X. K. Zhang

Phonon mode study of Si nanocrystals using micro‐Raman spectroscopy

First‐order Raman spectra of hydrogenated nanocrystalline silicon (nc:Si:H) films show unexpected features in their optical vibrational modes for crystallites with sizes ranging from 2 to 6 nm. Two size‐dependent spectral regions, one with the stronger intensity peaking at 505–509 cm−1 and another a shoulder‐like band between 512 and 517 cm−1, are clearly identified using a detailed line‐shape analysis and the strong phonon confinement model. The strong size dependence of the relative integrated intensities of the two bands suggests that the modification of the vibrational spectra can be attributed to an effect induced by the atomic vibrations from the near‐surface region of the nanocrystals.

Thermal analysis of the ferroelastic phase transitions of LaxPr1−xP5O14

The specific heat capacities of LaxPr1-xP5O14 crystals with x=0.9 and 0.7 were measured with a differential scanning calorimeter over the temperature range 350-450 K. A jump in the specific heat capacity at the phase transition temperature of 402 K and one thermal anomaly at about 385 K were found within experimental error. Factors contributing to the specific heat capacities were considered.

Elastic properties of amorphous hydrogenated silicon films

Abstract A Brillouin scattering study of elastic properties of a -Si: H film deposited onto a glass substrate is presented. From observed sound velocity of surface (Rayleigh) waves we have extracted the shear elastic constant C 44 to be (5.64 ± 0.05) × 10 11 day cm −2 . Due to the existence of the hydrogen atoms, the sound velocity of the surface (Rayleigh) waves is found to be 18% greater than the value of the amorphous silicon film.

STUDY OF THE RAYLEIGH ACOUSTIC MODE IN A-SI:H/A-SINX:H SUPERLATTICES

The Brillouin light‐scattering technique has been exploited for studying the Rayleigh acoustic mode in superlattice films consisting of alternate layers of a‐Si:H and a‐SiNx:H. The phase velocity of the Rayleigh mode has been measured on superlattices with different thicknesses of the a‐Si:H layer. The results obtained have been interpreted in terms of the effective medium model within the elastic continuum approximation; no anomalous behavior is observed, in contrast with previous ultrasonic measurements in superlattice with a minor nitrogen content.

BRILLOUIN-SCATTERING FROM SURFACE ACOUSTIC PHONONS IN A ZNSE-ZNSE1-XSX STRAINED-LAYER SUPERLATTICE

Brillouin scattering studies were performed on ZnSe‐ZnSe1−xSx strained‐layer superlattices with the composition x∼0.2. We have observed scattering from longitudinal guided modes (LGM) whose velocities are determined primarily by the C11 elastic constant. From the measured velocity of the first‐order LGM of the quasitransverse acoustic mode and of the quasilongitudinal acoustic mode, the C11 and C44 elastic constants in ZnSe‐ZnSe1−xSx superlattice film have been observed.

Raman scattering by acoustic phonons in Fibonacci Nb-Cu metallic superlattices

Abstract Raman scattering experiments have been performed on NbCu Fibonacci metallic superlattices. The Raman spectra are explained in terms of zone-folding effects and their frequencies are predicted by an elastic continuum model. The scattering intensities of folded doublets, however, are not accounted for by the standard photoelastic model.

Optical observations of the birefringence and the domains of LaxPr1-xP5O14 near the ferroelastic phase transitions

The changes in the optical properties of five samples of LaxPr1-xP5O14 with different La contents were measured for spontaneous birefringence and rotation angle of the ellipsoid indicatrix axes induced by the ferroelastic phase transitions. Two types of domain and their movement and reproduction were observed in a temperature range from Tc to Tc-100 degrees C. The transition temperature is found to vary linearly with La content. The experimental data are explained and fitted to an approximation considering high-order terms relating to the order parameter.

Anomalous behavior of phonons in NbCu metallic superlattices

Abstract Brillouin and Raman scattering investigations have been performed on NbCu metallic superlattices. Anomalies have been observed in the surface acoustic phonons and bulk acoustic phonons versus superlattice wavelength. These anomalies occur simultaneously at the same superlattice wavelength ≈ 19 A.

LONGITUDINAL PHONON MODES IN A ZNSE ZNSXSE1-X LATTICE-MISMATCHED SUPERLATTICE

Longitudinal acoustic and optical phonon modes of a ZnSe/ZnSxSe1−x (x≂0.20) lattice‐mismatched superlattice, prepared with atmospheric metal organic chemical vapor deposition method, have been investigated by light scattering measurements. Despite a lattice mismatch as large as 1% between the alternating layers, the measured longitudinal elastic constants are in agreement with the calculated values of an unstrained effective medium model. Furthermore, a correlative study was made by fitting the spectra to a spatial correlation model, which reproduces line shapes of the observed confined longitudinal‐optical modes without incorporating the strain effects. The results demonstrate that a combination of Brillouin and Raman spectroscopy provides a good method to determine accurately the elastic constants and strain information of the lattice‐mismatched superlattices and heterostructures.

Impact of postgrowth annealing on spin-glass ordering and exchange bias in SrMn3O6−δ nanoribbons

The magnetic properties of as-grown single-crystal SrMn3O6−δ nanoribbons synthesized by a simple molten-salt method can be significantly improved by a heat-treatment procedure, which enhances the ferromagnetism and spin-glass behavior. These enhanced properties might originate from a spin structure modulated by the thermodynamical equilibrium nonuniform oxygen distribution. Measurements of ac susceptibility prove the existence of a spin-glass phase in the postgrowth annealing SrMn3O6−δ nanoribbons, which differs from the only antiferromagnetic phase in the corresponding bulk material. Asymmetric magnetization hysteresis loops observed in applied magnetic fields H≤5 T are attributed to an exchange coupling between the spin-glass shell and the antiferromagnetic core of postgrowth annealing SrMn3O6−δ nanoribbons. Results suggest that the intrinsic phase inhomogeneity due to the surface effects of the nanostructural manganites may induce exchange anisotropy, which is of special interests for potential applica...