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Xu X, Zhang S, Zhu X, Guo J. Superconductivity enhancement in FeSe/SrTiO 3: a review from the perspective of electron-phonon coupling. J Phys Condens Matter 2020; 32:343003. [PMID: 32241002 DOI: 10.1088/1361-648x/ab85f0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Single-layer FeSe films grown on SrTiO3, with the highest superconducting transition temperature (T C) among all the iron-based superconductors, serves as an ideal platform for studying the microscopic mechanisms of high-T C superconductivity. The significant role of interfacial coupling has been widely recognized, while the precise nature of the T C enhancement remains open. In this review, we focus on the investigations of the interfacial coupling in FeSe/SrTiO3 from the perspective of electron-phonon coupling (EPC). The main content will include an overview of the experimental measurements associated with different theoretical models and arguments about the EPC. Especially, besides the discussions of EPC based on the measurements of electronic states, we will emphasize the analyses based on phonon measurements. A uniform picture about the nature of the EPC and its relation to the T C enhancement in FeSe/SrTiO3 has still not achieved, which should be the key for further studies aiming to the in-depth understanding of high-T C superconductivity and the discovery of new superconductors with even enhanced T C.
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Affiliation(s)
- Xiaofeng Xu
- Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
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Qiu W, Ma Z, Patel D, Sang L, Cai C, Shahriar Al Hossain M, Cheng Z, Wang X, Dou SX. The Interface Structure of FeSe Thin Film on CaF 2 Substrate and its Influence on the Superconducting Performance. ACS Appl Mater Interfaces 2017; 9:37446-37453. [PMID: 29019397 DOI: 10.1021/acsami.7b11853] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The investigations into the interfaces in iron selenide (FeSe) thin films on various substrates have manifested the great potential of showing high-temperature-superconductivity in this unique system. In present work, we obtain FeSe thin films with a series of thicknesses on calcium fluoride (CaF2) (100) substrates and glean the detailed information from the FeSe/CaF2 interface by using scanning transmission electron microscopy (STEM). Intriguingly, we have found the universal existence of a calcium selenide (CaSe) interlayer with a thickness of approximate 3 nm between FeSe and CaF2 in all the samples, which is irrelevant to the thickness of FeSe layers. A slight Se deficiency occurs in the FeSe layer due to the formation of CaSe interlayer. This Se deficiency is generally negligible except for the case of the ultrathin FeSe film (8 nm in thickness), in which the stoichiometric deviation from FeSe is big enough to suppress the superconductivity. Meanwhile, in the overly thick FeSe layer (160 nm in thickness), vast precipitates are found and recognized as Fe-rich phases, which brings about degradation in superconductivity. Consequently, the thickness dependence of superconducting transition temperature (Tc) of FeSe thin films is investigated and one of our atmosphere-stable FeSe thin film (127 nm) possesses the highest Tconset/Tczero as 15.1 K/13.4 K on record to date in the class of FeSe thin film with practical thickness. Our results provide a new perspective for exploring the mechanism of superconductivity in FeSe thin film via high-resolution STEM. Moreover, approaches that might improve the quality of FeSe/CaF2 interfaces are also proposed for further enhancing the superconducting performance in this system.
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Affiliation(s)
- Wenbin Qiu
- Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong , Squires Way, North Wollongong, New South Wales 2500, Australia
- Shanghai Key Laboratory of High Temperature Superconductors, Physics Department, Shanghai University , Shanghai 200444, People's Republic of China
| | - Zongqing Ma
- Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science & Engineering, Tianjin University , Tianjin 300072, People's Republic of China
| | - Dipak Patel
- Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong , Squires Way, North Wollongong, New South Wales 2500, Australia
| | - Lina Sang
- Shanghai Key Laboratory of High Temperature Superconductors, Physics Department, Shanghai University , Shanghai 200444, People's Republic of China
| | - Chuanbing Cai
- Shanghai Key Laboratory of High Temperature Superconductors, Physics Department, Shanghai University , Shanghai 200444, People's Republic of China
| | - Mohammed Shahriar Al Hossain
- Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong , Squires Way, North Wollongong, New South Wales 2500, Australia
| | - Zhenxiang Cheng
- Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong , Squires Way, North Wollongong, New South Wales 2500, Australia
| | - Xiaolin Wang
- Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong , Squires Way, North Wollongong, New South Wales 2500, Australia
| | - Shi Xue Dou
- Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong , Squires Way, North Wollongong, New South Wales 2500, Australia
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Abstract
Since the dramatic enhancement of the superconducting transition temperature (T c) was reported in a one-unit-cell FeSe film grown on a SrTiO3 substrate (1-UC FeSe/STO) by molecular beam epitaxy (MBE), related research on this system has become a new frontier in condensed matter physics. In this paper, we present a brief review on this rapidly developing field, mainly focusing on the superconducting properties of 1-UC FeSe/STO. Experimental evidence for high-temperature superconductivity in 1-UC FeSe/STO, including direct evidence revealed by transport and diamagnetic measurements, as well as other evidence from scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), are overviewed. The potential mechanisms of the enhanced superconductivity are also discussed. There are accumulating arguments to suggest that the strengthened Cooper pairing in 1-UC FeSe/STO originates from the interface effects, specifically the charge transfer and coupling to phonon modes in the TiO2 plane. The study of superconductivity in 1-UC FeSe/STO not only sheds new light on the mechanism of high-temperature superconductors with layered structures, but also provides an insight into the exploration of new superconductors by interface engineering.
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Affiliation(s)
- Ziqiao Wang
- International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, People's Republic of China
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Song CL, Zhang HM, Zhong Y, Hu XP, Ji SH, Wang L, He K, Ma XC, Xue QK. Observation of Double-Dome Superconductivity in Potassium-Doped FeSe Thin Films. Phys Rev Lett 2016; 116:157001. [PMID: 27127981 DOI: 10.1103/physrevlett.116.157001] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/06/2015] [Indexed: 06/05/2023]
Abstract
We report on the emergence of two disconnected superconducting domes in alkali-metal potassium- (K-)doped FeSe ultrathin films grown on graphitized SiC(0001). The superconductivity exhibits hypersensitivity to K dosage in the lower-T_{c} dome, whereas in the heavily electron-doped higher-T_{c} dome it becomes spatially homogeneous and robust against disorder, supportive of a conventional Cooper-pairing mechanism. Furthermore, the heavily K-doped multilayer FeSe films all reveal a large superconducting gap of ∼14 meV, irrespective of film thickness, verifying the higher-T_{c} superconductivity only in the topmost FeSe layer. The unusual finding of a double-dome superconducting phase is a step towards the mechanistic understanding of superconductivity in FeSe-derived superconductors.
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Affiliation(s)
- Can-Li Song
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
- Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
| | - Hui-Min Zhang
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
| | - Yong Zhong
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
| | - Xiao-Peng Hu
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
| | - Shuai-Hua Ji
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
- Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
| | - Lili Wang
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
- Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
| | - Ke He
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
- Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
| | - Xu-Cun Ma
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
- Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
| | - Qi-Kun Xue
- State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
- Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
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Seo JJ, Kim BY, Kim BS, Jeong JK, Ok JM, Kim JS, Denlinger JD, Mo SK, Kim C, Kim YK. Superconductivity below 20 K in heavily electron-doped surface layer of FeSe bulk crystal. Nat Commun 2016; 7:11116. [PMID: 27050161 PMCID: PMC4823826 DOI: 10.1038/ncomms11116] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2015] [Accepted: 02/23/2016] [Indexed: 11/30/2022] Open
Abstract
A superconducting transition temperature (Tc) as high as 100 K was recently discovered in one monolayer FeSe grown on SrTiO3. The discovery ignited efforts to identify the mechanism for the markedly enhanced Tc from its bulk value of 8 K. There are two main views about the origin of the Tc enhancement: interfacial effects and/or excess electrons with strong electron correlation. Here, we report the observation of superconductivity below 20 K in surface electron-doped bulk FeSe. The doped surface layer possesses all the key spectroscopic aspects of the monolayer FeSe on SrTiO3. Without interfacial effects, the surface layer state has a moderate Tc of 20 K with a smaller gap opening of 4.2 meV. Our results show that excess electrons with strong correlation cannot induce the maximum Tc, which in turn reveals the need for interfacial effects to achieve the highest Tc in one monolayer FeSe on SrTiO3. Thin FeSe film on SrTiO3 substrate becomes a superconductor with a transition temperature over 100 K, yet the origin remains controversial. Here, Seo et al. show superconductivity below 20 K on the electron-doped surface of an FeSe crystal, suggesting a decisive role of interfacial effects in the enhancement of superconductivity.
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Affiliation(s)
- J J Seo
- Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea.,Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, South Korea
| | - B Y Kim
- Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea.,Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - B S Kim
- Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, South Korea.,Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea
| | - J K Jeong
- Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea
| | - J M Ok
- Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea
| | - Jun Sung Kim
- Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea
| | - J D Denlinger
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - S-K Mo
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - C Kim
- Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, South Korea.,Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea
| | - Y K Kim
- Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, South Korea.,Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.,Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea
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Yang S, Sobota JA, Leuenberger D, Kemper AF, Lee JJ, Schmitt FT, Li W, Moore RG, Kirchmann PS, Shen ZX. Thickness-Dependent Coherent Phonon Frequency in Ultrathin FeSe/SrTiO₃ Films. Nano Lett 2015; 15:4150-4154. [PMID: 26027951 DOI: 10.1021/acs.nanolett.5b01274] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Ultrathin FeSe films grown on SrTiO3 substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. By using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO3 films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump-probe delay for 1-unit-cell, 3-unit-cell, and 60-unit-cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A1g phonon. The dominant mechanism for the phonon softening in 1-unit-cell thick FeSe films is a substrate-induced lattice strain. Our results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.
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Affiliation(s)
- Shuolong Yang
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
- ‡Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, United States
| | - Jonathan A Sobota
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
| | - Dominik Leuenberger
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
- ‡Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, United States
| | | | - James J Lee
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
- ‡Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, United States
| | - Felix T Schmitt
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
- ‡Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, United States
| | - Wei Li
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
- ‡Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, United States
| | - Rob G Moore
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
- ‡Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, United States
| | - Patrick S Kirchmann
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
| | - Zhi-Xun Shen
- †Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
- ‡Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, United States
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