Superconducting NdBa2Cu3O7−δ Thin Films Grown on Bare (1\overline 102) Sapphire by Pulsed Laser Deposition

Superconducting NdBa₂Cu₃O_7– (NBCO) thin films with T _c=89 K have been successfully grown on bare (1 02) sapphire substrates by pulsed laser deposition (PLD). The X-ray diffraction results show that the as-grown NBCO films are all c-axis oriented with no observable second phase. The c-axis parameter decreases monotonically with increasing the deposition temperature (T _s), suggesting that the corresponding degradation of T _c for T _s>790°C might not originate from oxygen deficiency. It is conceived that the film-substrate reaction that occurring at the interface may cause Ba-deficiency and hence enhance Nd-Ba antisite substitutions.     

Synthesis of thermoplastic polyurethane and its physical and shape memory properties

In this article, thermoplastic polyurethane (PU) with a shape memory property was synthesized. First, the PU prepolymer was prepared by reacting poly(tetramethylene glycol) with 4,4′‐diphenylmethane diisocyanate, then extended with various extenders such as linear aliphatic 1,4‐butanediol, benzoyl‐type 4,4‐bis(4‐hydroxyhexoxy)‐isopropylane and naphthalate‐type bis(2‐phenoxyethanol)‐sulfone or naphthoxy diethanol. The experimental results showed that the tensile strength, elongation at break, and initial modulus at 300% of these copolymer films were in the range of 31–64 Mpa, 42%–614%, and 8.26–11.5 MPa, respectively. Thermal analysis showed that the glass‐transition temperature of these copolymers was in the range of ?73°C to ?50°C for the soft segment (T_gs) and 70°C–106°C for the hard segment (T_gh) and that the melting point was in the range of 14.6°C–24.2°C for the soft segment and 198°C–206°C for the hard segment. The extender with a benzoyl or naphthalate group was better able to promote its shape memory property than was the regular polyurethane. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 607–615, 2006     

Quasiparticle Dynamics in FeSe Superconductors Studied by Femtosecond Spectroscopy

The ultrafast quasiparticle dynamics in FeSe single crystals were measured by using dual-color transient reflectivity measurements (ΔR/R) from 4.4 to 290 K. In general, the typical ΔR/R of FeSe includes two significant components. One is the relaxation of photoinduced quasiparticles, which has been used to estimate the electron–phonon coupling strength (λ=0.16). The other is the oscillation component due to the acoustic phonon. Moreover, the acoustic phonon’s energy estimated from the period of oscillation in ΔR/R markedly shrinks around 90 K, which is the so-called phonon softening.     

Selective growth of well-aligned carbon nanotubes by APCVD

Well-aligned good-quality carbon nanotube (CNT) array was grown on silicon substrate by atmospheric pressure chemical vapor deposition (APCVD) through SiO₂ masking. First, the patterned substrate was pretreated with NH₃ and then CNTs were synthesized at 800 °C using Ni as the catalyst, acetylene (C₂H₂) as the carbon source material and N₂ as the carrier gas. Effects of the NH₃-pretreatment time, the flow ratio of [C₂H₂]/[NH₃] and the CNT growth time on the qualities of CNT array were analyzed in detail. It was found that good-quality CNTs with an average length of around 15 μm could be grown by pretreating the Si substrate with NH₃ for 10 min and then conducting the CNT growth with a flow ratio of [C₂H₂]/[NH₃] = 30/100. Furthermore, the field emission property of CNT array was investigated using a diode structure. It was found that the turn-on electric field decreased with increasing CNT length. The turn-on electric field as low as about 2 V/μm with an emission current density of 10 μA/cm² was achieved for a CNT-array diode with the tube length near 18 μm. For the same device, the emission current density could be elevated to 10 mA/cm² with the applied voltage of 3.26 V/μm.     

Miniaturized 3 GHz cross-coupled planar microwave filters

Cross-coupled planar microwave filters were fabricated using YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconducting thin films deposited on both sides of a LaAlO/sub 3/ single crystal substrate by pulsed laser deposition. The hairpin resonator geometry allows one to miniaturize the 3-GHz filters by compacting the entire filter onto a substrate with dimensions 0.5 cm /spl times/ 1 cm. By adjusting the relative spacing among the resonators to establish adequate couplings, filters with very sharp rejection skirt and extremely small insertion loss in passband were obtained.     

Doping-Dependent Phase Diagram of Ca-Doped YBCO Observed by Femtosecond Spectroscopy

The quasiparticle relaxation dynamics in a single (001) Ca-doped YBCO (Y_0.7Ca_0.3Ba₂Cu₃O_7??? ) superconducting thin film was probed by using the femtosecond time-resolved spectroscopy. The (001) Y_0.7Ca_0.3Ba₂Cu₃O_7??? thin film was prepared on a (100)STO substrate by pulsed laser deposition. Through controlling the oxygen pressure and the annealing temperature within quartz tube, the doping level (0.08<p<0.22) in the phase diagram could be widely tuned from the overdoped region to the underdoped region with varying the hole concentration (p) in a single sample. In the overdoped region, two different components (positive/negative) were obviously identified in the transient reflectivity curves (??R/R). The negative component in ??R/R emerges at T<T _c and quickly defeated by the development of the positive component. The dramatic change in the positive component of ??R/R arises well below T _c. However, this anomaly change does not appear in the underdoped region. Only the positive ??R/R was clearly observed at whole temperatures. These results indicate that the scenario of high-T _c superconductivity in the overdoped region would be different from the underdoped region.     

Electron-Boson Coupling Constant of Oxygen-Deficient Y0.7Ca0.3Ba2Cu3O7?�� Measured by Ultrafast Spectroscopy

The ultrafast dynamics in an (001) oxygen-deficient Y_0.7Ca_0.3Ba₂Cu₃O_7??? thin film were measured by using the ultrafast spectroscopy. According to the model of time-dependent distributions of electrons in the case of electron-phonon relaxation, the electron-boson coupling constant ?? could be directly obtained from the relaxation time of the transient reflectivity change (??R/R). For the overdoped case with T _c=61.3 K and the underdoped case with T _c=65.5 K, the relaxation time of ??R/R is around 4.32 ps and 3.66 ps, respectively, which correspond to ????0.06. However, the ?? become larger (??0.15) in the optimal doped region with T _c=82.1 K while the relaxation time of ??R/R shrinks to ??1.51 ps. These results indicate that the ?? is strongly hole-doping dependent, which may govern the T _c of cuprate superconductors.     

Ultrafast Dynamics in Optimally Doped YBa2Cu3O7??? Observed with White Light Pump-probe Spectroscopy

The quasiparticle relaxation dynamics in optimally doped (001) YBa₂Cu₃O_7??? thin films were measured at various temperatures by white light pump-probe technique. The 2D ultrafast spectroscopy with broadband (??200?nm) and high time-resolution (??9 fs) was performed by the non-collinear optical parametric amplifier (NOPA). The relaxation time and the amplitude of the transient transmissivity changes (??T/T) are wavelength-dependent. The maximum ??T/T appears at around 625 nm (??1.98 eV) which corresponds to the transition between the Fermi level and the UHB (upper Hubbard band). Furthermore, the oscillations with ??150, ??320, ??500 cm^?1 have been clearly observed in the Fourier transformed graph of the 2D ultrafast spectroscopy, which are consistent with the phonon modes in Raman-scattering spectra.     

Extinction of a premixed flame by a large variation in axial velocity

Unsteady flame propagation in a tube is examined by introducing a mean velocity variation larger than the burning velocity to a stabilized flame for a period longer than the reaction time scale. In our previous work, stabilized propane-air flames were classified as either one-dimensional or two-dimensional flames. The eventual extinction during the velocity increase was categorized as either acoustic extinction or boundary layer extinction. In this work, the effects of a nonunity Lewis number were estimated through experiments with a methane-air flame; the eventual extinction during the velocity decrease was investigated in more detail; and the growth of the extinction boundary layer was analyzed with a transient one-dimensional model of the flame stretch. In our experiments, the Lewis number did not affect the existence or characteristics of the critical velocity and the characteristic time for boundary layer extinction. An additional critical velocity was found, however, for acoustic extinction when the Lewis number was smaller than unity. In the transient one-dimensional model, the velocity transition along the flame was calculated with a continuity equation and an axial momentum equation. The spatial gradient of the burning velocity and the extinction criterion were simplified with the experimental results and some theoretical studies. The analysis shows that the unsteady flame stretch at the flame edge during a large axial velocity variation is the prevailing cause of the growth of the extinction boundary layer. These results provide some evidence that flame stretch affects the behavior of the flame edge; they also suggest the cause of the finger flame. The findings help explain the unsteady behavior of premixed flames near a flammability limit.