DGIST Scholar: Yu-Shiba-Rusinov 상태를 이용한 초전도성 물질의 자기적 바닥상태에 대한 연구

Title

Yu-Shiba-Rusinov 상태를 이용한 초전도성 물질의 자기적 바닥상태에 대한 연구

Alternative Title

Study of magnetic ground states of superconducting materials using Yu-Shiba-Rusinov state

DGIST Authors

Song, Sang Yong ; Seo, Jungpil ; Kim, Tae-Hwan

Advisor

서정필

Co-Advisor(s)

Kim, Tae-Hwan

Issued Date

2020

Awarded Date

2020/08

Citation

Song, Sang Yong. (2020). Yu-Shiba-Rusinov 상태를 이용한 초전도성 물질의 자기적 바닥상태에 대한 연구. doi: 10.22677/thesis.200000332743

Type

Thesis

Description

주사 터널링 현미경, Yu-Shiba-Rusinov 상태, 1차원 위상 초전도체, 철 기반 초전도체, 자기적 특성

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.

Table Of Contents

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

URI

http://dgist.dcollection.net/common/orgView/200000332743
http://hdl.handle.net/20.500.11750/12182

DOI

10.22677/thesis.200000332743

Degree

Doctor

Department

Department of Emerging Materials Science

Publisher

DGIST

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