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Tunneling study of Co-substituted Bi2Sr2CaCu2O8
PHYSICA Physica C 282-287 (1997) 1489-1490
ELSEVIER
Tunneling study of Co-substituted Bi2Sr2CaCu208 N. Tsudaa, T. Arao a, T. Hosokawa a, Y. Shiina a, N. Matsuda a and D. Shimadab aDepartment of Applied Physics, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan bDepartment of Physics, Shizuoka University, 836 Ohya, Shizuoka 422, Japan Tunneling conductance was measured for Co-substituted Bi2Sr2CaCu2Os/ITO junctions. The results are analyzed by using the spectral function of the electron-phonon interaction for Co-free substance. The magnetic impurity is assumed only to break the pair. For fitting the experimental conductance, an inhomogeneous gap and a temperature-dependent barrier strength must be assumed. 1. I N T R O D U C T I O N The spectral function a 2 F of the electron-phonon interaction was determined in [1] for Bi2Sr2CaCu208 (Bi2212). Correspondence of a'2F with the phonon spectrum [2,3] is satisfactory [1,4]. The aim of this study is to see how well the magnetic impurity effect is explained by the conventional phonon
r.¢)
E
'
64
80C
~
I
,
~
40(3
theory with a use of this dr2F. 2. EXPERIMENTAL
20(3
METHOD
(b~ 45K-
Single crystals of Bi2Sr2CaCu2_xCoxO8 (Cox, x = 0.02, and 0.06) were grown by the usual self-flux method. X is a nominal value. The resistive transition temperatures Tc's were 81 K at zero resistance with a transition width of 3 K for Co0.02, and 75 K and 9 K for Co0.06. The tunnel junction was fabricated by sputtering ITO(In203:SnO2) on a plane parallel with the c-axis at 523 K. The crystal was cut in air 5 minutes before the sputtering. The current flowed macroscopically perpendicular to the c-axis. Electrons flow into the cuprate under a positive bias voltage. The contact area was typically
0
0
'
1~0
'
Voltage / m V Fig. 1 Differential conductance for Co0.06 (a) and Co0.02 (b). The origin of the ordinate is for the lowest line, and it was shifted every 10 mS (a) or 100 mS (b) for the others. The dashed line is a shifted 75 K line (a) or a smeared one over 50 mV (b). the decrease in Tc as 2 meV (Co0.02) and 7 meV (Co0.06) [5]. For the former, Tc is calculated to be
0.15 x 0.1 mm 2, and the junction resistance was 46
83.4 K, and the latter 73.7 K. When Z-p-I is 0, Tc is
f) (Co0.02) and 84 f) (Co0.06). After Au was evaporated onto r i o and the cuprate, four Cu wires were connected with silver paint. The measuring method is found in [1].
87.3 K [1]. Figure 1 shows dI/dV at various temperatures. Zero-bias conductance peak (ZBP) appears at higher temperatures. The ZBP will be extrinsic for the following reasons: The strength of ZBP differs from junction to junction, and there is even a case in which ZBP is scarcely discernible while a gap structure is clearer[l]. Just below To d//dV(0) is
3. RESULTS
AND
DISCUSSION
The pair breaking rate 2-p-1 was determined from 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. Pll S0921-4534(97)00851-4
1490
N. Tsuda et aL/Physica C 282-287 (1997) 1489-1490
larger than that at T>Te. These behaviors are exp-
~
lained when the barrier strength parameter Z [6] is nearly zero at Tc and increases as T decreases. The experimental dl/dV of Co0.06 can be fitted to that at 75 K above about 150 mV by subtracting a certain amount of constant conductance, typically a few % of that at 200 mV, as shown in Fig. 1. The normalization was done by using thus fitted conductance. For Co0.02, d(dl/dV)/dV at V>100 mV is temperature dependent, and the subtraction method cannot be applied. Therefore, we smeared dI/dV with widths 30 mV to 50 mV, and normalized dl/dV by the smeared ones at each temperature. Thus normalized conductance depends on the smearing width. Figure 2 shows the normalized dI/dV and the calculated dl/dV. For the calculation the tunneling matrix element was assumed to be constant. What are to be discussed are the sharpness of the edge peak and its position. For Co0.06 the experimental peak is as broad as the calculated peak except at 4.4 K. This broadening could neither be explained by an increase in z'p-1 below 10 K nor a decrease in Z. Therefore the excess broadness may be due to a distribution in the gap value at the contact area. The distribution in the gap value is also inferred for Co0.02. On the calculated dI/dV the phonon structures are discernible, for instance at 50 mV, but they are scarcely observable in the experimental dl/dV. The distribution in the gap value might mask the phonon structures. As for the separation between the edge peaks, 2Zlpp, the experimental one becomes narrower than the calculated one as temperature increases. Such a narrowing may occur when Z decreases. It must be noted that this extrinsic narrowing in Zlpp is different from the narrowing in the gap discussed in [7]. The temperature dependence of dl/dV for Co0.02 is similar to that for Co0.06 though the ambiguity due to the normalization procedure is large. Finally, 2Zlpp/kBTc is 6.54-0.4, 5.94-0.4, and 5.0__.0.3 for Bi2212 [1], Co0.02, and Co0.06, while a theoretical value is 5.84, 5.56, and 5.26, respectively. Boekholt et al. also reported a similar decrease in the ratio [8].
'
I
•
I
"
I
I
I
~
I
I
" .....
0
,0
,00,50
J
,60
Voltage/mV
Fig. 2 The normalized experimental dl/dV (solid line) and the calculated dI/dV (dashed line). (a) Co0.06, Z = 0.5, (b) Co0.02, Z - 0.8. 4. C O N C L U S I O N Tunneling conductance for Co substituted Bi2212 was observed and analyzed by the usual phonon theory. The magnetic impurity was assumed only to break a pair. For fitting the experimental conductance, gap must be assumed to be inhomogeneous at the contact and the barrier strength must weaken as temperature increases. REFERENCES 1. D. Shimada, Y. Shiina, A. Mottate, Y. Ohyagi and N. Tsuda, Phys. Rev. B51 (1995) 16495. 2. B. Renker, F. Gompf, D. Ewert, P. Adelmann, H. Schmidt, E. Gering and H. Mutka, Z. Phys. B77 (1989) 65. 3. F. W. de Wette, private communication. 4. D. Shimada, Y. Shiina, N. Miyakawa and N. Tsuda, Physica B219/220 (1996) 192. 5. Y. Shiina, N. Matsuda and Y. Oi Nakamura, Physica C212 (1993) 173. 6. G. E. Blonder, M. Tinkham and T. M. Klapwijk, Phys. Rev. B25 (1982) 4515. 7. Y. Shiina, D. Shimada, A. Mottate, Y. Ohyagi and N. Tsuda, J. Phys. Soc. Jpn. 64(1995) 2577. 8. M. Boekholt, Th. Bollmeier, L. Buschmann, M. Fleuster and G. Giintherodt, Physica C198 (1992) 33.
ELSEVIER
Tunneling study of Co-substituted Bi2Sr2CaCu208 N. Tsudaa, T. Arao a, T. Hosokawa a, Y. Shiina a, N. Matsuda a and D. Shimadab aDepartment of Applied Physics, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan bDepartment of Physics, Shizuoka University, 836 Ohya, Shizuoka 422, Japan Tunneling conductance was measured for Co-substituted Bi2Sr2CaCu2Os/ITO junctions. The results are analyzed by using the spectral function of the electron-phonon interaction for Co-free substance. The magnetic impurity is assumed only to break the pair. For fitting the experimental conductance, an inhomogeneous gap and a temperature-dependent barrier strength must be assumed. 1. I N T R O D U C T I O N The spectral function a 2 F of the electron-phonon interaction was determined in [1] for Bi2Sr2CaCu208 (Bi2212). Correspondence of a'2F with the phonon spectrum [2,3] is satisfactory [1,4]. The aim of this study is to see how well the magnetic impurity effect is explained by the conventional phonon
r.¢)
E
'
64
80C
~
I
,
~
40(3
theory with a use of this dr2F. 2. EXPERIMENTAL
20(3
METHOD
(b~ 45K-
Single crystals of Bi2Sr2CaCu2_xCoxO8 (Cox, x = 0.02, and 0.06) were grown by the usual self-flux method. X is a nominal value. The resistive transition temperatures Tc's were 81 K at zero resistance with a transition width of 3 K for Co0.02, and 75 K and 9 K for Co0.06. The tunnel junction was fabricated by sputtering ITO(In203:SnO2) on a plane parallel with the c-axis at 523 K. The crystal was cut in air 5 minutes before the sputtering. The current flowed macroscopically perpendicular to the c-axis. Electrons flow into the cuprate under a positive bias voltage. The contact area was typically
0
0
'
1~0
'
Voltage / m V Fig. 1 Differential conductance for Co0.06 (a) and Co0.02 (b). The origin of the ordinate is for the lowest line, and it was shifted every 10 mS (a) or 100 mS (b) for the others. The dashed line is a shifted 75 K line (a) or a smeared one over 50 mV (b). the decrease in Tc as 2 meV (Co0.02) and 7 meV (Co0.06) [5]. For the former, Tc is calculated to be
0.15 x 0.1 mm 2, and the junction resistance was 46
83.4 K, and the latter 73.7 K. When Z-p-I is 0, Tc is
f) (Co0.02) and 84 f) (Co0.06). After Au was evaporated onto r i o and the cuprate, four Cu wires were connected with silver paint. The measuring method is found in [1].
87.3 K [1]. Figure 1 shows dI/dV at various temperatures. Zero-bias conductance peak (ZBP) appears at higher temperatures. The ZBP will be extrinsic for the following reasons: The strength of ZBP differs from junction to junction, and there is even a case in which ZBP is scarcely discernible while a gap structure is clearer[l]. Just below To d//dV(0) is
3. RESULTS
AND
DISCUSSION
The pair breaking rate 2-p-1 was determined from 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. Pll S0921-4534(97)00851-4
1490
N. Tsuda et aL/Physica C 282-287 (1997) 1489-1490
larger than that at T>Te. These behaviors are exp-
~
lained when the barrier strength parameter Z [6] is nearly zero at Tc and increases as T decreases. The experimental dl/dV of Co0.06 can be fitted to that at 75 K above about 150 mV by subtracting a certain amount of constant conductance, typically a few % of that at 200 mV, as shown in Fig. 1. The normalization was done by using thus fitted conductance. For Co0.02, d(dl/dV)/dV at V>100 mV is temperature dependent, and the subtraction method cannot be applied. Therefore, we smeared dI/dV with widths 30 mV to 50 mV, and normalized dl/dV by the smeared ones at each temperature. Thus normalized conductance depends on the smearing width. Figure 2 shows the normalized dI/dV and the calculated dl/dV. For the calculation the tunneling matrix element was assumed to be constant. What are to be discussed are the sharpness of the edge peak and its position. For Co0.06 the experimental peak is as broad as the calculated peak except at 4.4 K. This broadening could neither be explained by an increase in z'p-1 below 10 K nor a decrease in Z. Therefore the excess broadness may be due to a distribution in the gap value at the contact area. The distribution in the gap value is also inferred for Co0.02. On the calculated dI/dV the phonon structures are discernible, for instance at 50 mV, but they are scarcely observable in the experimental dl/dV. The distribution in the gap value might mask the phonon structures. As for the separation between the edge peaks, 2Zlpp, the experimental one becomes narrower than the calculated one as temperature increases. Such a narrowing may occur when Z decreases. It must be noted that this extrinsic narrowing in Zlpp is different from the narrowing in the gap discussed in [7]. The temperature dependence of dl/dV for Co0.02 is similar to that for Co0.06 though the ambiguity due to the normalization procedure is large. Finally, 2Zlpp/kBTc is 6.54-0.4, 5.94-0.4, and 5.0__.0.3 for Bi2212 [1], Co0.02, and Co0.06, while a theoretical value is 5.84, 5.56, and 5.26, respectively. Boekholt et al. also reported a similar decrease in the ratio [8].
'
I
•
I
"
I
I
I
~
I
I
" .....
0
,0
,00,50
J
,60
Voltage/mV
Fig. 2 The normalized experimental dl/dV (solid line) and the calculated dI/dV (dashed line). (a) Co0.06, Z = 0.5, (b) Co0.02, Z - 0.8. 4. C O N C L U S I O N Tunneling conductance for Co substituted Bi2212 was observed and analyzed by the usual phonon theory. The magnetic impurity was assumed only to break a pair. For fitting the experimental conductance, gap must be assumed to be inhomogeneous at the contact and the barrier strength must weaken as temperature increases. REFERENCES 1. D. Shimada, Y. Shiina, A. Mottate, Y. Ohyagi and N. Tsuda, Phys. Rev. B51 (1995) 16495. 2. B. Renker, F. Gompf, D. Ewert, P. Adelmann, H. Schmidt, E. Gering and H. Mutka, Z. Phys. B77 (1989) 65. 3. F. W. de Wette, private communication. 4. D. Shimada, Y. Shiina, N. Miyakawa and N. Tsuda, Physica B219/220 (1996) 192. 5. Y. Shiina, N. Matsuda and Y. Oi Nakamura, Physica C212 (1993) 173. 6. G. E. Blonder, M. Tinkham and T. M. Klapwijk, Phys. Rev. B25 (1982) 4515. 7. Y. Shiina, D. Shimada, A. Mottate, Y. Ohyagi and N. Tsuda, J. Phys. Soc. Jpn. 64(1995) 2577. 8. M. Boekholt, Th. Bollmeier, L. Buschmann, M. Fleuster and G. Giintherodt, Physica C198 (1992) 33.