Superconducting junctions of InAs semiconductor nanowires

도용주,Jorden A. van Dam,Erik P. A. M. Bakkers,Leo P. Kouwenhoven 한국초전도학회 2008 Progress in superconductivity Vol.9 No.2

InAs semiconductor nanowires can provide a promising plat ting quantum circuit, which exploits tunable supercurrent under the operation of gate voltage. We report temperature and magnetic field dependence of the nanowire superconducting junctions, which is in agreement with the proximity-effect theory of superconductor-normal metal-superconductor weak link. Superconducting coherence length of the InAs nanowire is estimated from the fit and magnetic-field dependence of the critical current and the subgap structure of dI/dV is discussed as well.

Coherent motion of fluxons in stacked intrinsic Josephson junctions of $Bi_2$$Sr_2$Ca$Cu_2$$O_{8+x}$ single crystals

도용주,장현식,장동인,이후종,김진희,김규태,이우,최진호,Doh, Yong-Joo,Chang, Hyun-Sik,Chang, Dong-In,Lee, Hu-Jong,Kim, Jinhee,Kim, Kyu-Tae,Lee, Woo,Choy, Jin-Ho The Korean Superconductivity Society 2001 Progress in superconductivity Vol.3 No.1

We studied the flux-flow current-voltage characteristics of microwave-generated fluxons formed in serially stacked intrinsic Josephson junctions fabricated on$ HgI_2$-intercalated $Bi_2$$Sr_2$$CaCu_2$O/8+x/(Bi2212) single crystals. With increasing the irradiation power of 73$\square$76 GHz microwave, the supercurrent branch became resistive and split into multiple sub -branches. Each sub-branch represented a specific mode of collective motion of Josephson fluxons. We also observed similar branch splitting In a mesa prepared on an underdoped Bi2212 single crystal in a static magnetic field.

Quantum Interference Eects in InAs Semiconductor Nanowires

도용주,Aarnoud L. Roest,Erik P. A. M. Bakkers,Silvano De Franceschi,Leo P. Kouwenhoven 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.54 No.1

We report quantum interference eects in InAs semiconductor nanowires strongly coupled to superconducting electrodes. In the normal state, universal conductance uctuations are investigated as a function of the magnetic eld, the temperature, the bias and the gate voltage. The results are found to be in good agreement with theoretical predictions for weakly disordered one-dimensional conductors. In the superconducting state, the uctuation amplitude is enhanced by a factor up to ~1.6, which is attributed to a doubling of the charge transport via Andreev re ection. At a temperature of 4.2 K, well above the Thouless temperature, conductance uctuations are almost entirely suppressed and the nanowire conductance exhibits anomalous quantization in steps of e^2/h. We report quantum interference eects in InAs semiconductor nanowires strongly coupled to superconducting electrodes. In the normal state, universal conductance uctuations are investigated as a function of the magnetic eld, the temperature, the bias and the gate voltage. The results are found to be in good agreement with theoretical predictions for weakly disordered one-dimensional conductors. In the superconducting state, the uctuation amplitude is enhanced by a factor up to ~1.6, which is attributed to a doubling of the charge transport via Andreev re ection. At a temperature of 4.2 K, well above the Thouless temperature, conductance uctuations are almost entirely suppressed and the nanowire conductance exhibits anomalous quantization in steps of e^2/h.

나노하이브리드 조셉슨 소자 연구

도용주 한국초전도저온학회 (구 한국초전도저온공학회) 2013 초전도와 저온공학 Vol.15 No.2

$SmFeAsO_{0.85}$ 단결정의 c-축 전도 특성

박재현,도용주,이현숙,조병기,이후종,Park, Jae-Hyun,Doh, Yong-Joo,Lee, Hyun-Sook,Cho, B.K.,Lee, Hu-Jong 한국초전도학회 2010 Progress in superconductivity Vol.11 No.2

Electrical transport properties were measured on $SmFeAsO_{0.85}$ single crystals along the c-axis for various temperatures and magnetic fields. For the measurements a mesa structure was fabricated on the surface of the single crystals. Samples showed a metallic temperature dependence of resistance and current-voltage curves without hysteretic multiple branch splitting that is usually observed in tunneling Josephson junctions. In addition, in ab-planar magnetic fields, samples did not show the Fraunhofer-type field modulation of the critical current. All these features indicate that the c-axis transport characteristics of $SmFeAsO_{0.85}$ single crystals are explained by the anisotropic bulk superconductivity rather than Josephson tunneling.

Gate-tunable Supercurrent in Graphene-based Josephson Junction

정동찬,이길호,도용주,이후종,Jeong, D.,Lee, G.H.,Doh, Y.J.,Lee, H.J. The Korean Superconductivity Society 2011 Progress in superconductivity Vol.13 No.1

Mono-atomic-layer graphene is an interesting system for studying the relativistic carrier transport arising from a linear energy-momentum dispersion relation. An easy control of the carrier density in graphene by applying an external gate field makes the system even more useful. In this study, we measured the Josephson current in a device consisting of mono-layer graphene sheet sandwiched between two closely spaced (~300 nm) aluminum superconducting electrodes. Gate dependence of the supercurrent in graphene Josephson junction follows the gate dependence of the normal-state conductance. The gate-tunable and relatively large supercurrent in a graphene Josephson junction would facilitate our understanding on the weak-link behavior in a superconducting-normal metal-superconducting (SNS) type Josephson junction.

Above-gap Conductance Anomaly Studied in Superconductor-Graphene-Superconductor Josephson Junctions

Jae-Hyun Choi,이후종,도용주 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.1

We investigated the electrical transport properties of superconductor-graphene-superconductor (SGS) Josephson junctions. At low voltage bias, we observed the conventional proximity-coupled Josephson effect, such as supercurrent flow through graphene, a sub-gap structure of differential conductance due to Andreev reflection, and a periodic modulation of the critical current Ic when a perpendicular magnetic field H is applied to the graphene. For high bias above the superconducting gap voltage, however, we observed an anomalous jump of the differential conductance, the voltage position of which is sensitive to the backgate voltage Vg. Our extensive study with varying Vg, temperature, and H reveals that the above-gap structure takes place at a characteristic power P*, irrespective of Vg, for a given junction. The temperature and the H dependences of P are well explained by an increase in the electron temperature in graphene.

(Bi0.84Sb0.16)2Se3 위상절연체 나노선의 양자 전자 수송

임흥순,김홍석,김남희,도용주 한국물리학회 2018 새물리 Vol.68 No.10

위상 절연체와 저온 초전도체가 서로 접합을 이룰 때, 접합 계면에서 마요라나 페르미온과 같은 특이 준입자 상태를 형성한다는 것이 이론적으로 알려져 있다. 우리는 (Bi$_{0.84}$Sb$_{0.16}$)$_2$Se$_3$ 위상절연체 나노선 위에 알루미늄 (Al) 초전도 전극을 형성함으로써 위상 절연체의 위상 표면 상태에 의한 양자 전도 특성을 확인하고 Al 전극에 의한 초전도 근접 효과를 측정하기 위한 실험을 수행하였다. 위상 절연체 나노선 소자 표면에 수직한 방향으로 자기장을 인가하였을 때, 위상 절연체 나노선 소자 표면에 수직한 방향으로 자기장을 인가하였을 때, 자기 저항이 감소하는 것을 관측하였는데 이 결과는 위상 표면 상태에 의한 반약국재 현상으로 이해되며 위상절연체의 표면에서 발생한 것임을 실험적으로 뒷받침한다. 위상 절연체 나노선 소자를 Al 전극의 초전도 임계 온도 이하로 냉각시킬 경우, 위상 절연체와 초전도 전극 사이 계면에서 발생하는 안드레이 반사에 의한 전기 전도도 증가 현상을 관측할 수 있었는데, 이는 초전도 근접 효과가 잘 이루어 지고 있음을 보여준다. 이러한 실험 결과는 위상 양자 소자 연구에 필수적인 위상 절연체-초전도체 접합을 제작하는데 유용하게 사용될 것으로 생각된다. A topological insulator (TI) in contact with a conventional superconductor can provide a useful platform for creating exotic quasi-particles such as Majorana fermions. Here, we report the quantum electronic transport properties of (Bi$_{0.84}$Sb$_{0.16}$)$_2$Se$_3$ TI nanowire in contact with superconducting electrodes. When a magnetic field is applied perpendicular to the substrate, a negative magneto-conductance behavior is observed as a result of a weak antilocalization (WAL) effect in the topological surface states. In the superconducting state of the Al electrodes on top of the TI nanowire, a conductance enhancement is observed due to Andreev reflection at the interface between the TI nanowire and the superconducting electrode. Our experimental results should be useful in developing TI-based superconducting junctions, which are essential ingredients for topological quantum computing technology.

Electrical detection of spin-polarized current in topological insulator Bi1.5Sb0.5Te1.7Se1.3

황태하,김홍석,김호일,김준성,도용주 한국물리학회 2019 Current Applied Physics Vol.19 No.8

Spin-momentum locked (SML) topological surface state (TSS) provides exotic properties for spintronics applications. The spin-polarized current, which emerges owing to the SML, can be directly detected by performing spin potentiometric measurement. We observed spin-polarized current using a bulk insulating topological insulator (TI), Bi1.5Sb0.5Te1.7Se1.3, and Co as the ferromagnetic spin probe. The spin voltage was probed with varying the bias current, temperature, and gate voltage. Moreover, we observed non-local spin-polarized current, which is regarded as a distinguishing property of TIs. The spin-polarization ratio of the non-local current was larger than that of the local current. These findings could reveal a more accurate approach to determine spinpolarization ratio at the TSS.

Characterizing Pb-based superconducting thin films

박상일,김홍석,이준성,도용주 한국초전도.저온공학회 2014 한국초전도저온공학회논문지 Vol.16 No.4

We report on the superconducting and structural characteristics of Pb-based alloy (Pb0.9In0.1, Pb0.8In0.2 and Pb0.85Bi0.15) thin films,depending on the film deposition rate. The maximum critical magnetic field strength of Pb0.85Bi0.15 is almost six times larger thanthat of Pb0.9In0.1, and more rapid growth of the film enhances the critical magnetic field strength even for the same alloy material. Scanning electron microscopy inspection indicates that lower deposition rate condition is vulnerable to the formation of voidstructure in the film. Topographic images using atomic force microscopy are useful to optimize the deposition condition for thegrowth of smooth superconducting film. Our work can be utilized for future studies on hybrid superconducting devices usinglow-dimensional nanostructures.