Woo Il YANG

Publications (3)3.7 Total impact

• Article: Microwave surface resistance of thick MgB2 films on c-plane sapphire: a study on the depth profile of the surface resistance

  Ho Sang Jung, Woo Il Yang, Jae Hun Lee, Won Kyung Seong, Won Nam Kang, Man Soon Cho, Kee Nam Choo, Sang Young Lee
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  ABSTRACT: A depth profile of the surface resistance (RS) is obtained at 8.6 GHz for a thick MgB2 film both in the superconducting state and in the normal state, for which 1 µm thick MgB2 films are grown in situ on a c-plane sapphire substrate using the hybrid physical–chemical vapor deposition (HPCVD). A critical temperature TC of 40.4 K, a value higher than that of 39 K for MgB2 single crystals, is observed for the pristine 1 µm thick MgB2 film due to the thermal strain caused by its epitaxial nature. The depth profile of the effective RS for the 1 μm thick MgB2 film shows a drastic increase in the effective RS at low temperatures for films having thicknesses of 600 nm or less. The results for the x-ray diffraction and the normal-state resistivity provide evidence for the existence of Mg-rich phases in the lower part of the pristine MgB2 film. Our results show that thick MgB2 films grown using HPCVD could have very low RS despite the existence of Mg-rich phases in the lower part of the MgB2 films and that microwave technique could provide a sensitive method for investigating the homogeneity of thick MgB2 films.
  Superconductor Science and Technology 02/2012; 25(3):035016. · 2.66 Impact Factor
• Article: Effects of Proton Irradiation on the Microwave Surface Resistance of YBa2Cu3O7−δ Films

  Jae Hun Lee, Woo Il Yang, Eun Kyu Park, Sang Young Lee
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  ABSTRACT: Proton irradiations on superconducting bulk materials and thin films have enabled to enhance the critical current density and the upper critical field in the mixed state due to increased vortex pinning. We investigated the microwave surface resistance of proton-irradiated (YBCO) films grown on (LAO) substrates. The intrinsic was measured at 8.5 GHz at temperatures of 7-90 K with the effects of the finite film thickness taken into account. Significant reduction in the of YBCO films were observed at temperatures of 40 K to 70 K when the films were irradiated with 20 MeV proton for 10 minutes to the fluence of with the value of 100 of the proton-irradiated one at 60 K appearing significantly lower than the corresponding one of 140 before irradiation. However, at temperatures below 20 K, the of the irradiated YBCO films was higher than that of the YBCO film before irradiation with the residual of the former appearing higher than that of the latter. No significant changes in the of YBCO films were observed when the films were irradiated with 20 MeV proton for 30-120 minutes to the fluences of to . The reduction in the of proton-irradiated YBCO films at 40-70 K was attributed to enhanced impurity scattering rate of compared to a value of for unirradiated YBCO.
  IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
• Article: Microwave Properties of Sapphire Resonators with a Gap and Their Applicability for Measurements of the Intrinsic Surface Impedance of Thin Superconductor Films

  Sang Young LEE, Jae Hun LEE, Woo Il YANG, John H. CLAASSEN
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  ABSTRACT: A dielectric resonator with a gap between the top plate and the rest has been useful for measuring the penetration depth (λ) of superconductor films, a parameter essential for obtaining the intrinsic microwave surface resistance ( R s ) of thin superconductor films. We investigated effects of a gap on the microwave properties of TE 0ml -mode sapphire resonators with a gap between the top plate and the rest of the resonator. Regardless of a 10 μm-gap in TE 0ml -mode sapphire resonators, variations of the TE 0ml -mode resonant frequency on temperature (Δ f 0 ) as well as TE 0ml -mode unloaded Q remained almost the same due to lack of axial currents inside the resonator and negligible radiation effects. The λ of YBa 2 Cu 3 O 7−δ (YBCO) films obtained from a fit to the temperature-dependent Δ f 0 appeared to be 195 nm at 0 K and 19.3 GHz, which was well compared with the corresponding value of 193 nm at 10 kHz measured by the mutual inductance method. The intrinsic R s of YBCO films on the order of 1 mΩ, and the tan δ of sapphire on the order of 10−8 at 15 K and 40 GHz could be measured simultaneously using sapphire resonators with a 10 μm-gap.