양자나노과학연구단 주사터널현미경을이용한초전도체연구 DOI: 10.3938/PhiT.27.032 채중석 국양 Uncovering Superconductivity Pairing Glues by Using Scanning Tunneling Microscopy Jungseok CHAE and Young KUK The discovery of high-tc superconductors in 1986 has attracted much attention in the physics community, but the non-bcs type pairing mechanism is still not understood yet. The discovery of iron based superconductor in 2006 was another milestone in superconductor history because of the possible coexistence of spin ordering and superconductivity. Despite intensive studies, nanoscale or atomic-scale measurement will be the key to understanding the microscopic pairing mechanism in such materials. Because of the layered nature of those exotic superconducting materials, scanning tunneling microscopy (STM) can be easily adopted. STM is a powerful tool for investigating local electronic structure and superconducting properties at atomic scale on surfaces. In this article, we will introduce our recent effort on epitaxial growth of such materials and our recent STM study to unveil the secrets of superconductivity. 고온초전도체가발견된 1986년이후, 고온초전도체에대한수많은관심과연구에도불구하고 BCS 타입이아닌초전도물질의페어링메커니즘은아직명확히밝혀지지않고있다. 초전도연구에서또다른이정표적인발견은 2006년철을 저자약력 채중석박사는 2010년서울대학교에서고체실험물리분야박사학위를취득하였다. 이후 2010 년부터미국국립표준기술연구소 (NIST) 에서박사후연구원으로일했다. 2017년부터기초과학연구원 (IBS) 양자나노과학연구단 (QNS) 에서연구원으로일하고있다. 국양교수는 1981년미국펜실베니아주립대학에서고체실험물리분야박사학위를취득하였다. 이후미국 AT & T Bell 연구소의연구원으로일했으며, 1991년부터서울대학교교수로재직하였다. 2018년부터기초과학연구원 (IBS) 양자나노과학연구단 (QNS) 에서석좌교수로일하고있다. 기반으로한물질에서발견된초전도현상으로자기현상과초전도현상의공존가능성으로인해많은주목을받고있다. 현재까지의많은연구에도불구하고, 나노미터나원자단위의연구는이러한물질에서의초전도현상으로밝힐중요한정보를제공할것으로기대된다. 고온초전도체나철기반의초전도체는모두특정한원자층에서초전도현상을보이는것으로이해되고있으며, 원자수준에서표면의전자구조나초전도현상을측정할수있는장비인주사터널링현미경 (scanning tunneling microscopy, STM) 은이러한물질들을연구하는강력한도구가되고있다. 이기고에서는초전도물질의증착과 STM 을이용하여초전도의비밀을밝히기위한현재까지의연구성과를소개하고자한다. 도입초전도현상이처음발견된이후, Bardeen-Cooper-Schrieffer (BCS) 의이론은전자와포논의상호작용에의해두개의전자가쌍을이루는성질에의해성공적으로초전도현상을설명할수있었다. 그러나두개의전자가쌍을이루게하는매개로서의보존을직접측정하는것은어려움이있다. 역사적으로 BCS 페어링에역할을하는보존모드는두판사이의터널링분광법에의측정되었다. 터널링분광측정은 Fermi 에너지근처에서초전도에너지갭뿐만아니라갭바깥쪽의전자상태밀도를동시에측정가능한방법이다. 주석이나인듐과달리납박막의분광측정결과작은 kink가측정되었으며, 이결과는 BCS 이론을전자와포논간의강한상호작용으로확대한 Eliashberg의이론으로설명되었다. McMillan과 Rowell은 Eliashberg의이론을토대로납박막실험결과로부터초전도현상과직접연관된커플링상수 ( 2 ( )) 와포논의상태밀도 (F( )) 의곱을끌어내는데성공하였다. 초전도의역사에서 BCS 이론을뛰어넘는두가지, 구리산화물고온초전도체및철기반초전도체, 획기적인발견이있었다. 구리산화물초전도체에서는전자와포논의상호작용이 BSC형초전도체와비슷하거나더작게측정되었지만상전이온도는훨씬높다. 또한이방성초전도갭, 도핑의존양자위 22 물리학과첨단기술 JULY/AUGUST 2018
상전이, 전하또는스핀밀도파의존재, 동위원소및변형효과등특유의성질을갖는다. 최근에는철기반초전도체가발견되어자성과초전도현상의공존가능성으로인해많은관심을받고있다. 자성과초전도성은서로경쟁하는성질을갖는것으로널리알려져있다. 예를들어, 철기반의초전도체물질중가장단순한구조를갖는 FeSe에서는, Fermi 에너지근처에서 M 포인트주변의전자포켓과 포인트주변에홀포켓이존재하는전자구조를가진다. Cleaved 된표면에서, 전자와홀밴드간의상호전이가 STM으로측정되었으며, 이는초전도현상의 s± 대칭성및스핀 fluctuation 의증거가된다. 또한 FeSe 단결정에서중성자산란실험에의해직접적으로스핀여기현상이측정되기도하였다. 구리산화물초전도체에서는이차원의 CuO층과철기반초전도체에서는 Fe pnictide 또는 Fe chalcogenide 층에서만존재할수있다는사실은주목할만하다. 매우최근에, 8 K로측정된 FeSe 단결정의상전이온도와달리 STO 기판상에자란원자단층의 FeSe 막에서초전도상전이온도가 100 K로측정되었다. 이박막은평면터널링분광실험에적용하기에는너무얇다. 구조의단순성과표면으로의노출로인해 STM은 STO에서자란원자단층의 FeSe 박막을연구하는강력한도구가될것이다. 우리그룹의연구활동 Fig. 1. SEM image of Ir tip before (a) and after (b) FIB sharpening. (Scale bars: 10 μm) 먼저표면의초전도체에서보존모드를탐색하기위해주사터널링분광법 (scanning tunneling spectroscopy, STS) 에서분광강도를신중하게조사하였다. 분자진동모드의여기, 자기원자에대한스핀여기및표면포논모드를측정하는기능을제공하는비탄성터널링분광법 (inelastic tunneling spectroscopy, IETS) 와관련된요소에중점을두었다. IETS 에서는샘플의전자상태밀도에비례하는터널링전류의 1차미분 (di/dv) 을측정하는대신에, 비탄성터널링의여기가 peak 또는 dip으로명확하게나타나는터널링전류의 2차미분 (d 2 I/dV 2 ) 이측정된다. d 2 I/dV 2 신호는잡음에보다민감하며기술적으로 d 2 I/dV 2 스펙트럼을향상시키는것이어려움이있다. 이론적으로, STM-IETS 신호는비탄성터널링채널을개방하는팁아래의원자의에너지이완에의존한다. 따라서날카로운 STM 팁의존재는 IETS 신호를향상시킬수있다. 깨끗한 Cu(100) 표면과산소가흡착된 Cu(100) 표면에서표면의포논모드를측정하기위해 focused ion beam(fib) 를사용하여날카로운 STM 팁을준비하였다. 그림 1은화학적으로에칭된팁과 FIB 로준비한날카로운팁의주사전자현미경이미지를보여주고있다. 날카로운팁을사용하여신호대잡음비가향상된 STM- IETS 신호의대칭선택감도를입증하였다. 그림 2(a) 와같이깨끗한 Cu(100) 표면에서측정된 STM- IETS 스펙트럼을기준으로산소노출후포논모드의변화를관찰하였다. 깨끗한 Cu(100) 을측정한후, 산소를노출시켜정돈된산소흡착층을성장시켰다. 그림 2(b) 에서보듯이산소가흡착된 Cu(100) 표면의 STM 지형이미지는 (2 2 2)R45 -O와 ( 2 2)R45 -O의두가지표면재구성을보여주었다. 깨끗한 Cu(100) 표면에서 3.7, 19.0 및 27.9 mev 의피크가그림 2(c) 와같이측정되었다. 산소가흡착된 Cu(100) 표면에서 3.7 mev의피크가측정되었는데, 이는 Γ점근처의 out-of-plane의편광표면포논모드로부터유래되었다. 또한, 이전에전자에너지손실분광법 (electron energy loss spectroscopy, EELS) 에의해측정된 (2 2 2)R45 -O 표면의 13.5 mev 에서의피크가 STM-IETS 를사용하여처음으로측정되었다. 이에너지는 real space에서 Cu의누락행의방향과평행한포논모드의대칭에의해설명될수있다. 대신, 깨끗한 Cu(100) 표면에서측정된 19.0 mev 에서의피크는 (2 2 2)R45 -O 표면에서억제되었다. 이는이모드가 Cu 누락열의방향에수직이기때문으로이해된다. 물리학과첨단기술 JULY/AUGUST 2018 23
양자나노과학연구단 Fig. 2. (a) STM topographic image of clean Cu(100) surface. (scale bar: 30 nm) (b) High resolution image of copper oxide surface. Cu missing rows in (2 2x 2)R45 -O region (square at center) is clearly visible. (scale bar: 5 nm) (c) STM-IETS spectra obtained on (2 2x 2)R45 -O surface (blue and red) and on bare Cu(100) surface (black). Fig. 3. (a) Topographic images 3.3 nm 3.3 nm area obtained at Vtip = +0.1 V. (b) Atomic model for a (2 2 2 2)R45 structure. (c) Topographic image of BFCA surface. The exposed FeAs area is indicated by B. (d) Tunneling spectra of marked positions in (c). 우리 그룹에서는 In-situ로 필름을 성장하여 측정하는 기술 troscopy, ARPES) 측정 및 밀도 함수 이론(density functional 을 갖추고 있다. 높은 상전이 온도의 초전도체는 주로 단결정 theory, DFT) 연구는 노출된 Ba층이 As-Fe-As층의 초전도 특 성장 시료를 사용하여 연구되었지만, 초전도 박막을 성장시키 성을 차단한다는 것을 제안했다. 그림 3(c)의 A 에서 측정된 기 위해 펄스 레이저 증착(pulsed laser deposition, PLD)을 터널링 스펙트럼에서 그림 3(d)와 같이 Ba 표면에 의한 스크리 도입함으로써 다양한 고온 초전도 박막에 대해 층별 제어가 가 닝 효과가 관측되었다. 때때로 표면의 Ba 원자가 제거된 그림 능해졌다. 초고진공 환경에서 PLD를 사용하여 네 가지 원자의 3(c)의 B 와 같은 영역을 발견할 수 있었다. 이 영역의 터널링 조합 물질인 Ba(Fe1-x Cox)2As2 ( 0.08) (BFCA)를 성공적으 스펙트럼은 6 mev의 초전도 에너지 갭과 함께 보다 명확한 로 성장시켰으며, 표면 구조 및 물리적 특성의 차이를 STM을 coherence peak의 특성을 보여 주었다. 이용하여 측정하였다. 이 박막의 표면은 단결정과 비슷한 결정 앞서 언급했듯이 노출된 초전도 층은 스캐닝 탐침 현미경을 구조를 가지고 있는 것으로 알려져 있지만, 잘 알려진 단결정 사용하여 페어링 메커니즘을 이해하는 데 필수적이다. TiO2 종 시료의 표면과 달리 (2 2 2 2)R45º 재구성 구조가 그림 3 단면의 균일하고 깨끗한 STO(100) 기판 위에 단 원자층의 (a)와 (b)와 같이 측정되었다. 더욱이, 단결정의 BFCA 표면상의 FeSe 필름을 성장시켰다. 그림 4(a)와 (b)는 STO에서 자란 단 터널링 스펙트럼은 불명확한 coherence peak을 갖는 d-wave 원자층의 FeSe 필름의 STM 지형 이미지를 보여주며 표면 상 타입의 V자형의 초전도 갭을 갖는 것으로 보고되었다. 최근의 단에는 Se 원자의 정사각형 격자가 측정되었다. 확대된 원자 각도 분해 광전자 분광법(angle resolved photoemission spec- 이미지는 서로 다른 STM 팁에 의해 대칭의 둥근 모양과 긴 타 24 물리학과 첨단기술 JULY/AUGUST 20 1 8
Fig. 4. (a) Topographic image of FeSe monolayer on STO with tip #1. (scale bar: 2 nm) (inset) magnified image with symmetric shape of atoms. (scale bar: 1nm) (b) Topographic image of FeSe monolayer on STO with tip #2. (scale bar: 2 nm) (inset) magnified image with elongated shape of atoms. (scale bar: 1 nm) (c) tunneling spectra using the tip#1 (red) and tip #2 (blue). 원형 모양으로 측정된 원자의 모양이 변하는 것을 보여준다. 제공할 수 있지만 여전히 논쟁 중이다. 이 차이가 STM 팁의 꼭지점에서 원자들의 기하학적 차이에 의 해 기인한다고 이해된다. 이 두 가지 팁은 14 mev의 동일 전 망 한 초전도 에너지 갭이 측정되더라도 그림 4(c)와 같이 다른 분광 특성이 측정된다. 이전의 ARPES 측정은 도핑에 의존하는 QNS에서 우리 그룹은 원자 규모의 양자 재료, 특히 초전도 초전도 갭과 초전도 상전이 온도를 측정하고 단 원자층의 재료의 특성을 탐구한다. 우리는 고해상도의 STM을 이용하여 FeSe 박막에서 홀 밴드가 페르미 준위에서 -100 mev까지 이 초전도체의 초전도 페어링과 관련된 보존 모드를 연구한다. 현 동시킴을 보여주었다. 또한, FeSe 층상의 전자와 STO 표면상 재 스핀 극성을 갖는 STM 팁을 사용하여, 원자 수준에서 자기 의 계면 포논 사이의 강한 상호 작용에 기인한 복제 밴드의 존 특성과 초전도성 사이의 관계를 조사하고 있다. 우리의 노력은 재가 측정되었다. STO의 계면 포논은 100 mev의 에너지를 오랫동안 이해되지 못했던 초전도체의 페어링 메커니즘을 이해 가진 Fuchs-Kliewer(FK) 포논으로, 동위 원소 효과는 초전도 할 뿐만 아니라 기초적인 물리학적 이해에도 기여하고자 한다. 페어링에서 STO에 대한 포논의 역할에 대한 더 많은 증거를 물리학과 첨단기술 JULY/AUGUST 20 1 8 25
양자나노과학연구단 Uncovering Superconductivity Pairing Glues by Using Scanning Tunneling Microscopy Jungseok Chae and Young Kuk The discovery of high-tc superconductors in 1986 has attracted much attention in the physics community, but the non-bcs type pairing mechanism is still not understood yet. The discovery of iron based superconductor in 2006 was another milestone in superconductor history because of the possible coexistence of spin ordering and superconductivity. Despite intensive studies, nanoscale or atomic-scale measurement will be the key to understanding the microscopic pairing mechanism in such materials. Because of the layered nature of those exotic superconducting materials, scanning tunneling microscopy (STM) can be easily adopted. STM is a powerful tool for investigating local electronic structure and superconducting properties at atomic scale on surfaces. In this article, we will introduce our recent effort on epitaxial growth of such materials and our recent STM study to unveil the secrets of superconductivity. Introduction to Authors Introduction Jungseok Chae received his Ph.D. in Solid State Physics at Seoul National University, Korea, in 2010. Since then, he has worked as a post-doctoral researcher at National Institute of Standard and Technology (NIST), USA. Since 2017, he is working as a research fellow at the Center for Quantum Nanoscience, Institute for Basic Science. Young Kuk received his Ph.D. in Solid State Physics at Pennsylvania State University, USA, in 1981. Since then, he has worked as a researcher at AT&T Bell laboratories, USA. Since 1991, he has worked as a professor at Seoul National University. He is working as a distinguished professor at the Center for Quantum Nanoscience, Institute for Basic Science since 2018. Ever since the first discovery of superconductivity, Bardeen-Cooper-Schrieffer (BCS) theory successfully explained the superconducting properties by the electron-electron pairing mediated by electron-phonon interactions. However, the direct measurement of the bosonic mode as a pairing glue in superconductor has been a challenging subject. Historically, the bosonic modes incorporating BSC paring were measured by tunneling spectra in planner tunnel junctions. In a tunnel junction experiment, the superconducting gap near Fermi energy and the electronic density of state (DOS) outside the gap can be simultaneously measured. Small kinks were observed in di/dv spectra of Pb film unlike those of Sb, In and Al. Eliashberg theory explained the observation with the BCS theory with strong electron-phonon coupling. McMillan and Rowell successfully explained the Pb tunnel junction spectrum and derived the product between coupling constant ( 2 ( )) and phononic DOS (F ( )) incorporating the pairing mechanism. In the history of superconductivity, there has been two breakthrough discoveries beyond the BCS theory: cuprate high Tc superconductors and iron-based superconductors. In cuprate superconductors, the electron-phonon interactions were measured to be similar or smaller but their Tc's are much higher than BSC-type superconductors. It has peculiar properties such as anisotropic superconducting gap, doping dependent quantum phase transition, existence of charge or spin density wave order, isotope and strain effect etc. More recently, iron-based superconductor has been discovered and has attracted much attention due to the coexistence of the superconductivity and the magnetism. It was commonly accepted that the magnetism and superconductivity are competing properties. For example, in bulk FeSe, which has the simplest structure among iron-based superconductor families, the electron pockets are located around M points and the hole 22 물리학과첨단기술 JULY/AUGUST 2018
pocket is located around point near the Fermi energy. On cleaved surface, interband transition between electron and hole pockets was measured by scanning tunneling microscopy (STM), which showed s± wave symmetry and spin fluctuation. Direct spin excitation at bulk FeSe is also measured by neutron scattering experiment. It is noteworthy to mention that those exotic superconductivity can exist only on 2-dimensional CuO plane in cuprate superconductors and Fe pnictide or chalcogenide plane in iron based superconductors. Very recently, the superconductivity on 1 monolayer (ML) FeSe film on STO substrate was measured to be 100 K, unlike the bulk FeSe was measured to be 8 K. This system is too thin to apply planar tunnel junction experiment. Because of the structural simplicity and exposure to a surface, STM is a powerful tool to study 1 ML FeSe on STO. Research activities in our group To explore the bosonic modes in superconductors on the surface, we carefully examined the spectroscopic strength in scanning tunneling spectroscopy (STS). We focused on factors related to inelastic tunneling spectroscopy (IETS), which provides the capability to measure the excitation of molecular vibrational modes, spin excitation on magnetic atoms, and surface phonon modes. In IETS, instead of measuring first derivative of tunneling current (di/dv), which is proportional to electronic DOS of the sample, second derivative of tunneling current (d 2 I/dV 2 ) is monitored because the excitation of inelastic tunneling shows more clearly as peaks or dips in d 2 I/dV 2 signal. Naturally, d 2 I/dV 2 signal is more sensitive in noise and it was technically challenging to improve d 2 I/dV 2 spectrum. Theoretically, the STM-IETS signal is dependent on the relaxation energy of the atom under the tip, which opens the inelastic tunneling channels. Therefore, the presence of a sharp STM tip can enhance the IETS signal. We prepared an atomically sharp STM tips using focused ion beam (FIB) sharpening to measure the surface phonon modes on clean Cu(100) surface and oxygen-adsorbed Cu(100) surfaces. Figure 1 shows scanning electron microscopy images of chemically etched tip and FIB sharpened tip afterward. By using a sharp tip, we proved that symmetry selective sensitivity of STM-IETS signal with improved signal-to-noise Fig. 1. SEM image of Ir tip before (a) and after (b) FIB sharpening. (Scale bars: 10 μm) ratio. A clean Cu(100) surface was prepared as shown in Fig. 2(a) and STM-IETS spectra were taken as a reference to observe the change in phonon mode peaks after oxygen exposure. After measuring clean Cu(100), oxygen was exposed to grow the ordered oxygen-adsorbed layer. STM topographic image of the oxygen-adsorbed Cu(100) surface, as shown in Fig. 2(b), revealed two different surface reconstructions, (2 2 2)R45 -O and ( 2 2)R45 -O regions. On the clean Cu(100) surface, peaks at 3.7, 19.0, and 27.9 mev were resolved as shown in Fig. 2(c). On oxygen-adsorbed Cu(100) surface, peak at 3.7 mev was measured, which is originated from the out-of-plane polarized surface phonon mode near Γ point. Furthermore, peak at 13.5 mev on the (2 2 2)R45 -O surface, which was previously resolved by electron energy loss spectroscopy measurement, was measured for the first time using STM-IETS. The resolution of this peak can be explained by the symmetry of the phonon mode, which is parallel to the 물리학과첨단기술 JULY/AUGUST 2018 23
양자나노과학연구단 Fig. 2. (a) STM topographic image of clean Cu(100) surface. (scale bar: 30 nm) (b) High resolution image of copper oxide surface. Cu missing rows in (2 2x 2)R45 -O region (square at center) is clearly visible. (scale bar: 5 nm) (c) STM-IETS spectra obtained on (2 2x 2)R45 -O surface (blue and red) and on bare Cu(100) surface (black). Fig. 3. (a) Topographic images 3.3 nm 3.3 nm area obtained at Vtip = + 0.1 V. (b) Atomic model for a (2 2 2 2)R45º structure. (c) Topographic image of BFCA surface. The exposed FeAs area is indicated by B. (d) Tunneling spectra of marked positions in (c). direction of the Cu missing rows in real space. Instead, the surface structures and the physical properties in-situ peak at 19.0 mev, which was resolved on the clean by using STM measurements. The films are known to have Cu(100) surface, was suppressed on the (2 2 2)R45 -O similar crystal structures on surface, but unlike the surface, because this mode is perpendicular to the direc- cleaved bulk sample, well-ordered (2 2 2 2)R45º re- tion of Cu missing rows in real space. constructed structure was measured as shown in Fig. 3(a) In our group, we are fully equipped with in-situ film and (b). Moreover, tunneling spectrum on cleaved BFCA growth technique. Many high transition-temperature super- surfaces have been reported to have the V-shape of the conductors have been studied using bulk-grown samples, d-wave type superconducting gap with unclear coherence however, the introduction of pulsed laser deposition (PLD) peaks. A recent angle resolved photon emission spectro- to grow superconducting films has enabled layer-by-layer scopy (ARPES) measurement and density functional theory control on various high-tc superconducting thin films. We (DFT) study have suggested that an exposed Ba layer successfully grew a quaternary material, Ba(Fe1-xCox)2As2 screens the superconducting property of the As-Fe-As layer. ( 0.08) (BFCA), using PLD under ultrahigh vacuum We also observed the screening effect by the Ba surface as (UHV) environment and we investigated the differences in shown in Fig. 3(d) measured at A in Fig. 3(c). Occasion- 24 물리학과 첨단기술 JULY/AUGUST 20 1 8
Fig. 4. (a) Topographic image of FeSe monolayer on STO with tip #1. (scale bar: 2 nm) (inset) magnified image with symmetric shape of atoms. (scale bar: 1nm) (b) Topographic image of FeSe monolayer on STO with tip #2. (scale bar: 2 nm) (inset) magnified image with elongated shape of atoms. (scale bar: 1nm) (c) tunneling spectra using the tip#1 (red) and tip #2 (blue). ally, we can find areas such as B in Fig. 3(c), where the between electrons on FeSe layer and interfacial phonons on surface Ba atoms are removed, the tunneling spectrum STO surface was probed by ARPES measurement. The in- shows a more pronounced superconducting gap feature terfacial phonon on STO is thought to be Fuchs-Kliewer with 6 mev and clearer coherence peaks as shown in (FK) phonons with the energy at 100 mev. Isotope effect Fig. 3(d). can provide more evidence about the role of phonons on As previously mentioned, exposed superconducting layer is an essential to understand the pairing mechanism using STO in superconductor pairing, however, it is still in debate. scanning probe microscopy. We grew 1 ML FeSe film on a clean STO(100) substrate with uniform TiO2-terminated Perspective surface. Figure 4(a) and (b) show STM topographic images of as-grown 1 ML FeSe film on STO revealing Se square Our group in QNS explores the properties of quantum lattice at the top of the surface. Magnified atomic images materials at atomic scale, especially superconducting in insets shows different atomic shapes with different STM materials. We study bosonic modes that involved in super- tips: symmetric round shape vs elongated oval shape. We conductor pairing in superconductor with high resolution. believe that this difference is originated from different apex Using spin-polarized STM tip, we are investigating the rela- geometry in STM tips. Surprisingly, those two tips measure tionship between magnetic properties and superconductiv- different spectroscopic characters as shown in Fig. 3(c) ity in atomic scale. Our efforts will not only understand the even though the same superconducting energy gap of 14 pairing mechanism of superconductors that have not been mev is measured. Previously, angle resolved photoemission understood for a long time, but also contribute to funda- spectroscopy measured doping dependent superconducting mental physics. gap and Tc and the shift of the hole band to 100 mev from the Fermi level on 1 ML FeSe film. Moreover, the ex- *영문 원고는 온라인에서만 보실 수 있습니다. istence of replica band originated from strong interactions 물리학과 첨단기술 JULY/AUGUST 20 1 8 25