Won Nam Kang

We fabricated high-quality c axis-oriented epitaxial MgB2 thin films using a pulsed laser deposition technique. The thin films grown on (1 i 0 2) Al2O3 substrates have a transition temperature of 39 kelvin. The critical current density in zero field is approximately 6 x 10(6) amperes per cubic centimeter at 5 kelvin and approximately 3 x 10(5) amperes per cubic centimeter at 35 kelvin, which suggests that this compound has potential for electronic device applications, such as microwave devices a

We have measured the longitudinal resistivity and the Hall resistivity in the $\mathrm{ab}$ plane of highly c-axis-oriented ${\mathrm{MgB}}_{2}$ thin films. In the normal state, the Hall coefficient ${(R}_{H})$ behaves as ${R}_{H}\ensuremath{\sim}T$ with increasing temperature (T) up to 130 K and then deviates from that linear T dependence at higher temperatures. The ${T}^{2}$ dependence of the cotangent of the Hall angle is only observed above 130 K. The mixed-state Hall effect reveals no sign

We have measured the thermopower of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$ as a function of oxygen deficiency. We found that both the magnitude and the temperature dependence of the thermopower strongly depend upon the oxygen deficiency. As the oxygen deficiency increases, the magnitude at 273 K increases by more than a factor of 90 and the large positive peak observed at the superconducting transition temperature moves toward a higher-temperature region wi

The mixed-state Hall resistivity ${\ensuremath{\rho}}_{\mathrm{xy}}$ and the longitudinal resistivity ${\ensuremath{\rho}}_{\mathrm{xx}}$ in superconducting ${\mathrm{MgB}}_{2}$ thin films have been investigated as a function of the magnetic field over a wide range of current densities from ${10}^{2}$ to ${10}^{4} {\mathrm{A}/\mathrm{c}\mathrm{m}}^{2}.$ We observe a universal Hall scaling behavior with a constant exponent $\ensuremath{\beta}$ of $2.0\ifmmode\pm\else\textpm\fi{}0.1$ in ${\ensurem

The triple sign reversal in the mixed-state Hall effect has been observed in ion-irradiated ${\mathrm{HgBa}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{6}$ thin films. The negative dip at the third sign reversal is more pronounced for higher fields, which is opposite to the case of the first sign reversal near ${T}_{c}$ in most high-${T}_{c}$ superconductors. These observations can be explained by a recent prediction in which the third sign reversal is attributed to the energy derivative of the density

We have investigated the flux pinning force in MgB2 thick films with columnar structures. Three types of thick lms with different grain sizes were prepared on c-cut Al2O3 substrates by using the hybrid physical-chemical vapor deposition method at 520, 540 and 560 ℃. For all of the samples, the pinning force densities showed an unusual double-peak behavior with two types of irreversible fields, H*, separating the peaks. This double-peak behavior of the pinning forces did not obey any flux pinning

We have synthesized c-axis-oriented MgB2 thin films on (1 0 2) Al2O3 substrates by using a pulsed laser deposition technique and have measured the longitudinal and the Hall resistivites in the ab-plane direction. The as-grown thin films show Tc of 39.2 K with a sharp transition width of ∼0.15 K. In the normal state, the Hall coefficient (RH) behaves as RH ∼ T with increasing temperature (T) up to 130 K and then deviates from that linear T-dependence at higher temperatures. The T2 dependence of t

We have grown MgB2 thick films on c-cut Al2O3 substrates at low temperatures of 520 -600 C by using hybrid physical-chemical vapor deposition (HPCVD). To obtain a high magnesium vapor pressure at low temperatures, we used a unique susceptor having a cap. The as-grown MgB2 films showed superconducting transition temperatures of 39.5 -40 K with a sharp transition width of about 0.2 K. As the growth temperature was increased from 520 to 600 C, the film thickness was decreased from 3.7 to 2.0 m. X-r