1. Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China 2. School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
Superconducting metal dichalcogenides (MDCs) present several similarities to the other layered superconductors like cuprates. The superconductivity in atomically thin MDCs has been demonstrated by recent experiments, however, the investigation of the superconductivity intertwined with other orders are scarce. Investigating the pseudogap in atomic layers of MDCs may help to understand the superconducting mechanism for these true two-dimensional (2D) superconducting systems. Herein we report a pseudogap opening in the tunneling spectra of thin layers of SnSe2 epitaxially grown on highly oriented pyrolytic graphite (HOPG) with scanning tunneling microscopy/spectroscopy (STM/STS). A significant V-shaped pseudogap was observed to open near the Fermi level (EF) in the STS. And at elevated temperatures, the gap gradually evolves to a shallow dip. Our experimental observations provide direct evidence of a pseudogap state in the electron-doped SnSe2 atomic layers on the HOPG surface, which may stimulate further exploration of the mechanism of superconductivity at 2D limit in MDCs.
K. S. Novoselov, A. Mishchenko, A. Carvalho, and A. H. Castro Neto, 2D materials and van der Waals heterostructures, Science 353(6298), 461 (2016) https://doi.org/10.1126/science.aac9439
2
P. Chen, Y. H. Chan, X. Y. Fang, Y. Zhang, M. Y. Chou, S. K. Mo, Z. Hussain, A. V. Fedorov, and T. C. Chiang, Charge density wave transition in single-layer titanium diselenide, Nat. Commun. 6(1), 1 (2015) https://doi.org/10.1038/ncomms9943
3
P. Chen, W. W. Pai, Y. H. Chan, A. Takayama, C. Z. Xu, A. Karn, S. Hasegawa, M. Y. Chou, S. K. Mo, A. V. Fedorov, and T. C. Chiang, Emergence of charge density waves and a pseudogap in single-layer TiTe2, Nat. Commun. 8(1), 1 (2017) https://doi.org/10.1038/s41467-017-00641-1
4
M. M. Ugeda, A. J. Bradley, Y. Zhang, S. Onishi, Y. Chen, W. Ruan, C. Ojeda-Aristizabal, H. Ryu, M. T. Edmonds, H. Z. Tsai, A. Riss, S. K. Mo, D. H. Lee, A. Zettl, Z. Hussain, Z. X. Shen, and M. F. Crommie, Characterization of collective ground states in single-layer NbSe2, Nat. Phys. 12(1), 92 (2016) https://doi.org/10.1038/nphys3527
5
X. X. Xi, Z. F. Wang, W. W. Zhao, J. H. Park, K. T. Law, H. Berger, L. Forró, J. Shan, and K. F. Mak, Ising pairing in superconducting NbSe2 atomic layers, Nat. Phys. 12(2), 139 (2016) https://doi.org/10.1038/nphys3538
6
Y. Cao, A. Mishchenko, G. L. Yu, E. Khestanova, A. P. Rooney, E. Prestat, A. V. Kretinin, P. Blake, M. B. Shalom, C. Woods, J. Chapman, G. Balakrishnan, I. V. Grigorieva, K. S. Novoselov, B. A. Piot, M. Potemski, K. Watanabe, T. Taniguchi, S. J. Haigh, A. K. Geim, and R. V. Gorbachev, Quality heterostructures from twodimensional crystals unstable in air by their assembly in inert atmosphere, Nano Lett. 15(8), 4914 (2015) https://doi.org/10.1021/acs.nanolett.5b00648
7
E. Navarro-Moratalla, J. O. Island, S. Mañas-Valero, E. Pinilla-Cienfuegos, A. Castellanos-Gomez, J. Quereda, G. Rubio-Bollinger, L. Chirolli, J. A. Silva-Guillén, N. Agraït, G. A. Steele, F. Guinéa, H. S. van der Zant, and E. Coronado, Enhanced superconductivity in atomically thin TaS2, Nat. Commun. 7(1), 1 (2016) https://doi.org/10.1038/ncomms11043
8
J. T. Ye, Y. J. Zhang, R. Akashi, M. S. Bahramy, R. Arita, and Y. Iwasa, Superconducting dome in a gatetuned band insulator, Science 338(6111), 1193 (2012) https://doi.org/10.1126/science.1228006
9
J. M. Lu, O. Zheliuk, I. Leermakers, N. F. Q. Yuan, U. Zeitler, K. T. Law, and J. T. Ye, Evidence for twodimensional Ising superconductivity in gated MoS2, Science 350(6266), 1353 (2015) https://doi.org/10.1126/science.aab2277
10
Y. Saito, Y. Nakamura, M. S. Bahramy, Y. Kohama, J. T. Ye, Y. Kasahara, Y. Nakagawa, M. Onga, M. Tokunaga, T. Nojima, Y. Yanase, and Y. Iwasa, Superconductivity protected by spin-valley locking in ion-gated MoS2, Nat. Phys. 12(2), 144 (2016) https://doi.org/10.1038/nphys3580
11
C. Pépin, V. S. de Carvalho, T. Kloss, and X. Montiel, Pseudogap, charge order, and pairing density wave at the hot spots in cuprate superconductors, Phys. Rev. B. 90(19), 195207 (2014) https://doi.org/10.1103/PhysRevB.90.195207
12
T. Wu, H. Mayaffre, S. Krämer, M. Horvatić, C. Berthier, W. N. Hardy, R. X. Liang, D. A. Bonn, and M. H. Julien, Incipient charge order observed by NMR in the normal state of YBa2Cu3Oy, Nat. Commun. 6(1), 6438 (2014) https://doi.org/10.1038/ncomms7438
13
J. J. Wen, H. Huang, S. J. Lee, H. Jang, J. Knight, Y. S. Lee, M. Fujita, K. M. Suzuki, S. Asano, S. A. Kivelson, C. C. Kao, and J. S. Lee, Observation of two types of chargedensity- wave orders in superconducting La2−xSrxCuO4, Nat. Commun. 10(1), 3269 (2019) https://doi.org/10.1038/s41467-019-11167-z
14
B. Loret, N. Auvray, Y. Gallais, M. Cazayous, A. Forget, D. Colson, M. H. Julien, I. Paul, M. Civelli, and A. Sacuto, Intimate link between charge density wave, pseudogap and superconducting energy scales in cuprates, Nat. Phys. 15(8), 15771 (2019) https://doi.org/10.1038/s41567-019-0509-5
15
S. V. Borisenko, A. A. Kordyuk, A. N. Yaresko, V. B. Zabolotnyy, D. S. Inosov, R. Schuster, B. Büchner, R. Weber, R. Follath, L. Patthey, and H. Berger, Pseudogap and charge density waves in two dimensions, Phys. Rev. Lett. 100(19), 196402 (2008) https://doi.org/10.1103/PhysRevLett.100.196402
16
D. V. Evtushinsky, A. A. Kordyuk, V. B. Zabolotnyy, D. S. Inosov, B. Büchner, H. Berger, L. Patthey, R. Follath, and S. V. Borisenko, Pseudogap-driven sign reversal of the Hall effect,Phys. Rev. Lett. 100(23), 236402 (2008) https://doi.org/10.1103/PhysRevLett.100.236402
17
S. V. Borisenko, A. A. Kordyuk, V. B. Zabolotnyy, D. S. Inosov, D. Evtushinsky, B. Büchner, A. N. Yaresko, A. Varykhalov, R. Follath, W. Eberhardt, L. Patthey, and H. Berger, Two energy gaps and Fermi-surface “arcs” in NbSe2, Phys. Rev. Lett. 102(16), 166402 (2009) https://doi.org/10.1103/PhysRevLett.102.166402
18
A. Soumyanarayanan, M. M. Yee, Y. He, J. van Wezel, D. J. Rahn, K. Rossnagel, E. W. Hudson, M. R. Norman, and J. E. Hoffman, Quantum phase transition from triangular to stripe charge order in NbSe2, Proc. Natl. Acad. Sci. USA 110(5), 1623 (2013) https://doi.org/10.1073/pnas.1211387110
19
Y. Umemoto, K. Sugawara, Y. Nakata, T. Takahashi, and T. Sato, Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D–2D crossover in monolayer VSe2, Nano Res. 12(1), 165 (2019) https://doi.org/10.1007/s12274-018-2196-4
20
Z. J. Li, Y. C. Zhao, K. J. Mu, H. Shan, Y. Q. Guo, J. J. Wu, Y. Q. Su, Q. R. Wu, Z. Sun, A. D. Zhao, X. F. Cui, C. Z. Wu, and Y. Xie, Molecule-confined engineering toward superconductivity and ferromagnetism in two-dimensional superlattice, J. Am. Chem. Soc. 139(45), 16398 (2017) https://doi.org/10.1021/jacs.7b10071
21
Y. H. Zhou, B. W. Zhang, X. L. Chen, C. C. Gu, C. An, Y. Zhou, K. M. Cai, Y. F. Yuan, C. H. Chen, H. Wu, R. R. Zhang, C. Y. Park, Y. M. Xiong, X. W. Zhang, K. Y. Wang, and Z. R. Yang, Pressure-induced metallization and robust superconductivity in pristine 1T-SnSe2, Adv. Electron. Mater. 4(8), 1800155 (2018) https://doi.org/10.1002/aelm.201800155
22
J. W. Zeng, E. Liu, Y. J. Fu, Z. Y. Chen, C. Pan, C. Y. Wang, M. Wang, Y. J. Wang, K. Xu, S. H. Cai, X. X. Yan, Y. Wang, X. W. Liu, P. Wang, S. J. Liang, Y. Cui, H. Y. Hwang, H. T. Yuan, and F. Miao, Gate-induced interfacial superconductivity in 1T-SnSe2, Nano Lett. 18(2), 1410 (2018) https://doi.org/10.1021/acs.nanolett.7b05157
23
Y. M. Zhang, J. Q. Fan, W. L. Wang, D. Zhang, L. L. Wang, W. Li, K. He, C. L. Song, X. C. Ma, and Q. K. Xue, Observation of interface superconductivity in a SnSe2/epitaxial graphene van der Waals heterostructure, Phys. Rev. B 98(22), 220508 (2018) https://doi.org/10.1103/PhysRevB.98.220508
24
Z. B. Shao, Z. G. Fu, S. J. Li, Y. Cao, Q. Bian, H. G. Sun, Z. Y. Zhang, H. Gedeon, X. Zhang, L. J. Liu, Z. W. Cheng, F. W. Zheng, P. Zhang, and M. H. Pan, Strongly compressed few-layered SnSe2 films grown on a SrTiO3 substrate: The coexistence of charge ordering and enhanced interfacial superconductivity, Nano Lett. 19(8), 5304 (2019) https://doi.org/10.1021/acs.nanolett.9b01766
25
P. Yu, X. C. Yu, W. L. Lu, H. Lin, L. F. Sun, K. Z. Du, F. C. Liu, W. Fu, Q. S. Zeng, Z. X. Shen, C. H. Jin, Q. J. Wang, and Z. Liu, Fast photoresponse from 1T tin diselenide atomic layers, Adv. Funct. Mater. 26(1), 137 (2016) https://doi.org/10.1002/adfm.201503789
26
Y. B. Zhang, V. W. Brar, F. Wang, C. Girit, Y. Yayon, M. Panlasigui, A. Zettl, and M. F. Crommie, Giant phononinduced conductance in scanning tunnelling spectroscopy of gate-tunable graphene, Nat. Phys. 4(8), 627 (2008) https://doi.org/10.1038/nphys1022
27
D. W. Shen, Y. Zhang, L. X. Yang, J. Wei, H. W. Ou, J. K. Dong, B. P. Xie, C. He, J. F. Zhao, B. Zhou, M. Arita, K. Shimada, H. Namatame, M. Taniguchi, J. Shi, and D. L. Feng, Primary role of the barely occupied states in the charge density wave formation of NbSe2, Phys. Rev. Lett. 101(22), 226406 (2008) https://doi.org/10.1103/PhysRevLett.101.226406
28
K. C. Rahnejat, C. A. Howard, N. E. Shuttleworth, S. R. Schofield, K. Iwaya, C. F. Hirjibehedin, C. Renner, G. Aeppli, and M. Ellerby, Charge density waves in the graphene sheets of the superconductor CaC6, Nat. Commun. 2(1), 1 (2011) https://doi.org/10.1038/ncomms1574
29
Ø. Fischer, M. Kugler, I. Maggio-Aprile, C. Berthod, and C. Renner, Scanning tunneling spectroscopy of hightemperature superconductors, Rev. Mod. Phys. 79(1), 353 (2007) https://doi.org/10.1103/RevModPhys.79.353
30
T. Kondo, R. Khasanov, T. Takeuchi, J. Schmalian, and A. Kaminski, Competition between the pseudogap and superconductivity in the high-Tccopper oxides, Nature 457(7227), 296 (2009) https://doi.org/10.1038/nature07644
31
M. J. Lawler, K. Fujita, J. Lee, A. R. Schmidt, Y. Kohsaka, C. K. Kim, H. Eisaki, S. Uchida, J. C. Davis, J. P. Sethna, and E. A. Kim, Intra-unit-cell electronic nematicity of the high-Tccopper-oxide pseudogap states, Nature 466(7304), 347 (2010) https://doi.org/10.1038/nature09169
32
K. W. Zhang, C. L. Yang, B. Lei, P. C. Lu, X. B. Li, Z. Y. Jia, Y. H. Song, J. Sun, X. H. Chen, J. X. Li, and S. C. Li, Unveiling the charge density wave inhomogeneity and pseudogap state in 1T-TiSe2, Sci. Bull. 63(7), 426 (2018) https://doi.org/10.1016/j.scib.2018.02.018
33
A. Damascelli, Z. Hussain, and Z. X. Shen, Angleresolved photoemission studies of the cuprate superconductors, Rev. Mod. Phys. 75(2), 473 (2003) https://doi.org/10.1103/RevModPhys.75.473
34
M. Hashimoto, I. M. Vishik, R. H. He, T. P. Devereaux, and Z. X. Shen, Energy gaps in high-transitiontemperature cuprate superconductors, Nat. Phys. 10(7), 483 (2014) https://doi.org/10.1038/nphys3009
35
X. Montiel, T. Kloss, C. Pépin, S. Benhabib, Y. Gallais, and A. Sacuto, ηcollective mode as A1g Raman resonance in cuprate superconductors, Phys. Rev. B 93(2), 024515 (2016) https://doi.org/10.1103/PhysRevB.93.024515
36
T. Shimada, F. S. Ohuchi, and B. A. Parkinson, Work function and photothreshold of layered metal dichalcogenides, Jpn. J. Appl. Phys. 1(33), 2696 (1994) https://doi.org/10.1143/JJAP.33.2696
37
H. Ago, T. Kugler, F. Cacialli, K. Petritsch, R. H. Friend, W. R. Salaneck, Y. Ono, T. Yamabe, and K. Tanaka, Work function of purified and oxidised carbon nanotubes, Synth. Met. 103(1–3), 2494 (1999) https://doi.org/10.1016/S0379-6779(98)01062-5
