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In-plane tunneling spectroscopy in Bi2Sr2CaCu2O8+δ–SiO–Ag planar junctions
Physica C 357±360 (2001) 205±207
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In-plane tunneling spectroscopy in Bi2Sr2CaCu2O8d±SiO±Ag planar junctions I. Shigeta a,*, T. Uchida a, Y. Tominari a, T. Arai a, F. Ichikawa b, T. Fukami c, T. Aomine a, V.M. Svistunov d a
Department of Chemistry and Physics of Condensed matter, Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan b Department of Physics, Faculty of Science, Kumamoto University, Kumamoto 860-8555, Japan c Department of Materials Science and Engineering, Himeji Institute of Technology, Himeji 671-2201, Japan d A. Galkin Physico-Technical Institute, National Academy of Sciences of Ukraine, 340114 Donetsk, Ukraine Received 16 October 2000; accepted 15 January 2001
Abstract We report the experimental results of quasiparticle tunneling using Bi2 Sr2 CaCu2 O8d ±SiO±Ag planar junctions. We have measured the {0 0 1}-, {1 0 0}- and {1 1 0}-oriented junctions. For the in-plane ab oriented junctions, a reproducible zero bias conductance peak has been observed. We have analyzed the data by using the extended Andreev bound states model for d-wave superconductors and have considered the roughness eect by averaging over in-plane directions. The experimental results are consistent with the theoretical calculations. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.25.Fy; 74.50.+r; 74.72.Hs Keywords: Zero bias conductance peak; Andreev re¯ection; Roughness eect; d-wave superconductor
1. Introduction A zero bias conductance peak (ZBCP) has been often observed in experimental tunneling spectra from junctions on high-Tc superconductors. Recently, the ZBCP has been theoretically explained by the Andreev bound states (ABS) model based on a d-wave pairing symmetry [1]. In recent research on in-plane directions, most of ZBCP measurements which were examined with the ABS model were mainly reported on oriented
*
Corresponding author. Fax: +81-92-642-2553. E-mail address: [email protected] (I. Shigeta).
YBa2 Cu3 O7 d (YBCO) thin ®lms [2], because an ideal oriented surface was obtained for oriented YBCO thin ®lms as compared to other sample preparations. On the other hand, for Bi2 Sr2 CaCu2 O8d (BSCCO), it was hard to obtain an ideal oriented surface except for the cleavage surface of single crystals. Thus, although there are only a few reports of in-plane tunneling experiments on BSCCO, these results have varied from a gaplike structure to the ZBCP [3,4]. We have also reported the ZBCP in the conductance of in-plane directions [5] and our junctions retain surface roughness. Here, we analyze the data of the in-plane tunneling of BSCCO by taking into consideration a roughness eect of the interface, because the
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 0 7 - 6
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I. Shigeta et al. / Physica C 357±360 (2001) 205±207
importance of this eect seems not to be negligible and recent theoretical studies also support the roughness eect [6]. 2. Results and discussion The tunnel junctions were fabricated ex situ on the single crystal with Tc of about 87 K by the same procedure in Ref. [5]. The typical conductance spectrum tunneling into the {0 0 1} interface has a V-shaped gap structure as expected for the d-wave pairing symmetry. In our experiments Dp±p de®ned to be the peak-to-peak value of the energy gap has a value of 21.7 meV. On the other hand, Fig. 1 shows the typical dynamic conductance versus voltage characteristic of the {1 1 0}-oriented junction. Concerning the spectra with the ZBCP, there was no great dierence for spectral shapes between the {1 0 0}- and {1 1 0}-oriented junctions. The ZBCP disappears at the {1 0 0} or {0 1 0} interface for the dx2 y 2 pairing model. Hence, it is possible that the junctions with the {1 0 0} or {1 1 0} interface include other tunneling directions in the microscopic scale because of the roughness of the interface. The polished surface of in-plane junctions was observed with scanning electron microscope (SEM) and this surface had the roughness of submicron order. However, we can assume that our results for the {1 0 0} or {1 1 0} interface mainly include the in-plane tunneling instead of the out-of-plane tunneling [5]. Thus, the ZBCP is
Fig. 1. Typical experimental spectrum of the {1 1 0}-oriented junction has the enhanced ZBCP.
an intrinsic property for the in-plane tunneling, and this strongly suggests that the pairing symmetry of BSCCO is the d-wave pairing one. We obtained an angle between an a-axis and a normal vector of the interface is 27:0° from the ®tting result [5], and this suggests that a current only with a particular angle mainly in¯uences tunnel conductance. However, as mentioned above, there was no great dierence for spectral shapes among in-plane directions and the ZBCP height depends on this angle. Hence, the tunneling current has rather distribution of any angles than a particular angle. Thus, as an extended simple model, we introduce the roughness eect by averaging over in-plane directions because the CuO2 plane has strong two-dimensional superconductivity. A dynamic conductance r eV is written by using an expectation value ri eV ; a which is same as that of Eq. (2) in Ref. [5]. r eV
rS eV ; rN eV
ri eV
1 p
Z
p=2b p=2b
1
dae
c a b2
ri eV ; a;
2
where c relates to the spread of tunneling directions in k-space, a is an angle between an a-axis of the single crystal and a local normal vector of the rough interface and b is an angle between an a-axis and a normal vector of the cut surface, for example, the {1 0 0} or {1 1 0} surface. The angles a and b are illustrated in Fig. 2. The subscript i is replaced with S for a superconducting state or N for a normal state. Spectral ®ts of the ABS model to our data are shown in Fig. 3. The solid circles represent the normalized experimental spectrum, which normalized by a parabolic background, for the {1 1 0}-oriented junction in Fig. 1. At T 5:0 K, the peak height is 2.1 times higher than the background and the full-width at half maximum is 6.5 meV. Two calculated results which are ®tted to the ZBCP for the {1 1 0}-oriented junction are shown in this ®gure. In the ®rst case that junction has ideal and ¯at interface, we calculated using r eV rS eV ; a=rN eV ; a at a 45:0° for Eq. (1) in Ref. [5]. The broken line indicates this result
I. Shigeta et al. / Physica C 357±360 (2001) 205±207
Fig. 2. Schematic illustration of the interface with the roughness. The quantities a and b are illustrated as the visible angles.
207
using Eqs. (1) and (2) at b 45:0° is indicated by the solid line in Fig. 3. In this case, the energy gap has the value of D0 19:4 meV. Thus, for 2D0 =kB Tc a value of about 5.2 was obtained. By averaging over a, the value of D0 tended to increase in ®tting results and this was close to the value of Dp±p of the {0 0 1} junction. We could also obtain similar ®tting results for other background, for example, linear background, and for other tunnel junctions with the ZBCP. These results seem to support rather the roughness eect than the suppression of the superconductivity, because fabricated junctions actually retain the submicron order roughness and D0 of the {1 1 0}-oriented junction coincides with Dp±p of the {0 0 1}-oriented junction. The latter model then describes the more real condition of the in-plane tunnel junctions with the rough interface. 3. Conclusion
Fig. 3. A comparison of calculated spectra with experimental one. Solid circles represent the normalized experimental spectrum in Fig. 1. The solid line and the broken line represent the calculations of the extended ABS model and the ABS model, respectively, where C is a broadening parameter, Z a potential height and k relates to the spread of tunneling directions in kspace.
with the energy gap D0 9:8 meV. Thus, for 2D0 =kB Tc a value of about 2.6 was obtained. This value is smaller than the value expected from the BCS theory. This result seems to suggest the possibility of the suppression of the superconductivity at the interface due to polishing the surface. In the next case, taking into consideration the surface observation of junctions with SEM and the experimental results of the existence of the ZBCP in both of in-plane directions, the calculation needs to introduce the roughness eect. This calculation
We have observed the reproducible ZBCP only in the {1 0 0}- and {1 1 0}-oriented junctions of BSCCO single crystals. As shown in Fig. 1, the ZBCP is an intrinsic property of the in-plane tunnel directions. Our experimental results agree with the extended ABS model which was based on the d-wave pairing symmetry. We have considered the roughness eect of the interface by averaging over in-plane directions. Acknowledgements We are grateful to T. Kawae and A. Nakagawa for support and advice during the experiments. We would like to thank Y. Tanaka, M. Suzuki, S. Tanaka, M. Taira and A. Suzuki for helpful discussions. References [1] [2] [3] [4] [5] [6]
Y. Tanaka, S. Kashiwaya, Phys. Rev. Lett. 74 (1995) 3451. I. Iguchi, et al., Phys. Rev. B 62 (2000-II) R6131. A.K. Gupta, K.W. Ng, Phys. Rev. B 58 (1998-II) R8901. S. Sinha, K.W. Ng, Phys. Rev. Lett. 80 (1998) 1296. I. Shigeta, et al., J. Phys. Soc. Jpn. 69 (2000) 2743. Y. Tanuma, et al., Phys. Rev. B 60 (1999) 9817.
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In-plane tunneling spectroscopy in Bi2Sr2CaCu2O8d±SiO±Ag planar junctions I. Shigeta a,*, T. Uchida a, Y. Tominari a, T. Arai a, F. Ichikawa b, T. Fukami c, T. Aomine a, V.M. Svistunov d a
Department of Chemistry and Physics of Condensed matter, Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan b Department of Physics, Faculty of Science, Kumamoto University, Kumamoto 860-8555, Japan c Department of Materials Science and Engineering, Himeji Institute of Technology, Himeji 671-2201, Japan d A. Galkin Physico-Technical Institute, National Academy of Sciences of Ukraine, 340114 Donetsk, Ukraine Received 16 October 2000; accepted 15 January 2001
Abstract We report the experimental results of quasiparticle tunneling using Bi2 Sr2 CaCu2 O8d ±SiO±Ag planar junctions. We have measured the {0 0 1}-, {1 0 0}- and {1 1 0}-oriented junctions. For the in-plane ab oriented junctions, a reproducible zero bias conductance peak has been observed. We have analyzed the data by using the extended Andreev bound states model for d-wave superconductors and have considered the roughness eect by averaging over in-plane directions. The experimental results are consistent with the theoretical calculations. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.25.Fy; 74.50.+r; 74.72.Hs Keywords: Zero bias conductance peak; Andreev re¯ection; Roughness eect; d-wave superconductor
1. Introduction A zero bias conductance peak (ZBCP) has been often observed in experimental tunneling spectra from junctions on high-Tc superconductors. Recently, the ZBCP has been theoretically explained by the Andreev bound states (ABS) model based on a d-wave pairing symmetry [1]. In recent research on in-plane directions, most of ZBCP measurements which were examined with the ABS model were mainly reported on oriented
*
Corresponding author. Fax: +81-92-642-2553. E-mail address: [email protected] (I. Shigeta).
YBa2 Cu3 O7 d (YBCO) thin ®lms [2], because an ideal oriented surface was obtained for oriented YBCO thin ®lms as compared to other sample preparations. On the other hand, for Bi2 Sr2 CaCu2 O8d (BSCCO), it was hard to obtain an ideal oriented surface except for the cleavage surface of single crystals. Thus, although there are only a few reports of in-plane tunneling experiments on BSCCO, these results have varied from a gaplike structure to the ZBCP [3,4]. We have also reported the ZBCP in the conductance of in-plane directions [5] and our junctions retain surface roughness. Here, we analyze the data of the in-plane tunneling of BSCCO by taking into consideration a roughness eect of the interface, because the
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 0 7 - 6
206
I. Shigeta et al. / Physica C 357±360 (2001) 205±207
importance of this eect seems not to be negligible and recent theoretical studies also support the roughness eect [6]. 2. Results and discussion The tunnel junctions were fabricated ex situ on the single crystal with Tc of about 87 K by the same procedure in Ref. [5]. The typical conductance spectrum tunneling into the {0 0 1} interface has a V-shaped gap structure as expected for the d-wave pairing symmetry. In our experiments Dp±p de®ned to be the peak-to-peak value of the energy gap has a value of 21.7 meV. On the other hand, Fig. 1 shows the typical dynamic conductance versus voltage characteristic of the {1 1 0}-oriented junction. Concerning the spectra with the ZBCP, there was no great dierence for spectral shapes between the {1 0 0}- and {1 1 0}-oriented junctions. The ZBCP disappears at the {1 0 0} or {0 1 0} interface for the dx2 y 2 pairing model. Hence, it is possible that the junctions with the {1 0 0} or {1 1 0} interface include other tunneling directions in the microscopic scale because of the roughness of the interface. The polished surface of in-plane junctions was observed with scanning electron microscope (SEM) and this surface had the roughness of submicron order. However, we can assume that our results for the {1 0 0} or {1 1 0} interface mainly include the in-plane tunneling instead of the out-of-plane tunneling [5]. Thus, the ZBCP is
Fig. 1. Typical experimental spectrum of the {1 1 0}-oriented junction has the enhanced ZBCP.
an intrinsic property for the in-plane tunneling, and this strongly suggests that the pairing symmetry of BSCCO is the d-wave pairing one. We obtained an angle between an a-axis and a normal vector of the interface is 27:0° from the ®tting result [5], and this suggests that a current only with a particular angle mainly in¯uences tunnel conductance. However, as mentioned above, there was no great dierence for spectral shapes among in-plane directions and the ZBCP height depends on this angle. Hence, the tunneling current has rather distribution of any angles than a particular angle. Thus, as an extended simple model, we introduce the roughness eect by averaging over in-plane directions because the CuO2 plane has strong two-dimensional superconductivity. A dynamic conductance r eV is written by using an expectation value ri eV ; a which is same as that of Eq. (2) in Ref. [5]. r eV
rS eV ; rN eV
ri eV
1 p
Z
p=2b p=2b
1
dae
c a b2
ri eV ; a;
2
where c relates to the spread of tunneling directions in k-space, a is an angle between an a-axis of the single crystal and a local normal vector of the rough interface and b is an angle between an a-axis and a normal vector of the cut surface, for example, the {1 0 0} or {1 1 0} surface. The angles a and b are illustrated in Fig. 2. The subscript i is replaced with S for a superconducting state or N for a normal state. Spectral ®ts of the ABS model to our data are shown in Fig. 3. The solid circles represent the normalized experimental spectrum, which normalized by a parabolic background, for the {1 1 0}-oriented junction in Fig. 1. At T 5:0 K, the peak height is 2.1 times higher than the background and the full-width at half maximum is 6.5 meV. Two calculated results which are ®tted to the ZBCP for the {1 1 0}-oriented junction are shown in this ®gure. In the ®rst case that junction has ideal and ¯at interface, we calculated using r eV rS eV ; a=rN eV ; a at a 45:0° for Eq. (1) in Ref. [5]. The broken line indicates this result
I. Shigeta et al. / Physica C 357±360 (2001) 205±207
Fig. 2. Schematic illustration of the interface with the roughness. The quantities a and b are illustrated as the visible angles.
207
using Eqs. (1) and (2) at b 45:0° is indicated by the solid line in Fig. 3. In this case, the energy gap has the value of D0 19:4 meV. Thus, for 2D0 =kB Tc a value of about 5.2 was obtained. By averaging over a, the value of D0 tended to increase in ®tting results and this was close to the value of Dp±p of the {0 0 1} junction. We could also obtain similar ®tting results for other background, for example, linear background, and for other tunnel junctions with the ZBCP. These results seem to support rather the roughness eect than the suppression of the superconductivity, because fabricated junctions actually retain the submicron order roughness and D0 of the {1 1 0}-oriented junction coincides with Dp±p of the {0 0 1}-oriented junction. The latter model then describes the more real condition of the in-plane tunnel junctions with the rough interface. 3. Conclusion
Fig. 3. A comparison of calculated spectra with experimental one. Solid circles represent the normalized experimental spectrum in Fig. 1. The solid line and the broken line represent the calculations of the extended ABS model and the ABS model, respectively, where C is a broadening parameter, Z a potential height and k relates to the spread of tunneling directions in kspace.
with the energy gap D0 9:8 meV. Thus, for 2D0 =kB Tc a value of about 2.6 was obtained. This value is smaller than the value expected from the BCS theory. This result seems to suggest the possibility of the suppression of the superconductivity at the interface due to polishing the surface. In the next case, taking into consideration the surface observation of junctions with SEM and the experimental results of the existence of the ZBCP in both of in-plane directions, the calculation needs to introduce the roughness eect. This calculation
We have observed the reproducible ZBCP only in the {1 0 0}- and {1 1 0}-oriented junctions of BSCCO single crystals. As shown in Fig. 1, the ZBCP is an intrinsic property of the in-plane tunnel directions. Our experimental results agree with the extended ABS model which was based on the d-wave pairing symmetry. We have considered the roughness eect of the interface by averaging over in-plane directions. Acknowledgements We are grateful to T. Kawae and A. Nakagawa for support and advice during the experiments. We would like to thank Y. Tanaka, M. Suzuki, S. Tanaka, M. Taira and A. Suzuki for helpful discussions. References [1] [2] [3] [4] [5] [6]
Y. Tanaka, S. Kashiwaya, Phys. Rev. Lett. 74 (1995) 3451. I. Iguchi, et al., Phys. Rev. B 62 (2000-II) R6131. A.K. Gupta, K.W. Ng, Phys. Rev. B 58 (1998-II) R8901. S. Sinha, K.W. Ng, Phys. Rev. Lett. 80 (1998) 1296. I. Shigeta, et al., J. Phys. Soc. Jpn. 69 (2000) 2743. Y. Tanuma, et al., Phys. Rev. B 60 (1999) 9817.