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Resonance tunneling phenomena by periodic potential in type-II superconductor
Lee, Yeong Seon,Kang, Byeongwon The Korea Institute of Applied Superconductivity a 2014 한국초전도저온공학회논문지 Vol.16 No.1
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.
Resonance tunneling phenomena by periodic potential in type-II superconductor
Yeong Seon Lee,Byeongwon Kang 한국초전도저온공학회 2014 초전도와 저온공학 Vol.16 No.1
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 Nd1.85Ce0.15CuOx 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.
송병헌,Oh Jun-Yung,Park Han-Seok,Kang Byeongwon 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.81 No.8
We fabricated epitaxial La1−xSrxMnO3 (LSMO) thin flms on a (001) SrTiO3 (STO) substrate via pulsed laser deposition by varying the Sr doping contents. We used several characterization techniques to investigate the structural, magnetic, and electronic transport properties of the epitaxial LSMO and evaluate its applicability as a bufer layer for GdBa2Cu3O7−δ (GdBCO) superconducting flms. The structural analysis indicated that the best lattice match with the STO substrate was obtained in LSMO with x=0.2, and the lattice constant decreased as the Sr content increased. We found that the magnetic properties of the LSMO were governed by the Mn ion proportion, and LSMO with x=0.3 exhibited the strongest ferromagnetism because it had the highest ratio of Mn3+. The electronic transport properties, including the metal–insulator transition temperature, depended signifcantly on the Sr content. A GdBCO flm on top of LSMO (x=0.3) exhibited the best superconducting properties, probably owing to strong ferromagnetism and good metallic behavior of LSMO as well as the optimal Cu–O bond length. The results of this study may serve as a reference for investigating the development of LSMO bufer layers for high-temperature superconductor cuprates.
Jun-Yung Oh,Dong-Seok Yang,Byeongwon Kang 한국초전도저온학회 2022 한국초전도저온공학회논문지 Vol.24 No.2
We studied the effect of substrate-induced strain state on the superconducting transition in GdBa2Cu3O7-x(GdBCO)/La0.7Sr0.3MnO3 (LSMO) bilayers deposited on a LaAlO3 (LAO) substrate. The stain state of LSMO is controlled byincreasing the thickness from 20 nm to 80 nm. Analyses on the extended X-ray absorption fine structure (EXAFS) measurementsreveal difference in the direction of MnO6 octahedral distortion depending on the LSMO thickness, which leads to a difference inanisotropy of magnetization of LSMO layer. The superconducting transitions of our system are strongly correlated with themagnetic anisotropy accompanied by the MnO6 octahedron distortion in a specific direction. This result suggests the possibility ofimproving the superconducting transition in the GdBCO/LSMO bilayer system by controlling the degree of competition betweensuperconductivity and ferromagnetism via adjusting strain state in the LSMO layer
Putri, Witha B. K.,Tran, Duc Hai,Kang, Byeongwon,Ranot, Mahipal,Jae Hak Lee,Nam Hoon Lee,Kang, Won Nam IEEE 2014 IEEE transactions on magnetics Vol.50 No.6
<P>We have fabricated the superconducting MgB<SUB>2</SUB> films grown on three different crystalline SiC-buffer layers of different thickness by means of the hybrid physical-chemical vapor deposition technique and pulsed laser deposition method. Significant changes in the microstructural and superconducting properties of MgB<SUB>2</SUB> films with addition of crystalline SiC-buffer layers were observed. The microstructural analyses of MgB<SUB>2</SUB> films revealed that columnar grains were formed perpendicularly to the substrates, thus, enhancing all critical current density values at 5 and 20 K for all SiC-buffered-MgB<SUB>2</SUB> films. The scaling behavior of the flux pinning force shows a magnetic field-dependent feature with different pinning mechanisms, from which one can infer that there are additional pinning centers that exist in the samples and cannot be interpreted by a simple superposition of different types of elementary pinning sources.</P>
Tran, Duc H.,Putri, Witha B. K.,Byeongwon Kang,Nam Hoon Lee,Won Nam Kang IEEE 2014 IEEE transactions on magnetics Vol.50 No.6
<P>The critical current density (J<SUB>c</SUB>) of pulsed-laser-deposited GdBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-δ</SUB> (GdBCO) superconducting films with film thickness (d) ranging from 0.2 to 1.5 μm was studied as functions of temperature (T) and held (H). A thickness dependence of both the self-held and the in-held J<SUB>c</SUB>s at 77 K showed that the J<SUB>c</SUB>s were observed to decrease with increasing film thickness as an expression of J<SUB>c</SUB> ~ d<SUP>-1/2</SUP>, which was predicted by the collective pinning theory of uncorrelated defects. However, the in-held J<SUB>c</SUB> at d ~0.6 μm was found to suddenly increase in the general trend of decreasing J<SUB>c</SUB>-d, which probably suggested a change in the pinning mechanism at that film thickness. The temperature dependence of the in-held J<SUB>c</SUB> in the GdBCO films was specifically studied at the low and the intermediate-held regions. A characteristic held H* separating the held-independent and the held-dependent J<SUB>c</SUB> regions was found to decrease with increasing both T and d. The value of α of the J<SUB>c</SUB> ~ H<SUP>α</SUP> dependence in the intermediate-held region was estimated to be α ~0.48-0.52 for both the thin- and the thick-film regions, which is similar to the theoretical value of α = 0.5 for pinning by sparse point-like defects in REBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-δ</SUB> (RE: rare-earth elements) superconducting films. The value of α ~ 0.35 estimated for the 0.6 μm thick GdBCO film might suggest that effective pinning induced via nanosized dislocations formed on the surface of the GdBCO film. Based on the application of the theoretical models, it could be concluded that the dominant pinning mechanism in the GdBCO films is pinning by sparse uncorrelated defects.</P>
Terahertz time domain spectroscopy of GdBCO superconducting thin films
박형렬,Gangseon Ji,Woongkyu Park,이형택,Chang-Yun Song,서충원,Minjo Park,Byeongwon Kang,Kyungwan Kim,김대식 한국초전도.저온공학회 2019 한국초전도저온공학회논문지 Vol.21 No.1
We present terahertz optical properties of GdBa2Cu3O7-x (GdBCO) superconducting thin films. GdBCO films with a thickness of about 105 nm were grown on a LaAlO3 (LAO) single crystal substrate using a conventional pulsed laser deposition (PLD) technique. Using an Ar ion milling system, the thickness of the GdBCO film was reduced to 58 nm, and its surface was also smoothened. Terahertz (THz) transmission spectra through two different GdBCO films are measured over the range between 0.2 and 1.5 THz using THz time domain spectroscopy. Interestingly, the THz transmission of the thinner GdBCO film has been increased to six times larger than that of the thicker one, while the thinner film is still maintaining its superconducting property at below 90 K.