Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor.advisor | 서정필 | - |
| dc.contributor.author | Song, Sang Yong | - |
| dc.date.accessioned | 2020-08-06T06:17:05Z | - |
| dc.date.available | 2020-08-06T06:17:05Z | - |
| dc.date.issued | 2020 | - |
| dc.identifier.uri | http://dgist.dcollection.net/common/orgView/200000332743 | en_US |
| dc.identifier.uri | http://hdl.handle.net/20.500.11750/12182 | - |
| dc.description | 주사 터널링 현미경, Yu-Shiba-Rusinov 상태, 1차원 위상 초전도체, 철 기반 초전도체, 자기적 특성 | - |
| dc.description.abstract | In this paper, we studied Yu-Shiba-Rusinov state in quantum confiment system and the local mag-netism of FeSe thin film using scanning tunneling microscipy (STM). Magnetic impurity on conventional BCS superconductor leads impurity bound state which is called Yu-Shiba-Rusinov state. YSR states can hybridize with nearest atoms in the magnetic atomic chain, which form the YSR band. In the special condition, YSR band leads topological quantum phase transition, resulting in the Majorana zero mode (MZM) at the end of the chain. In this paper, we artifi-cially changed the density of state (DOS) at Fermi level (EF) by using the Ar-induced nano-cavities (AIC) in Pb(111) substrate. We confirmed that the YSR energy is influenced by DOS at EF, and it is re-stricted by the strength and sign of local impurity potential. In addition, we found that the ratio between YSR energy and Kondo energy is anomalously changed on AICs. And, we succeed to grow the Fe atomic chains on Pb(111) substrate. This system provides the platform to study the relationship between YSR band and DOS at EF. In addition, we successfully grew FeSe film on Pb(111) which is S-wave superconductor. We demonstrated that the FeSe film possesses the S-wave superconductivity due to the proximity effect. The fact that S-wave Cooper pairs only respond to the magnetic impurity could realize the detection of local magnetism on FeSe film. Furthermore, we used the superconducting tip that improved considerable energy and spatial resolution. As a result, we clearly detected the local magnetism near the boundaries, intrinsic defects, and even near the non-magnetic impurities. The discovery of the induced magnetism near nonmagnetic ad-atoms brings significantly perceptual changes to the results of many experiments and analyzes that have been performed to confirm the pairing symmetry of FeSe superconductors. In addition, the induced magnetism indicates response to impurities near the magnetic quantum critical point. Morover, we found that the distribution of magnetism around the nonmagnetic atom has C2 sym-metry. This indicates that the FeSe film has a collinear anti-ferromagnetic order, according to the theo-retical description. |
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| dc.description.statementofresponsibility | N | - |
| dc.description.tableofcontents | List of Contents Abstract i List of contents ii List of figures vi 1. Introduction 1 2. Basic Theory 3 2.1 Tunneling effect 3 2.1.1 Tunneling probability 3 2.1.2 Normal-insulator-normal (NIN) junction 5 2.2 BCS superconductivity 7 2.2.1 BCS theory 7 2.2.2 Superconducting order parameter 9 2.2.3 Critical temperature 10 2.2.4 Density of states 11 2.2.5 Superconductor-insulator-normal (SIN) junction 12 2.2.6 Superconductor-insulator-superconductor (SIS) junction 14 2.3 Kondo effect 15 2.3.1 Kondo model 15 2.3.2 Anderson model 16 2.3.3 Fano function 18 2.3.4 Frota function 19 2.4 Yu-Shiba-Rusinov state 20 2.4.1 Classical spin model 20 2.4.2 Anderson impurity model 23 2.5 Spin excitations 25 2.5.1 Spin excitations in tunneling process 25 3. Experimental methods 28 3.1 Scanning tunneling microscopy 28 3.1.1 Home built STM 28 3.1.2 STM head 29 3.1.3 Preparation chamber 30 3.1.4 Superconducting tip 31 4. Tuning of density of states at EF on Pb(111) 32 4.1 Introduction 32 4.2 Quantum well state on Pb(111) 33 4.2.1 Introduction 33 4.2.2 Quantum confinement effect and AICs 34 4.2.3 Variation of superconductivity near SBs 36 4.2.4 Phonon peaks near SBs 37 4.2.5 Superconductivity versus the DOS at EF 39 4.2.6 Superconductivity versus phonon peaks 41 4.2.7 Superconductivity versus the size of SBs 41 4.2.8 Discussion 42 4.3 PbSe layers on Pb(111) 48 4.3.1 Introduction 48 4.3.2 PbSe layers on Pb(111) 49 4.3.3 Co impurities on PbSe layers/Pb(111) 52 4.3.4 MnPc molecules on PbSe layers/Pb(111) 55 4.3.5 Discussion 58 5. Tuning of Yu-Shiba-Rusinov state using density of states at EF 62 5.1 Introduction 62 5.2 Mn-phthalocyanine island on Pb(111) 63 5.2.1 Introduction 63 5.2.2 MnPc on Pb(111) 63 5.2.3 YSR state versus DOS at EF 67 5.2.4 Kondo effect versus the DOS at EF 69 5.2.5 discussion 69 5.3 Co on 1ML PbSe/Pb(111) 71 5.3.1 Introduction 71 5.3.2 Co on 1ML PbSe/Pb(111) 71 5.3.3 YSR wave function versus DOS at EF 72 5.3.4 Discussion 74 5.4 Fe atomic chains on Pb(111) 80 5.4.1 Introduction 80 5.4.2 AICs in Pb(110) and Pb(111) 83 5.4.3 Fe dimer on Pb(111) and 1MLPbSe/Pb(111) 84 5.4.4 Fe atomic wire on Pb(111) 85 5.5 Conclusion 88 6. Local magnetism of FeSe film 91 6.1 Introduction 91 6.2 Argument for pairing symmetry 94 6.2.1 introduction 94 6.2.2 Properties of 3UC FeSe in Pb(111) 95 6.2.3 Fe vacancy, Se vacancy and defect 98 6.2.4 magnetic and non-magnetic adatoms 100 6.2.5 1UC FeSe on PbSe/Pb(111) 103 6.2.6 Boundaries 105 6.3 Magnetic order of 3UC FeSe in Pb(111) 108 6.3.1 Local magnetic order around Ag atom 108 6.3.2 Local magnetic order around Fe vacancy 114 6.4 Conclusion 116 Reference 118 Summary in Korean 135 |
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| dc.format.extent | 145 | - |
| dc.language | eng | - |
| dc.publisher | DGIST | - |
| dc.title | Yu-Shiba-Rusinov 상태를 이용한 초전도성 물질의 자기적 바닥상태에 대한 연구 | - |
| dc.title.alternative | Study of magnetic ground states of superconducting materials using Yu-Shiba-Rusinov state | - |
| dc.type | Thesis | - |
| dc.identifier.doi | 10.22677/thesis.200000332743 | - |
| dc.description.alternativeAbstract | 본 논문에서는 주사 터널링 현미경(scanning tunneling microscopy, STM)을 이용하여 1차원 위상초전도체(1D topological superconductor)와 철 기반 초전도체중의 하나인 FeSe박막의 성질을 연구했다. 전통적인 초전도체(conventional superoconductor)인 Bardeen-Cooper-Schrieffer(BCS) 초전도체 위에 자성을 띈 원자가 흡착되면, 특정한 조건에서 Cooper 쌍이 반응하여 얽매인 상태(bound state)인 Yu-Shiba-Rusibov (YSR) 상태를 형성한다. 이러한 자성을 띈 입자가 일렬로 결합하게 되면 YSR 상태가 혼성화(hybridization)되어서 YSR 대역(YSR band)을 이루게 되고, 특정한 조건에서 1차원 위상초전도체로 상전이를 일으켜, 양 끝에 마요라나 제로 모드(Majorana zero mode, MZM)를 형성한다. 본 실험에서는 Pb(111)시료 내부에 인위적으로 아르곤(Ar) 거품을 형성시켜서 만든 양자 우물(quantum well state)을 이용하여, 단 원자 또는 단 분자의 YSR 에너지가 패르미 준위의 전자 밀도에 반응하는 것을 확인했으며, 불순물의 국소 쿨롱 전위차(Coulomb potential)의 세기와 부호에 상당한 영향을 받고 있음을 확인했다. 또한, 양자 우물이 형성된 지역에서 콘도 에너지와 YSR상태 에너지의 비율이 비정상적으로 변했음을 인지했다. 이 현상은 YSR상태의 상전이 조건이 변화하였음을 시사한다. 그리고 Pb(111) 표면 위에 철 원자 선(iron atomic wire)의 성장에 성공하였으며, 원자 선 연구에 대한 기틀을 준비했다. 또한 우리는 BCS초전도체인 Pb(111) 기판에 FeSe 박막을 성장시켰으며, 초전도 기판의 근접 효과(Proximity effect)에 의해서 FeSe 박막이 BCS초전도화가 되었음을 확인했다. 근접 효과에 의해 유도된 BCS초전도체의 Cooper쌍이 자성에만 반응한다는 사실이 FeSe 박막의 자성을 측정할 수 있게 했다. 이에 더하여, 초전도 팁을 사용하여 공간과 에너지 분해능을 상당한 수준으로 향상시켰다. FeSe 박막의 결함과 경계 면에서 자성이 검출되었고, 심지어는 인위적으로 흡착시킨 비자성 불순물의 주변에서 자성이 유도되고 있음을 확인했다. 유도된 자성의 발견은, FeSe초전도체의 전자쌍 대칭성(pairing symmetry)을 확인하기위해 수행되어왔던 많은 실험과 분석결과에 상당한 인식의 변화를 가져다준다. 그리고 비자성 원자 주변에서 유도되는 자성은FeSe 박막이 자기적 불안정성(magnetic imstability)을 가지고 있음을 말한다. 또한 그 분포가 C2 대칭성을 가지고 있음을 확인하였으며, 이를 통해 FeSe박막이 동일 선형 반강자성 순서(collinear anti-ferromagnetic order)를 띄고 있음을 밝혔다. |
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| dc.description.degree | Doctor | - |
| dc.contributor.department | Department of Emerging Materials Science | - |
| dc.contributor.coadvisor | Kim, Tae-Hwan | - |
| dc.date.awarded | 2020/08 | - |
| dc.publisher.location | Daegu | - |
| dc.description.database | dCollection | - |
| dc.citation | XT.MD송52Y 202008 | - |
| dc.date.accepted | 7/23/20 | - |
| dc.contributor.alternativeDepartment | 신물질과학전공 | - |
| dc.embargo.liftdate | 8/31/21 | - |
| dc.contributor.affiliatedAuthor | Song, Sang Yong | - |
| dc.contributor.affiliatedAuthor | Seo, Jungpil | - |
| dc.contributor.affiliatedAuthor | Kim, Tae-Hwan | - |
| dc.contributor.alternativeName | 송상용 | - |
| dc.contributor.alternativeName | Seo, Jungpil | - |
| dc.contributor.alternativeName | 김태환 | - |
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