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FeSe0.5Te0.5 single crystals with superconducting critical temperature of 13.5 K are investigated by scanning tunneling microscopy/spectroscopy (STM/STS) measureflents in detail. STM image on the top surface shows an atomically resolved square lattice consisted by white and dark spots with a constant of about 3.73 0.03 which is consistent with the lattice constant 3.78 . The Se and Te atoms with a height difference of about 0.35 are successfully identified since the sizes of the two kinds of atoms are different. The tunneling spectra show very large zero-bias conductance value and asymmetric coherent peaks in the superconducting state. According to the positions of coherence peaks, we determine the superconducting gap 2 = 5.5 meV, and the reduced gap 2/kBTc = 4.9 is larger than the value predicted by the weak-coupling BCS theory. The zero-bias conductance at 1.7 K only have a decrease of about 40% compared with the normal state conductance, which may originate from some scattering and broadening mechanism in the material. This broadening effect will also make the superconducting gap determined by the distance between the coherence peaks larger than the exact gap value. The asymmetric structure of the tunneling spectra near the superconducting gap is induced by the hump on the background. This hump appears at temperature more than twice the superconducting critical temperature. This kind of hump has also been observed in other iron pnictides and needs further investigation. A possible bosonic mode outside the coherence peak with a mode energy of about 5.5 meV is observed in some tunneling spectra, and the ratio between the mode energy and superconducting transition temperature /kBTc 4.7 is roughly consistent with the universal ratio 4.3 in iron-based superconductors. The high-energy background of the spectra beyond the superconducting gaps shows a V-shape feature. The slopes of the differential conductance spectra at high energy are very different in the areas of Te-atom cluster and Se-atom cluster, and the difference extends to the energy of more than 300 meV. The differential conductance mapping has very little information about the quasi-particle interference of the superconducting state, which may result from the other strong scattering mechanism in the sample.
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Keywords:
- iron-based superconductor /
- scanning tunneling spectrum /
- unconventional superconductivity
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[1] Kamihara Y, Watanabe T, Hirano M, Hosono H 2008 J. Ame. Chem. Soc. 130 3296
[2] Ren Z A, Lu W, Yang J, Yi W, Shen X L, Li Z C, Che G C, Dong X L, Sun L L, Zhou F, Zhao Z X 2008 Chin. Phys. Lett. 25 2215
[3] Chen X H, Wu T, Wu G, Liu R H, Chen H, Fang D F 2008 Nature 453 761
[4] Jiang H, Sun Y L, Xu Z A, Cao G H 2013 Chin. Phys. B 22 087410
[5] Luo H Q 2014 Physics 43 430 (in Chinese) [罗会仟 2014 物理 43 430]
[6] Scalapino D J 2012 Rev. Mod. Phys. 84 1383
[7] Wen H H, Li S L 2011 Annu. Rev. Condens.Matter Phys. 2 121
[8] Stewart G R 2011 Rev. Mod. Phys. 83 1589
[9] Zhao Z X, Yu L 2013 Basic reflearch on the physical properties of iron-based superconductor (Shanghai: Shanghai Scientific and Technical Publishers) p163 (in Chinese) [赵忠贤, 于渌 2013 铁基超导体物性基础研究(上海: 上海科学技术出版社) 第163页]
[10] Shen B, Yang H, Wang Z S, Han F, Zeng B, Shan L, Ren C, Wen H H 2011 Phy. Rev. B 84 184512
[11] Cai P Zhou X D, Ruan W, Wang A F, Chen X H, Lee D H, Wang Y Y 2013 Nature Commun 4 1596
[12] Zhou X D, Cai P, Wang Y Y 2013 Chin. Phys. B 22 087413
[13] Yi M Zhang Y, Liu Z K, Ding X X, Chu J H Kemper A F, N. Plonka N Moritz B, Hashimoto M, Mo S K, Hussain Z, Devereaux T P Fisher I R, Wen H H, Shen Z X Lu D H 2013 Nature Common. 5 3711
[14] Fernandes R M, Chubukov A V Schmalian J 2014 Nature Phys. 10 97
[15] Yu S L and Li J X 2013 Chin. Phys. B 22 087411
[16] Mazin I I, Singh D J, Johannes M D, Du M H 2008 Phys. Rev. Lett. 101 057003
[17] Kuroki K, Onari S, Arita R, Usui H, Tanaka Y, Kontani H, Aoki H 2008 Phys. Rev. Lett. 101 087004
[18] Yang H, Wang Z Y, Fang D L, Deng Q, Wang Q H, Xiang Y Y, Yang Y, Wen H H 2013 Nature Common. 4 2749
[19] Zhu X Y, Han F, Mu G, Cheng P, Shen B, Zeng B, Wen H H 2009 Phys. Rev. B 79 220512
[20] Bao W, Qiu Y, Huang Q, Green M A, Zajdel P, Fitzsimmons M R, Zhernenkov M, Chang S, Fang M H, Qian B, Vehstedt E K, Yang J H, Pham H M, Spinu L, Mao Z Q 2009 Phys. Rev. Lett. 102 247001
[21] Wang Q Yan, Li Z, Zhang W H, Zang Z C, Zhang J S, Li W, Ding H, Ou Y B, Deng P, Chang K, Wen J, Song C L, He K, Jia J F, Ji S H, Wang Y Y, Wang L L, Chen X, Ma X C, Xue Q K 2012 Chin. Phys. Lett. 29 037402
[22] Zhang W H, Sun Y, Zhang J S, Li F S, Guo M H, Zhao Y F, Zhang H M, Peng J P, Xing Y, Wang H C, Fujita T, Hirata A, Li Z, Ding H, Tang C J, Wang M, Wang Q Y, He K, Ji S H, Chen X, Wang J F, Xia Z C, Li L, Wang Y Y, Wang J, Wang L L, Chen M W, Xue Q K, Ma X C 2014 Chin. Phys. Lett. 31 017401
[23] Kasahara S, Watashige T, Hanaguri T, Kohsaka Y, Yamashita T, Shimoyama Y, Mizukami Y, Endo R, Ikeda H, Aoyama K, Terashima T, Uji S, Wolf T, Hilbert von Löhneysen H V, Shibauchi T, Matsuda Y 2014 PNAS 111 16309
[24] Liu T J, Hu J, Qian B, Fobes D, Mao Z Q, Bao W, Reehuis M, Kimber S A J Prokeš K Matas S, Argyriou D N, Hiess A, Rotaru A, Pham H, Spinu L, Y. Qiu Y Thampy V, Savici A T, Rodriguez J A Broholm C 2010 Nature Mater 9 716
[25] Hanaguri T, Niitaka S Kuroki K Takagi H 2010 Science 328 474
[26] Lin W Z Li Q, Sales B C Jesse S Sefat A S Kalinin S V, Pan M H 2013 ACS Nano 7 2634
[27] Wen J S, Xu G Y, Xu Z J, Lin Z W, Li Q, Chen Y, Chi S X, Gu G D, Tranquada J M2010 Phys. Rev. B 81 100513(R)
[28] Wang Z Y 2014 Ph. D. Dissertation (Beijing: Institute of Physics, CAS) (in Chinese) [王震宇 2014 博士学位论文(北京: 中科院物理研究所)]
[29] Sun Y, Tsuchiya Y, Taen T, Yamada T, Pyon S, Sugimoto A, Ekino T, Shi Z X, Tamegai T 2014 Sci. Rep. 4 4585
[30] Yang H, Wang Z Y, Fang D L, Li S, Kariyado T, Chen G F, Ogata M, Das T, Balatsky A V, Wen H H 2012 Phys. Rev. B 86 214512
[31] Zhou X D, Cai P, Wang A F, Ruan W, Ye C, Chen X H, You Y Z, Weng Z Y, Wang Y Y 2012 Phys. Rev. Lett. 109 037002
[32] Wang Z Y, Fang D L, Deng Q, Yang H, Ren C, Wen H H 2014 Phys. Rev. B 89 214515
[33] Wang Z Y, Yang H, Fang D L, Shen B, Wang Q H, Shan L, Zhang C L, Dai P C, Wen H H 2013 Nature Phys. 9 42
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