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Optical studies of high-Tc oxide superconductors
Physica 148B (1987) 282-284 North-Holland, Amsterdam
O P T I C A L S T U D I E S OF H I G H - T c O X I D E S U P E R C O N D U C T O R S
S. SUGAI, S. UCHIDA*, H. TAKAGI*, K. KITAZAWA*, S. TANAKA*, M. SATO** and S. HOSOYA*** Department of Physics, Faculty of Science, Osaka University, Machikaneyama, Toyonaka 560, Japan *Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113, Japan **Institute for Molecular Science, Myodaiji, Okazaki 444, Japan *** The Research Institute for Iron, Steel and Other Metals, Tohoku University, Katahira, Sendai 980, Japan Received 3 August 1987 Revised manuscript received 17 August 1987
Oxygen deficiency effects on the superconductivity of YBa2Cu3O 7. ~ are investigated by the electrical resistivity and the infrared reflection spectroscopy. On increasing the quenching temperature the sample changes to a semiconductor from a metal. The superconducting gap of the slowly cooled sample is anisotropic and the minimum gap is 2A/kaT~ ~ 1.6. The anisotropy of the sample quenched from 600°C is very small in the proximity-coupled superconducting region and the
2 A / k , T c ~ 1.
Recent remarkable increase of Tc in oxide superconductors, (Lal_xMx)2CuO4 and YBa 2Cu307_ ~ [1,2], has stimulated enormous experimental and theoretical studies. Existence of many kinds of structures and oxygen deficiency of these materials give variety of superconducting, metallic and semiconducting properties. We present the results of infrared reflection spectroscopy for the oxygen deficiency effects on the superconductivity of YBazCu307_ ~. The sample annealed at low temperatures has orthorhombic structure with C u - O linear chains [3]. The oxygen deficiency 6 is about 0.07 [4]. With increasing temperature the oxygen deficiency increases monotonically and the structure changes into tetragonal at about 575-670°C [57]. In the tetragonal phase the linear chains disappear and oxygen atoms distribute randomly in the C u - O layers. The oxygen deficiency 6 is about 0.70 at 950°C in air [4]. The occupation factor of oxygen is about 93% for samples annealed below 400°C, about 58% on the chains at the phase transition temperature and only about 15% at 950°C in air. Samples were prepared using reagents of Y 2 0 3 , BaCO 3 and CuO with 99.9% purity. The mixed powder was calcined at 900°C for 20 h and 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
then pulverized. The reactant powder was pressed into a pellet of a diameter of 10 mm and a thickness of 2mm. The pellet was calcined again at 950°C for 24 h followed by annealing at various temperatures for 12 h in air and quenching into liquid nitrogen. Fig. 1 shows the resistivity of Y B a 2 C u 3 0 7 _ ~ samples quenched from 950, 800,700 and 600°C, and cooled slowly in a furnace after annealing at 350°C. The slowly cooled sample shows a superconducting transition at 92 K. The sample quenched from 600°C shows aresistivity drop by double steps and undergoes a superconducting state at 61 K. The temperature region between 92 and 61 K is a proximity-coupled superconducting region [8]. The sample quenched from 700°C shows a resistivity increase below 90 K followed by a superconducting transition at 46K. The resistivity of the sample quenched from 800°C decreases at first and then increases below 105 K. The samples quenched from 800 and 950°C do not show the superconducting transition down to 4.2 K. The difference of the temperature-dependent resistivity with other experiment [9] is due to the difference of the quenching speed. Fig. 2 shows the Im(e), imaginary part of the dielectric constant, of the YBa2Cu3Ov_ ~ sample
S. Sugai et al. / Optical studies o f high-T c oxide superconductors 10
283
200 ;
7-6
YBa2Cu3OT_
6
350 *C SC
i i i i
td
'~ 100 %%
950"C O "~
10 -1
800 "C O 0
I
0 m.-
i0_2
I
~
100
200
300 400 500 ENERGY ( c m - = )
600
700
Fig. 2. Ira(e) of the slowly cooled sample at 65 K (solid curve), 79 K (dot-dashed curve) and 105 K (dashed curve). The Im(e) was calculated by the Kramers-Kronig transformation from the reflection spectra of the samples cooled in a furnace after annealing at 350°C.
700 "c o
350 °C SC
10-3
10-4
I I J I
0
I
100 200 TEMPERATURE (K)
300
Fig. 1. Electrical resistivity for the samples quenched from 950, 800, 700, 600°C and cooled slowly in a furnace after annealing at 350°C.
cooled slowly in a furnace after annealing at 350°C. The Im(e) was calculated from the reflection spectra by the K r a m e r s - K r o n i g transformation. The reflectivity increases at low energies on approaching the T c and stays 100% at low energies below the T c. The small change of reflectivity is enhanced in the Im(e), when the reflectivity is close to 100%. The reflectivity curves of the non-superconducting state (105 K) and the superconducting state ( 7 9 K ) are crossing near 250 c m - 1. The calculated Im(e) at 79 K are smaller than that at 105 K in the whole spectral range of fig. 2. It is due to the anisotropic superconductivity [10]. The ratio of the Ira(e) at 79 K to that at 105 K starts to decrease at about 450cm - 1 The Im(e) at 7 9 K reaches the maximum at about 160 cm -] excluding the phonon peaks and goes to zero at 70 cm -]. The ratio of the Im(e) at
65 K to that at 79 K starts to decrease at about 300cm 1. The Im(e) at 65 K reaches the maximum at about 175 c m -1 and goes to zero at 84cm 1. The minimum gap 2 A / k B T c extrapolated to 0 K is 1.7 for the measurement at 79 K and 1.6 at 65 K. The peaks of the Im(e) are close to the superconducting energy gap 2A, about 142cm -I at 7 9 K and about 187cm -I at 6 5 K , calculated by the weak coupled BCS model. Fig. 3 shows the Im(e) of the sample quenched from 600°C. The spectra resemble those by Bonn et al. [11]. This sample is a mixture of the 92 K superconductor phase and a low-temperature superconductor phase with the Tc about 60 K. At temperatures between 92 and 61 K the induced pair potential in the non-superconducting material has a finite value lower than that of the 92 K superconductor. The energies of I m ( e ) = 0 are 4 2 c m -1 at 7 9 K and 4 6 c m 1 at 65K. The decrease of Im(e) in a wide energy range is not observed in the superconducting state compared to the non-superconducting state. It suggests the almost extinction of the anisotropy [10] observed in the slowly cooled YBa2Cu3OT_ ~ sample. In the superconducting phase a broad band appears near 430cm -1, the peak at 316cm -1 shifts to 310cm I and the peak at 193cm 1 to 190cm -l.
284
S. Sugai et al. / Optical studies o f h i g h - T oxide superconductors
200
c o o l e d s a m p l e is a l m o s t missing in t h e p r o x i m i t y c o u p l e d r e g i o n of t h e s a m p l e q u e n c h e d f r o m 600°C.
, t
YBa2Cu307-6 ',
600
*C Q
i
' it I
References
Ii,x
'~
~100
i o
0
//
' 100
'
, 200
'
' 300
'
' 400
'
. . . . 500 600
700
ENERGY ( c m - J )
Fig. 3. Im(e) of the sample quenched from 600°C at 65 K (solid curve), 79K (dot-dashed curve) and 105K (dashed curve). T h e s a m p l e q u e n c h e d f r o m 700°C d o e s n o t s h o w t h e sign o f s u p e r c o n d u c t i n g g a p d o w n to 66 K in the i n f r a r e d s p e c t r a . O n l y t h e i n c r e a s e of reflectivity b e l o w 4 0 c m 1 is o b s e r v e d at 6 6 K . The superconducting energy gap, 2A/kBT c = 3 . 7 - 5 . 6 , o b t a i n e d b y t u n n e l i n g s p e c t r o s c o p y is always larger than the gap, 2A/kBT c --1.6-3.4, m e a s u r e d b y i n f r a r e d s p e c t r o s c o p y [12]. This d i f f e r e n c e has b e e n e x p l a i n e d b y t h e a n i s o t r o p i c p a i r i n g [10]. T h e p r e s e n t e x p e r i m e n t s h o w s t h a t t h e m i n i m u m g a p m e a s u r e d b y t h e i n f r a r e d spect r o s c o p y is 2 A / k B T c ~ 1.6 for t h e s a m p l e c o o l e d slowly a n d - 1 for t h e s a m p l e q u e n c h e d f r o m 600°C. T h e a n i s o t r o p y o b s e r v e d in the slowly
[1] J.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1987) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, EH. Hot, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. B 58 (1987) 908. [3] F. Izumi, H. Asano, T. Ishigaki, E. TakayamaMuromachi, Y. Uchida, N. Watanabe and T. Nishikawa, Jpn. J. Appl. Phys. 26 (1987) L649. [4] K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) L1228. [5] E. Takayama-Muromachi, Y. Uchida, K. Yukino, T. Tanaka and K. Kato, Jpn. J. Appl. Phys. 26 (1987) L665. E. Takayama-Muromachi, Y. Uchida, M, Ishii, T. Tanaka and K. Kato, Jpn. J. Appl. Phys. 26 (1987) L1156, [6] S. Sueno, 1. Nakai, F.P. Okamura and A. Ono, Jpn. J. Appl. Phys. 26 (1987) L842. {7] K. Yukino, T. Sato, S. Ooba, M. Ohta, F.P. Okamura and A. Ono, Jpn, J. Appl. Phys. 26 (1987) L869. [8} D.J. Resnick, J.C. Garland, J.T. Boyd, S. Shoemaker and R.S. Newrock, Phys. Rev. Lett. 47 (1981) 1542. [91 H. Sawada, T. Iwazumi, Y. Saito, Y. Abe, H. Ikeda and R. Yoshizaki, Jpn. J. Appl. Phys. 26 (1987) L1054. [10] H. Ebisawa, Y. Isawa and S. Maekawa, Jpn. J. Appl. Phys. 26 (1987) L992. [11] D.A. Bonn, J.E. Greedan, C.V. Stager, T. Timusk, M.G. Doss, S.L. Herr, K. Kamarfis and D.B. Tanner, Phys. Rev. Lett. 58 (1987) 2249. [12] J.R. Kirtley, R.T. Collins, Z. Schlesinger, W.J. Gallagher, R.L. Sandstrom, T.R. Dinger and D.A. Chance, Phys. Rev. B 35 (1987) 8846.
O P T I C A L S T U D I E S OF H I G H - T c O X I D E S U P E R C O N D U C T O R S
S. SUGAI, S. UCHIDA*, H. TAKAGI*, K. KITAZAWA*, S. TANAKA*, M. SATO** and S. HOSOYA*** Department of Physics, Faculty of Science, Osaka University, Machikaneyama, Toyonaka 560, Japan *Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113, Japan **Institute for Molecular Science, Myodaiji, Okazaki 444, Japan *** The Research Institute for Iron, Steel and Other Metals, Tohoku University, Katahira, Sendai 980, Japan Received 3 August 1987 Revised manuscript received 17 August 1987
Oxygen deficiency effects on the superconductivity of YBa2Cu3O 7. ~ are investigated by the electrical resistivity and the infrared reflection spectroscopy. On increasing the quenching temperature the sample changes to a semiconductor from a metal. The superconducting gap of the slowly cooled sample is anisotropic and the minimum gap is 2A/kaT~ ~ 1.6. The anisotropy of the sample quenched from 600°C is very small in the proximity-coupled superconducting region and the
2 A / k , T c ~ 1.
Recent remarkable increase of Tc in oxide superconductors, (Lal_xMx)2CuO4 and YBa 2Cu307_ ~ [1,2], has stimulated enormous experimental and theoretical studies. Existence of many kinds of structures and oxygen deficiency of these materials give variety of superconducting, metallic and semiconducting properties. We present the results of infrared reflection spectroscopy for the oxygen deficiency effects on the superconductivity of YBazCu307_ ~. The sample annealed at low temperatures has orthorhombic structure with C u - O linear chains [3]. The oxygen deficiency 6 is about 0.07 [4]. With increasing temperature the oxygen deficiency increases monotonically and the structure changes into tetragonal at about 575-670°C [57]. In the tetragonal phase the linear chains disappear and oxygen atoms distribute randomly in the C u - O layers. The oxygen deficiency 6 is about 0.70 at 950°C in air [4]. The occupation factor of oxygen is about 93% for samples annealed below 400°C, about 58% on the chains at the phase transition temperature and only about 15% at 950°C in air. Samples were prepared using reagents of Y 2 0 3 , BaCO 3 and CuO with 99.9% purity. The mixed powder was calcined at 900°C for 20 h and 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
then pulverized. The reactant powder was pressed into a pellet of a diameter of 10 mm and a thickness of 2mm. The pellet was calcined again at 950°C for 24 h followed by annealing at various temperatures for 12 h in air and quenching into liquid nitrogen. Fig. 1 shows the resistivity of Y B a 2 C u 3 0 7 _ ~ samples quenched from 950, 800,700 and 600°C, and cooled slowly in a furnace after annealing at 350°C. The slowly cooled sample shows a superconducting transition at 92 K. The sample quenched from 600°C shows aresistivity drop by double steps and undergoes a superconducting state at 61 K. The temperature region between 92 and 61 K is a proximity-coupled superconducting region [8]. The sample quenched from 700°C shows a resistivity increase below 90 K followed by a superconducting transition at 46K. The resistivity of the sample quenched from 800°C decreases at first and then increases below 105 K. The samples quenched from 800 and 950°C do not show the superconducting transition down to 4.2 K. The difference of the temperature-dependent resistivity with other experiment [9] is due to the difference of the quenching speed. Fig. 2 shows the Im(e), imaginary part of the dielectric constant, of the YBa2Cu3Ov_ ~ sample
S. Sugai et al. / Optical studies o f high-T c oxide superconductors 10
283
200 ;
7-6
YBa2Cu3OT_
6
350 *C SC
i i i i
td
'~ 100 %%
950"C O "~
10 -1
800 "C O 0
I
0 m.-
i0_2
I
~
100
200
300 400 500 ENERGY ( c m - = )
600
700
Fig. 2. Ira(e) of the slowly cooled sample at 65 K (solid curve), 79 K (dot-dashed curve) and 105 K (dashed curve). The Im(e) was calculated by the Kramers-Kronig transformation from the reflection spectra of the samples cooled in a furnace after annealing at 350°C.
700 "c o
350 °C SC
10-3
10-4
I I J I
0
I
100 200 TEMPERATURE (K)
300
Fig. 1. Electrical resistivity for the samples quenched from 950, 800, 700, 600°C and cooled slowly in a furnace after annealing at 350°C.
cooled slowly in a furnace after annealing at 350°C. The Im(e) was calculated from the reflection spectra by the K r a m e r s - K r o n i g transformation. The reflectivity increases at low energies on approaching the T c and stays 100% at low energies below the T c. The small change of reflectivity is enhanced in the Im(e), when the reflectivity is close to 100%. The reflectivity curves of the non-superconducting state (105 K) and the superconducting state ( 7 9 K ) are crossing near 250 c m - 1. The calculated Im(e) at 79 K are smaller than that at 105 K in the whole spectral range of fig. 2. It is due to the anisotropic superconductivity [10]. The ratio of the Ira(e) at 79 K to that at 105 K starts to decrease at about 450cm - 1 The Im(e) at 7 9 K reaches the maximum at about 160 cm -] excluding the phonon peaks and goes to zero at 70 cm -]. The ratio of the Im(e) at
65 K to that at 79 K starts to decrease at about 300cm 1. The Im(e) at 65 K reaches the maximum at about 175 c m -1 and goes to zero at 84cm 1. The minimum gap 2 A / k B T c extrapolated to 0 K is 1.7 for the measurement at 79 K and 1.6 at 65 K. The peaks of the Im(e) are close to the superconducting energy gap 2A, about 142cm -I at 7 9 K and about 187cm -I at 6 5 K , calculated by the weak coupled BCS model. Fig. 3 shows the Im(e) of the sample quenched from 600°C. The spectra resemble those by Bonn et al. [11]. This sample is a mixture of the 92 K superconductor phase and a low-temperature superconductor phase with the Tc about 60 K. At temperatures between 92 and 61 K the induced pair potential in the non-superconducting material has a finite value lower than that of the 92 K superconductor. The energies of I m ( e ) = 0 are 4 2 c m -1 at 7 9 K and 4 6 c m 1 at 65K. The decrease of Im(e) in a wide energy range is not observed in the superconducting state compared to the non-superconducting state. It suggests the almost extinction of the anisotropy [10] observed in the slowly cooled YBa2Cu3OT_ ~ sample. In the superconducting phase a broad band appears near 430cm -1, the peak at 316cm -1 shifts to 310cm I and the peak at 193cm 1 to 190cm -l.
284
S. Sugai et al. / Optical studies o f h i g h - T oxide superconductors
200
c o o l e d s a m p l e is a l m o s t missing in t h e p r o x i m i t y c o u p l e d r e g i o n of t h e s a m p l e q u e n c h e d f r o m 600°C.
, t
YBa2Cu307-6 ',
600
*C Q
i
' it I
References
Ii,x
'~
~100
i o
0
//
' 100
'
, 200
'
' 300
'
' 400
'
. . . . 500 600
700
ENERGY ( c m - J )
Fig. 3. Im(e) of the sample quenched from 600°C at 65 K (solid curve), 79K (dot-dashed curve) and 105K (dashed curve). T h e s a m p l e q u e n c h e d f r o m 700°C d o e s n o t s h o w t h e sign o f s u p e r c o n d u c t i n g g a p d o w n to 66 K in the i n f r a r e d s p e c t r a . O n l y t h e i n c r e a s e of reflectivity b e l o w 4 0 c m 1 is o b s e r v e d at 6 6 K . The superconducting energy gap, 2A/kBT c = 3 . 7 - 5 . 6 , o b t a i n e d b y t u n n e l i n g s p e c t r o s c o p y is always larger than the gap, 2A/kBT c --1.6-3.4, m e a s u r e d b y i n f r a r e d s p e c t r o s c o p y [12]. This d i f f e r e n c e has b e e n e x p l a i n e d b y t h e a n i s o t r o p i c p a i r i n g [10]. T h e p r e s e n t e x p e r i m e n t s h o w s t h a t t h e m i n i m u m g a p m e a s u r e d b y t h e i n f r a r e d spect r o s c o p y is 2 A / k B T c ~ 1.6 for t h e s a m p l e c o o l e d slowly a n d - 1 for t h e s a m p l e q u e n c h e d f r o m 600°C. T h e a n i s o t r o p y o b s e r v e d in the slowly
[1] J.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1987) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, EH. Hot, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. B 58 (1987) 908. [3] F. Izumi, H. Asano, T. Ishigaki, E. TakayamaMuromachi, Y. Uchida, N. Watanabe and T. Nishikawa, Jpn. J. Appl. Phys. 26 (1987) L649. [4] K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) L1228. [5] E. Takayama-Muromachi, Y. Uchida, K. Yukino, T. Tanaka and K. Kato, Jpn. J. Appl. Phys. 26 (1987) L665. E. Takayama-Muromachi, Y. Uchida, M, Ishii, T. Tanaka and K. Kato, Jpn. J. Appl. Phys. 26 (1987) L1156, [6] S. Sueno, 1. Nakai, F.P. Okamura and A. Ono, Jpn. J. Appl. Phys. 26 (1987) L842. {7] K. Yukino, T. Sato, S. Ooba, M. Ohta, F.P. Okamura and A. Ono, Jpn, J. Appl. Phys. 26 (1987) L869. [8} D.J. Resnick, J.C. Garland, J.T. Boyd, S. Shoemaker and R.S. Newrock, Phys. Rev. Lett. 47 (1981) 1542. [91 H. Sawada, T. Iwazumi, Y. Saito, Y. Abe, H. Ikeda and R. Yoshizaki, Jpn. J. Appl. Phys. 26 (1987) L1054. [10] H. Ebisawa, Y. Isawa and S. Maekawa, Jpn. J. Appl. Phys. 26 (1987) L992. [11] D.A. Bonn, J.E. Greedan, C.V. Stager, T. Timusk, M.G. Doss, S.L. Herr, K. Kamarfis and D.B. Tanner, Phys. Rev. Lett. 58 (1987) 2249. [12] J.R. Kirtley, R.T. Collins, Z. Schlesinger, W.J. Gallagher, R.L. Sandstrom, T.R. Dinger and D.A. Chance, Phys. Rev. B 35 (1987) 8846.