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Influence of defects on charge–density–wave and superconductivity in 1T-TaS2 and 2H-TaS2 systems
Physica C 492 (2013) 64–67
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Physica C journal homepage: www.elsevier.com/locate/physc
Influence of defects on charge–density–wave and superconductivity in 1T-TaS2 and 2H-TaS2 systems L.J. Li a, W.J. Lu a, Y. Liu a, Z. Qu b, L.S. Ling b, Y.P. Sun a,b,⇑ a b
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 29 January 2013 Received in revised form 1 May 2013 Accepted 8 June 2013 Available online 14 June 2013 Keywords: Superconductivity Charge–density wave Defect
a b s t r a c t We grew pure 1T/2H-TaS2 single crystals via the chemical vapor transport (CVT) method and introduced defects into the samples by quenching. We systematically investigated the influence of defects on charge–density–wave (CDW) and superconductivity (SC) in 1T and 2H-TaS2. The defects can induce SC to 1T-TaS2 system, and inhibit the commensurate CDW (CCDW) and Mott insulator phase, the supercon) is about 2.5 K and the zero resistivity temperature (T zero ) is about ducting transition temperature (T Onset c c is increased to about 2.7 K. The 0.7 K. For 2H-TaS2, the defects nearly have no effect on CDW, while T Onset c magnetization hysteresis loops show typical type-II superconductor behaviors for both 1T and 2H-TaS2 systems. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Layered transition-metal dichalcogenides (TMDC’s) with the type of MX2 (M is the transition metal, X = S, Se, Te) have been extensively studied for their rich electronic properties. Each layer of TMDC’s consists of a hexagonal transition metal sheet sandwiched by two similar chalcogen sheets, which can be regarded as stacking of covalent coupling X–M–X sandwiches, and the coupling between sandwiches being of weak van der Waals type. TaS2 is a typical material of the TMDCs with a quasi-two-dimensional character [1,2], which has two basic structures that are defined by the different orientation of stacking chalcogen sheets. One is 1T-TaS2 with Ta in octahedral coordination with S atoms, and the other one is 2H-TaS2 with Ta in trigonal prismatic coordination with S atoms [3,4]. For 1T-TaS2, it presents multifarious charge–density–wave (CDW) phase transitions accompanied by changes in lattice parameters and the anomaly in the temperature dependence of resistivity, that is in-commensurate (IC), nearly commensurate (NC), or commensurate (C) [2,5–8]. Many experiments and theoretical calculation indicate that both defects and impurities act as pinning centers for the CDW [9] and have strong influence on the properties [10–13]. For example, it is failing to establish long-range CDW order and it inhibits the NC–C and Mott–Hubbard transitions which are triggered by the CDW phase transition [8,14]; and the ⇑ Corresponding author at: Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China. Tel.: +86 551 559 2757; fax: +86 551 559 1434. E-mail addresses: [email protected] (W.J. Lu), [email protected] (Y.P. Sun). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.06.002
metal–insulator transition temperature (TMI) is related to the growth conditions, such as growth temperature and atmosphere [15]. Zwick et al. reported that the random defects can destroy the long-range phase coherence, and inhibit the metal–insulator transition [14]. Clerc et al. pointed out that the lattice distortion can enhance the electron–phonon interaction from the temperature dependence of the electronic structure of 1T-TaS2 by considering the results obtained on angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) [16]. In addition, Xu et al. found that the small amount of defects not only suppress the Mott transition, but also lead to the appearance of superconductivity (SC) at ambient pressure below T = 2.1 K [17]. For 2H-TaS2, it undergoes an in-plane CDW transition at about 78 K and a superconducting transition at about 0.8 K [3,18–22], while there is no previous experimental or theoretical report about the effect of defects on the SC and CDW of 2H-TaS2 system until now. In the present work, we report the influence of defects on the CDW and SC in both 1T and 2H-TaS2 systems.
2. Experiment 1T and 2H-TaS2 single crystals were grown via the chemical vapor transport (CVT) method with iodine as a transport agent. The pure 1T-TaS2 single crystal is silver gray, mirror-like plates; while the pure 2H-TaS2 single crystal is black, wrinkled, mirror-like plates (we call them as p-1T/2H-TaS2 for short). In order to obtain the defective 1T/2H-TaS2 single crystals (we refer to these samples as d-1T/2H-TaS2), we kept the evacuated quartz tubes that were filled with the obtained p-1T-TaS2 and p-2H-TaS2 single crystals
L.J. Li et al. / Physica C 492 (2013) 64–67
at 760 °C and 900 °C for 2 days, respectively, and then the tubes were directly quenched in the cool water (about 0 °C). The dark gray, loose d-1T-TaS2 single crystals and the black, less wrinkled d-2H-TaS2 single crystals are obtained (shown Fig. 1a). The crystal structure and phase purity were examined by X-ray diffraction (XRD) method with Cu Ka radiation (k = 1.5418 Å). The composition of the d-TaS2 single crystal was determined by using energy dispersive X-ray spectroscopy (EDS) in a JEOLJSM-6500 scanning electron microscope. Physical property measurements were performed in a Quantum Design Physical Property Measurement System (PPMS) and Magnetic Property Measurement System (MPMS). 3. Results and discussion Fig. 1a shows the photos of the d-1T-TaS2, p-1T-TaS2, d-2H-TaS2 and p-2H-TaS2 single crystals. Compared with the pure single
Fig. 1. (a) Photographs of d-1T-TaS2, p-1T-TaS2, d-2H-TaS2, and p-2H-TaS2, (b) EDS pattern of d-1T-TaS2 single crystal, the inset table shows the content radio of the Ta and S, (c) XRD patterns for d- and p-1T-TaS2 single crystals and (d) powder XRD patterns for crushed d- and p-1T-TaS2 single crystals.
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crystal, the color of d-1T-TaS2 single crystal is a little darker, and the plate is less mirror-like, and the color of d-2H-TaS2 single crystal is the same black as the pure one, while the plate is less wrinkled. Fig. 1b shows the EDS pattern of the d-1T-TaS2 single crystal, and the ratio between Ta and S elements is determined by EDS. Taking account of the possible inhomogeneity existing in the samples, multiple points on crystal surface were selected and analyzed. The atomic concentrations of different analyzed regions are similar, and the average ratio between Ta and S is about 1: 1.76 (see the inset table of Fig. 1b). It means that some Sulfur vacancies are introduced into 1T-TaS2 system by quenching. The single crystal XRD patterns of both d-1T-TaS2 and p-1T-TaS2 show that the orientation of the crystal surface is (0 0 l) plane (shown in Fig. 1c). Fig. 1d shows the powder XRD patterns for the crushed d- and p-1T-TaS2 single crystals, it indicates that all the peaks can be well indexed to the 1T structure. The lattice parameters are obtained by fitting the powder XRD patterns using the Rietica software [2], and the obtained lattice constants are a = b = 3.359 Å, c = 5.842 Å for p-1T-TaS2, and a = b = 3.352 Å, c = 5.833 Å for d-1T-TaS2. It means that the Sulfur vacancies lead lattice shrinkage to 1T-TaS2 system. Fig. 2a shows the H = 0 resistivity data for p-1T-TaS2 and d-1TTaS2 single crystals as a function of temperature for the current flowing in the ab plane. At high temperatures, the resistivity step corresponding to the NCCDW transition occurring at 355 K for the host p-1T-TaS2 shifts to about 315 K for d-1T-TaS2. Upon lowering the temperature, the CCDW transition resistivity step occurring at around 200 K disappears for d-1T-TaS2, and an abrupt decrease of resistivity appears at around 25 K. We contribute such transitions to Sulfur vacancies. For 1T-TaS2 system, in the CCDW phase, the Ta atoms are grouped into David-star clusters consisting of two 6-atom rings which contract towards the central atom, so the Sulfur vacancies may influence the Ta atoms’ position in the lattice and destroy the Ta atoms’ superstructure which is related to CCDW order. Based on this, the Mott insulating state which is due to the CCDW superstructure and the Ta-5d electrons’ correlation can be suppressed. Going on decreasing the temperature, another abrupt resistivity decrease happens at around 2.5 K. In order to identify if such abrupt resistivity decreases are superconducting transitions, we measure the low-temperature range q–T curve under 0.1 T magnetic field (shown in fig. 2b). It is clearly shown that no change happens for the high temperature resistivity drop occurring at 25 K, while the low temperature one shifts to lower temperature. Such result indicates that the first transition is not related to superconducting transition, while the second one is the superconducting transition, so the superconducting transition temperature T Onset is c 2.5 K. What the first transition meaning needs to be investigated in future. Keeping on decreasing the temperature, the resistivity decreases to zero when T = 0.7 K (shown in fig. 2c). Fig. 2d shows the temperature dependence of thermoelectric power (S) for d1T-TaS2 and p-1T-TaS2 single crystals sample. The sign of S in the normal phase T > TCDW is negative, and it changes from negative to positive at both NCCDW and CCDW transitions. The S–T curves indicate that the defects suppress the CCDW transition and shift the NCCDW transition to lower temperature for 1T-TaS2 system. It is consistent with the resistivity measurement results. Fig. 3 shows the temperature dependence of magnetic susceptibility for d-1T-TaS2 down to 0.5 K with the 10 Oe applied field parallel to the surface of the single crystal (H//ab). The diamagnetism is observed under T 0.7 K, which further confirms the existence of superconductivity at the low temperature region for the d-1TTaS2 single crystal. The smaller magnetization value as shown in the field-cooling (FC) curve is likely due to the complicated magnetic flux pinning effects [23]. The inset of Fig. 3 shows the magnetization hysteresis loop for d-1T-TaS2 at 0.5 K as H//ab, and the
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Fig. 2. (a) In-plane resistivity (qab) for p- and d-1T-TaS2 single crystals at H = 0 T, (b) the enlargement of low-temperature range resistivity under H = 0 T and H = 0.1 T, (c) the superconducting transition on an enlarged scale and (d) the thermopower of d- and p-1T-TaS2 single crystals.
of hexagonal. The lattice parameters are c = 12.085 Å for p-2HTaS2, and c = 12.068 Å for d-2H-TaS2. So the slight lattice shrinkage happens in d-2H-TaS2 system which is the same as that in d-1T-TaS2 sample. Fig. 5a shows the temperature dependence of the in-plane resistivity qab for p-2H-TaS2 and d-2H-TaS2 single crystals, although the conductivity becomes much worse, the CDW transition temperature denoted by arrow and dashed line is nearly unaffected by the defects, and Tc rises to about 2.76 K (see the inset of Fig. 5a). It might be because such Sulfur vacancies is not enough to influence CDW superstructure where six Ta atoms are close to the central Ta atom, and CDW coexists with superconductivity as the host material p-2H-TaS2. Fig. 5b shows the temperature dependence of magnetic susceptibility for d-2H-TaS2 at 10 Oe as H//ab, and the diamagnetism appears below 2.5 K which further confirms the existence of the superconductivity. The inset shows the hysteresis loop of d-2H-TaS2 at T = 2 K for Hab, which also reveals its type-II superconductor. Fig. 3. Field cooled (FC) and zero field cooled (ZFC) magnetic susceptibilities of d1T-TaS2 single crystal at 10 Oe, and the arrow points out the zero resistivity temperature (T zero ¼ 0:7 K). The inset shows the hysteresis loop of d-1T-TaS2 at c T = 0.5 K for Hab.
shape of the M–H curves indicates that d-1T-TaS2 is a typical typeII superconductor. In order to know if there are some Sulfur vacancies in d-2H-TaS2 sample, the p- and d-2H-TaS2 single crystals are analyzed with the EDS (shown in fig. 4a). It shows that no extra elements appear, and the radio of Ta: S is about 1: 1.73 for d-2H-TaS2, it means that Sulfur vacancies also exist in d-2H-TaS2 sample. Fig. 4b presents the X-ray diffraction patterns at room temperature for p- and d-2HTaS2 single crystals, it shows that the orientation of the crystals’ surface are both (0 0 l) plane, and they are indexed on the basis
4. Conclusion The p-1T/2H-TaS2 single crystals were successfully grown via CVT method, and the defects were introduced into the samples by quenching. We investigated the influence of defects on CDW and SC in 1T/2H-TaS2 single crystals systematically. We found that the Sulfur vacancies are introduced into 1T/2H-TaS2 system, and the defects lead slight lattice shrinkage to both d-1T-TaS2 and d2H-TaS2 systems. For 1T-TaS2, the defects suppress the CCDW and Mott insulator ground states and induce superconductivity to 1T-TaS2 system, and the NCCDW transition temperature shifts to 315 K from 350 K. The superconducting transition temperature (T Onset ) is 2.5 K, and the zero resistivity temperature (T zero ) is c c 0.7 K. For 2H-TaS2, the defects do not affect the CDW transition, while T Onset is raised to about 2.7 K. The magnetization hysteresis c
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Fig. 4. (a) EDS pattern of d-2H-TaS2 single crystal, the inset table shows the content radio of Ta and S and (b) XRD patterns for d- and p-2H-TaS2 single crystal samples.
loops show a typical type-II superconductor behavior for both d1T/2H-TaS2 systems. Acknowledgments This work is supported by the National Key Basic Research under Contract No. 2011CBA00111, and the National Nature Science Foundation of China Under Contract Nos. U1232139, 11274311, 11104279 and 51102240. References [1] [2] [3] [4] [5] [6] [7] [8]
J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (1969) 193. J.A. Wilson, F.J. Di Salvo, S. Mahajan, Adv. Phys. 24 (1975) 117. L.F. Mattheiss, Phys. Rev. B 8 (1973) 3719. A. Meetsma, G.A. Wiegers, R.J. Haange, J.L. De Boer, Acta Crystallogr. Sect. C: Cryst. Struct. Commun. C46 (1990) 1598. K. Nakanishi, H. Shiba, J. Phys. Soc. Jpn. 43 (1977) 1839. X.L. Wu, C.M. Lieber, Phys. Rev. Lett. 64 (1990) 1150. B. Burk, R.E. Thomson, A. Zettl, J. Clarke, Phys. Rev. Lett. 66 (1991) 3040. P. Fazekas, E. Tosatti, Philos. Mag. B 39 (1979) 229.
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Fig. 5. (a) In-plane resistivity (qab) for p- and d-2H-TaS2 single crystals, the inset shows the low temperature superconducting transition on an enlarged scale; (b) Magnetic susceptibility for d-2H-TaS2 single crystal, the inset shows the M (H) hysteresis loop of d-2H-TaS2 at T = 2 K as Hab.
[9] X.L. Wu, P. Zhou, C.M. Lieber, Phys. Rev. Lett. 61 (1988) 2604. [10] T. Endo, S. Nakao, W. Yamaguchi, T. Hasegawa, K. Kitazawa, Solid State Commun. 116 (2000) 47. [11] F.J. Di Salvo, J.A. Wilson, B.G. Bagley, J.V. Waszczak, Phys. Rev. B 12 (1975) 2220. [12] H. Mutka, L. Zuppiroli, P. Molinié, J.C. Bourgouin, Phys. Rev. B 23 (1981) 5030. [13] W.L. McMillan, Phys. Rev. B 12 (1975) 1187. [14] F. Zwick, H. Berger, I. Vobornik, G. Margaritondo, L. Forró, C. Beeli, M. Onellion, G. Panaccione, A. Taleb-Ibrahimi, M. Grioni, Phys. Rev. Lett. 81 (1998) 1058. [15] R. Inada, Y. Onuki, S. Tanuma, J. Phys. Soc. Jpn. 52 (1983) 3536. [16] F. Clerc, C. Battaglia, M. Bovet, L. Despont, C. Monney, H. Cercellier, M.G. Garnier, P. Aebi, Phys. Rev. B 74 (2006) 155114. [17] P. Xu, J.O. Piatek, P. -H. Lin, B. Sipos, H. Berger, L. Forró, H.M. Rønnow, M. Grioni, Phys. Rev. B 81 (2010) 172503. [18] J.M.E. Harper, T.H. Geballe, F.J. DiSalvo, Phys. Rev. B 15 (1977) 2943. [19] J.P. Tidman, O. Singh, A.E. Curzon, R.F. Frindt, Phil. Mag. 30 (1974) 1191. [20] G.A. Scholz, O. Singh, R.F. Frindt, A.E. Curzon, Solid State Commun. 44 (1982) 1455. [21] M.H. van Maaren, H.B. Harland, Phys. Lett. A 29 (1969) 571. [22] S. Nagata, T. Aochi, T. Abe, S. Ebisu, T. Hagino, Y. Seki, K. Tsutsumi, J. Phys. Chem. Solids 53 (1992) 1259. [23] D.E. Prober, M.R. Beasley, R.E. Schwall, Phys. Rev. B 15 (1977) 5245.
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Influence of defects on charge–density–wave and superconductivity in 1T-TaS2 and 2H-TaS2 systems L.J. Li a, W.J. Lu a, Y. Liu a, Z. Qu b, L.S. Ling b, Y.P. Sun a,b,⇑ a b
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 29 January 2013 Received in revised form 1 May 2013 Accepted 8 June 2013 Available online 14 June 2013 Keywords: Superconductivity Charge–density wave Defect
a b s t r a c t We grew pure 1T/2H-TaS2 single crystals via the chemical vapor transport (CVT) method and introduced defects into the samples by quenching. We systematically investigated the influence of defects on charge–density–wave (CDW) and superconductivity (SC) in 1T and 2H-TaS2. The defects can induce SC to 1T-TaS2 system, and inhibit the commensurate CDW (CCDW) and Mott insulator phase, the supercon) is about 2.5 K and the zero resistivity temperature (T zero ) is about ducting transition temperature (T Onset c c is increased to about 2.7 K. The 0.7 K. For 2H-TaS2, the defects nearly have no effect on CDW, while T Onset c magnetization hysteresis loops show typical type-II superconductor behaviors for both 1T and 2H-TaS2 systems. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Layered transition-metal dichalcogenides (TMDC’s) with the type of MX2 (M is the transition metal, X = S, Se, Te) have been extensively studied for their rich electronic properties. Each layer of TMDC’s consists of a hexagonal transition metal sheet sandwiched by two similar chalcogen sheets, which can be regarded as stacking of covalent coupling X–M–X sandwiches, and the coupling between sandwiches being of weak van der Waals type. TaS2 is a typical material of the TMDCs with a quasi-two-dimensional character [1,2], which has two basic structures that are defined by the different orientation of stacking chalcogen sheets. One is 1T-TaS2 with Ta in octahedral coordination with S atoms, and the other one is 2H-TaS2 with Ta in trigonal prismatic coordination with S atoms [3,4]. For 1T-TaS2, it presents multifarious charge–density–wave (CDW) phase transitions accompanied by changes in lattice parameters and the anomaly in the temperature dependence of resistivity, that is in-commensurate (IC), nearly commensurate (NC), or commensurate (C) [2,5–8]. Many experiments and theoretical calculation indicate that both defects and impurities act as pinning centers for the CDW [9] and have strong influence on the properties [10–13]. For example, it is failing to establish long-range CDW order and it inhibits the NC–C and Mott–Hubbard transitions which are triggered by the CDW phase transition [8,14]; and the ⇑ Corresponding author at: Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China. Tel.: +86 551 559 2757; fax: +86 551 559 1434. E-mail addresses: [email protected] (W.J. Lu), [email protected] (Y.P. Sun). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.06.002
metal–insulator transition temperature (TMI) is related to the growth conditions, such as growth temperature and atmosphere [15]. Zwick et al. reported that the random defects can destroy the long-range phase coherence, and inhibit the metal–insulator transition [14]. Clerc et al. pointed out that the lattice distortion can enhance the electron–phonon interaction from the temperature dependence of the electronic structure of 1T-TaS2 by considering the results obtained on angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) [16]. In addition, Xu et al. found that the small amount of defects not only suppress the Mott transition, but also lead to the appearance of superconductivity (SC) at ambient pressure below T = 2.1 K [17]. For 2H-TaS2, it undergoes an in-plane CDW transition at about 78 K and a superconducting transition at about 0.8 K [3,18–22], while there is no previous experimental or theoretical report about the effect of defects on the SC and CDW of 2H-TaS2 system until now. In the present work, we report the influence of defects on the CDW and SC in both 1T and 2H-TaS2 systems.
2. Experiment 1T and 2H-TaS2 single crystals were grown via the chemical vapor transport (CVT) method with iodine as a transport agent. The pure 1T-TaS2 single crystal is silver gray, mirror-like plates; while the pure 2H-TaS2 single crystal is black, wrinkled, mirror-like plates (we call them as p-1T/2H-TaS2 for short). In order to obtain the defective 1T/2H-TaS2 single crystals (we refer to these samples as d-1T/2H-TaS2), we kept the evacuated quartz tubes that were filled with the obtained p-1T-TaS2 and p-2H-TaS2 single crystals
L.J. Li et al. / Physica C 492 (2013) 64–67
at 760 °C and 900 °C for 2 days, respectively, and then the tubes were directly quenched in the cool water (about 0 °C). The dark gray, loose d-1T-TaS2 single crystals and the black, less wrinkled d-2H-TaS2 single crystals are obtained (shown Fig. 1a). The crystal structure and phase purity were examined by X-ray diffraction (XRD) method with Cu Ka radiation (k = 1.5418 Å). The composition of the d-TaS2 single crystal was determined by using energy dispersive X-ray spectroscopy (EDS) in a JEOLJSM-6500 scanning electron microscope. Physical property measurements were performed in a Quantum Design Physical Property Measurement System (PPMS) and Magnetic Property Measurement System (MPMS). 3. Results and discussion Fig. 1a shows the photos of the d-1T-TaS2, p-1T-TaS2, d-2H-TaS2 and p-2H-TaS2 single crystals. Compared with the pure single
Fig. 1. (a) Photographs of d-1T-TaS2, p-1T-TaS2, d-2H-TaS2, and p-2H-TaS2, (b) EDS pattern of d-1T-TaS2 single crystal, the inset table shows the content radio of the Ta and S, (c) XRD patterns for d- and p-1T-TaS2 single crystals and (d) powder XRD patterns for crushed d- and p-1T-TaS2 single crystals.
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crystal, the color of d-1T-TaS2 single crystal is a little darker, and the plate is less mirror-like, and the color of d-2H-TaS2 single crystal is the same black as the pure one, while the plate is less wrinkled. Fig. 1b shows the EDS pattern of the d-1T-TaS2 single crystal, and the ratio between Ta and S elements is determined by EDS. Taking account of the possible inhomogeneity existing in the samples, multiple points on crystal surface were selected and analyzed. The atomic concentrations of different analyzed regions are similar, and the average ratio between Ta and S is about 1: 1.76 (see the inset table of Fig. 1b). It means that some Sulfur vacancies are introduced into 1T-TaS2 system by quenching. The single crystal XRD patterns of both d-1T-TaS2 and p-1T-TaS2 show that the orientation of the crystal surface is (0 0 l) plane (shown in Fig. 1c). Fig. 1d shows the powder XRD patterns for the crushed d- and p-1T-TaS2 single crystals, it indicates that all the peaks can be well indexed to the 1T structure. The lattice parameters are obtained by fitting the powder XRD patterns using the Rietica software [2], and the obtained lattice constants are a = b = 3.359 Å, c = 5.842 Å for p-1T-TaS2, and a = b = 3.352 Å, c = 5.833 Å for d-1T-TaS2. It means that the Sulfur vacancies lead lattice shrinkage to 1T-TaS2 system. Fig. 2a shows the H = 0 resistivity data for p-1T-TaS2 and d-1TTaS2 single crystals as a function of temperature for the current flowing in the ab plane. At high temperatures, the resistivity step corresponding to the NCCDW transition occurring at 355 K for the host p-1T-TaS2 shifts to about 315 K for d-1T-TaS2. Upon lowering the temperature, the CCDW transition resistivity step occurring at around 200 K disappears for d-1T-TaS2, and an abrupt decrease of resistivity appears at around 25 K. We contribute such transitions to Sulfur vacancies. For 1T-TaS2 system, in the CCDW phase, the Ta atoms are grouped into David-star clusters consisting of two 6-atom rings which contract towards the central atom, so the Sulfur vacancies may influence the Ta atoms’ position in the lattice and destroy the Ta atoms’ superstructure which is related to CCDW order. Based on this, the Mott insulating state which is due to the CCDW superstructure and the Ta-5d electrons’ correlation can be suppressed. Going on decreasing the temperature, another abrupt resistivity decrease happens at around 2.5 K. In order to identify if such abrupt resistivity decreases are superconducting transitions, we measure the low-temperature range q–T curve under 0.1 T magnetic field (shown in fig. 2b). It is clearly shown that no change happens for the high temperature resistivity drop occurring at 25 K, while the low temperature one shifts to lower temperature. Such result indicates that the first transition is not related to superconducting transition, while the second one is the superconducting transition, so the superconducting transition temperature T Onset is c 2.5 K. What the first transition meaning needs to be investigated in future. Keeping on decreasing the temperature, the resistivity decreases to zero when T = 0.7 K (shown in fig. 2c). Fig. 2d shows the temperature dependence of thermoelectric power (S) for d1T-TaS2 and p-1T-TaS2 single crystals sample. The sign of S in the normal phase T > TCDW is negative, and it changes from negative to positive at both NCCDW and CCDW transitions. The S–T curves indicate that the defects suppress the CCDW transition and shift the NCCDW transition to lower temperature for 1T-TaS2 system. It is consistent with the resistivity measurement results. Fig. 3 shows the temperature dependence of magnetic susceptibility for d-1T-TaS2 down to 0.5 K with the 10 Oe applied field parallel to the surface of the single crystal (H//ab). The diamagnetism is observed under T 0.7 K, which further confirms the existence of superconductivity at the low temperature region for the d-1TTaS2 single crystal. The smaller magnetization value as shown in the field-cooling (FC) curve is likely due to the complicated magnetic flux pinning effects [23]. The inset of Fig. 3 shows the magnetization hysteresis loop for d-1T-TaS2 at 0.5 K as H//ab, and the
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Fig. 2. (a) In-plane resistivity (qab) for p- and d-1T-TaS2 single crystals at H = 0 T, (b) the enlargement of low-temperature range resistivity under H = 0 T and H = 0.1 T, (c) the superconducting transition on an enlarged scale and (d) the thermopower of d- and p-1T-TaS2 single crystals.
of hexagonal. The lattice parameters are c = 12.085 Å for p-2HTaS2, and c = 12.068 Å for d-2H-TaS2. So the slight lattice shrinkage happens in d-2H-TaS2 system which is the same as that in d-1T-TaS2 sample. Fig. 5a shows the temperature dependence of the in-plane resistivity qab for p-2H-TaS2 and d-2H-TaS2 single crystals, although the conductivity becomes much worse, the CDW transition temperature denoted by arrow and dashed line is nearly unaffected by the defects, and Tc rises to about 2.76 K (see the inset of Fig. 5a). It might be because such Sulfur vacancies is not enough to influence CDW superstructure where six Ta atoms are close to the central Ta atom, and CDW coexists with superconductivity as the host material p-2H-TaS2. Fig. 5b shows the temperature dependence of magnetic susceptibility for d-2H-TaS2 at 10 Oe as H//ab, and the diamagnetism appears below 2.5 K which further confirms the existence of the superconductivity. The inset shows the hysteresis loop of d-2H-TaS2 at T = 2 K for Hab, which also reveals its type-II superconductor. Fig. 3. Field cooled (FC) and zero field cooled (ZFC) magnetic susceptibilities of d1T-TaS2 single crystal at 10 Oe, and the arrow points out the zero resistivity temperature (T zero ¼ 0:7 K). The inset shows the hysteresis loop of d-1T-TaS2 at c T = 0.5 K for Hab.
shape of the M–H curves indicates that d-1T-TaS2 is a typical typeII superconductor. In order to know if there are some Sulfur vacancies in d-2H-TaS2 sample, the p- and d-2H-TaS2 single crystals are analyzed with the EDS (shown in fig. 4a). It shows that no extra elements appear, and the radio of Ta: S is about 1: 1.73 for d-2H-TaS2, it means that Sulfur vacancies also exist in d-2H-TaS2 sample. Fig. 4b presents the X-ray diffraction patterns at room temperature for p- and d-2HTaS2 single crystals, it shows that the orientation of the crystals’ surface are both (0 0 l) plane, and they are indexed on the basis
4. Conclusion The p-1T/2H-TaS2 single crystals were successfully grown via CVT method, and the defects were introduced into the samples by quenching. We investigated the influence of defects on CDW and SC in 1T/2H-TaS2 single crystals systematically. We found that the Sulfur vacancies are introduced into 1T/2H-TaS2 system, and the defects lead slight lattice shrinkage to both d-1T-TaS2 and d2H-TaS2 systems. For 1T-TaS2, the defects suppress the CCDW and Mott insulator ground states and induce superconductivity to 1T-TaS2 system, and the NCCDW transition temperature shifts to 315 K from 350 K. The superconducting transition temperature (T Onset ) is 2.5 K, and the zero resistivity temperature (T zero ) is c c 0.7 K. For 2H-TaS2, the defects do not affect the CDW transition, while T Onset is raised to about 2.7 K. The magnetization hysteresis c
L.J. Li et al. / Physica C 492 (2013) 64–67
Fig. 4. (a) EDS pattern of d-2H-TaS2 single crystal, the inset table shows the content radio of Ta and S and (b) XRD patterns for d- and p-2H-TaS2 single crystal samples.
loops show a typical type-II superconductor behavior for both d1T/2H-TaS2 systems. Acknowledgments This work is supported by the National Key Basic Research under Contract No. 2011CBA00111, and the National Nature Science Foundation of China Under Contract Nos. U1232139, 11274311, 11104279 and 51102240. References [1] [2] [3] [4] [5] [6] [7] [8]
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Fig. 5. (a) In-plane resistivity (qab) for p- and d-2H-TaS2 single crystals, the inset shows the low temperature superconducting transition on an enlarged scale; (b) Magnetic susceptibility for d-2H-TaS2 single crystal, the inset shows the M (H) hysteresis loop of d-2H-TaS2 at T = 2 K as Hab.
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