J-Y Lin

Ultrafast thermoelastic dynamics of HoMnO(3) single crystals derived from femtosecond optical pump-probe spectroscopy

In this paper, the photo-induced ultrafast thermoelastic dynamics of hexagonal HoMnO3 single crystals has been studied by using optical pump–probe spectroscopy. The thermoelastic effect in the ab-plane of hexagonal HoMnO3, associated with giant magnetoelastic coupling around the Neel temperature (TN), is intimately correlated to the emergence of a negative component in the transient reflectivity changes (ΔR/R) obtained in temperature-dependent pump–probe experiments. Moreover, the variation in the oscillation period in ΔR/R caused by a strain pulse propagating along the c-axis exhibits an abrupt drop around TN, presumably due to the antiferromagnetic-ordering-induced changes in ferroelectricity.

A three-intensity technique for polarizer-sample-analyser photometric ellipsometry and polarimetry

This work presents a novel three-intensity-measurement technique to determine the ellipsometric parameters and in a polarizer-sample-analyser photometric ellipsometer. This technique can be employed to correct the azimuthal misalignment of the analyser with respect to the plane of incidence. By performing two sets of measurements with this technique with the polarizers azimuth at and , respectively, we can simultaneously determine the azimuthal deviation of the polarizer and further improve the ellipsometric measurements. Applying this technique in the transmission mode allows us to obtain the phase retardation and the optical axis of a waveplate at the same time.

Microstructure and magnetic properties of oxidized titanium nitride thin films in situ grown by pulsed laser deposition

Different oxidation states of titanium nitride thin films, including pure TiN(hu20090u20090), TiN1−xOx(hu20090u20090), Ti2O3(0u20090u2009l) and pure anatase TiO2(0u20090u2009l), were prepared by pulsed laser deposition with various oxygen pressures using a TiN target. Elaborative evolutions of the crystal and electronic structures of the obtained films were examined systematically by x-ray diffraction and x-ray absorption spectroscopy. We found that the Ti2O3(0u20090u2009l) film, which was prepared at oxygen pressures , exhibited the maximum room temperature ferromagnetism (RTFM) behaviour. The bound magnetic polaron model is used to clarify the origin of RTFM in these films.

Ultrafast dynamics and phonon softening in Fe1+ySe1?xTex single crystals

The ultrafast quasiparticle dynamics of Fe1+ySe1?xTex single crystals were investigated by dual-color transient reflectivity measurements (?R/R) from 4.3 to 290?K. The electron?phonon coupling strength ??(=0.16?0.01) and the temperature-dependent energy of longitudinal-acoustic phonons were, respectively, obtained from the relaxation time of a fast component and the period of an oscillation component in ?R/R. Such a small ? demonstrates the unconventional origin of superconductivity in FeSe. Moreover, the temperature-dependent ?R/R exhibits anomalous changes at Tc, 90?K and 230?K, unambiguously revealing the phase transition as well as the phonon softening via the magnetoelastic effect.

Cross-sectional shape modulation of physical properties in ZnO and Zn1−xCoxO nanowires

We have prepared comparable-diameter ZnO and Zn1- xCoxO nanowires with both circular and hexagonal cross-sections. The average diameters are ~113 and ~134u2009nm for cylindrical and hexagonal nanowires, respectively. The as-grown nanowires have been characterized via structure, electrical conductivity and photoluminescence (PL) spectrum measurements. Pure ZnO nanowires were Co-ion implanted to make magnetic Zn1- xCoxO nanowires for magnetization studies. Bumpier edge surfaces on a nanometre scale, higher densities of stacking faults and a bending feature along the growth direction have been found in cylindrical ZnO nanowires. As compared with hexagonal nanowires, we have observed relatively higher conductivities in cylindrical nanowires, which implied large numbers of shallow donors existing in the latter nanowires. The cylindrical ZnO nanowires also displayed intensified green defect emission and considerably more stacking faults in the crystalline structure. In addition, we have found increased magnetization and stronger ferromagnetic ordering in cylindrical than in hexagonal Zn1-xCoxO nanowires, and have experimentally identified that the point defects of either Zn interstitials or O vacancies played governing roles in ferromagnetism. We conclude that the cross-sectional shape effect originating a varied point defect density can profoundly modulate the structural, electrical, optical as well as magnetic properties of ZnO and Zn1-xCoxO nanowires.

Superconducting fluctuations and the Nernst effect in high-T c superconductors

The comprehensive. T/ measurements and the consequent resistivity curvature mapping (RCM) on Y0:7Ca0:3Ba2Cu3O7 thin films (doping levels pD 0:08‐0.21) elucidate a phase diagram for the whole doping range. This phase diagram further strengthens a view that the ‘normal’ phase in hole-doped cuprates should be divided into a strong superconducting (SC) fluctuation phase and the ‘real’ normal phase in which there is no significant influence of SC. The temperature of superconducting fluctuations Tf as a function of p was calculated using the Ginzburg‐Landau model for layered superconductors. Comparisons between Tf and the Nernst temperature establish the origin of the Nernst effect as SC fluctuations. Some of the details in. T/ cannot be fully understood by the existing models and call for a more sophisticated theory of carrier dynamics in cuprates. (Some figures may appear in colour only in the online journal)

Magnetic transition anisotropies in orthorhombic LuMnO3 and HoMnO 3 multiferroic thin films

We have successfully prepared the b-axis-oriented orthorhombic LuMnO3 (LuMO) and HoMnO3 (HMO) thin films by pulsed laser deposition on (110)-LaAlO3 substrates. The nearly perfect alignment between the film growth orientation and the substrate allows us to study the magnetic transitions along the respective crystal orientation, which has displayed marked anisotropic behaviours. In particular, with the largest ionic size difference between Lu and Ho for the family of RMnO3 displaying the E-type AFM, the effects of lattice distortion on the magnetic transition are compared.

Polarization-dependent x-ray absorption spectroscopy of hexagonal and orthorhombic TbMnO3 thin films

Pure phase TbMnO3 manganite thin films with hexagonal (h-TMO) and orthorhombic (o- TMO) crystal structures were prepared by pulsed laser deposition. The distinctive orientation alignments between film and substrate obtained here have allowed us to perform the x-ray absorption near edge spectroscopy (XANES) measurements with the electric field applied along the three major crystallographic directions. The XANES results, as expected, display significantly different spectral features for the h-TMO and o-TMO films. In addition, the XANES spectra also exhibit strong polarization dependence at O K and Mn L edges for both samples.

Controllable subwavelength-ripple and -dot structures on Y Ba2Cu3O7 induced by ultrashort laser pulses

The evolution of periodic ripples and dots on an Y Ba2Cu3O7 surface irradiated by single-beam and dual-beam femtosecond laser pulses is investigated. The density and size of the dots can be controlled by the number of pulses. The spatial period of these ripples does not show a close relation to the pulse number. However, the orientation and geometrical period of the ripples are strongly dependent on the polarization and incident angle of the laser pulses, respectively. The formation processes are also discussed based on the dependence.

Coherent acoustic phonons in hexagonal HoMnO3 probed by femtosecond spectroscopy

We have observed the coherent acoustic phonons in the hexagonal manganite HoMnO3 single crystals using wavelength-dependent femtosecond pump-probe spectroscopy. The oscillatory component of the photoinduced transient reflectivity change (ΔR/R) at various pump-probe wavelengths is consistent with the model of strain pulse propagation. Furthermore, the oscillation period of the oscillation is strong temperature- and wavelength-dependent. In particular, the period of all oscillations in ΔR/R for various wavelengths are very similar in the vicinity of the Neel temperature (TN), which associates with the appearance of the antiferromagnetic (AFM) long-range ordering.