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Tunneling characteristics of I- and HgI2-intercalated Bi2Sr2CaCu2O8+x single crystals
Physica B 284}288 (2000) 1844}1845
Tunneling characteristics of I- and HgI -intercalated Bi Sr CaCu O single crystals >V
Minhyea Lee , Hyun-Sik Chang , Yong-Joo Doh , Hu-Jong Lee *, Woo Lee, Jin-Ho Choy, Dong Han Ha Department of Phyisics, Pohang University of Science and Technology, Pohang, 790-784, South Korea Department of Chemistry, Seoul National University, Seoul 151-742, South Korea Korea Research Institute of Standards and Science, Taejon 305-600, South Korea
Abstract We compared c-axis tunneling characteristics of small stacked intrinsic Josephson junctions prepared on the surface of pristine, I-, and HgI -intercalated Bi Sr CaCu O (Bi2212) single crystals. Relation between the interlayer coupling and W the in-plane gap, estimated from I}< characteristics, seems to contradict to the interlayer pair tunneling mechanism of high-¹ superconductivity. 2000 Elsevier Science B.V. All rights reserved. Keywords: Bi Sr CaCu O high-¹ superconductors; Hole doping; Intercalation; Interlayer pair tunneling W
Bi Sr CaCu O (Bi2212) high-¹ superconduc >V tors, consisting of thin superconducting CuO planes separated by non-superconducting bu!er layers, exhibit intrinsic Josephson e!ects between neighboring CuO planes. c-axis tunneling properties were measured on small stacks of intrinsic Josephson junctions fabricated on the surface of pristine, I- and HgI -intercalated Bi2212 single crystals. I}< characteristics of all the specimens show multiple quasiparticle branches with welldeveloped gap features, indicating Josephson coupling between neighboring CuO planes. Tunneling character istics of the intercalated samples can be explained by the combination of weakened interlayer coupling and a change of doping level in CuO planes. But the behav ior of HgI -Bi2212 seems to contradict to the prediction of the interlayer pair tunneling (IPT) theory [1]. Pristine Bi2212 specimen was almost optimally doped. Stage-I intercalated single crystals were synthesized by vapor transport reaction method [2] on optimally doped pristine crystals. Fabrication method of small stacks and
* Corresponding author. E-mail address: [email protected] (H.-J. Lee)
more details on measurements are described in Ref. [3]. The junction area was 1200}1500 lm and the height of a stack was &400 As , containing &30 intrinsic Josephson junctions. XRD analysis revealed that inter-bilayer distance in our I- and HgI -Bi2212 specimens was expanded by &3.6 and &7.1 As , respectively, compared to the pristine one. Fig. 1 shows R(¹) curves of our specimens, each one normalized by the value at 290 K. As observed previously [4,5], the R(¹) curve changes from strongly semiconducting-like behavior for the pristine specimen to almost metallic for I-Bi2212. The R(¹) curve of HgI Bi2212 has a little semiconducting nature. The transition temperatures ¹ , de"ned as zero-resistance temper atures, are 82, 73, and 76.8 K for pristine, I-, and HgI Bi2212 specimens, respectively. Fig. 2 shows the I}< curves of our three specimens. The critical current I of I-Bi2212 is &2.5 mA, close to the value &2.9 mA for the pristine specimen. But I for HgI -Bi2212 is drasti cally reduced to &0.5 mA. The variation in ¹ itself can be explained by the change in the doping level of specimens by intercalation, as expressed by the empirical relation [6], ¹ /¹ "1!82.6( p!0.16). Here, ¹ is the ¹ at the optimal hole doping and p is the average hole
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 8 6 4 - 1
M. Lee et al. / Physica B 284}288 (2000) 1844}1845
1845
Fig. 1. R versus ¹ data of Bi2212 (line), I-Bi2212 ("lled circle), and HgI -Bi2212 (open circle) specimens.
number per Cu site in a CuO plane. It is widely accepted that an iodine atom in I-Bi2212 serves as a hole donor to nearby CuO planes [7]. Thus, iodine intercalation cha nges the optimally doped pristine specimen to slightly overdoped in I-Bi2212, which causes a decrease in ¹ . This hole-doping e!ect in I-Bi2212 is known to be responsible for the metallic behavior of the R(¹) curve and may restore I , in spite of the c-axis lattice expansion. A HgI molecule, on the other hand, is more inert and a less e!ective hole donor compared to iodine, resulting in the smaller depression of ¹ but the drastic reduction of I corresponding to weakened interlayer coupling. The hole-doping e!ect, however, is not entirely negligible as inferred from the reduced semiconducting behavior of R(¹). None the less, as in Fig. 2(c), the increase of < of HgI -Bi2212, compared to pristine Bi2212, is in contradiction to the IPT theory. < corres ponds to the gap of a CuO bilayer and the theory predicts a suppression of the gap as the interlayer coupling is weakened. Acknowledgements This work was supported by BSRI, MARC, and POSTECH under Contract Nos. 1RB9811401, 1MC9801301, and 1UD9900801, respectively.
Fig. 2. I}< curves showing clear quasiparticle branches for the three samples.
References [1] P.W. Anderson, The Superconductivity Theory of High-¹ Cuprate, Princeton University Press, Priceton, 1997. [2] J.H. Choy et al., J. Am. Chem. Soc. 116 (1994) 11 564. [3] N. Kim et al., Phys. Rev. B 59 (1999) 14 639. [4] X.-D. Xiang et al., Phys. Rev. Lett. 68 (1992) 530. [5] A. Fijiwara et al., Phys. Rev. B 52 (1995) 15 598. [6] G.V.M. Williams et al., Phys. Rev. B 54 (1996) R6909. [7] T. Stoto et al., Phys. Rev. B 51 (1995) 16 220.
Tunneling characteristics of I- and HgI -intercalated Bi Sr CaCu O single crystals >V
Minhyea Lee , Hyun-Sik Chang , Yong-Joo Doh , Hu-Jong Lee *, Woo Lee, Jin-Ho Choy, Dong Han Ha Department of Phyisics, Pohang University of Science and Technology, Pohang, 790-784, South Korea Department of Chemistry, Seoul National University, Seoul 151-742, South Korea Korea Research Institute of Standards and Science, Taejon 305-600, South Korea
Abstract We compared c-axis tunneling characteristics of small stacked intrinsic Josephson junctions prepared on the surface of pristine, I-, and HgI -intercalated Bi Sr CaCu O (Bi2212) single crystals. Relation between the interlayer coupling and W the in-plane gap, estimated from I}< characteristics, seems to contradict to the interlayer pair tunneling mechanism of high-¹ superconductivity. 2000 Elsevier Science B.V. All rights reserved. Keywords: Bi Sr CaCu O high-¹ superconductors; Hole doping; Intercalation; Interlayer pair tunneling W
Bi Sr CaCu O (Bi2212) high-¹ superconduc >V tors, consisting of thin superconducting CuO planes separated by non-superconducting bu!er layers, exhibit intrinsic Josephson e!ects between neighboring CuO planes. c-axis tunneling properties were measured on small stacks of intrinsic Josephson junctions fabricated on the surface of pristine, I- and HgI -intercalated Bi2212 single crystals. I}< characteristics of all the specimens show multiple quasiparticle branches with welldeveloped gap features, indicating Josephson coupling between neighboring CuO planes. Tunneling character istics of the intercalated samples can be explained by the combination of weakened interlayer coupling and a change of doping level in CuO planes. But the behav ior of HgI -Bi2212 seems to contradict to the prediction of the interlayer pair tunneling (IPT) theory [1]. Pristine Bi2212 specimen was almost optimally doped. Stage-I intercalated single crystals were synthesized by vapor transport reaction method [2] on optimally doped pristine crystals. Fabrication method of small stacks and
* Corresponding author. E-mail address: [email protected] (H.-J. Lee)
more details on measurements are described in Ref. [3]. The junction area was 1200}1500 lm and the height of a stack was &400 As , containing &30 intrinsic Josephson junctions. XRD analysis revealed that inter-bilayer distance in our I- and HgI -Bi2212 specimens was expanded by &3.6 and &7.1 As , respectively, compared to the pristine one. Fig. 1 shows R(¹) curves of our specimens, each one normalized by the value at 290 K. As observed previously [4,5], the R(¹) curve changes from strongly semiconducting-like behavior for the pristine specimen to almost metallic for I-Bi2212. The R(¹) curve of HgI Bi2212 has a little semiconducting nature. The transition temperatures ¹ , de"ned as zero-resistance temper atures, are 82, 73, and 76.8 K for pristine, I-, and HgI Bi2212 specimens, respectively. Fig. 2 shows the I}< curves of our three specimens. The critical current I of I-Bi2212 is &2.5 mA, close to the value &2.9 mA for the pristine specimen. But I for HgI -Bi2212 is drasti cally reduced to &0.5 mA. The variation in ¹ itself can be explained by the change in the doping level of specimens by intercalation, as expressed by the empirical relation [6], ¹ /¹ "1!82.6( p!0.16). Here, ¹ is the ¹ at the optimal hole doping and p is the average hole
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 8 6 4 - 1
M. Lee et al. / Physica B 284}288 (2000) 1844}1845
1845
Fig. 1. R versus ¹ data of Bi2212 (line), I-Bi2212 ("lled circle), and HgI -Bi2212 (open circle) specimens.
number per Cu site in a CuO plane. It is widely accepted that an iodine atom in I-Bi2212 serves as a hole donor to nearby CuO planes [7]. Thus, iodine intercalation cha nges the optimally doped pristine specimen to slightly overdoped in I-Bi2212, which causes a decrease in ¹ . This hole-doping e!ect in I-Bi2212 is known to be responsible for the metallic behavior of the R(¹) curve and may restore I , in spite of the c-axis lattice expansion. A HgI molecule, on the other hand, is more inert and a less e!ective hole donor compared to iodine, resulting in the smaller depression of ¹ but the drastic reduction of I corresponding to weakened interlayer coupling. The hole-doping e!ect, however, is not entirely negligible as inferred from the reduced semiconducting behavior of R(¹). None the less, as in Fig. 2(c), the increase of < of HgI -Bi2212, compared to pristine Bi2212, is in contradiction to the IPT theory. < corres ponds to the gap of a CuO bilayer and the theory predicts a suppression of the gap as the interlayer coupling is weakened. Acknowledgements This work was supported by BSRI, MARC, and POSTECH under Contract Nos. 1RB9811401, 1MC9801301, and 1UD9900801, respectively.
Fig. 2. I}< curves showing clear quasiparticle branches for the three samples.
References [1] P.W. Anderson, The Superconductivity Theory of High-¹ Cuprate, Princeton University Press, Priceton, 1997. [2] J.H. Choy et al., J. Am. Chem. Soc. 116 (1994) 11 564. [3] N. Kim et al., Phys. Rev. B 59 (1999) 14 639. [4] X.-D. Xiang et al., Phys. Rev. Lett. 68 (1992) 530. [5] A. Fijiwara et al., Phys. Rev. B 52 (1995) 15 598. [6] G.V.M. Williams et al., Phys. Rev. B 54 (1996) R6909. [7] T. Stoto et al., Phys. Rev. B 51 (1995) 16 220.