• 한국초전도학회:학술대회논문집
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• 한국초전도학회 2009년도 Korea Superconductivity Society Meeting 2009
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• pp.13-13
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• 2009

• Progress in Superconductivity
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• 제3권1호
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• pp.31-35
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• 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.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2009년도 제38회 동계학술대회 초록집
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• pp.390-390
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• 2010

Superconducting proximity effects of Nb/Si(111) were investigated with scanning tunneling microscopy(STM) and scanning tunneling spectroscopy(STS). A highly-doped($0.002\;{\omega}{\diamondsuit}cm$) Si wafer pieces were used as substrate and Nb source was thermally evaporated onto the atomically clean silicon substrate. The temperature of the silicon sample was held at $600^{\circ}C$ during the niobium deposition. And the sample was annealed at $600^{\circ}C$ for 30 minutes additionally. Volmer-Weber growth mode is preferred in Nb/Si(111) at the sample temperature of $600^{\circ}C$. With proper temperature and annealing time, we can obtain Nb islands of lateral size larger than Nb coherence length(~38nm). And outside of the islands, bare Si($7{\times}7$) reconstructed surface is exposed due to the Volmer-Weber Growth mode. STS measurement at 5.6K showed that Nb island have BCS-like superconducting gap of about 2mV around the Fermi level and the critical temperature is calculated to be as low as 6.1K, which is lower than that of bulk niobium, 9.5K. This reduced value of superconducting energy gap indicates suppression of superconductivity in nanostructures. Moreover, the superconducting state is extended out of the Nb island, over to bare Si surface, due to the superconducting proximity effect. Spatially-resolved scanning tunneling spectroscopy(SR-STS) data taken over the inside and outside of the niobium island shows gradually reduced superconducting gap.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1998년도 하계학술대회 논문집 A
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• pp.91-93
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• 1998

In this paper 30 kVA superconducting generator's transient-state electromagnetic analysis by FEM is described. The transient-state analysis by moving air gap technique was performed to analyze its 3 phase sudden short circuit characteristics. External circuit components were connected to generator model with end-winding resistance and inductance.

• 대한전기학회논문지:전기기기및에너지변환시스템부문B
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• 제48권4호
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• pp.167-173
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• 1999

The use of high-Tc superconducting synchronous motor in power machinery has many advantages such as reduced power dissipation, size and weight. This paper presents the ac loss simulation in the rotor having an high-Tc superconducting field winding using Ag sheathed Bi-2223. The analysis was conducted with an equivalent model of the high-Tc superconducting motor with flux damper under transition condition during which the load varies from 0 watt to 250watts and from 250watts to 500watts. The simulation results show that the transient state lasts for about 3 seconds, and the ac losses decreased exponentially from the initial value above 20mW.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1999년도 하계학술대회 논문집 A
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• pp.232-234
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• 1999

A 30kVA superconducting generator (S.G.) has been designed, developed and successfully tested in KERI before the end of last year. The design is based on 2-dimensional electromagnetic field analysis of cylindrical coordinate. The field winding of rotor has been wound with superconducting wire of Nb-Ti alloy and tested after assembled with other rotor components. The stator has air-gap type armature windings which allow higher terminal voltage and more sinusoidal voltage waveform than conventional iron cored machines. Steady-state open & short-circuit test and lamp load test have been conducted also. The results of tests are given in this paper and compared with design quantities. Moreover the cooling scheme and characteristics of test system is included.

• Journal of Astronomy and Space Sciences
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• 제21권4호
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• pp.481-492
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• 2004

Nb/Al Superconducting Tunnel Junction(STJ) 소자를 제작하여 I-V 특성곡선을 측정하고 제작된 STJ 소자의 초전도체 특성 및 성능 파라미터 값들을 구하였다. 크기가 각각 20, 40, 60, 그리고 $80{\mu}m$인 4종류의 STJ소자를 제작하였으며 각 소자는 총 5층의 Nb/A1/AlOx/Al/Nb 다결정(polycrystalline) 박막으로 구성된 SIS(Superconductor Insulate. Superconductor) 방식의 조셉슨 접합 구조를 갖는다. 이 연구에서 제작한 STJ 소자는 $Tanner^{TM}$ L-Edit 8.3 프로그램으로 설계하였으며 한국표준과학연구원의 SQUID 제조실험실에서 제작하였다. 5층의 STJ 박막은 DC magnetron sputtering, reactive ion etching, CVD(Chemical Vapor Deposition) 장비를 이용해 생성되었다. 제작된 STJ 소자는 액체헬륨으로 냉각(4K)시킨 후 I-V 특성곡선을 측정하여 초전도 특성을 확인하였고, STJ 소자의 성능을 결정하는 파라미터인 energy gap, normal resistance, normal resistivity, dynamic resistance, dynamic resistivity, 그리고 quality factor를 계산하였다. Nb/Al STJ 소자의 FWHM 에너지 분해능 계산 결과, 순수 Nb STJ 소자보다 $11\%$ 우수한 에너지 분해능 특성을 확인하였다.

• Progress in Superconductivity
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• 제7권1호
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• pp.1-5
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• 2005

A pseudogap in the normal-state quasiparticle density of states of $high-T_c$ superconductors has been revealed in many different kinds of experiments. The existence of the pseudogap and the superconducting gap, and the correlation between them has attracted considerable attention because they are believed to be a key to understanding the mechanism of the $high-T_c$ superconductivity. The interlayer tunneling spectroscopy, excluding the surface-dependent effect, is one of the most accurate means to examine the electron spectral characteristics both in the superconducting and the normal states. In this study, a new constant-temperature intrinsic tunneling spectroscopic technique, excluding the overheating effect using the in-situ temperature monitoring combined with the digital proportional-integral-derivative control, is introduced. The implication on the $high-T_c$ superconductivity of the detailed temperature dependencies of the observed spectral weight in $Bi_{2}Sr_{2}CaCu_{2}O_{8+x}\;high-T_c$ material for overdoped and underdoped levels is discussed.

• Progress in Superconductivity
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• 제3권2호
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• pp.140-145
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• 2002

An enhancement of the c-axis differential conductance around the zero-bias voltage near the superconducting transition temperature $T_{c}$ has been observed in $Au/Bi_2$$Sr_2$$CaCu_2$$O_{8+x}$ junctions. We attribute such an enhancement to the Andreev reflection (AR) between the surface Cu-O bilayer with suppressed superconductivity and the next superconducting Cu-O bilayer. The continuous evolution of the differential conductance, from gap like depression to an AR-like peak structure, around the zero-bias voltage points to weakening of the barrier strength of the nonsuperconducting layer between adjacent Cu-O bilayers as temperature approaches $T_{c}$ from below. The peak structure disappeared just below the bulk $T_{c}$ value of underdoped Bi2212 single crystals, whereas it survived up to ~1 K above $T_{c}$ in Junctions prepared on slightly overdosed crystals. According to a recently proposed theoretical consideration, a wider temperature range of the AR a bone $T_{c}$ is expected in the underdoped regime when phase-incoherent preformed pairs emerge in the pseudogap state. Our result is in contradiction to the preformed pair scenario. scenario.o.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2004년도 하계학술대회 논문집 B
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• pp.1118-1120
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• 2004

For the first time in Korea, a 100hp high Tc superconducting(HTS) motor has been designed, developed and successfully tested recently. The design was based on 2-dimensional electromagnetic field analysis of cylindrical coordinate. The field winding of rotor was wound with Bi-2223 tapes and tested after assembled with other rotor components. The stator had air-gap type armature windings which allow higher terminal voltage and more sinusoidal voltage waveform than conventional iron cored machines. Steady state open & short-circuit test and resistive load test were conducted also in generator mode. The fabrication and test results of this machine were discussed in this paper.