T. M. Uen

Preparation and characterization of some tin oxide films

Tin oxide films were successfully prepared by the reactive evaporation method with a d.c. glow discharge of oxygen. The structure and composition of the films were characterized by Mossbauer spectroscopy, X-ray diffraction and scanning electron microscopy. The valency of tin atoms in the films determined by Mossbauer spectroscopy is mostly divalent in the samples prepared without discharge, but mostly tetravalent in the samples prepared with discharge. Measurements of electrical conductivity and Hall mobility were also carried out and the data were found to correlate with the concentration ratio [Sn4+]/[Sn2+] determined from Mossbauer spectroscopy.

Temperature dependence of transport properties of evaporated indium tin oxide films

Abstract Indium tin oxide films with high conductivity and transparency were successfully prepared by evaporating an alloy of indium and tin covered with In 2 O 3 powder. At room temperature the films have high carrier mobilities of about 60 cm 2 V -1 s -1 . Conductivities as high as 5 × 10 3 Ω -1 cm -1 and transmittance values of greater than 90% in the visible region of light were obtained. The carrier mobility was found to be inversely proportional to temperature in the high temperature range and independent of temperature in the low temperature range. The temperature dependence of the carrier conductivity indicated that the carrier excitation energy was less than 8.6 × 10 -6 eV. Mossbauer spectroscopy showed that the tin in all the high quality films was tetravalent.

Quasiparticle Dynamics and Phonon Softening in FeSe Superconductors

Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (ΔR/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ΔR/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe.

Monophasic TiO2 films deposited on SrTiO3(100) by pulsed laser ablation

Single phase TiO2 thin films, of either rutile or anatase structure, have been prepared on SrTiO3(STO)(100) substrates by in situ pulsed laser deposition (PLD). Thermodynamically unfavorable, for films deposited on STO(100) substrate directly, pure anatase TiO2(00l) films were formed even when a rutile TiO2(110) substrate was used as a target. On the other hand, pure rutile TiO2(110) films were obtained by oxidizing PLD TiN films in-situ at temperatures higher than 700 °C. The optimized deposition conditions for preparing TiN and TiO2 films were reported. The crystalline structure, surface morphology, and electronic structure of these films were examined. A mechanism of the process of film formation is also proposed.

Enhancement of critical current density in direct‐current‐sputtered TlBa2Ca2Cu3O9±δ superconducting thin films

A multistep postannealing scheme has been developed for preparing nearly single phased, c‐axis oriented TlBa2Ca2Cu3Ox (Tl‐1223) superconducting thin films fabricated by the direct‐current‐sputtering process. Films obtained by the present process have shown, for the first time in this system, a critical current density (Jc) above 106 A/cm2 at 77 K with a zero‐resistance transition temperature Tc0≊110 K. The order of magnitude enhancement in Jc is attributed to the improvement of film morphology which, in turn, removed most of the weak links encountered previously.

Optimization of depositing Y1Ba2Cu3O7-δ superconducting thin films by excimer laser ablation with Co2 laser-heated substrates

Abstract The optimum deposition conditions for growing Y 1 Ba 2 Cu 3 O 7 -δ superconducting thin films in situ by using xenon, Nd:YAG, and KrF excimer lasers with CO 2 laser-heated substrates were studied.The dependences of the T co and microstructure of the films on the deposition parameters such as substrate temperature, oxygen partial pressure, laser energy density and repetition rates have been investigated systematically. It was found that both the surface mobility on the substrate and the reaction temperatures for the particulates arriving at the substrates are the key factors for obtaining high-quality films. High-quality Y 1 Ba 2 Cu 3 O 7 -δ thin films with mirror-like surface morphology, T co ≈90K, and nearly perfect c -axis oriented normal to the film surface were routinely grown in situ on SrTiO 3 substrates with the following deposition conditions: substrate temperature at 670°C, oxyegen partial pressure of 0.1 torr, laser energy density of 4J/cm 2 per pulse, and repition rate of 10hz. More significantly, it too only about 3 min to grow a 0.3μm thick film and less than 50 s to down the film to room temperature after deposition by the present process. Subsequent processes, such as slow cooling in oxygen atmosphere or any further heat treatments, were unnecessary for the present process. The unique advantage of the present process further offered us good oppurtunities to delineate the effects of each deposition parameter on the detailed film growth mechanisms without having to deal with the complications introduced by prolonged post-deposition treatments. The details of all the interesting features observed will be presented and their physical implications will be discussed.

Ultrafast photoinduced mechanical strain in epitaxial BiFeO3 thin films

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76 km/s.

Magnetic ordering anisotropy in epitaxial orthorhombic multiferroic YMnO3 films

Orthorhombic YMnO3 thin films with (200), (020), and (001) orientations were, respectively, obtained by pulsed laser deposition on SrTiO3(110), LaAlO3(110), and SrTiO3(001) substrates. The results demonstrate that the strain between film and substrate can serve as an alternative in transforming the thermodynamically stable hexagonal YMnO3 into the orthorhombic phase, which previously could be obtained with high-pressure high temperature syntheses and epitaxy-stabilized thin film processes. More importantly, these films allow us, for the first time, to unambiguously disclose the intrinsic magnetic property along different crystallographic orientations. Our results show that, although the antiferromagnetic (AFM) ordering remains the same, there is an additional spin reordering transition which is very much dependent on the crystallographic orientation along which the measuring field was applied and on the in-plane crystallographic alignment of the films. Detailed analyses indicate that the origin of the obs...

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.

Fluctuation-enhanced conductivity in YBa2Cu3Ox (6.5 ≤ x ≤ 6.95) and Tl2Ba2Ca2Cu3O10±δ superconducting thin films

Abstract The fluctuation-induced conductivity (Δ σ ( T )) near T c of a sole YBa 2 Cu 3 O x (YBCO) film with various precisely controlled oxygen contents (6.5≤ x ≤6.95) was studied and compared with those obtained from its Tl-based counterparts. For YBCO films with x >6.7, Δσ( T ) displays a distinct 3D to 2D crossover as the temperature approaches T c . On the other hand, as T → T c , Δ σ ( T ) of both Tl-based and YBCO film with x ≤6.7 exhibits a reversed 2D to 3D crossover. It is suggestive that the coupling between the CuO 2 layers may have changed significantly with reducing oxygen.