제주대학교 Repository

A Study of high-Tc Superconductor Submicron Josephson Junction Devices

Metadata Downloads
Abstract
The non-linear current-voltage characteristics of Josephson effect phenomenon is always being focused by scientists and engineers. Using Josephson junction phenomenon with high-T c superconductors gives a wide scope of applications. Depending on in-plane size, the application of Josephson junctions varied from the terahertz (THz) oscillator to the single electron transistors. This thesis is focused on the study of submicron Josephson junctions. The unit cell of layered high-T c superconductors show intrinsic Josephson junctions (IJJs) phenomenon. The Cooper pair tunneling in the IJJs can be affected by quantum phase fluctuations when the resistance of a submicron stack is in the range of quantum resistance. To observe this quantum effect, we have fabricated stacks of IJJs of various in-plane area from 4 µm2 down to 0.16 µm2 in Bi2Sr2CaCu2O8+δ single crystal whiskers using three-dimensional focused ion beam (3-D FIB) etching technique. For stacks of in-plane area less than 1 µm2 a strong suppression in critical current density (Jc) was noticed in current-voltage characteristics at 30 K. The possible mechanisms for this suppression of Jc are discussed and the data analysis points to the quantum phase fluctuations as the most likely mechanism of this effect. We have achieved quantum fluctuations at 30 K first time ever reported for Bi2Sr2CaCu2O8+δ (Bi-2212) single crystal whisker.
A-axis oriented thin films of YBa2Cu3O7 are potentially superior to c-axis films for sandwich-type junction applications because of the larger coherence length in a-axis direction. Thus growth of thin epitaxial insulators or normal barriers on a-axis films, followed by another a-axis superconductor, is an important goal. Considering this phenomenon, we have studied a submicron stack of YBa2Cu3O7 and PrBa2Cu3O7 (Y123/Pr123) multi layered thin film Josephson junction. The dimensions of submicron stack is about 300 nm × 200 nm with the height of about 200 nm. The submicron stack shows the transition temperature about 84 K and critical current of about 0.12 mA at 30 K. We noticed the suppression in critical current as the effect of external microwave at different power. As we increase the power, the superconducting state was suppressed and resulted in the suppression of the critical current. However, we have not observed any voltage steps in current-voltage characteristics with external microwave irradiation.
Further we have fabricated a superconducting quantum interface device SQUID using focused ion beam etching technique. The microwave induced voltage steps are observed in I-V characteristics. The super current branch become resistive above a certain microwave power and the value of J c was suppressed as we increased the microwave power. The power dependence of voltage steps shows the number of Josephson junctions contributing to the vortices-flow varies with the power of microwave. The formation of superconducting-semiconducting-superconducting -like Josephson junction is confirmed by Ambegaokar - Baratoff theory.
조셉슨 효과의 비선형 전류-전압 특성은 지금까지 많은 과학자와 엔지니어들에게 연구 및 실험의 대상이 되어오고 있다. 고온 초전도체를 이용하여 제작한 조셉슨접합에 의한 양자현상을 이용하는 것은 매우 다양하고 폭 넓은 응용분야를 가지고 있다. 면내 (in-plane)크기에 따른 특성에 의존하는 조셉슨 접합은 THz발진기에서부터 단전자트랜지스터에 이르기까지 응용분야가 매우 다양하다.
본 연구에서는 서브마이크론크기의 자연적으로 형성된 고온초전도체의 적층구조를 가공한 고유조셉슨접합(intrinsic Josephson junction)을 제작하고 그 기초물성 및 응용에 관한 연구를 수행하였다. 적층구조 고온초전도체의 단위층간에 수직방향으로 전류가 인가되면, 직렬로 연결된 층의 수에 비례한 전류-전압특성을 나타내는데, 이러한 접합을 고유조셉슨접합 혹은 고유조셉슨현상이라고 부른다.
고유조셉슨접합내의 쿠퍼쌍 턴넬은 적층구조접합의 저항이 양자저항보다도 크게 되면 양자페이즈(quantum phase)의 요동에 의한 영향을 받게 된다. 양자현상의 영향을 관찰하기 위하여 Bi2Sr2CaCu2O8+δ (Bi-2212) 수염상단결정에 입체적인(3-D) 집속이온빔가공을 이용하여 4 μm2 에서 0.16 μm2 에 이르는 다양한 면내크기의 고유조셉슨접합을 제작하였다. 적층구조의 면내크기가 1μm2 보다 작을 때, 30 K의 전류-전압특성에서 임계전류밀도 (Jc)가 크게 줄어드는 것이 발견되었다. 본 논문에서는 Jc 의 줄어듬 (suppression)에 대한 여러 가지 가능한 메커니즘에 대하여 논의하였고, 양자페이즈요동 (fluctuations)의 가능성의 관점에서 분석되었다.
본 논문에서는 Bi-2212 수염상단결정의 30 K에서의 양자요동에 관하여 처음으로 보고 하였다. a-축배향 YBa2Cu3O7 박막은 샌드위치접합의 응용에 주로 사용되는 c-축 박막보다 코히어런스길이가 길기 때문에 소자응용에 있어서 보다 많은 가능성을 가지고 있음으로, a-축배향 초전도박막에 교차적으로 성장되는 a-축배향 에피탁셜 절연박막 혹은 일반금속 장벽층을 성장하는 것은 중요한 연구 목표중의 하나이다. 이러한 관점에서, 본 연구에서는 YBa2Cu3O7 와 PrBa2Cu3O7 (Y123/Pr123)가 교차적으로 구성되는 다층의 서브마이크론 적층구조를 제작하고 연구를 수행하였다. 제작한 적층구조의 크기는 약 300 nm × 200 nm 이고 높이는 약 200 nm 이며, 임계온도는 84 K이고, 30 K에서 0.12 mA의 임계전류를 나타내었다. 이 제작된 접합에서는 외부의 여러 가지 크기의 고주파파워에 따른 임계전류의 억압을 관찰하였다.
고주파 출력을 증가시킴에 따라 초전도 상태는 억제되고, 임계전류가 줄어드는 것을 발견하였지만, 전류-전압 특성에서 외부 고주파조사에 따른 교류조셉슨효과에 의한 샤피로스텝은 관찰 할 수 없었다. 또한. 집속이온빔 가공 기술을 이용하여 두 개의 접합과 하나의 홀로 구성되는 초전도 양자 간섭계(SQUID)형상의 소자를 제작하였으며, 고주파유기 전압스텝을 I-V특성상에서 관찰하였다.
초전도전류는 마이크로출력의 증가에 따라 저항성을 가지게 되고, Jc 는 마이크로출력의 증가시킴에 따라 억제되었다. 관찰된 전압스텝의 파워의존성은 고주파파워에 따른 많은 수의 조셉슨접합이 볼텍스플로 (vortices-flow)현상에 기여함을 보여준다. 제작된 접합의 초전도체-반도체-초전도체 (S-S'-S)와 같이 거동하는 조셉슨접합전류의존성은 Ambegaokar-Baratoff이론에 근거하여 근사함을 확인하였다.
Author(s)
Shrikant Saini
Issued Date
2011
Awarded Date
2011. 7
Type
Dissertation
URI
http://dcoll.jejunu.ac.kr/jsp/common/DcLoOrgPer.jsp?sItemId=000000005466
Alternative Author(s)
사이니 스리칸트
Affiliation
제주대학교
Department
대학원 기계공학과
Table Of Contents
1. Introduction ......................................................................................................................... 2
1.1 Introduction ....................................................................................................................... 2
1.2 Josephson Junction ............................................................................................................ 2
1.2.1 The Josephson effect ...................................................................................................... 2
1.3 The Quantum Regime ....................................................................................................... 6
1.3.1 The Coulomb charging energy ...................................................................................... 6
1.3.2 Tunneling resistance ...................................................................................................... 6
1.3.3 The Josephson energy .................................................................................................... 6
1.4 The RCSJ Model ............................................................................................................... 7
1.5 Intrinsic Josephson junction ……………………………………………………………. 9
1.6 SSeS type Josephson junction .......................................................................................... 9
1.7 Applications...................................................................................................................... 11
1.7.1 The Coulomb blockade ................................................................................................ 12
1.7.2 Tera hertz applications ................................................................................................. 14
1.8 Conclusions ..................................................................................................................... 15
2. Growth of Bi-2212 single crystal whiskers ....................................................................... 19
2.1 Introduction ..................................................................................................................... 19
2.2 The growth process ......................................................................................................... 20
2.3 Summary ......................................................................................................................... 22
3. Fabrication Technique ...................................................................................................... 25
3.1 Introduction ..................................................................................................................... 25
3.2 TheFocusedIonBeam ....................................................................................................... 25
3.2.1 Operation of Focused Ion Beam ................................................................................... 28
3.3 Fabrication of Josephson junctions ................................................................................. 30
3.4 Summary ......................................................................................................................... 30
4. Intrinsic Josephson junctions Stack of Bi-2212 ................................................................ 32
4.1 Introduction ..................................................................................................................... 32
4.2 Experimental details ........................................................................................................ 35
4.3 Results and discussion ..................................................................................................... 37
4.4 Conclusions ..................................................................................................................... 41
5. Quantum fluctuation induced suppression of critical current density in Submicron IJJs ......................................................................................................................................... 43
5.1 Introduction ..................................................................................................................... 43
5.2 Experimental details ........................................................................................................ 44
5.3 Results and discussion ..................................................................................................... 45
5.4 Conclusions ..................................................................................................................... 50
6. Characteristics of submicron Josephson junctions of Y123/Pr123 multi layered thin films ....................................................................................................................................... 53
6.1 Introduction ..................................................................................................................... 54
6.2 Experimental details ........................................................................................................ 56
6.2.1 Growth of a-axis oriented multi layered thin films …………………….…..……... 56
6.2.2 Fabrication of submicron stack of a-axis oriented multi layered thin films........................................................................................................................................ 56
6.3 Results and discussion ..................................................................................................... 59
6.4 Conclusion........................................................................................................................ 68
7. Microwave irradiation on a-axis oriented Y123/Pr123 nano-SQUID ............................... 70
7.1 Introduction ..................................................................................................................... 70
7.2 Experimental details ........................................................................................................ 71
7.2.1 Growth of multi layered thin films of a-axis oriented Y123/Pr123 ………..………71
7.2.2 Fabrication of SQUID structure in a-axis oriented Y123/Pr123 ………….………..74
7.3 Results and discussions ................................................................................................... 74
7.4 Conclusions ..................................................................................................................... 79
8. Summary ........................................................................................................................... 82
Degree
Doctor
Publisher
제주대학교 대학원
Citation
Shrikant Saini. (2011). A Study of high-Tc Superconductor Submicron Josephson Junction Devices
Appears in Collections:
Faculty of Applied Energy System > Mechanical Enginering
공개 및 라이선스
  • 공개 구분공개
파일 목록

Items in Repository are protected by copyright, with all rights reserved, unless otherwise indicated.