Search Results (8 total)

We investigate the presence of secondary phases in La-doped Bi-2201 thin films grown by laser ablation. The cation ratios in the target material, the oxygen pressure, and the substrate temperature during the deposition are the main parameters determining the presence of diluted intergrowth and/or polytype aggregates. A statistical model of random intergrowth is used to analyze the x-ray diffraction (XRD) anomalies caused by hidden defects and to characterize the latter. A detailed structural XRD refinement on oriented aggregates allows us to identify the guest phase as a Bi deficient phase, Bi-1201. The occurrence of this particular embedded polytype is accompanied by a global Bi deficiency introduced in the films by the growing process and/or by the annealing treatment. The presence of La favors the Bi-1201 formation mostly as La-rich c-axis oriented aggregates. Bi excess in the target material improves considerably the crystallographic structure of Bi-2201, avoids intergrowth formation, but does not prevent the phase separation of Bi-1201 in La-doped thin films. We also investigate the influence of the deposition parameters on the type of intergrowth as well as their variation with La doping. This work introduces a specific methodology for optimizing the growth of thin films grown by laser ablation, which applies to layered oxides that admit polytypes with close formation enthalpies in their phase diagram.

Optical-field-ionization x-ray lasers in an optically preformed plasma waveguide for pure xenon, krypton, and argon gases, respectively, are achieved. In addition to the 46.9 nm main lasing line for Ne-like argon, the 45.1 and 46.5 nm lasing lines are also observed, indicative of the strong enhancement effect and the large gas density in the plasma waveguide. With this technique multispecies parallel x-ray lasing is also demonstrated in a Kr-Ar mixed-gas waveguide. Extensive experimental results including the pump-energy dependence, the density dependence, and the effects of parameters that control the waveguide fabrication are reported and discussed.

Dramatic enhancement of optical-field-ionization collisional-excitation x-ray lasing is achieved by using an optically preformed plasma waveguide. With a 9-mm-long pure krypton plasma waveguide prepared by using the axicon-ignitor-heater scheme, lasing at 32.8 nm is enhanced by 400 folds relative to the case without the plasma waveguide. An output level of 8×10¹⁰  photon/shot is reached at an energy conversion efficiency of 2×10^-6. The same method is used to achieve x-ray lasing in a gas jet for the high-threshold low-gain transition at 46.9 nm in neonlike argon.

We propose a simple model for the nucleation of random intercalates during the growth of high-temperature superconductor (HTSC) films by pulsed laser deposition (PLD). The model predicts a very particular spatial distribution of defects: a Markovian-like sequence of displacements along the growth direction (c axis), as well as a two-component in-plane correlation function, characteristic of self-organized intercalates. A model for x-ray diffraction (XRD) on such structures is also developed and accounts for both c-axis and in-plane anomalies observed in XRD experiments. The method presented in this work constitutes a useful characterization tool in the optimization of deposition parameters for the growth of HTSC films.

Optical-field-ionization collisional-excitation x-ray lasers in xenon and krypton clustered gas jets were experimentally investigated in detail. Dependence of x-ray lasing on atom density, laser focus position, and laser polarization ellipticity was mapped out to find the optimal lasing conditions, and a tomographic measurement technique was used to provide clear pictures of the x-ray amplification process.

By using deflectometry of a longitudinal probe pulse and reflective interferometry of a transverse probe pulse to resolve the spatiotemporal distribution of the preformed plasma, we characterize and control the plasma density distribution near the target surface for the development of solid-target x-ray lasers. We show that the use of prepulses in an ignitor-heater scheme can increase the scale length of the preformed plasma and how the effect varies with target materials. Many important issues crucial to x-ray lasing such as electron density distribution, electron temperature, and the optimal timing between pumping pulses can be resolved with these methods.

The potential use of two planar superconducting elliptical undulators—a vertically wound racetrack coil structure and a staggered array structure—to generate a circularly polarized hard x-ray source was investigated. The magnetic poles and wires of the up and down magnet arrays were rotated in alternating directions on the horizontal plane, an elliptical field is generated to provide circularly polarized light in the electron-storage ring and the energy-recovery linac accelerator. Rapid switching between right- and left-circularly polarized radiations is performed using two undulators with oppositely rotated wires and poles. Given a periodic length of 15 mm and a gap of 5 mm, the magnetic-flux densities in the elliptical undulator are B_z=1.2   T (B_x=0.6   T) and B_z=0.35   T (B_x=0.15   T) in the planar vertically wound racetrack coil and the staggered structure with poles rotated by 35° and 25°, respectively. In maximizing the merit of the flux and the width of the effective field region in the two superconducting elliptical undulators, the trade-off rotation angles of the coils and poles are 20° and 5°, for vertically wound racetrack coil and staggered undulators, respectively.

The low-temperature specific heat C(T,H) of the superconductor MgCNi₃ has been measured in detail. ΔC/γ_nT_c=1.97 is estimated from the anomaly at T_c. At low temperatures, the electronic contribution in the superconducting state follows C_es/γ_nT_c≈7.96 exp(-1.46T_c/T). The magnetic-field dependence of γ(H) is found to be linear with respect to H. T_c estimated from the McMillan formula agrees well with the observed value. All the specific-heat data appear to be consistent with each other within the moderate-coupling BCS context. It is amazing that such a superconductor unstable to ferromagnetism behaves so conventionally. The Debye temperature Θ_D=287 K and the normal state γ_n=33.6 mJ/mol K² are determined for the present sample.