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 single crystal by Brillouin scattering*

In this paper, a complete set of elastic, piezoelectric, and dielectric constants of high-quality tetragonal poled 0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 single crystal grown by the modified flux method is determined using high-resolution Brillouin scattering. A comparison is made between the results obtained by a hybrid method combining ultrasonic and resonant techniques and the results obtained by the Brillouin scattering. The elastic, piezoelectric, and dielectric constants obtained by the two methods are similar. The Brillouin spectrum consists of one longitudinal and two transverse acoustic phonon modes, and the variations of the Brillouin shifts, the full widths at half maximum, and the scattering intensities of these modes with scattering angle θ are investigated. In particular, the transverse acoustic phonon mode at the low-frequency becomes markedly soft from 28.2 GHz to 18.4 GHz and broadens gradually with the increase of θ, while its intensity decreases gradually as compared with that of the original one. The possible origins of the results are discussed.

Elastic and Piezoelectric Properties of Ce:BaTiO3 Single Crystals

We investigate magnetic and crystalline microstructures of melt-spun (Fe0.675Pt0.325)(100-x)B-x (x = 12, 14, 16, 18, 20) nanocomposite ribbons after optimal thermal treatment using a magnetic force microscope. The magnetic microstructures are characterized by darker spots adjacent to brighter ones in a sub-micro scale and in random distribution. It is found that the strength of the exchange coupling interaction between the crystals in the 10-100nm scale, implied by the maximum value (delta M)(max) of the Henkel plot, could be roughly described by the ratio of the average width of the magnetic spots (w) over bar to the average crystal size (D) over bar for the ribbons. Moreover, we find that the intrinsic coercivity H-j(c) of the ribbons is sensitive to their crystal sizes, and the smaller (D) over bar, the higher jHc. Finally, by using roughness analysis, the curve of the root mean square values of the phase shift of the magnetic force images versus the boron content x is obtained, which is qualitatively consistent with that of the magnetization sigma(12)kOe of the ribbons versus x.

Analytical Results for Frequency-Weighted Kuramoto-Oscillator Networks*

The Kuramoto model for an ensemble of coupled oscillators provides a paradigmatic example of non-equilibrium transitions between an incoherent and a synchronized state. A frequency-weighted network of Kuramoto oscillators is proposed, where the oscillators are asymmetrically coupled with the weights depending on their own native frequencies. Moreover, the characteristics of the whole network can be described by a single weighting exponent β. To obtain some analytical results, we focus on three special values of the weighting exponent β. Obviously, the network of oscillators in connection with the heterogeneous coupling scheme turns out to exhibit richer dynamics. Our findings indicate that the weighting exponents should be of importance to affect the networks synchronization ability.

Influences of KrF laser irradiation on the structure and luminescence of ZnO single crystal

In this paper, we investigate the laser irradiation of ZnO single crystals and its influence on photoluminescence. Our study shows that the photoluminescence of ZnO single crystals strongly depends on surface morphologies. The ultraviolet emissions of laser treated-ZnO under 200 mJ/cm2 become stronger, whereas for those deteriorated by irradiation above 200 mJ/cm2, the green emissions centred at 2.53 eV are significantly enhanced with a red-shift to 2.19 eV, probably due to the changes in the charge states of the defects. Enhanced yellow-green emissions are well resolved into four peaks at around 1.98, 2.19, 2.36, and 2.53 eV due to a shallow irradiation depth. Possible origins are proposed and discussed.