Shiro Takano

High-Tc and high-Jc SmFeAs(O,F) films on fluoride substrates grown by molecular beam epitaxy

Superconducting thin films of SmFeAs(O,F) were prepared by molecular beam epitaxy on fluoride substrates. In our process, F-free SmFeAsO films were grown first, and F was subsequently introduced to the films by diffusion from an overlayer of SmF3. By this simple process, record high Tc, namely, Tcon (Tcend) = 57.8 K (56.4 K) was obtained in a film on CaF2. Furthermore, the films on CaF2 showed high critical current density over 1 MA/cm2 in the self-field at 5 K. The correlation between superconductivity and epitaxial strain in SmFeAs(O,F) films is discussed.

Molecular Beam Epitaxy Growth of Superconducting Sr1-xKxFe2As2 and Ba1-xKxFe2As2

We report on the molecular beam epitaxy growth of nonsuperconducting SrFe2As2 and BaFe2As2 and superconducting Sr1-xKxFe2As2 and Ba1-xKxFe2As2 thin films. SrFe2As2 and BaFe2As2 films were obtained rather easily at the growth temperatures of 540–600 °C, which are not much different from those for GaAs growth. However, superconducting Sr1-xKxFe2As2 and Ba1-xKxFe2As2 films cannot be obtained at the same growth temperatures as elemental K is highly volatile. The key to incorporating K into films is low-temperature growth (≤350 °C) in reduced As flux. The resultant films showed good superconducting properties: Tcon (Tcend) = 33.2 K (30.0 K) and 38.3 K (35.5 K) for Sr1-xKxFe2As2 and Ba1-xKxFe2As2 thin films respectively.

Measurement of the Bunch Length on the UVSOR Storage Ring

We have measured the bunch length in single-bunch operation of the UVSOR (Ultraviolet Synchrotron Orbital Radiation) storage ring in a beam current region from 0.1 to 70 mA at energies ranging from 500 to 750 MeV. The momentum compaction factor of the present operation point is determined experimentally from the bunch length and the synchrotron oscillation frequency measured simultaneously at a low beam current. The bunch length increases gradually as the beam current increases, and bunch lengthening is larger when the beam energy is lower. This behavior of bunch lengthening agrees well with a prediction of the potential-well distortion theory. Based on the theory, the effective longitudinal coupling impedance is estimated as a function of bunch length.

Molecular beam epitaxy growth of superconducting Ba 1-xK xFe 2As 2 and SmFeAs(O,F) films

We report the molecular beam epitaxy (MBE) growth of the iron-based superconductors, Ba1-xKxFe2As2 and SmFeAs(O,F). In the growth of Ba1-xKxFe2As2 films, the key to incorporating volatile K in films is low-temperature (≤350 °C) growth in reduced As flux. The highest Tc thus far obtained is Tcon (Tcend) = 38.0 K (35.8 K). In the growth of superconducting SmFeAs(O,F), we adopted two methods. In the first method, we first grew pristine SmFeAsO films, and subsequently introduced F into the films by diffusion from an overlayer of SmF3. In the second method, we grew as-grown superconducting SmFeAs(O,F) films by coevaporating Sm, SmF3, Fe, and As. Thus far, better results have been obtained by the first F diffusion method. The films prepared by F diffusion showed Tcon (Tcend) = 56.5 K (55.3 K), whereas the as-grown films showed Tcon (Tcend) = 51.5 K (48.0 K).

As-Grown Superconducting SmFeAs(O,F) Thin Films by Molecular Beam Epitaxy

As-grown superconducting thin films of SmFeAs(O,F) with Tc higher than 50 K were successfully grown by molecular beam epitaxy. In the growth of the films, we codeposited Sm, SmF3, Fe, and As in oxygen atmosphere. The most crucial factor for obtaining superconducting films has turned out to be the substrate choice. We have found that CaF2-buffered LaAlO3 is much more suitable for the growth of SmFeAs(O,F) films than bare LaAlO3 or CaF2 and yielded nearly single-phased SmFeAs(O,F) films with Tcon (Tcend) ~ 52 K (50 K).

Gain measurement of a free electron laser with an optical klystron on the UVSOR storage ring

We have conducted experiments to measure the gain of a free electron laser (FEL) with an optical klystron (OK) on the UVSOR storage ring. A conventional undulator has been remodeled into the OK in order to increase the gain. The FEL gain at a wavelength of 488 nm has been measured as a function of Nd, which is the number of wavelengths of light passing over an electron in a dispersive section of the OK. The measured gain curve is in good agreement with that calculated according to Madeys theorem from a measured spectrum of spontaneous radiation. The measured instantaneous peak gain is 0.4% for an average beam current of 10 mA/bunch, which agrees well with the gain calculated theoretically. The gain with the OK is enhanced by a factor of five compared with that with the conventional undulator.

Gain Measurement of a Free Electron Laser on the UVSOR Storage Ring

We report the results of gain measurement of a free electron laser at a wavelength of 488 nm made on the UVSOR storage ring. In this experiment, a conventional transverse undulator made of permanent magnets is employed, and the storage ring is operated at the energy of 500 MeV in the single-bunch mode. The measured peak gain is 8×10-4 for the average beam current of 10 mA, which agrees well with gain estimated from theory.