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Reproducible superconducting gap on clean surfaces of BiSrCaCuO prepared by etching with a scanning tunneling microscope tip
Physica C 300 Ž1998. 26–32
Reproducible superconducting gap on clean surfaces of BiSrCaCuO prepared by etching with a scanning tunneling microscope tip Shinobu Matsuura a,) , Takahiro Taneda a,1, Wataru Yamaguchi a,2,c , Hiroharu Sugawara a,c , Tetsuya Hasegawa b,c , Koichi Kitazawa a,c a
Department of SuperconductiÕity, UniÕersity of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan b Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226, Japan c CREST, Japan Science and Technology, Japan Received 12 January 1998; accepted 4 February 1998
Abstract Tunneling properties on the basal plane of a high-Tc superconductor, Bi 2 Sr2 CaCu 2 O y , have been investigated by cryogenic scanning tunneling microscopyrspectroscopy ŽSTMrSTS.. The tunneling spectra obtained on the as-cleaved surfaces showed significant scattering, ranging from semiconducting to superconducting behavior, depending on the measurement position. On fresh surfaces prepared by tip apex etching procedure at 4.2 K, a clear superconducting gap structure was reproducibly observed with a flat background conductance curve. We have also experienced that during the tip scanning the background conductance suddenly changed from a flat shape to a V shape accompanied with discontinuity in an STM image, suggesting that the V-shaped tunneling conductance reflects the electronic nature of the tip apex. q 1998 Elsevier Science B.V. PACS: 73.40.Gk; 74.50.q r; 74.72.Hs Keywords: Scanning tunneling microscopy; Tunneling spectroscopy; Surface etching; Energy gap; Background conductance
1. Introduction Scanning tunneling spectroscopy ŽSTS., the spectroscopic version of scanning tunneling microscopy ŽSTM., possesses a potential capability for directly probing a quasiparticle density of states ŽDOS. near )
Corresponding author. Fax: q81-03-3815-5632; e-mail: [email protected]. 1 Now at Sumitomo Electric Industries. 2 Now at National Industrial Research Institute in Nagoya.
the Fermi level with high resolutions both in energy and in spatial distribution. This unique information is expected to give us important clues to elucidate the pairing nature of high temperature superconductors ŽHTS’s.. The cleaved ab plane of Bi 2 Sr2 CaCu 2 O y ŽBi2212. has been subjected to STMrSTS measurements at cryogenic temperatures because smooth surfaces can be easily prepared by cleavage. However, the STS data on Bi2212 reported so far disagree significantly, and are still under controversy w1–11x. Even at 4.2 K, semiconducting tunneling behavior
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 0 9 6 - 3
S. Matsuura et al.r Physica C 300 (1998) 26–32
has been frequently encountered. Renner and Fisher w3x observed both superconducting and semiconducting gap structures on the ab planes of Bi2212 under the condition that Bi atomic corrugations were visible by STM, claiming that observed distinct spectra were due to different oxygen contents in the surface. Hasegawa et al. w6x reported that the tunneling characteristics varied continuously from superconducting to semiconducting behavior as a function of STM tip–sample distance. Furthermore, the shape of background conductance outside the superconducting gap structure, representing the low energy excitations in the normal state, also differs from one measurement to another. That is, some observed V-shaped w4– 6,8,10x or parabolic conductance curves w9x, while others obtained flat background conductance w2,3x. Experimental inconsistency described above seems to be partly due to deference in oxygen content in the surface BiO layer w3x, which determines the carrier concentration, and thus, the critical temperature ŽTc . near the surface layers. In order to overcome this problem of surface degradation, various surface preparation techniques have been attempted at low temperatures w9,11–13x, where oxygen diffusion is expected to be sufficiently slow. Hancotte et al. w9x have mechanically cleaved a Bi2212 single crystal at 4.2 K in a He gas atmosphere. However, the obtained superconducting gap values showed a wide scattering in the range of 2 Drk B Tc s 6.4–7.8, suggesting that the mechanical cleavage takes place preferably between the adjacent two BiO layers incorporating with atomic scale defects w9x. Thus, more sophisticated in situ surface treatment is desired. Some of the layered materials including van der Waals bonds, such as transition metal dichalcogenides, have a crystallographic nature that surfaces can be etched off by STM tip scanning in a layerby-layer manner under certain experimental conditions including atmosphere, temperature, bias voltage and set-point current w14–18x. This tip apex etching process enables us to expose a smooth surface on which a bulk electronic nature appears. Here, we report on the STS measurements on the basal planes of Bi2212 prepared by the tip apex etching at 4.2 K. We reproducibly observed a superconducting gap structure with a flat background over a wide scanning area up to 500 = 500 nm2 . In addition, we
27
experienced sudden appearances of V-shaped background conductance during the STS measurement. This phenomenon is understandable by assuming that a semiconducting particle is attached on the tip apex.
2. Experimental Single crystals of Bi2212 were grown by the floating zone method. The as-grown crystals were further annealed in air at 8008C for 3 days to improve the crystal homogeneity. The superconducting transition temperature was evaluated to be 88 K using a SQUID susceptometer. Magnetic hysteresis curve of the specimen showed the second peak effect around 300 Oe at 25 K, implying that it is slightly underdoped. The STM instrument utilized in this study was laboratory-constructed. The STM head part, consisting of a PZT tube, STM needle and sample holder, was sealed in a small Cu vessel of 32 mm in diameter and 94 mm in length. The Cu vessel was installed in a cylindrical stainless chamber, which was directly immersed into liquid He. The Bi2212 specimens cleaved in a glove box filled with an inert gas were set to the STM head without exposing them to air. All STMrSTS measurements were performed in a He gas atmosphere Ž; 20 mbar. at 4.2 K. Tip scanning for both topological STM measurement and surface etching was done in the constant current mode. Mechanically sharpened PtrIr wire was used as an STM tip throughout the present study.
3. Results and discussion Fig. 1a and b are STM images obtained on the as-cleaved surfaces of Bi2212 at room temperature and 4.2 K, respectively. Both of the images clearly demonstrate the modulation structure along the b-axis with an interval of ; 2.4 nm, originating from the periodic displacement of Bi atoms. This implies that the as-cleaved surface is covered with the BiO layer. Tunneling spectra taken at 4.2 K on the as-cleaved surfaces varied significantly, depending on the measurement position, and they can be classified into three groups, as pointed out by Kaneko w11x. Fig. 2
28
S. Matsuura et al.r Physica C 300 (1998) 26–32
Since all measurement processes, i.e., sample cleavage, sample setting and STMrSTS operation, are done in an inert atmosphere, surface contamination from ambient air can be ruled out as the cause for irreproducible tunneling characteristics. It is likely that oxygen atoms in the topmost BiO layer are easily released when the sample was placed in a reducing atmosphere in the glove box and the STM chamber at room temperature, resulting in the local destruction of superconductivity near the sample surface. Consequently, a semiconducting character was observed on the oxygen-deficient surface where superconductivity is degraded. In order to establish the optimum experimental conditions for preparing the fresh surface at low temperature by tip-induced surface etching, we repeated the STM scanning at a variety of bias conditions at 4.2 K. As a consequence, it was found that the oxygen-deficient semiconducting layers could be efficiently removed, without causing damage on the tip apex, when the bias voltage was set to ; 50 mV
Fig. 1. STM images Ž54 nm=54 nm. of as-cleaved surfaces of Bi2212 taken at Ža. room temperature and Žb. 4.2 K. The sample bias voltages and the set-point currents are Ža. 990 mV and 0.23 nA, and Žb. 800 mV and 0.27 nA, respectively.
exhibits typical examples of the three d IrdV vs. sample bias voltage, Vsample , curves. Most of the tunneling spectra revealed a semiconducting character with V-shaped or parabolic differential conductance, as indicated in Fig. 2a. Although we sometimes observed a clear superconducting gap structure, the normal state conductance outside the superconducting gap was found to be either flat or Vshaped with respect to the bias voltage, as shown in Fig. 2b and c, respectively.
Fig. 2. Typical tunneling spectra observed on the as-cleaved surfaces of Bi2212 at 4.2 K. Spectra are characterized by Ža. a semiconducting gap Žb. a superconducting gap with flat background, and Žc. a superconducting gap with V-shaped background. The sample bias voltages and the set-point currents for Ža. are 45 mV and 2.51 nA, for Žb. 49 mV and 2.23 nA, and for Žc. 56 mV and 1.64 nA, respectively.
S. Matsuura et al.r Physica C 300 (1998) 26–32
and the tunneling current was higher than 2 nA. The typical scanning speed was approximately 50 msrline and 1 ; 3 unit cell layers were removed at a single scan under the conditions described above. Fig. 3a is an STM image of the etched surface Ždark region. together with the original as-cleaved surface Žbright region.. The step height is ; 6.0 nm, which is approximately twice of the c-parameter, 3.0 nm. Moreover, the modulation structure could be seen on the etched surface as well as on the as-cleaved one. These observations indicate that the etched surface is also terminated at the BiO plane. For the layer-by-layer surface etching by scanning probes, Parkinson w14x proposed four possibilities; Ži. field evaporation, Žii. mechanical contact, Žiii. Joule
29
heating and Živ. surface activation by an electric field andror a current flow. In this study, the tip apex etching in Bi2212 was accelerated when the bias voltage was set to be less than 50 mV. Thus, one can exclude the conventional field evaporation as an etching mechanism of Bi2212. Since STM image did not change and was reproducible after etching, it seems unlikely that there is a strong mechanical interaction between the tip and the sample surface during etching. Joule heating andror surface activation are possible candidates for describing the relatively mild surface etching on Bi2212 achieved in this study. Fig. 3 compares the tunneling spectra taken on the as-cleaved and the etched surfaces. As can be seen,
Fig. 3. Ža. STM image including an etched surface of Bi2212 and Žb. a cross sectional profile of a–b in Ža.. Žc. Tunneling spectra observed on the as-cleaved ŽA. and the etched surfaces ŽB.. The sample bias voltage and the set-point current for Ža. are 990 mV and 0.71 nA, respectively. The spectroscopic measurement positions are marked as crosses in Ža..
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S. Matsuura et al.r Physica C 300 (1998) 26–32
the former shows semiconducting behavior similar to Fig. 2a, while the latter is characterized by a definite superconducting gap. This gives a clear evidence that a fresh BiO plane with enough oxygen content was exposed on the topmost etched surface. The etching reaction hardly progressed on the dark region in Fig. 3a. In other words, the as-cleaved surface layers in which oxygen was partially released, two unit cell layers in case of Fig. 3a, can be selectively removed by the tip scanning under the present bias parameters. Fig. 4 plots 20 tunneling spectra observed at different spatial locations over the etched surface of 500 = 500 nm2 . Notably, a superconducting gap structure was reproducibly observed, regardless of the measurement position. The gap value, 2 D, estimated from peak to peak separation is ; 75 meV Ž2 Drk B Tc ; 9.9., which is relatively large compared to the literature data, 2 Drk B Tc s 6.4–12 w2–5,8–10x. This may be because our sample is in the slightly underdoped regime w19,20x. Another interesting feature seen from Fig. 4 is the rather flat background conductance which is asymmetric with Vsample s 0 and slightly decreases with increasing Vsample . One may also notice a prominent dip structure only in the filled state. Renner and Fisher w3x observed the similar remarkable feature of background spectra in a He gas atmosphere after cleaving sample surfaces in an UHV atmosphere at room temperature, concluding that these features represent intrinsic bulk nature of
Fig. 4. Twenty tunneling spectra observed over 500=500 nm2 area of etched surface at 4.2 K. The sample bias voltage and the set-point current are 50 mV and 2.62 nA, respectively.
Bi2212. The present results of spectra obtained on etched surfaces support their conclusion. During an STS measurement, we have experienced the abrupt change of the background conductance shape which synchronized with the discontinuity in the STM image, as exhibited in Fig. 5, where site-specified tunneling spectra were recorded simultaneously with topographic STM imaging. The background spectra were rather flat before the occurrence of the discontinuity in the topographic image, labeled by an arrow in Fig. 5a. After that, however, V-shaped background suddenly appeared. We further repeated the STS measurements on the same area as Fig. 5a, and confirmed that the V-shaped background was reproducibly observed. Taking into account that the tunneling spectrum is expressed as a convolution of both the tip and the sample electronic states, the V-shaped background seems to reflect the electronic state of the tip apex. One possible explanation for the sudden change in the background is that during the STS measurement the tip apex picked up a semiconducting particle with a V-shaped density of states ŽDOS. distribution, which may originate from oxygen-deficient surface layers. It is also possible that the V-shaped conductance dose not correspond to the intrinsic DOS of a semiconducting particle. The resonant tunneling model proposed by Halbritter w21x can also give an interpretation consistent with the experimental data. In this model, the localized states located near the semiconducting surface mediate the resonant tunneling, leading to a power-law dependence of differential conductance s Ž V . on bias voltage V, i.e., s Ž V . A < V < ny 2rŽ nq1., where n is the number of localized states involved in the resonant tunneling process. The V-shaped tunneling conductance, reported by a considerable number of research groups w3,5,9–11x, has been argued in terms of the anomalous normal state property of HTS’s w1,22–26x. For example, the strong correlation theories predict the V-shaped tunneling DOS function with excitation energy w22–24x. The marginal Fermi liquid model is phenomenologically based on the V-shaped DOS in the normal states w25x. Furthermore, Kirtley and Scalapino w26x postulated the inelastic tunneling picture, where the tunneling electrons excite or de-excite an antiferromagnetic spin fluctuation localized on the Cu sites. However, the present STS observation suggests us
S. Matsuura et al.r Physica C 300 (1998) 26–32
31
that the quasiparticle DOS function of HTS is essentially independent of excitation energy, and that the V-shaped DOS is related with the electronic nature of oxygen-deficient layers as pointed by Renner and Fisher w3x. 4. Summary We have established an experimental condition to produce fresh surfaces of Bi2212 by removing oxygen-deficient surface layers with the STM tip apex at 4.2 K. On the etched surfaces, a superconducting gap structure was reproducibly observed with rather flat background conductance, while on the as-cleaved surfaces semiconducting behavior was mostly encountered. These results indicate the usefulness of tip induced etching as a real in situ surface preparation technique at low temperatures, where oxygen diffusion can be neglected. We have also experienced the abrupt change in background conductance accompanied with the discontinuity in the STM image, suggesting that the tip electronic states are responsible for the V-shaped tunneling characteristics outside the superconducting gap structure. References
Fig. 5. STS data obtained on the etched surface of Bi2212 at 4.2 K. Ža. STM image including discontinuity Žmarked as an arrow., Žb. tunneling spectra observed before the discontinuity ŽA., tunneling spectra observed after the discontinuity ŽB.. The sample bias voltage and the set-point current are 56 mV and 1.64 nA, respectively.
w1x T. Hasegawa, H. Ikuta, K. Kitazawa, Physical Properties of High Temperature Superconductors III, in: D.M. Ginsberg ŽEd.. World Scientific, Chap. 7, Singapore, 1992, p. 553. w2x T. Hasegawa, M. Nantoh, K. Kitazawa, Jpn. J. Appl. Phys. 30 Ž1991. L276. w3x Ch. Renner, Ø. Fisher, Phys. Rev. B 51 Ž1995. 9208. w4x H. Murakami, R. Aoki, J. Phys. Soc. Jpn. 64 Ž1995. 1287. w5x M. Oda, C. Manabe, M. Ido, Phys. Rev. B 53 Ž1996. 2253. w6x T. Hasegawa, M. Nantoh, A. Takagi, W. Yamaguchi, M. Ogino, M. Kawasaki, J.P. Gong, H. Koinuma, K. Kitazawa, Physica B 197 Ž1994. 617. w7x K. Kitazawa, Science 271 Ž1996. 313. w8x J. Kane, Q. Chen, K.-W. Ng, H.-J. Tao, Phys. Rev. Lett. 72 Ž1994. 128. w9x H. Hancotte, D.N. Davydov, M. Ye, R. Deltour, Phys. B 204 Ž1995. 206. w10x P. Mallet, D. Roditchev, W. Sacks, D. Defourneau, J. Klein, ´ Phys. Rev. B 54 Ž1996. 13324. w11x S. Kaneko, Doctorate thesis, Tokyo Institute of Technology, Tokyo, 1997. w12x H. Bando, H. Tokumoto, A. Zettl, K. Kajimura, Ultramicroscopy 42–44 Ž1992. 1627. w13x H.L. Edwards, J.T. Markert, A.L. de Lozanne, J. Vac. Sci. Technol. B 12 Ž1994. 1886.
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w14x B. Parkinson, J. Am. Chem. Soc. 112 Ž1990. 7498. w15x W. Yamaguchi, O. Shiino, H. Sugawara, T. Hasegawa, K. Kitazawa, Appl. Surf. Sci. 119 Ž1997. 67. w16x I. Heyvaert, E. Osquiguil, C. van Haesendonck, Y. Bruynseraede, Appl. Phys. Lett. 61 Ž1992. 111. w17x A.P. Volodin, J. Aarts, Physica C 235–240 Ž1994. 1909. w18x U. Geyer, G. von Minnigerode, J. Appl. Phys. 76 Ž1994. 7774. w19x M. Oda, K. Hoya, R. Kubota, C. Manabe, N. Momono, T. Nakano, M. Ido, Physica C 282–287 Ž1997. 1499.
w20x Ch. Renner, B. Revaz, J.-Y. Genoud, K. Kadowaki, Ø. Fisher, to be published in Phys. Rev. Lett. w21x J. Halbritter, Phys. Rev. B 46 Ž1992. 14861. w22x P.W. Anderson, Z. Zou, Phys. Rev. Lett. 60 Ž1988. 132. w23x K. Flensberg, P. Hedegard, ˚ M.B. Pedersen, Phys. Rev. B 38 Ž1988. 841. w24x K. Flensberg, P. Hedegard, ˚ Physica C 160 Ž1989. 89. w25x C.M. Varma, P.B. Littlewood, S. Schmitt-Rink, E. Abrahams, A.E. Ruckenstein, Phys. Rev. Lett. 63 Ž1989. 1996. w26x J.R. Kirtley, D.J. Scalapino, Phys. Rev. Lett. 65 Ž1990. 798.
Reproducible superconducting gap on clean surfaces of BiSrCaCuO prepared by etching with a scanning tunneling microscope tip Shinobu Matsuura a,) , Takahiro Taneda a,1, Wataru Yamaguchi a,2,c , Hiroharu Sugawara a,c , Tetsuya Hasegawa b,c , Koichi Kitazawa a,c a
Department of SuperconductiÕity, UniÕersity of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan b Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226, Japan c CREST, Japan Science and Technology, Japan Received 12 January 1998; accepted 4 February 1998
Abstract Tunneling properties on the basal plane of a high-Tc superconductor, Bi 2 Sr2 CaCu 2 O y , have been investigated by cryogenic scanning tunneling microscopyrspectroscopy ŽSTMrSTS.. The tunneling spectra obtained on the as-cleaved surfaces showed significant scattering, ranging from semiconducting to superconducting behavior, depending on the measurement position. On fresh surfaces prepared by tip apex etching procedure at 4.2 K, a clear superconducting gap structure was reproducibly observed with a flat background conductance curve. We have also experienced that during the tip scanning the background conductance suddenly changed from a flat shape to a V shape accompanied with discontinuity in an STM image, suggesting that the V-shaped tunneling conductance reflects the electronic nature of the tip apex. q 1998 Elsevier Science B.V. PACS: 73.40.Gk; 74.50.q r; 74.72.Hs Keywords: Scanning tunneling microscopy; Tunneling spectroscopy; Surface etching; Energy gap; Background conductance
1. Introduction Scanning tunneling spectroscopy ŽSTS., the spectroscopic version of scanning tunneling microscopy ŽSTM., possesses a potential capability for directly probing a quasiparticle density of states ŽDOS. near )
Corresponding author. Fax: q81-03-3815-5632; e-mail: [email protected]. 1 Now at Sumitomo Electric Industries. 2 Now at National Industrial Research Institute in Nagoya.
the Fermi level with high resolutions both in energy and in spatial distribution. This unique information is expected to give us important clues to elucidate the pairing nature of high temperature superconductors ŽHTS’s.. The cleaved ab plane of Bi 2 Sr2 CaCu 2 O y ŽBi2212. has been subjected to STMrSTS measurements at cryogenic temperatures because smooth surfaces can be easily prepared by cleavage. However, the STS data on Bi2212 reported so far disagree significantly, and are still under controversy w1–11x. Even at 4.2 K, semiconducting tunneling behavior
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 0 9 6 - 3
S. Matsuura et al.r Physica C 300 (1998) 26–32
has been frequently encountered. Renner and Fisher w3x observed both superconducting and semiconducting gap structures on the ab planes of Bi2212 under the condition that Bi atomic corrugations were visible by STM, claiming that observed distinct spectra were due to different oxygen contents in the surface. Hasegawa et al. w6x reported that the tunneling characteristics varied continuously from superconducting to semiconducting behavior as a function of STM tip–sample distance. Furthermore, the shape of background conductance outside the superconducting gap structure, representing the low energy excitations in the normal state, also differs from one measurement to another. That is, some observed V-shaped w4– 6,8,10x or parabolic conductance curves w9x, while others obtained flat background conductance w2,3x. Experimental inconsistency described above seems to be partly due to deference in oxygen content in the surface BiO layer w3x, which determines the carrier concentration, and thus, the critical temperature ŽTc . near the surface layers. In order to overcome this problem of surface degradation, various surface preparation techniques have been attempted at low temperatures w9,11–13x, where oxygen diffusion is expected to be sufficiently slow. Hancotte et al. w9x have mechanically cleaved a Bi2212 single crystal at 4.2 K in a He gas atmosphere. However, the obtained superconducting gap values showed a wide scattering in the range of 2 Drk B Tc s 6.4–7.8, suggesting that the mechanical cleavage takes place preferably between the adjacent two BiO layers incorporating with atomic scale defects w9x. Thus, more sophisticated in situ surface treatment is desired. Some of the layered materials including van der Waals bonds, such as transition metal dichalcogenides, have a crystallographic nature that surfaces can be etched off by STM tip scanning in a layerby-layer manner under certain experimental conditions including atmosphere, temperature, bias voltage and set-point current w14–18x. This tip apex etching process enables us to expose a smooth surface on which a bulk electronic nature appears. Here, we report on the STS measurements on the basal planes of Bi2212 prepared by the tip apex etching at 4.2 K. We reproducibly observed a superconducting gap structure with a flat background over a wide scanning area up to 500 = 500 nm2 . In addition, we
27
experienced sudden appearances of V-shaped background conductance during the STS measurement. This phenomenon is understandable by assuming that a semiconducting particle is attached on the tip apex.
2. Experimental Single crystals of Bi2212 were grown by the floating zone method. The as-grown crystals were further annealed in air at 8008C for 3 days to improve the crystal homogeneity. The superconducting transition temperature was evaluated to be 88 K using a SQUID susceptometer. Magnetic hysteresis curve of the specimen showed the second peak effect around 300 Oe at 25 K, implying that it is slightly underdoped. The STM instrument utilized in this study was laboratory-constructed. The STM head part, consisting of a PZT tube, STM needle and sample holder, was sealed in a small Cu vessel of 32 mm in diameter and 94 mm in length. The Cu vessel was installed in a cylindrical stainless chamber, which was directly immersed into liquid He. The Bi2212 specimens cleaved in a glove box filled with an inert gas were set to the STM head without exposing them to air. All STMrSTS measurements were performed in a He gas atmosphere Ž; 20 mbar. at 4.2 K. Tip scanning for both topological STM measurement and surface etching was done in the constant current mode. Mechanically sharpened PtrIr wire was used as an STM tip throughout the present study.
3. Results and discussion Fig. 1a and b are STM images obtained on the as-cleaved surfaces of Bi2212 at room temperature and 4.2 K, respectively. Both of the images clearly demonstrate the modulation structure along the b-axis with an interval of ; 2.4 nm, originating from the periodic displacement of Bi atoms. This implies that the as-cleaved surface is covered with the BiO layer. Tunneling spectra taken at 4.2 K on the as-cleaved surfaces varied significantly, depending on the measurement position, and they can be classified into three groups, as pointed out by Kaneko w11x. Fig. 2
28
S. Matsuura et al.r Physica C 300 (1998) 26–32
Since all measurement processes, i.e., sample cleavage, sample setting and STMrSTS operation, are done in an inert atmosphere, surface contamination from ambient air can be ruled out as the cause for irreproducible tunneling characteristics. It is likely that oxygen atoms in the topmost BiO layer are easily released when the sample was placed in a reducing atmosphere in the glove box and the STM chamber at room temperature, resulting in the local destruction of superconductivity near the sample surface. Consequently, a semiconducting character was observed on the oxygen-deficient surface where superconductivity is degraded. In order to establish the optimum experimental conditions for preparing the fresh surface at low temperature by tip-induced surface etching, we repeated the STM scanning at a variety of bias conditions at 4.2 K. As a consequence, it was found that the oxygen-deficient semiconducting layers could be efficiently removed, without causing damage on the tip apex, when the bias voltage was set to ; 50 mV
Fig. 1. STM images Ž54 nm=54 nm. of as-cleaved surfaces of Bi2212 taken at Ža. room temperature and Žb. 4.2 K. The sample bias voltages and the set-point currents are Ža. 990 mV and 0.23 nA, and Žb. 800 mV and 0.27 nA, respectively.
exhibits typical examples of the three d IrdV vs. sample bias voltage, Vsample , curves. Most of the tunneling spectra revealed a semiconducting character with V-shaped or parabolic differential conductance, as indicated in Fig. 2a. Although we sometimes observed a clear superconducting gap structure, the normal state conductance outside the superconducting gap was found to be either flat or Vshaped with respect to the bias voltage, as shown in Fig. 2b and c, respectively.
Fig. 2. Typical tunneling spectra observed on the as-cleaved surfaces of Bi2212 at 4.2 K. Spectra are characterized by Ža. a semiconducting gap Žb. a superconducting gap with flat background, and Žc. a superconducting gap with V-shaped background. The sample bias voltages and the set-point currents for Ža. are 45 mV and 2.51 nA, for Žb. 49 mV and 2.23 nA, and for Žc. 56 mV and 1.64 nA, respectively.
S. Matsuura et al.r Physica C 300 (1998) 26–32
and the tunneling current was higher than 2 nA. The typical scanning speed was approximately 50 msrline and 1 ; 3 unit cell layers were removed at a single scan under the conditions described above. Fig. 3a is an STM image of the etched surface Ždark region. together with the original as-cleaved surface Žbright region.. The step height is ; 6.0 nm, which is approximately twice of the c-parameter, 3.0 nm. Moreover, the modulation structure could be seen on the etched surface as well as on the as-cleaved one. These observations indicate that the etched surface is also terminated at the BiO plane. For the layer-by-layer surface etching by scanning probes, Parkinson w14x proposed four possibilities; Ži. field evaporation, Žii. mechanical contact, Žiii. Joule
29
heating and Živ. surface activation by an electric field andror a current flow. In this study, the tip apex etching in Bi2212 was accelerated when the bias voltage was set to be less than 50 mV. Thus, one can exclude the conventional field evaporation as an etching mechanism of Bi2212. Since STM image did not change and was reproducible after etching, it seems unlikely that there is a strong mechanical interaction between the tip and the sample surface during etching. Joule heating andror surface activation are possible candidates for describing the relatively mild surface etching on Bi2212 achieved in this study. Fig. 3 compares the tunneling spectra taken on the as-cleaved and the etched surfaces. As can be seen,
Fig. 3. Ža. STM image including an etched surface of Bi2212 and Žb. a cross sectional profile of a–b in Ža.. Žc. Tunneling spectra observed on the as-cleaved ŽA. and the etched surfaces ŽB.. The sample bias voltage and the set-point current for Ža. are 990 mV and 0.71 nA, respectively. The spectroscopic measurement positions are marked as crosses in Ža..
30
S. Matsuura et al.r Physica C 300 (1998) 26–32
the former shows semiconducting behavior similar to Fig. 2a, while the latter is characterized by a definite superconducting gap. This gives a clear evidence that a fresh BiO plane with enough oxygen content was exposed on the topmost etched surface. The etching reaction hardly progressed on the dark region in Fig. 3a. In other words, the as-cleaved surface layers in which oxygen was partially released, two unit cell layers in case of Fig. 3a, can be selectively removed by the tip scanning under the present bias parameters. Fig. 4 plots 20 tunneling spectra observed at different spatial locations over the etched surface of 500 = 500 nm2 . Notably, a superconducting gap structure was reproducibly observed, regardless of the measurement position. The gap value, 2 D, estimated from peak to peak separation is ; 75 meV Ž2 Drk B Tc ; 9.9., which is relatively large compared to the literature data, 2 Drk B Tc s 6.4–12 w2–5,8–10x. This may be because our sample is in the slightly underdoped regime w19,20x. Another interesting feature seen from Fig. 4 is the rather flat background conductance which is asymmetric with Vsample s 0 and slightly decreases with increasing Vsample . One may also notice a prominent dip structure only in the filled state. Renner and Fisher w3x observed the similar remarkable feature of background spectra in a He gas atmosphere after cleaving sample surfaces in an UHV atmosphere at room temperature, concluding that these features represent intrinsic bulk nature of
Fig. 4. Twenty tunneling spectra observed over 500=500 nm2 area of etched surface at 4.2 K. The sample bias voltage and the set-point current are 50 mV and 2.62 nA, respectively.
Bi2212. The present results of spectra obtained on etched surfaces support their conclusion. During an STS measurement, we have experienced the abrupt change of the background conductance shape which synchronized with the discontinuity in the STM image, as exhibited in Fig. 5, where site-specified tunneling spectra were recorded simultaneously with topographic STM imaging. The background spectra were rather flat before the occurrence of the discontinuity in the topographic image, labeled by an arrow in Fig. 5a. After that, however, V-shaped background suddenly appeared. We further repeated the STS measurements on the same area as Fig. 5a, and confirmed that the V-shaped background was reproducibly observed. Taking into account that the tunneling spectrum is expressed as a convolution of both the tip and the sample electronic states, the V-shaped background seems to reflect the electronic state of the tip apex. One possible explanation for the sudden change in the background is that during the STS measurement the tip apex picked up a semiconducting particle with a V-shaped density of states ŽDOS. distribution, which may originate from oxygen-deficient surface layers. It is also possible that the V-shaped conductance dose not correspond to the intrinsic DOS of a semiconducting particle. The resonant tunneling model proposed by Halbritter w21x can also give an interpretation consistent with the experimental data. In this model, the localized states located near the semiconducting surface mediate the resonant tunneling, leading to a power-law dependence of differential conductance s Ž V . on bias voltage V, i.e., s Ž V . A < V < ny 2rŽ nq1., where n is the number of localized states involved in the resonant tunneling process. The V-shaped tunneling conductance, reported by a considerable number of research groups w3,5,9–11x, has been argued in terms of the anomalous normal state property of HTS’s w1,22–26x. For example, the strong correlation theories predict the V-shaped tunneling DOS function with excitation energy w22–24x. The marginal Fermi liquid model is phenomenologically based on the V-shaped DOS in the normal states w25x. Furthermore, Kirtley and Scalapino w26x postulated the inelastic tunneling picture, where the tunneling electrons excite or de-excite an antiferromagnetic spin fluctuation localized on the Cu sites. However, the present STS observation suggests us
S. Matsuura et al.r Physica C 300 (1998) 26–32
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that the quasiparticle DOS function of HTS is essentially independent of excitation energy, and that the V-shaped DOS is related with the electronic nature of oxygen-deficient layers as pointed by Renner and Fisher w3x. 4. Summary We have established an experimental condition to produce fresh surfaces of Bi2212 by removing oxygen-deficient surface layers with the STM tip apex at 4.2 K. On the etched surfaces, a superconducting gap structure was reproducibly observed with rather flat background conductance, while on the as-cleaved surfaces semiconducting behavior was mostly encountered. These results indicate the usefulness of tip induced etching as a real in situ surface preparation technique at low temperatures, where oxygen diffusion can be neglected. We have also experienced the abrupt change in background conductance accompanied with the discontinuity in the STM image, suggesting that the tip electronic states are responsible for the V-shaped tunneling characteristics outside the superconducting gap structure. References
Fig. 5. STS data obtained on the etched surface of Bi2212 at 4.2 K. Ža. STM image including discontinuity Žmarked as an arrow., Žb. tunneling spectra observed before the discontinuity ŽA., tunneling spectra observed after the discontinuity ŽB.. The sample bias voltage and the set-point current are 56 mV and 1.64 nA, respectively.
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