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Two dimensional superconducting properties in single crystalline Bi2Sr2CaCu2O8+σ
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Physica C 185-189 (1991) 1811-1812 North-Holland
TWO DIMENSIONAL SUPERCONDUCTING PROPERTIES IN SINGLE CRYSTALLINE Bi2Sr2CaCu2Os~ Kazuo KADOWAKI National Research Institute for Metals, Tsukuba Laboratory 1-2-1, Sengen, Tsukuba-Shi, Ibaraki 305, JAPAN High precision measurements of angular dependences of the resistivity in high quality single crystalline Bi2Sr2CaCu2Os+8 have been performed to study unusual vortex dynamics in magnetic fields. The results show evidence of strong superconducting fluctuations, which is in strong contrast with the previous understanding based on the vortex motion driven by the Lorentz force interaction, and which is responsible for the dissipation in the reversible magnetization region. It is also shown that the two dimensionality associated with the inherent layered electronic structure in this system plays an essential role for the origin of the unusual vortex dynamics. 1. INTRODUCTION Recent growing interest in the vortex dynamics in high Te superconductors originates from the unusual experimental phenomena in the mixed state in magnetic fields. The essence of these phenomena result from dissipation in the superconducting mixed state in those compounds. Large field induced resistivityl), strong relaxation of the diamagnetic moment2), existence of the unusual phase boundary named the irreversibility line3), etc. are the typical examples. It is obvious that the weak pinning energy in the vortex state is partly responsible for those phenomena, but the enigma extracted from a piece of incompatible experimental evidence can not be accounted for by the traditional flux motion type of approach. Study of transport properties provides direct means to investigate the mechanism of dissipation. In spite of a large number of works previously done on this problem, the results in most cases have been treated to relate the traditional ideas to those phenomena. We have focussed on this problem in order to better understand based on high quality single crystalline Bi2Sr2CaCu208+8. Similar work previously done in YBa2Cu307& has already been reported4). 2. SAMPLE PREPARATION It is emphasized that for this study high quality single crystals are indispensable to reveal features hidden behind the effects of impurities. Single crystals of Bi2Sr2CaCu208+8 have been grown by the furnace heated by infrared light converged by the two ellipsoidal gold plated mirrors. The heat is created by the two halogen lamps illuminated at the focal position of the mirrors. Single crystals were characterized thoroughly prior to measurements by X-ray Laue method, LEED (Low Energy Electron Diffraction), SEM (Scanning Electron Microscope), HREM (High Resolution Electron Microscope), neutron fascan method, etc. in order to check the quality. The inclusions and intergrowths of neighboring phases such as Bi2Sr2CuO6+8 and Bi2Sr2Ca2Cu3Ol0+8, which are common in this system and are often inevitably incorporated especially in the flux-grown single crystals, were carefully estimated by HREM to be less than 0.1 percent.
I00
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........
,
........
i
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,
........
10
m
~oO ,.t
B//bc-plane
o
o
0.1 m
0.01
o
B=IT
a
B=2T
tx
B=3T
.
0.001 0.001
0.01
0.1 B
perp
1
,
,
, **,LI
10
fr)
Fig. 1 The logarithmic plot of the field induced b-axis resistivity on single crystalline Bi2Sr2CaCu2084~5 as a function of the effective perpendicular field. Superconductivity with transition temperatures of 80-90 K in as-grown crystals, depending on the oxygen doping level, was detected by ac susceptibility and the width was less than 0.1 K. 3. RESULTS AND DISCUSSION One of the outstanding phenomena of the resistivity in magnetic fields is the broadening of the resistive wansition in a wide temperature range. This field effect is mare pronounced in Bi2Sr2CaCu208+6 than in YBa2Cu307-~). From the measurements performed in all possible configurations between the current, the crystallographic axes and the applied field orientationsS), it is observed that the broadening depends only on the field direction with respect to the crystallographic axis and there is no correlation between the measuring current and the applied field directions. With this fact as well as several characteristics from the additional results, it is concluded that the resistivity
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights rese~'ed.
1812
K. Kadowaki / Two dimensional superconducting properties
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TEMPERATURE (K)
Fig. 2 The field effect of the in-plane resistivity on single crystalline Bi2Sr2CaCu208+8 in the vicinity of Tc. The open and filled symbols correspond to the data obtained by the present work and Palstra's datat) in fields of 0T (o), IT (t3), 3T (~) and 5T (A) and 0T (.), 2T (li), 5T ( , ) and 10T (&), respectively. induced by fields is not caused by the Lorentz force acting on the vortex. However, as is commonly investigated and reportedt-8), it is not inconsistent with an idea proposed by ICes et al.9) that the perpendicular component of the field is only effective to the resistivity due to highly two dimensional vortex state in this system, only if the in-plane resistivity is studied. This is shown in fig. 1, where the resistivity Pb is plotted as a function of the field component perpendicular to the ab-plane, which is determined by the field rotation diagram in the at-plane. Here, it is stressed that three curves shown in fig. 1 corresponding to three measurements in different magnetic fields coincide each other down to angles as small as 0.2 °, which is actually the experimental limit of the angular resolution. This fact means, at least down to this effective perpendicular field, that the resistivity is indeed induced by the perpendicular component only and that the parallel component of the field does not play any role for the resistivity. This point will be discussed further below. As a result of this analysis, the texture of the vortex is also considered to be highly anisotropic, resulting in an extremely two dimensional pancake-like structure. It is noted that the resistivity behavior in the field direction very close to the abplane observed here contradicts the results obtained in the thin filmsl0), where the deviation has already been found at angles as large as 10° from the ab-plane. They attributed this phenomenon to another type of dissipation occurring only in the field direction near the ab-plane. The reason for this discrepancy is not clear yet, though their experimental conditions on the current revel is as high as 67 A/cm2, whereas a typical current density of 0.5 A/cm2 is used in the present measurements. When the field is applied near the ab-plane, Pc shows a sharp dip at the field orientation exactly parallel to the abplane. As temperature is increased, this feature is more pronounced. This fact suggests that the origin of the dissipation is due to the field effect on the inferent nature of the two dimensional superconducting state, and is not due to the traditional flux motion by the Lorentz type of interaction.
This understanding is also supported by the fact that the normal state resistivity as well as the susceptibility shows a pronounced superconducting fluctuation effect. More comprehensive analyses will be published elsewherel 1). The significant effects due to the superconducting fluctuations on the dissipation, which have not been pointed out before is originated from the inherent highly two dimensional electronic structure. The two dimensionality in Bi2Sr2CaCu2Os~ has been estimated by various methods to be as high as 105 12). Shown in fig. 2, is an experimental result to deduce anisotropy of the coherence length in this compound. The resistivity, after careful calibration of the field effect of the thermometer, is hardly influenced by magnetic fields. This surprising result contrasts with the previous results6) indicated by the filled symbols in fig. 2. The effect, if any, estimated is IdBc2ab(T)/dTI > 100 T/K. Using a value for ~al~(0) = 20-23 A, the ~c(0)-value can be estimated to be 0.3 A or even shorter. It is noted that this value is in good agreement with the value deduced from the analysis of fluctuation conductivity. As a consequence, anisotropy of the coherence length amounts at least to ~b(0)/~(0) > 70. This value is compared with even higher values of 300-400 and 100-220 obtained by the anisotropic resistivityl2) and torque measurementsl3), respectively. These surprising values, though there are still large ambiguities in the values due to the severe experimental constraint, are becoming rather consistent with each other. ACKNOWLEDGEMENTS The author wish to thank Prof. J. J. M. Franse for discussing part of the work presented here. REFERENCES 1. T. T. M. Palstra, B. Batlogg, R. B. van Dover, L. F. Schneemeyer and J. V. Waszczak, Phys. Rev. B41 (1990) 6621. 2. Y. Yeshurun and A. P. Malozemoff, Phys. Rev. Lett. 60 (1899) 2202. 3. K. Kadowaki, N. J. Li, F. R. de Boer, P. H. Frings and J. J. M. Franse, Supercond. Sci. Technol. 4 (1991) s88-90. 4. K. Kadowaki, J. N. Li and J. J. M. Franse, Physica C170 (1990) 298. 5. K. Kadowaki, K. Togano, H. Maeda and J. J. M. Franse, Proc. 2nd Int. Symp. Phys. & Chem• Oxide Supercond. (PCOS'91), Jan. 16-18, 1991, Tokyo, JAPAN. 6. T. T. M. Palstra, B. Batlogg, L. F. Schneemeyer, R. B. van Dover and J. W. Waszczak, Phys. Rev. B38 (1988) 5102. 7 T T • x~ D,~l,,,,.n B . D,,,I,,,-~, e,k . . . . . . . . . and L , a ~ v ~ , L. F. o~,lnl~nl~y~x Waszczak, Phys. Rev. Lett. 61 (1988) 1662. 8. E. Zeldov, N. M. Amer, G. Koren, A. Gupta, R. J. Gambino and M. W. McElfresh, Phys. Rev. Lett. 62 (1989) 3093. 9. P. H. Kes, J. Aarts, V. M. Vinokur and C. J. van der Beek, Phys. Rev. Lett. 64 (1990) 1063. 10. Y. Iye, T. Tamegai and S. Nakamura, Physica C174 (1991) 227. 11. K. Kadowaki et al. to be published. 12. K. Kadowaki, A. A. Menovsky and J. J. M. Franse, Physica B 165&166 (1990) 1159. 13. K. Okuda, S. Kawamata, S. Noguchi, N. Itoh and K. Kadowaki, to be published in J. Phys. Soc. Jpn.
Physica C 185-189 (1991) 1811-1812 North-Holland
TWO DIMENSIONAL SUPERCONDUCTING PROPERTIES IN SINGLE CRYSTALLINE Bi2Sr2CaCu2Os~ Kazuo KADOWAKI National Research Institute for Metals, Tsukuba Laboratory 1-2-1, Sengen, Tsukuba-Shi, Ibaraki 305, JAPAN High precision measurements of angular dependences of the resistivity in high quality single crystalline Bi2Sr2CaCu2Os+8 have been performed to study unusual vortex dynamics in magnetic fields. The results show evidence of strong superconducting fluctuations, which is in strong contrast with the previous understanding based on the vortex motion driven by the Lorentz force interaction, and which is responsible for the dissipation in the reversible magnetization region. It is also shown that the two dimensionality associated with the inherent layered electronic structure in this system plays an essential role for the origin of the unusual vortex dynamics. 1. INTRODUCTION Recent growing interest in the vortex dynamics in high Te superconductors originates from the unusual experimental phenomena in the mixed state in magnetic fields. The essence of these phenomena result from dissipation in the superconducting mixed state in those compounds. Large field induced resistivityl), strong relaxation of the diamagnetic moment2), existence of the unusual phase boundary named the irreversibility line3), etc. are the typical examples. It is obvious that the weak pinning energy in the vortex state is partly responsible for those phenomena, but the enigma extracted from a piece of incompatible experimental evidence can not be accounted for by the traditional flux motion type of approach. Study of transport properties provides direct means to investigate the mechanism of dissipation. In spite of a large number of works previously done on this problem, the results in most cases have been treated to relate the traditional ideas to those phenomena. We have focussed on this problem in order to better understand based on high quality single crystalline Bi2Sr2CaCu208+8. Similar work previously done in YBa2Cu307& has already been reported4). 2. SAMPLE PREPARATION It is emphasized that for this study high quality single crystals are indispensable to reveal features hidden behind the effects of impurities. Single crystals of Bi2Sr2CaCu208+8 have been grown by the furnace heated by infrared light converged by the two ellipsoidal gold plated mirrors. The heat is created by the two halogen lamps illuminated at the focal position of the mirrors. Single crystals were characterized thoroughly prior to measurements by X-ray Laue method, LEED (Low Energy Electron Diffraction), SEM (Scanning Electron Microscope), HREM (High Resolution Electron Microscope), neutron fascan method, etc. in order to check the quality. The inclusions and intergrowths of neighboring phases such as Bi2Sr2CuO6+8 and Bi2Sr2Ca2Cu3Ol0+8, which are common in this system and are often inevitably incorporated especially in the flux-grown single crystals, were carefully estimated by HREM to be less than 0.1 percent.
I00
"i
........
,
........
i
........
,
........
10
m
~oO ,.t
B//bc-plane
o
o
0.1 m
0.01
o
B=IT
a
B=2T
tx
B=3T
.
0.001 0.001
0.01
0.1 B
perp
1
,
,
, **,LI
10
fr)
Fig. 1 The logarithmic plot of the field induced b-axis resistivity on single crystalline Bi2Sr2CaCu2084~5 as a function of the effective perpendicular field. Superconductivity with transition temperatures of 80-90 K in as-grown crystals, depending on the oxygen doping level, was detected by ac susceptibility and the width was less than 0.1 K. 3. RESULTS AND DISCUSSION One of the outstanding phenomena of the resistivity in magnetic fields is the broadening of the resistive wansition in a wide temperature range. This field effect is mare pronounced in Bi2Sr2CaCu208+6 than in YBa2Cu307-~). From the measurements performed in all possible configurations between the current, the crystallographic axes and the applied field orientationsS), it is observed that the broadening depends only on the field direction with respect to the crystallographic axis and there is no correlation between the measuring current and the applied field directions. With this fact as well as several characteristics from the additional results, it is concluded that the resistivity
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights rese~'ed.
1812
K. Kadowaki / Two dimensional superconducting properties
$0
"
"
"
I
"
"
"
I
"
"
"
I
"
"
"
I
"
"
"
~.'.F~."/
BiaSrsCaCu~Os.,s
,. ~ ¢
B//a, I//b
,% " g •
4,
0
80
'I,
4,
i,
82
•
I
84
86
88
90
TEMPERATURE (K)
Fig. 2 The field effect of the in-plane resistivity on single crystalline Bi2Sr2CaCu208+8 in the vicinity of Tc. The open and filled symbols correspond to the data obtained by the present work and Palstra's datat) in fields of 0T (o), IT (t3), 3T (~) and 5T (A) and 0T (.), 2T (li), 5T ( , ) and 10T (&), respectively. induced by fields is not caused by the Lorentz force acting on the vortex. However, as is commonly investigated and reportedt-8), it is not inconsistent with an idea proposed by ICes et al.9) that the perpendicular component of the field is only effective to the resistivity due to highly two dimensional vortex state in this system, only if the in-plane resistivity is studied. This is shown in fig. 1, where the resistivity Pb is plotted as a function of the field component perpendicular to the ab-plane, which is determined by the field rotation diagram in the at-plane. Here, it is stressed that three curves shown in fig. 1 corresponding to three measurements in different magnetic fields coincide each other down to angles as small as 0.2 °, which is actually the experimental limit of the angular resolution. This fact means, at least down to this effective perpendicular field, that the resistivity is indeed induced by the perpendicular component only and that the parallel component of the field does not play any role for the resistivity. This point will be discussed further below. As a result of this analysis, the texture of the vortex is also considered to be highly anisotropic, resulting in an extremely two dimensional pancake-like structure. It is noted that the resistivity behavior in the field direction very close to the abplane observed here contradicts the results obtained in the thin filmsl0), where the deviation has already been found at angles as large as 10° from the ab-plane. They attributed this phenomenon to another type of dissipation occurring only in the field direction near the ab-plane. The reason for this discrepancy is not clear yet, though their experimental conditions on the current revel is as high as 67 A/cm2, whereas a typical current density of 0.5 A/cm2 is used in the present measurements. When the field is applied near the ab-plane, Pc shows a sharp dip at the field orientation exactly parallel to the abplane. As temperature is increased, this feature is more pronounced. This fact suggests that the origin of the dissipation is due to the field effect on the inferent nature of the two dimensional superconducting state, and is not due to the traditional flux motion by the Lorentz type of interaction.
This understanding is also supported by the fact that the normal state resistivity as well as the susceptibility shows a pronounced superconducting fluctuation effect. More comprehensive analyses will be published elsewherel 1). The significant effects due to the superconducting fluctuations on the dissipation, which have not been pointed out before is originated from the inherent highly two dimensional electronic structure. The two dimensionality in Bi2Sr2CaCu2Os~ has been estimated by various methods to be as high as 105 12). Shown in fig. 2, is an experimental result to deduce anisotropy of the coherence length in this compound. The resistivity, after careful calibration of the field effect of the thermometer, is hardly influenced by magnetic fields. This surprising result contrasts with the previous results6) indicated by the filled symbols in fig. 2. The effect, if any, estimated is IdBc2ab(T)/dTI > 100 T/K. Using a value for ~al~(0) = 20-23 A, the ~c(0)-value can be estimated to be 0.3 A or even shorter. It is noted that this value is in good agreement with the value deduced from the analysis of fluctuation conductivity. As a consequence, anisotropy of the coherence length amounts at least to ~b(0)/~(0) > 70. This value is compared with even higher values of 300-400 and 100-220 obtained by the anisotropic resistivityl2) and torque measurementsl3), respectively. These surprising values, though there are still large ambiguities in the values due to the severe experimental constraint, are becoming rather consistent with each other. ACKNOWLEDGEMENTS The author wish to thank Prof. J. J. M. Franse for discussing part of the work presented here. REFERENCES 1. T. T. M. Palstra, B. Batlogg, R. B. van Dover, L. F. Schneemeyer and J. V. Waszczak, Phys. Rev. B41 (1990) 6621. 2. Y. Yeshurun and A. P. Malozemoff, Phys. Rev. Lett. 60 (1899) 2202. 3. K. Kadowaki, N. J. Li, F. R. de Boer, P. H. Frings and J. J. M. Franse, Supercond. Sci. Technol. 4 (1991) s88-90. 4. K. Kadowaki, J. N. Li and J. J. M. Franse, Physica C170 (1990) 298. 5. K. Kadowaki, K. Togano, H. Maeda and J. J. M. Franse, Proc. 2nd Int. Symp. Phys. & Chem• Oxide Supercond. (PCOS'91), Jan. 16-18, 1991, Tokyo, JAPAN. 6. T. T. M. Palstra, B. Batlogg, L. F. Schneemeyer, R. B. van Dover and J. W. Waszczak, Phys. Rev. B38 (1988) 5102. 7 T T • x~ D,~l,,,,.n B . D,,,I,,,-~, e,k . . . . . . . . . and L , a ~ v ~ , L. F. o~,lnl~nl~y~x Waszczak, Phys. Rev. Lett. 61 (1988) 1662. 8. E. Zeldov, N. M. Amer, G. Koren, A. Gupta, R. J. Gambino and M. W. McElfresh, Phys. Rev. Lett. 62 (1989) 3093. 9. P. H. Kes, J. Aarts, V. M. Vinokur and C. J. van der Beek, Phys. Rev. Lett. 64 (1990) 1063. 10. Y. Iye, T. Tamegai and S. Nakamura, Physica C174 (1991) 227. 11. K. Kadowaki et al. to be published. 12. K. Kadowaki, A. A. Menovsky and J. J. M. Franse, Physica B 165&166 (1990) 1159. 13. K. Okuda, S. Kawamata, S. Noguchi, N. Itoh and K. Kadowaki, to be published in J. Phys. Soc. Jpn.