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STS measurements on clean and stable surfaces of as-prepared Nd1Ba2Cu3Oy single crystals
PHYSlCA ELSEVIER
Physica C
263 (1996) 185-188
STM/STS measurements on clean and stable surfaces of as-prepared NdlBa2Cu3Oysingle crystals Wu Ting *, T. Egi, R. Itti, K. Kuroda, N. Koshizuka, S. Tanaka Superconductivity Research Laboratory, International Superconductivity Technology Center (ISTEC), 1-10-13 Shinonome, Koto-ku, Tokyo 135, Japan
Abstract The preliminary results of the surface characterization of Nd~Ba~Cu3Oy (Nd123) single crystals employing ultrahigh vacuum scanning tunneling microscopy/spectroscopy (UHV-STM/STS) at room temperature are reported. The natural surface termination layer is unambiguously determined. STS measurements show that the as-prepared surfaces of Nd123 single crystals are semiconductive and are clean and stable in air. Our results suggest that Nd123 single crystals are good candidates for surface-sensitive scientific research.
1. Introduction Many experimental tools have been used to study the physical properties of high-temperature superconductors (HTSC) since the famous breakthrough made by Bednorz and Miiller [1]. However, owing to the highly reactive nature of most of the HTSC to air, surface-sensitive experimental tools, such as, for instance, scanning tunneling microscopy/spectroscopy (STM/STS), have not been playing the roles they deserve in the field of HTSC so far. Basic questions regarding the natural termination layer of HTSC single crystals and thin films and the corresponding surface electronic properties are still unclear. Recently, we found that single crystals of NdlBa2Cu3Oy (Nd123), grown under a reduced oxygen partial pressure and followed by an appropriate thermal treatment, possess a superior surface quality in terms of stability, flatness and cleanness
* Corresponding author. Fax: +81 3 3536 5717.
[2]. This opens a new way for surface scientists to be effectively involved in the exciting research field of high-Tc superconductivity. In this paper, we report some preliminary results of ultrahigh vacuum (UHV) S T M / S T S measurements on the surfaces of as-prepared Nd123 single crystals. The natural termination layer of the surfaces of the single crystals is identified to be the CuO-chain layer. STS measurements reveal that the surfaces are not metallic as predicted by band structure calculations, but rather semiconductive. The results of STM and STS measurements indicate that the surfaces of Nd123 single crystals are clean and stable in air.
2. Experimental Single crystals of Nd123 were fabricated using an improved traveling solvent floating zone method [3]. The T~onset of the crystals was 86 K determined from the standard four-probe resistivity measurements. The
0921-4534/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 0 8 6 - X
186
Wu Ting et al./Physica C 263 (1996) 185-188
samples were thermally treated at an oxygen pressure of one atmosphere at 450°C for 4 days before being transferred into the UHV-STM unit. No surface protection technique was used during sample transportation. The total exposure time of each sample to air was around ten minutes. S T M / S T S measurements were performed employing an UHV-STM/STS system (JSTM-4000XV) at room temperature at a pressure around 10 -8 Pa. Home-made ultra-sharp tungsten tips were used for all S T M / S T S measurements. In order to have good STM/STS results, it was found that the in situ heating of the tunneling tips up to 1300°C was very crucial.
3. Results and discussion The surfaces of the as-prepared Nd123 single crystals were rough in general. They contained many huge steps, holes and cracks, typically as shown in Fig. 1 observed under a normal optical microscope. It was quite difficult to perform STM measurements on the surfaces. After many attempts, only in some small regions could we find some highly clean and flat surfaces [2]. High resolution STM scans were done on these atomically flat surfaces. Fig. 2 shows an atomic image obtained on the matrix of the a-b-plane of Nd123 single crystals. A quasi-s.quare lattice with average lattice spacings of 4 × 4 A2 can be seen from the image. It is surprising
Fig. 1. A typical optical image of the as-prepared surfaces of Nd i Ba2Cu 3Oysingle crystals.
Fig. 2. An atomic image observed on the surfaces of Nd~BazCu 3Or single crystals by UHV-STM at room temperature using topographic mode. The size of the image is 38 × 38 ,~2. The image was taken with a sample bias voltage of 1.8 V and tunneling current of 0.1 nA.
that the a- and b-directions of the two-dimensional network are not perpendicular to each other, as one would have expected for a superconducting 123based single crystal. This type of tilting has been observed on the as-prepared surfaces of YI Ba2Cu3Oy (Y123) thin films [4,5] and the surfaces of a Y123 single crystal [6] cleaved at temperatures below 20 K in ultrahigh vacuum. The detailed reason for causing such a tilting in the surface lattice is still unknown at present and is one of the topics for further investigation. In addition to the two-dimensional network, a superimposed one-dimensional character can be clearly seen from Fig. 2. This character helps us to unambiguously determine the surface termination layer of the crystals as the CuO-chain layer, since only the CuO-chain layer in the Nd 123 structure has this property. This implies that under the conditions we used for the atomic imaging of the surfaces of Nd 123 single crystals, only one of the atomic species of Cu and O is observed. This is quite similar to the case of the cleaved surfaces of Bi2Sr2CaCu2Oy (Bi2212) single crystals [7].
Wu Ting et al. / Physica C 263 (1996) 185-188
Now, the question is which atomic species is observed here, Cu or O? To answer this question, it is important to know the projected density of states (DOS) for the CuO-chain layer of the Nd123 structure. This information is not available in the literature. Therefore, we use the calculated results for Y123 as an approximation. From the band structure calculations [8] of Y123, we know that for the energy state at 1.8 eV about the Fermi surface, the contribution to the DOS from Cu is almost the same as that from O. So the atomic image shown in Fig. 2 mainly reflects the difference in the atomic radius between Cu and O. Since the CuO-chain layer is not insulating, we adopt the covalent radii of Cu and O for comparison. It is obvious that the covalent radius of Cu (1.17 :k) is much larger that of O (0.73 ,~,). Therefore, we conclude that the atoms observed here are the Cu atoms. We also perform I - V STS measurements on the surfaces. A typical curve is shown in Fig. 3. The surface is not metallic as expected from band structure calculations [8], but rather semiconductive with an energy gap of about 120 meV. This is not completely unexpected, since the band structure calculations were done by assuming a perfect three-dimen-
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o o 0 O 0 D 0 O
°
O
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~
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o
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--
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.-
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• • • I • , 0
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0.2 0.4 0.6 0.8 Distance between Tip and Sample Surface (nm)
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Fig. 4. A typical l - d STS m e a s u r e d on the surfaces o f N d i B a 2 C u 3 O y single crystals. The STS is a result o f the averaging over 128 spectra taken at one point. The dashed line represents the best exponential fit with f o r m u l a l(d)= 1.112e -°6657a.
sional lattice, while in our case it is quasi-two-dimensional. Furthermore, the effect of the replacement of Y by Nd was not taken into account in the band structure calculations, neither was the effect of the surface defect structures [9]. There are two broad peaks near + 600 mV in Fig. 3. At present, we do not know what is the origin of the two peaks. To confirm that the surfaces are stable in air, l - d STS measurements are performed. It has been demonstrated that such measurements can be used to check whether a surface of HTSC is degraded or not [10,11]. Fig. 4 shows the typical l - d STS measured on the matrix of Nd123 single crystals. An exponential dependence is seen, as it should be for a clean and stable surface. From Fig. 4, we can extract an effective barrier height of 0.4 eV which is consistent with the results obtained on surface-protected Y123 thin films [10] and on the low-temperature-UHVcleaved Y 123 single crystals [ 12].
.I,,.1,,
200
400
600
800
4. Conclusions
Voltage (mV) Fig. 3. A typical I - V STS measured on the surfaces of N d I B a 2 C u 3 O y single crystals. S a m p l e bias voltage is 2 V a n d tunneling current is 0.16 nA. The STS is a result o f the a v e r a g i n g over 128 spectra taken at one point.
The natural surface termination layer of NdlBa 2CU3Oy (Nd123) single crystals has been identified to be the CuO chain layer by ultrahigh vacuum scanning tunneling microscopy (UHV-STM). The surface
188
Wu Ting et al./Physica C 263 (1996) 185-188
is s e m i c o n d u c t i v e with an energy gap of about 120 meV. O u r preliminary results suggest that the surfaces of Nd123 single crystals are clean and stable in air and are good candidates for surface-sensitive scientific measurements. Further m e a s u r e m e n t s at low temperatures are in progress.
Acknowledgements This work was supported by the New E n e r g y and Industrial T e c h n o l o g y D e v e l o p m e n t Organization ( N E D O ) for the R & D of Industrial Science and T e c h n o l o g y Frontier Program. We thank H. U n o k i and Y. M a t s u m u r a for assistance in preparing Nd t Ba2Cu 3Oy single crystals.
References [I] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1986) 189. Wu Ting et al., to be published. T. Egi et al., to be published. M. Nantoh et al., J. Appl. Phys. 75 (1994) 5227. H.P. Lang, H. Haefke, G. Leemann and H.J. Giintherodt, Physica C 194 (1992) 81. H.L. Edwards et al., Phys. Rev. Lett. 73 (1994) 1154. Wu Ting et al., J. Mater. Res. 10(1995) 817. L.F. Mattheiss and D.R. Hamann, Solid State Commun. 63 (1987) 395. Wu Ting et al., Mod. Phys. Lett. B, to be published. T.G. Miller, M. McElfresh and R. Reifenberger, Phys. Rev. B 48 (1993) 7499. Wu Ting et al., J. Mater. Sci. Lett., to be published. H.L. Edwards et al., Phys. Rev. Lett. 69 (1992) 2967.
Physica C
263 (1996) 185-188
STM/STS measurements on clean and stable surfaces of as-prepared NdlBa2Cu3Oysingle crystals Wu Ting *, T. Egi, R. Itti, K. Kuroda, N. Koshizuka, S. Tanaka Superconductivity Research Laboratory, International Superconductivity Technology Center (ISTEC), 1-10-13 Shinonome, Koto-ku, Tokyo 135, Japan
Abstract The preliminary results of the surface characterization of Nd~Ba~Cu3Oy (Nd123) single crystals employing ultrahigh vacuum scanning tunneling microscopy/spectroscopy (UHV-STM/STS) at room temperature are reported. The natural surface termination layer is unambiguously determined. STS measurements show that the as-prepared surfaces of Nd123 single crystals are semiconductive and are clean and stable in air. Our results suggest that Nd123 single crystals are good candidates for surface-sensitive scientific research.
1. Introduction Many experimental tools have been used to study the physical properties of high-temperature superconductors (HTSC) since the famous breakthrough made by Bednorz and Miiller [1]. However, owing to the highly reactive nature of most of the HTSC to air, surface-sensitive experimental tools, such as, for instance, scanning tunneling microscopy/spectroscopy (STM/STS), have not been playing the roles they deserve in the field of HTSC so far. Basic questions regarding the natural termination layer of HTSC single crystals and thin films and the corresponding surface electronic properties are still unclear. Recently, we found that single crystals of NdlBa2Cu3Oy (Nd123), grown under a reduced oxygen partial pressure and followed by an appropriate thermal treatment, possess a superior surface quality in terms of stability, flatness and cleanness
* Corresponding author. Fax: +81 3 3536 5717.
[2]. This opens a new way for surface scientists to be effectively involved in the exciting research field of high-Tc superconductivity. In this paper, we report some preliminary results of ultrahigh vacuum (UHV) S T M / S T S measurements on the surfaces of as-prepared Nd123 single crystals. The natural termination layer of the surfaces of the single crystals is identified to be the CuO-chain layer. STS measurements reveal that the surfaces are not metallic as predicted by band structure calculations, but rather semiconductive. The results of STM and STS measurements indicate that the surfaces of Nd123 single crystals are clean and stable in air.
2. Experimental Single crystals of Nd123 were fabricated using an improved traveling solvent floating zone method [3]. The T~onset of the crystals was 86 K determined from the standard four-probe resistivity measurements. The
0921-4534/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 0 8 6 - X
186
Wu Ting et al./Physica C 263 (1996) 185-188
samples were thermally treated at an oxygen pressure of one atmosphere at 450°C for 4 days before being transferred into the UHV-STM unit. No surface protection technique was used during sample transportation. The total exposure time of each sample to air was around ten minutes. S T M / S T S measurements were performed employing an UHV-STM/STS system (JSTM-4000XV) at room temperature at a pressure around 10 -8 Pa. Home-made ultra-sharp tungsten tips were used for all S T M / S T S measurements. In order to have good STM/STS results, it was found that the in situ heating of the tunneling tips up to 1300°C was very crucial.
3. Results and discussion The surfaces of the as-prepared Nd123 single crystals were rough in general. They contained many huge steps, holes and cracks, typically as shown in Fig. 1 observed under a normal optical microscope. It was quite difficult to perform STM measurements on the surfaces. After many attempts, only in some small regions could we find some highly clean and flat surfaces [2]. High resolution STM scans were done on these atomically flat surfaces. Fig. 2 shows an atomic image obtained on the matrix of the a-b-plane of Nd123 single crystals. A quasi-s.quare lattice with average lattice spacings of 4 × 4 A2 can be seen from the image. It is surprising
Fig. 1. A typical optical image of the as-prepared surfaces of Nd i Ba2Cu 3Oysingle crystals.
Fig. 2. An atomic image observed on the surfaces of Nd~BazCu 3Or single crystals by UHV-STM at room temperature using topographic mode. The size of the image is 38 × 38 ,~2. The image was taken with a sample bias voltage of 1.8 V and tunneling current of 0.1 nA.
that the a- and b-directions of the two-dimensional network are not perpendicular to each other, as one would have expected for a superconducting 123based single crystal. This type of tilting has been observed on the as-prepared surfaces of YI Ba2Cu3Oy (Y123) thin films [4,5] and the surfaces of a Y123 single crystal [6] cleaved at temperatures below 20 K in ultrahigh vacuum. The detailed reason for causing such a tilting in the surface lattice is still unknown at present and is one of the topics for further investigation. In addition to the two-dimensional network, a superimposed one-dimensional character can be clearly seen from Fig. 2. This character helps us to unambiguously determine the surface termination layer of the crystals as the CuO-chain layer, since only the CuO-chain layer in the Nd 123 structure has this property. This implies that under the conditions we used for the atomic imaging of the surfaces of Nd 123 single crystals, only one of the atomic species of Cu and O is observed. This is quite similar to the case of the cleaved surfaces of Bi2Sr2CaCu2Oy (Bi2212) single crystals [7].
Wu Ting et al. / Physica C 263 (1996) 185-188
Now, the question is which atomic species is observed here, Cu or O? To answer this question, it is important to know the projected density of states (DOS) for the CuO-chain layer of the Nd123 structure. This information is not available in the literature. Therefore, we use the calculated results for Y123 as an approximation. From the band structure calculations [8] of Y123, we know that for the energy state at 1.8 eV about the Fermi surface, the contribution to the DOS from Cu is almost the same as that from O. So the atomic image shown in Fig. 2 mainly reflects the difference in the atomic radius between Cu and O. Since the CuO-chain layer is not insulating, we adopt the covalent radii of Cu and O for comparison. It is obvious that the covalent radius of Cu (1.17 :k) is much larger that of O (0.73 ,~,). Therefore, we conclude that the atoms observed here are the Cu atoms. We also perform I - V STS measurements on the surfaces. A typical curve is shown in Fig. 3. The surface is not metallic as expected from band structure calculations [8], but rather semiconductive with an energy gap of about 120 meV. This is not completely unexpected, since the band structure calculations were done by assuming a perfect three-dimen-
o n o O O '
o o 0 O 0 D 0 O
°
O
@
~
% o
¢J
o
O
o
0
.,1.,.1..
--
-800 -600 - 4 0 0 - 2 0 0
0
187
100 ~
D
10,
.-
10-2
[.-.
o
o o 10_3
• • • I • , 0
~
• I • • • I • • • I • • .
0.2 0.4 0.6 0.8 Distance between Tip and Sample Surface (nm)
1
Fig. 4. A typical l - d STS m e a s u r e d on the surfaces o f N d i B a 2 C u 3 O y single crystals. The STS is a result o f the averaging over 128 spectra taken at one point. The dashed line represents the best exponential fit with f o r m u l a l(d)= 1.112e -°6657a.
sional lattice, while in our case it is quasi-two-dimensional. Furthermore, the effect of the replacement of Y by Nd was not taken into account in the band structure calculations, neither was the effect of the surface defect structures [9]. There are two broad peaks near + 600 mV in Fig. 3. At present, we do not know what is the origin of the two peaks. To confirm that the surfaces are stable in air, l - d STS measurements are performed. It has been demonstrated that such measurements can be used to check whether a surface of HTSC is degraded or not [10,11]. Fig. 4 shows the typical l - d STS measured on the matrix of Nd123 single crystals. An exponential dependence is seen, as it should be for a clean and stable surface. From Fig. 4, we can extract an effective barrier height of 0.4 eV which is consistent with the results obtained on surface-protected Y123 thin films [10] and on the low-temperature-UHVcleaved Y 123 single crystals [ 12].
.I,,.1,,
200
400
600
800
4. Conclusions
Voltage (mV) Fig. 3. A typical I - V STS measured on the surfaces of N d I B a 2 C u 3 O y single crystals. S a m p l e bias voltage is 2 V a n d tunneling current is 0.16 nA. The STS is a result o f the a v e r a g i n g over 128 spectra taken at one point.
The natural surface termination layer of NdlBa 2CU3Oy (Nd123) single crystals has been identified to be the CuO chain layer by ultrahigh vacuum scanning tunneling microscopy (UHV-STM). The surface
188
Wu Ting et al./Physica C 263 (1996) 185-188
is s e m i c o n d u c t i v e with an energy gap of about 120 meV. O u r preliminary results suggest that the surfaces of Nd123 single crystals are clean and stable in air and are good candidates for surface-sensitive scientific measurements. Further m e a s u r e m e n t s at low temperatures are in progress.
Acknowledgements This work was supported by the New E n e r g y and Industrial T e c h n o l o g y D e v e l o p m e n t Organization ( N E D O ) for the R & D of Industrial Science and T e c h n o l o g y Frontier Program. We thank H. U n o k i and Y. M a t s u m u r a for assistance in preparing Nd t Ba2Cu 3Oy single crystals.
References [I] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1986) 189. Wu Ting et al., to be published. T. Egi et al., to be published. M. Nantoh et al., J. Appl. Phys. 75 (1994) 5227. H.P. Lang, H. Haefke, G. Leemann and H.J. Giintherodt, Physica C 194 (1992) 81. H.L. Edwards et al., Phys. Rev. Lett. 73 (1994) 1154. Wu Ting et al., J. Mater. Res. 10(1995) 817. L.F. Mattheiss and D.R. Hamann, Solid State Commun. 63 (1987) 395. Wu Ting et al., Mod. Phys. Lett. B, to be published. T.G. Miller, M. McElfresh and R. Reifenberger, Phys. Rev. B 48 (1993) 7499. Wu Ting et al., J. Mater. Sci. Lett., to be published. H.L. Edwards et al., Phys. Rev. Lett. 69 (1992) 2967.