- Email: [email protected]
STS Study of the Sb (111) Surface
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 71 (2015) 323 – 326
18th Conference on Plasma-Surface Interactions, PSI 2015, 5-6 February 2015, Moscow, Russian Federation and the 1st Conference on Plasma and Laser Research and Technologies, PLRT 2015, 18-20 February 2015
STM/STS study of the Sb (111) surface S.V. Chekmazov*, S.I. Bozhko, A.A. Smirnov, A.M. Ionov, A.A. Kapustin Institute of Solid State Physics RAS, Chernogolovka, Moscow district 142432, Russia
Abstract An Sb crystal is a Peierls insulator. Formation of double layers in the Sb structure is due to the shift of atomic planes (111) next but one along the C3 axis. Atomic layers inside the double layer are connected by covalent bonds. The interaction between double layers is determined mainly by Van der Waals forces. The cleave of an Sb single crystal used to be via break of Van der Waals bonds. However, using scanning tunneling microscopy (STM) and spectroscopy (STS) we demonstrated that apart from islands equal in thickness to the double layer, steps of one atomic layer in height also exist on the cleaved Sb (111) surface. Formation of “unpaired” (111) planes on the surface leads to a local break of conditions of Peierls transition. STS experiment reveals higher local density of states (LDOS) measured for “unpaired” (111) planes in comparison with those for the double layer. © Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the N ational R esearch Nuclear U niversity M E P hI (M oscow E ngineering Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)
P hys ics Institute)
. Keywords: scanning tunnelling microscopy/spectroscopy; topological insulator; Peierls transition; local density of states; antimony
1. Introduction Topological properties of electron surface states (SSs) of Sb (111) stir up considerable attention to investigations of the link between atomic and electronic structures of the surface of topological insulators Refs. Sugawara et al. (2006), (2007), Hsieh et al. (2009). Due to a strong spin-orbit interaction Refs. Sugawara et al. (2006), (2007), Molotkov and Tatarskij (1988), energy dispersion of SSs of Sb (111) is a linear function of momentum and they are
* Corresponding author. Tel.: +7-49652-2-83-45; fax: +7-496-522-8160. E-mail address: [email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.08.342
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S.V. Chekmazov et al. / Physics Procedia 71 (2015) 323 – 326
protected by time-reversal symmetry Refs. Hsieh et al. (2009), Kenjiro et al (2009), Narayan et al. (2012), Soumyanarayanan et al. (2013). i.e., the SSs of Sb (111) is similar to those of topological insulators Refs. Zhang et al. (2009), Chen et al. (2009), Yan and Zhang (2012), Franz and Molenkamp (2013). An Sb crystal is a Peierls insulator. Sb is a semimetal because of a small overlapping of the valence and conduction bands at M points of the Brillouin zone Ref. Molotkov and Tatarskij (1988). Peierls instability leads to deformation of a simple cubic lattice and “doubling” of the period along the trigonal axis Ref. Molotkov and Tatarskij (1988). There is a sequence of short and long bonds between the (111) planes Bengió et al. (2007). Strong spin-orbit interaction is comparable to the energy parameters of the Peierls transition Ref. Molotkov and Tatarskij (1988). Here we present the results of our investigations of the cleaved surface of Sb (111) by means of STM/STS technique. 2. Experiment Samples of Sb in a shape of slabs 5×5×2 mm3 in size were cut from a single crystal by a spark errossion technique. The [111] direction was along a short side of the sample. The experiments were conducted using Omicron VT AFM XA (Germany) scanning probe microscope. STM study of Sb was performed under ultra high vacuum conditions (p = 2 × 10-10 mbar) at room temperature. The samples were cleaved at p = 2 × 10 -8 mbar. STM probes were fabricated by means of mechanical cut technique Ref. Chen (2008) from Pt-Ir wire 0.3 mm in diameter. 3. Results and discussions All semimetals of group five (As, Sb, Bi) have a rhombohedral crystal structure ( R3 m ) which is obtained from a simple cubic lattice through a small deformation (Fig. 1(a)).
(b)
(a)
(c)
[111]
[ 211 1]
[01 1]
Fig. 1. Crystal structure of antimony: (a) Sb unit cell; side (b) and top (c) view of Sb (111).
There is a shift of planes (111) next but one along [111] and a small homogeneous deformation. The shift in atomic layers along [111] leads to a sequence of short and long bonds between the atomic layers, as shown in Fig. 1(b). The corresponding interplanar spacings are: d1 = 1.5 ± 0.1 Å and d2 = 2.3 ± 0.1 Å.
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S.V. Chekmazov et al. / Physics Procedia 71 (2015) 323 – 326
Fig. 2(a) shows STM image of Sb (111) surface after cleavage. (a)
(b)
(c)
Fig. 2. (a) STM image of Sb (111) surface (400 × 400 nm2) measured with the Pt-Ir tip at It = 200 pA, Ub = 0.6 V; (b) cross-section of STM image along purple line; (c) side view of the model of Sb (111) corresponding to Fig. 2(b) cross-section.
Fig. 3. LDOS of Sb (111) cleaved surface (It = 200 pA, Ub = -0.8 V, Um = 20 mV).
The surface is covered by large atomically flat terraces (Fig 2(a) insert) separated by steps with a heigth of 2.3 Å and 3.9 Å (Fig. 2(b)). Fig. 2(b) shows the cross-section of STM image along the purple line. Origin of the step 2.3 Å in height is demonstrated in Fig 2(b). The base atomic plane 1 presented in Fig. 2(c) by bilayer (marked in red and brown) is covered by an Sb monolayer (marked in blue) which is separated from the base atomic plane by 2.3 Å. To form a structure with such a step an Sb crystal has to be cleaved via breaking a short interplane bond. Thus terraces 2 and 3 are terminated by “unpaired” planes and separated by 3.9 Å in [111] direction. The bilayer structure (Fig. 2(c)) on the top of the base layer is realized at terasse 4. STM experiment performed in several points of the sample
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S.V. Chekmazov et al. / Physics Procedia 71 (2015) 323 – 326
surface reveals that the relative area of the sample, covered by terraces terminated by “unpaired” planes, is quite small.LDOS was studied by STS at different terraces on the surface of Sb (111). Typical STS spectra of differential conductivity in a vicinity of Ub=0 are presented in Fig. 3. The blue curve was obtained when STM tip was placed at the base plane or at terrace 4 of the surface. STS at terraces terminated by “unpaired” planes resulted in LDOS curve (marked in red) is shifted up in comparison with the blue curve. Peierls transition in Sb resulted in creating a bilayer structure, which causes an energy gap at the Fermi level. Therefore, one could simplistically expect an insignificant DOS at Fermi level. However, STS experiment demonstrates relatively high conductivity regardless of whether the conductivity was measured at bilayer or “unpaired” planes. Such an “enlarged” DOS obtained in the experiment is due to the contribution of electrons of bulk states into tunneling current. “Unpaired” planes at terraces 2 and 3 can be considered as destruction of the bilayer structure. It leads to a break of conditions of Peierls transition at the local area of the surface and hence to an increase of LDOS at Fermi level. 4. Conclusion In conclusion, it was found that cleaving of Sb crystals along (111) plane occurs by breaking both the short and the long bonds Sb-Sb. As a result, a relatively small area of the surface is covered by “unpaired” atomic planes. LDOS at Fermi level measured for the surface area covered by an “unpaired” atomic plane is higher in comparison with LDOS obtained at any other point of the surface. The difference can be explained in terms of a local break of conditions of Peierls transition. Acknowledgements The authors are very grateful to S.N. Molotkov for fruitful discussions. The work was partially supported by Russian Academy of Sciences (“New materials and Structures”, Program №24 “Basic technologies of Nanostructures and Nanomaterials”), and the Ministry of Education and Science of the Russian Federation. References Bengió, S. , Wells, J.W., Kim, T.K., Zampieri, G., Petaccia, L., Lizzit, S., Hofmann, Ph., 2007. Surface Science 601, 2908-2911. Chen, C. Julian, 2008. Introduction to scanning tunnelling microscopy, second ed., Oxford University Press, New York, pp. 313-328. Chen, Y.L. , Analytis, J.G., Chu, J.-H., Liu, Z.K., Mo, S.-K., Qi, X.L., Zhang, H.J., Lu, D.H., Dai, X., Fang, Z., Zhang, S.C., Fisher, I.R., Hussain, Z., Shen, Z.-X., 2009. Science 325, 178-181. Franz, Marcel and Molenkamp, Laurens, 2013. Topological Insulators, first ed., Elsevier, Department in Oxford, UK. Gomes, Kenjiro K., Ko, Wonhee, Mar, Warren, Chen, Yulin, Shen, Zhi-Xun, Manoharan, Hari C., 2009. Preprint at http://arxiv.org/abs/0909.0921v2. Hsieh, D. , Xia, Y., Wray, L., Qian, D., Pal, A., Dil, J.H., Osterwalder, J., Meier, F., Bihlmayer, G., Kane, C.L., Hor, Y.S., Cava, R.J., Hasan, M.Z., 2009. Science 323, 919-922. Molotkov, S.N. and Tatarskij, V.V, 1988. Physics, Chemistry and Mechanics of Surfaces 5, 17-27. Narayan, Awadhesh, Rungger, Ivan, Sanvito, Stefano, 2012. Physical Review B 86, 201402(R). Soumyanarayanan, Anjan, Yee, Michael M., He, Yang, Lin, Hsin, Gardner, Dillon R., Bansil, Arun, Lee, Young S., Hoffman, Jennifer E., 2013. Preprint at http://arxiv.org/abs/1311.1758v1. Sugawara, K., Sato, T., Souma, S., Takahashi, T., Arai, M., Sasaki, T., 2006. Physical Review Letters 96, 046411. Sugawara, K., Sato, T., Souma, S., Takahashi, T., Arai, M., Sasaki, T., 2007. Journal of Magnetism and Magnetic Materials 310, 2177-2179. Yan, Binghai and Zhang, Shou-Cheng, 2012. Rep. Prog. Phys. 75, 096501. Zhang, Haijun, Liu, Chao-Xing, Qi, Xiao-Liang, Dai, Xi, Fang, Zhong, Zhang, Shou-Cheng, 2009. Nature Physics 5, 438-442.
ScienceDirect Physics Procedia 71 (2015) 323 – 326
18th Conference on Plasma-Surface Interactions, PSI 2015, 5-6 February 2015, Moscow, Russian Federation and the 1st Conference on Plasma and Laser Research and Technologies, PLRT 2015, 18-20 February 2015
STM/STS study of the Sb (111) surface S.V. Chekmazov*, S.I. Bozhko, A.A. Smirnov, A.M. Ionov, A.A. Kapustin Institute of Solid State Physics RAS, Chernogolovka, Moscow district 142432, Russia
Abstract An Sb crystal is a Peierls insulator. Formation of double layers in the Sb structure is due to the shift of atomic planes (111) next but one along the C3 axis. Atomic layers inside the double layer are connected by covalent bonds. The interaction between double layers is determined mainly by Van der Waals forces. The cleave of an Sb single crystal used to be via break of Van der Waals bonds. However, using scanning tunneling microscopy (STM) and spectroscopy (STS) we demonstrated that apart from islands equal in thickness to the double layer, steps of one atomic layer in height also exist on the cleaved Sb (111) surface. Formation of “unpaired” (111) planes on the surface leads to a local break of conditions of Peierls transition. STS experiment reveals higher local density of states (LDOS) measured for “unpaired” (111) planes in comparison with those for the double layer. © Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the N ational R esearch Nuclear U niversity M E P hI (M oscow E ngineering Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)
P hys ics Institute)
. Keywords: scanning tunnelling microscopy/spectroscopy; topological insulator; Peierls transition; local density of states; antimony
1. Introduction Topological properties of electron surface states (SSs) of Sb (111) stir up considerable attention to investigations of the link between atomic and electronic structures of the surface of topological insulators Refs. Sugawara et al. (2006), (2007), Hsieh et al. (2009). Due to a strong spin-orbit interaction Refs. Sugawara et al. (2006), (2007), Molotkov and Tatarskij (1988), energy dispersion of SSs of Sb (111) is a linear function of momentum and they are
* Corresponding author. Tel.: +7-49652-2-83-45; fax: +7-496-522-8160. E-mail address: [email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.08.342
324
S.V. Chekmazov et al. / Physics Procedia 71 (2015) 323 – 326
protected by time-reversal symmetry Refs. Hsieh et al. (2009), Kenjiro et al (2009), Narayan et al. (2012), Soumyanarayanan et al. (2013). i.e., the SSs of Sb (111) is similar to those of topological insulators Refs. Zhang et al. (2009), Chen et al. (2009), Yan and Zhang (2012), Franz and Molenkamp (2013). An Sb crystal is a Peierls insulator. Sb is a semimetal because of a small overlapping of the valence and conduction bands at M points of the Brillouin zone Ref. Molotkov and Tatarskij (1988). Peierls instability leads to deformation of a simple cubic lattice and “doubling” of the period along the trigonal axis Ref. Molotkov and Tatarskij (1988). There is a sequence of short and long bonds between the (111) planes Bengió et al. (2007). Strong spin-orbit interaction is comparable to the energy parameters of the Peierls transition Ref. Molotkov and Tatarskij (1988). Here we present the results of our investigations of the cleaved surface of Sb (111) by means of STM/STS technique. 2. Experiment Samples of Sb in a shape of slabs 5×5×2 mm3 in size were cut from a single crystal by a spark errossion technique. The [111] direction was along a short side of the sample. The experiments were conducted using Omicron VT AFM XA (Germany) scanning probe microscope. STM study of Sb was performed under ultra high vacuum conditions (p = 2 × 10-10 mbar) at room temperature. The samples were cleaved at p = 2 × 10 -8 mbar. STM probes were fabricated by means of mechanical cut technique Ref. Chen (2008) from Pt-Ir wire 0.3 mm in diameter. 3. Results and discussions All semimetals of group five (As, Sb, Bi) have a rhombohedral crystal structure ( R3 m ) which is obtained from a simple cubic lattice through a small deformation (Fig. 1(a)).
(b)
(a)
(c)
[111]
[ 211 1]
[01 1]
Fig. 1. Crystal structure of antimony: (a) Sb unit cell; side (b) and top (c) view of Sb (111).
There is a shift of planes (111) next but one along [111] and a small homogeneous deformation. The shift in atomic layers along [111] leads to a sequence of short and long bonds between the atomic layers, as shown in Fig. 1(b). The corresponding interplanar spacings are: d1 = 1.5 ± 0.1 Å and d2 = 2.3 ± 0.1 Å.
325
S.V. Chekmazov et al. / Physics Procedia 71 (2015) 323 – 326
Fig. 2(a) shows STM image of Sb (111) surface after cleavage. (a)
(b)
(c)
Fig. 2. (a) STM image of Sb (111) surface (400 × 400 nm2) measured with the Pt-Ir tip at It = 200 pA, Ub = 0.6 V; (b) cross-section of STM image along purple line; (c) side view of the model of Sb (111) corresponding to Fig. 2(b) cross-section.
Fig. 3. LDOS of Sb (111) cleaved surface (It = 200 pA, Ub = -0.8 V, Um = 20 mV).
The surface is covered by large atomically flat terraces (Fig 2(a) insert) separated by steps with a heigth of 2.3 Å and 3.9 Å (Fig. 2(b)). Fig. 2(b) shows the cross-section of STM image along the purple line. Origin of the step 2.3 Å in height is demonstrated in Fig 2(b). The base atomic plane 1 presented in Fig. 2(c) by bilayer (marked in red and brown) is covered by an Sb monolayer (marked in blue) which is separated from the base atomic plane by 2.3 Å. To form a structure with such a step an Sb crystal has to be cleaved via breaking a short interplane bond. Thus terraces 2 and 3 are terminated by “unpaired” planes and separated by 3.9 Å in [111] direction. The bilayer structure (Fig. 2(c)) on the top of the base layer is realized at terasse 4. STM experiment performed in several points of the sample
326
S.V. Chekmazov et al. / Physics Procedia 71 (2015) 323 – 326
surface reveals that the relative area of the sample, covered by terraces terminated by “unpaired” planes, is quite small.LDOS was studied by STS at different terraces on the surface of Sb (111). Typical STS spectra of differential conductivity in a vicinity of Ub=0 are presented in Fig. 3. The blue curve was obtained when STM tip was placed at the base plane or at terrace 4 of the surface. STS at terraces terminated by “unpaired” planes resulted in LDOS curve (marked in red) is shifted up in comparison with the blue curve. Peierls transition in Sb resulted in creating a bilayer structure, which causes an energy gap at the Fermi level. Therefore, one could simplistically expect an insignificant DOS at Fermi level. However, STS experiment demonstrates relatively high conductivity regardless of whether the conductivity was measured at bilayer or “unpaired” planes. Such an “enlarged” DOS obtained in the experiment is due to the contribution of electrons of bulk states into tunneling current. “Unpaired” planes at terraces 2 and 3 can be considered as destruction of the bilayer structure. It leads to a break of conditions of Peierls transition at the local area of the surface and hence to an increase of LDOS at Fermi level. 4. Conclusion In conclusion, it was found that cleaving of Sb crystals along (111) plane occurs by breaking both the short and the long bonds Sb-Sb. As a result, a relatively small area of the surface is covered by “unpaired” atomic planes. LDOS at Fermi level measured for the surface area covered by an “unpaired” atomic plane is higher in comparison with LDOS obtained at any other point of the surface. The difference can be explained in terms of a local break of conditions of Peierls transition. Acknowledgements The authors are very grateful to S.N. Molotkov for fruitful discussions. The work was partially supported by Russian Academy of Sciences (“New materials and Structures”, Program №24 “Basic technologies of Nanostructures and Nanomaterials”), and the Ministry of Education and Science of the Russian Federation. References Bengió, S. , Wells, J.W., Kim, T.K., Zampieri, G., Petaccia, L., Lizzit, S., Hofmann, Ph., 2007. Surface Science 601, 2908-2911. Chen, C. Julian, 2008. Introduction to scanning tunnelling microscopy, second ed., Oxford University Press, New York, pp. 313-328. Chen, Y.L. , Analytis, J.G., Chu, J.-H., Liu, Z.K., Mo, S.-K., Qi, X.L., Zhang, H.J., Lu, D.H., Dai, X., Fang, Z., Zhang, S.C., Fisher, I.R., Hussain, Z., Shen, Z.-X., 2009. Science 325, 178-181. Franz, Marcel and Molenkamp, Laurens, 2013. Topological Insulators, first ed., Elsevier, Department in Oxford, UK. Gomes, Kenjiro K., Ko, Wonhee, Mar, Warren, Chen, Yulin, Shen, Zhi-Xun, Manoharan, Hari C., 2009. Preprint at http://arxiv.org/abs/0909.0921v2. Hsieh, D. , Xia, Y., Wray, L., Qian, D., Pal, A., Dil, J.H., Osterwalder, J., Meier, F., Bihlmayer, G., Kane, C.L., Hor, Y.S., Cava, R.J., Hasan, M.Z., 2009. Science 323, 919-922. Molotkov, S.N. and Tatarskij, V.V, 1988. Physics, Chemistry and Mechanics of Surfaces 5, 17-27. Narayan, Awadhesh, Rungger, Ivan, Sanvito, Stefano, 2012. Physical Review B 86, 201402(R). Soumyanarayanan, Anjan, Yee, Michael M., He, Yang, Lin, Hsin, Gardner, Dillon R., Bansil, Arun, Lee, Young S., Hoffman, Jennifer E., 2013. Preprint at http://arxiv.org/abs/1311.1758v1. Sugawara, K., Sato, T., Souma, S., Takahashi, T., Arai, M., Sasaki, T., 2006. Physical Review Letters 96, 046411. Sugawara, K., Sato, T., Souma, S., Takahashi, T., Arai, M., Sasaki, T., 2007. Journal of Magnetism and Magnetic Materials 310, 2177-2179. Yan, Binghai and Zhang, Shou-Cheng, 2012. Rep. Prog. Phys. 75, 096501. Zhang, Haijun, Liu, Chao-Xing, Qi, Xiao-Liang, Dai, Xi, Fang, Zhong, Zhang, Shou-Cheng, 2009. Nature Physics 5, 438-442.