Zhang Xing-Kui

Raman scattering in Fibonacci metallic superlattices

Raman scattering experiments from acoustic phonons in Fibonacci metallic Nb-Cu superlattices were performed. The main features of the experimental results show that the Raman spectrum is dominated by triplet peaks which originate from the density of states of longitudinal-acoustic (LA) phonons in the subbands of phonon dispersion. The expected rich structure of gaps in the phonon spectrum has been labeled by strongest diffraction wave vector factor nτp.

Domain Patterns and Phase Transitions in Ferroelectric Crystal KNbO3

We have studied the domain configurations and structures of KNbO3 crystal in orthorhombic and tetragonal phases by polarizing microscopy. The phase transition processes among orthorhombic, tetragonal and cubic phases and thermal hysteresis during phase transitions are observed directly. Domain-pattern changes and the stable domain patterns during the phase transitions are discussed.

The linear temperature behavior of resistivity and oxygen stoichiometry in YBa2Cu3O7-x

By employing a gettered annealing technique, we studied the relationship between the resistivity ρ(x, T) (20° C ≤ T ≤ 300° C) and oxygen content in the high-Tc superconductor YBa2Cu3O7-x ρ(x, T) is approximately linear for 0.0 ≤ x ≤ 0.6, and both the slope dρ/dT and the residual resistivity increase continuously with decreasing oxygen content. A qualitative analysis is given.

THE HIGH TEMPERATURE RESISTIVITY OF Ba2YCu3O7−x

The high temperature resistivity (ρ), thermogravimetry (TG) and derivative thermogravimetry (DTG) have been used to characterize superconductor Ba2YCu3O7−x (BYCO) in O2, air and N2. The resistivity is linear from room temperature to 350°C and then deviate from linearity with oxygen evolution, the derivative of resistivity dρ/dT increases abruptly near orthorhombic to tetragonal phase transition. These phenomena can give good explanations for a two-band Drude model.

BRILLOUIN LIGHT-SCATTERING INVESTIGATION OF ELASTIC ANOMALY FROM SUPERLATTICES

In this paper we report some new experimental results concerning the elastic properties of periodic and quasiperiodic superlattices studied by means of Brillouin spectroscopy. The observation of both bulk and surface acoustic phonons as well as the fitting using effective medium approximation on the phonon spectrum enables us to extract accurately some relevant effective elastic constants in both structures. We have verified that the elastic anomaly which occurs in the periodic structures is also present in quasiperiodic ones. These results imply that the interaction mechanism of Fermi electrons with reduced Brillouin zone boundary is not responsible for the observed elastic anomalies. The comparative study of these superlattices proves that the elastic anomaly is relevant directly to interfacial effects.

Brillouin Scattering from Metallic Ta/Al Fibonacci Multilayers

Surface phonon spectra from Ta/Al Fibonacci multilayers are studied via Brillouin scattering. Determination of the frequency positions of the Rayleigh wave and the intensity dip corresponding to the longitudinal threshold has enabled us to extract two effective elastic constants C44 and C11 of the whole structure. The 22.6% enhancement of the shear modulus is found in our experiment.

Vibrational spectra of quasi-periodic metallic superlattices

Raman scattering studies of several Cu/Nb quasi-periodic metallic superlattices fabricated by the magnetron sputtering technique are presented. The obtained room-temperature vibrational spectra of superlattice phonons were analysed using an elastic continuum model. For quasi-periodicity below about 5.0 nm a resonant-like TA phonon spectrum corresponding to the 514.5 nm excited line is observed, but the dominant Raman spectral feature of quasi-periodicity greater than about 14.0 nm resembles folded acoustic modes in periodic metallic superlattices. The calculation of the theoretical frequencies shows good agreement with the experimental data.