• 한국초전도학회:학술대회논문집
• /
• v.15
• /
• pp.9-9
• /
• 2005

• Progress in Superconductivity
• /
• v.9 no.2
• /
• pp.183-187
• /
• 2008

We have studied superconducting properties of $Mg_{1-x}Al_xB_2$ single crystals from reversible magnetization measurements. It was found that the upper critical fields $H_{c2}$ were decreased for both H // c and H // ab as Al is substituted for Mg. As a result, the large anisotropy of $H_{c2}$ observed in pure $MgB_2$, which is considered as one of the characteristics of two-gap superconductor, was decreased with Al doping. On the other hand, the irreversibility fields $H_{irr}$ were increased for x = 0.1 and were significantly decreased for x = 0.2. In contrary to the anisotropy of $H_{c2}$, the anisotropy of $H_{irr}$ was increased as Al concentration increases.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.17 no.10
• /
• pp.1061-1067
• /
• 2004

In this study, ferroelectric (Ba,Sr)TiO$_3$ and high temperature superconductor YBCO thin films were fabricated by PLD (Pulsed Laser Deposition) method and tuneable bandstop filters were implemented with two different IDC(Interdigital Capacitance) gap patterns, 20${\mu}{\textrm}{m}$ and 30${\mu}{\textrm}{m}$ using these two thin film layers. The resonant frequency was changed by DC bias voltage. By comparing measured results with simulation, the dielectric properties of ferroelectric thin film have been extracted. The permittivity was 820 ～ 900 at 30 K and had an acceptable error range but the loss tangent had a great difference, 0.018 in 30${\mu}{\textrm}{m}$ IDC gap pattern and 0.037 in 20 ${\mu}{\textrm}{m}$.

• Progress in Superconductivity and Cryogenics
• /
• v.16 no.1
• /
• pp.1-5
• /
• 2014

We calculated the resonance tunneling energy band in the BCS gap for Type-II superconductor in which periodic potential is generated by external magnetic flux. In this model, penetrating magnetic flux was assumed to be in a fixed lattice state which is not moving by an external force. We observed the existence of two subbands when we used the same parameters as for the $Nd_{1.85}Ce_{0.15}CuO_X$ thin film experiment. The voltages at which the regions of negative differential resistivity (NDR) started after the resonant tunneling ended were in a good agreement with the experimental data in the field region of 1 T - 2.2 T, but not in the high field regions. Discrepancy occurred in the high field region is considered to be caused by that the potential barrier could not be maintained because the current induced by resonant tunneling exceeds the superconducting critical current. In order to have better agreement in the low field region, more concrete designing of the potential rather than a simple square well used in the calculation might be needed. Based on this result, we can predict an occurrence of the electromagnetic radiation of as much difference of energy caused by the 2nd order resonant tunneling in which electrons transit from the 2nd band to the 1st band in the potential wells.

• Progress in Superconductivity
• /
• v.8 no.2
• /
• pp.143-151
• /
• 2007

Accurate measurements of the microwave surface resistance (Rs) of high temperature superconductor (HTS) films are important with regard to applications of HTS materials for wireless communications. As the surface resistance values of HTS films are usually extracted from the measured unloaded quality factor ($Q_0$) of resonators made of HTS films, it is essential to measure the resonator $Q_0$ with accuracy. The $TE_{011}\;mode\;Q_0$ of sapphire resonators with the endplates made of $YBa_2Cu_3O_{7-{\delta}}$(YBCO) film on $LaAlO_3$ is measured by using the S-parameter circle-fit method at a frequency of about 19.6 GHz and temperatures of 30 K to 90 K, which is compared with the measured values by using the Lorentzian-fit method. Good agreements are found between the two sets of $Q_0$ values measured by using the two different methods whether the resonator is used in a weak-coupling scheme or a strong-coupling scheme, showing reliability of both methods fur measuring the resonator $Q_0$ accurately. The $Q_0$ of sapphire resonators with a gap between the top plate and the rest of the resonator is also discussed.

• Progress in Superconductivity
• /
• v.3 no.1
• /
• pp.31-35
• /
• 2001

We investigated the temperature and magnetic field dependent optical properties of a$ MgB_2$ thin film in the far-infrared region. In the superconducting state, i.e. 5 K, we obtained the values of superconducting gap $2\Delta$ ~ 5.2 meV and $2\Delta$ $_{k}$ $B/T_{c}$ ~1.8. Although the value of$ 2\Delta$$B/T_{c}$ was nearly half of the BCS value, the $2\Delta$ seemed to follow the temperature dependence of the BCS formula. Under the magnetic field (H), the superconducting state became suppressed. Interestingly, we found that the normal state area fraction abruptly increased at low field but slowly increased at high field. It did not follow the H-dependences predicted for a s-wave superconductor (i.e. a linear dependence) nor for a s-wave one (i.e. $H^{1}$2/ dependence). We discussed the complex gap nature of $MgB_2$ in comparison with two gap and anisotropic s-wave scenarios.ios.