Showing papers by "Yoshishige Murakami published in 1996"

Terahertz radiation from superconducting YBa2Cu3O7−δ thin films excited by femtosecond optical pulses

Abstract: Ultrashort electromagnetic waves (600 fs width) from superconducting YBCO thin films have been observed by irradiating current‐biased samples with femtosecond optical laser pulses (80 fs width). The Fourier component of the pulse extends up to ∼2 THz. The characteristics of the radiation are studied and the radiation mechanism is ascribed to the ultrafast supercurrent modulation by the laser pulses, which induce the nonequilibrium superconductivity.

Ultrashort Electromagnetic Pulse Radiation from YBCO Thin Films Excited by Femtosecond Optical Pulse

Abstract: We have observed ultrashort electromagnetic pulse radiation from YBa2Cu3O7-δ thin-film dipole antennas. The supercurrent transient is created by the excitation of the supercarriers into quasiparticles with a femtosecond laser pulse, and freely propagated electromagnetic pulses are measured and characterized. A pulse with 0.5 ps full width at half-maximum was obtained, containing frequency components up to 2.0 THz. A femtosecond time-resolved characterization of the spectra revealed that they strongly depend on the excitation conditions, and the quasiparticle recombination time becomes longer with increase in the excitation intensity. It is also observed that the radiation power increases in proportion to the square of both the bias current and the laser power in the region of weak excitation, which is consistent with the classical theory based on a two-fluid model. In the region of strong excitation, deviation from the classical theory was observed.

Enhanced Sub-Picosecond Electromagnetic Radiation from YBa2Cu3O7-δ Thin-Film Bow-Tie Antennas Excited with Femtoseconds Laser Pulses

Abstract: We demonstrate the emission of sub-picosecond electromagnetic waves from YBa2Cu3O7-δ thin-film bow-tie antennas. Femtosecond laser-pulse irradiation creates a radiative supercurrent transient in the films, and freely propagated radiation is detected and characterized. The low-frequency component of the radiation was greatly enhanced by using bow-tie antennas instead of dipole antennas. We discuss the antenna effects on both the radiation spectra and the radiation power.

Atomic scale flattening and characterization of a YBCO film surface

Abstract: Surface flattening of YBCO films with Ar ion beam exposure was studied. Atomic force microscopy (AFM) observation revealed that the surface roughness decreases with increasing ion beam exposure time. The mechanism of flattening was explained by the angular-dependent etching characteristics. A flattening effect on an atomic scale with a mean roughness of 0.8 nm was observed. Electrical measurements and Raman spectroscopy indicated that superconductivity and crystallinity of the films exposed to the ion beam were damaged due to the ion bombardment.

Current control of magnetically coupled superconducting coils for nuclear fusion experimental system using H/spl infin/ control scheme

Abstract: Precise current control within 0.01% accuracy of superconducting coils composing the Large Helical Device (LHD), which is a nuclear fusion experimental system under construction in Japan, is needed under the condition with strong mutual coupling between coils and some disturbances such as plasma, vacuum vessel etc. Application of the H/spl infin/ control scheme with a two-degree-of-freedom controller was studied. Good control results were obtained from the experimental model system with two superconducting coils. Simulation results with the LHD's circuit parameters show the possibility of successful control results.

Novel dynamic diagnosis of femtosecond time-transient nonequilibrium state in optically excited YBCO thin films

Abstract: Terahertz emission from optically excited YBCO thin films has been interpreted in terms of a femtosecond time-transient quasiparticle dyanmics. The analysis of the emission waveforms based on Rothwarf-Tailor rate equations suggested that quasiparticle recombination time and pair breaking time are shorter than 1 ps and longer than 7 ps, respectively, except nearT c .

Design and Operating Characteristics of Superconducting Transformer with Simple Configuration

Abstract: A 40kVA three phase superconducting transformer has been developed and tested. In order to simplify its configuration, whole the transformer including windings and iron core was immersed in liquid helium. Reducing iron losses is the key to design such type of superconducting transformer. Iron loss characteristics of three types of core materials were measured under liquid nitrogen temperature. Among them, ultra fine crystallized metal (FINEMET) showed excellent low iron loss characteristics. Superconducting transformer made of such core and high Tc superconducting ac wire, whose whole the structure is cooled by liquid nitrogen, is very promising alternative.

Subpicosecond electromagnetic pulse generation from high-Tc superconducting thin films by femtosecond laser pulse excitation

Abstract: Ultrashort electromagnetic pulses with a halfwidth of 045 psec have been radiated from current biased superconducting YBCO films by exciting femtosecond laser pulses Characteristics of the radiation are studied in detail