4PO7-40 Low-Temperature Normal State of Bi2201 in a Wide Doping Range: Where Does the Metal to Insulator Crossover Take Place?

S. Ono 1, Yoichi Ando 1,2, T. Murayama 1,2, F.F. Balakirev 3, J.B. Betts 3, and G.S. Boebinger 3. 1 Central Research Institute of Electric Power Industry, Tokyo 201-8511, Japan. 2 Dept. of Physics, Science University of Tokyo, Tokyo 162-8601, Japan. 3 NHMFL, Los Alamos National Laboratory, Los Alamos, NM 87545.

Presenting Author: S. Ono

The in-plane normal-state resistivity r _ab is measured in Bi₂Sr_2-xLa_xCuO_6+d single crystals (La-doped Bi-2201) with 60-T pulsed magnetic fields. The single crystals of Bi₂Sr_2-xLa_xCuO_6+d are grown using a floating-zone technique from heavily underdoped to overdoped regions [1]. Under the 60-T field, it is observed that a metallic behavior gradually changes to an insulating one with decreasing carrier concentration, and this metal to insulator (MI) crossover is found to take place in the underdoped region, not at the optimum doping where the MI crossover has been reported to occur in La_2-xSr_xCuO₄ [2] or in Pr_2-xCe_xCuO_4+d [3]. In the "insulating" region (where dr /dT < 0), it is found that r _ab(T) increases as log(1/T) with decreasing temperature, as was previously reported for underdoped LSCO [4].

[1] Y. Ando and T. Murayama, Phys. Rev. B 60, R6991 (1999).

[2] G.S. Boebinger, Y. Ando et al., Phys. Rev. Lett. 77, 5417 (1996).

[3] P. Fournier et al., Phys. Rev. Lett. 81, 4720 (1998).

[4] Y. Ando, G.S. Boebinger et al., Phys. Rev. Lett. 75, 4662 (1995).

4PO7-41 34K Superconductivity of Bi2201 System

Yuheng Zhang, Jianwu Zhang, Gaojie Xu, and Zhiqiang Mao, Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China

Presenting Author: Y. Zhang

With the similar crystal structures, Bi2201 and Tl2201 phases exhibit surprisingly different superconducting transition temperatures (92K for Tl2201 and only 10K for Bi2201). We noted that Bi2201 phase possesses complicated incommensurate modulation structure, while Tl2201 phase does not show any modulation characteristic and this microstructural discrepancy may be of key importance to the difference of T_c of the two phases. The T_c(zero) more than 30K of Bi2201 phase can not be reached and only can be improved up to 27K by decreasing its carrier concentration through La doping. Here we reported that the T_c of Bi2201 can be improved to T_c(zero)4K from T_c<10K while the modulation wavelength increases from 5.0b to 8.6b through suppressing the modulation structure by doping Pb into Bi2201 phase and adjusting the carrier concentration to an optimal doping level by annealing. The improvement of the T_c in our case is mainly attributed to the suppression of the modulation structure.

4PO7-42 Iodine intercalation effect on the critical fluctuation of Bi-2212 single crystal

Jin-Tae Kim 1, M.-S. Kim 1, S.H. Chung 2, D.H. Ha 2, Sung-Ik Lee 1, Y.K. Park 2, and J.-C. Park 2. 1 National Creative Research Initiative Center for Superconductivity, Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea. 2 Korea Research Institute of Standards and Science, P.O. Box 102, Yusong, Taejon 305-606, Korea.

Presenting Author: J.-T. Kim

The magnetization of Bi₂Sr₂CaCu₂O₈ (Bi-2212) single crystal has been measured as a function of magnetic field H and temperature T. The extracted fluctuation part of the magnetization of the Bi-2212 crystal exhibits a critical behavior consistent with three-dimensional XY model. The static critical exponent nu is 0.769 which is larger than the value 0.669 of YBa₂Cu₃O_7-d , indicating that weaker interlayer coupling increases the value of nu.

The Bi-2212 single crystal has been successfully intercalated with iodine and its magnetization has been measured to see how the intercalation influences the critical fluctuation. The effect of the iodine intercalation is such that iodine is located between Bi-O layers and increases the c-axis length of the crystal, therefore interlayer coupling strength is weakened. The extracted fluctuation part of the magnetization of the iodine intercalated sample shows the 3D-XY scaling behavior with an increased static critical exponent nu of 0.99.

The fluctuation magnetization of the iodine intercalated Bi-2212 obeys the 3D-XY scaling model with the increased value of the static critical exponent nu = 0.99. The weaker is the interlayer coupling, the value of the static critical exponent nu is larger.

4PO7-43 Macroscopic phase separation in extremely hole-underdoped Bi₂Sr_2-xLa_xCuO_6+y single crystals

W.L. Yang 1, H.H. Wen 2, P. Li 2, H. Chen 2, F. Wu 2, C. Dong 2, J.W. Xiong 2, Y.M. Ni 2, and Z.X. Zhao 2. 1 National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Science and Center for Condensed Matter Physics, Beijing 100080. 2 National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Science and Center for Condensed Matter Physics, Beijing 100080.

Presenting Author: W.L. Yang

Structural, magnetic, and Raman scattering measurements have been performed on a series of high quality Bi₂Sr_2-xLa_xCuO_6+y single crystals. a superconducting phase with T_c of 36K, which is the highest T_c reported in this system, has been discovered for the first time in heavily La-doped (hole-underdoped) samples with x ³ 0.8.

X-ray diffraction pattern (XRD) and electron diffraction patterns from transmission electron microscopy (TEM) confirmed the excellent crystallinity, and the actual composition of La was determined by not only Energy-Dispersive X-ray (EDX) analysis but also the c-axis parameter determined from XRD. All the data coincide with each other and show that these samples, which T_c is as high as 36K, are extremely hole-underdoped ones which normally do not show superconductivity.

Furthermore, annealing results show that this superconducting phase is not caused by chemical imperfection but by oxygen dopants. In addition, relatively stronger anti-ferromagnetic correlation is observed in these samples by Raman scattering measurements. Therefore, we attribute the appearance of this high T_c superconducting phase to the macroscopic phase separation induced by the compromise between the hole kinetic energy and anti-ferromagnetic ordering.

4PO7-44 Charge-transfer Induced by Pb-doping and Annealing in Bi-2212 phase superconductor

Lei Shi, Qiuzhao Dong and Yuheng Zhang, Structure Research Laboratory, University of Science and Technology of China, Academia Sinica, Hefei, Anhui 230026, P.R. China

Presenting Author: L. Shi

Aliovalent cation substitution in cuprate has been one of the basic methods to investigate the mechanisms of high-T_c superconductor. In this paper, a systematic study of Pb-doping and annealing effects in vacuum (2x10^-3 Pa) on T_c and microstructure of Bi_2-xPb_xSr₂CaCu₂Od compounds (esp. Bi_1.6Pb_0.4Sr₂CaCu₂Od ) has been carried out. By x-ray diffraction, resistivity, and TEM measurements, it reveals that the structure belongs to Bi-2212 phase, and the modulated period becomes longer and finally disappears with increasing the content of Pb. Meanwhile, the c and b axes almost keep unchanged while Bi is replaced by Pb. It is evident, however, that with annealing temperature increasing, c and b axes parameters monotonously increase while the a-axis parameter is unchanged. By the curve of resistivity-temperature, the T_c is found to increase after annealing. There is an optimum T_c with annealing temperature increasing and the optimum T_c of Bi_2-xPb_xSr₂CaCu₂Od (x=0~0.5) almost keeps the same, which reveals that the carriers concentrations p changes from overdoped to underdoped. Combining the above results with the Raman scattering measurements, it is suggested that a charge-transfer between carrier group (BiO-SrO) and conducting CuO₂ plane is induced by Pb-doping and annealing in vacuum. The change of the microstructure and superconductivity of Pb-doped Bi-2212 phase is related to the charge transfer.

4PO7-45 Origin and characterisation of peak effect in pure and Pb doped Bi-2212 single crystals

X.L. Wang 1, J. Horvat 1, H.K. Liu 1, S.X. Dou 1, G. Heine 2, and W. Lang 2. 1 Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia. 2 Institut fuer Materialphysik, University of Vienna, Kopernikusgasse 15, A-1060 Wien, Austria.

Presenting Author: X.L. Wang

The peak effect in pure and Pb doped Bi2212 single crystals with different oxygen doping levels were studied by measuring M-H loops in a temperature range from 5 K to T_c. The peak effect in pure Bi2212 crystals was obtained only for crystals with oxygen optimum doping and overdoping but not observed for oxygen underdoping crystals. For Pb-doped Bi2212 crystals, the peak effect appeared at higher field than in pure crystals and persisted up to T_c.

Formation and decomposition of Bi⁵⁺ rich clusters, which caused the reduction of c-axis and r _c, were proposed to be responsible for the appearance and disappearance of peak effect for the pure crystals. Co-existence of Pb⁴⁺ - and Bi⁴⁺ rich clusters caused the strong peak effect in Pb doped Bi2212 crystals.

The peak effect for both pure and Pb-doped crystals was characterised by plotting (H_max-H_min)/H_max vs T/T_c, where H_max and H_min represents the field in which the magnetisation reaches maximum and starts to increase around secondary peak position. Results showed that the evolution of the peak effect with temperature in the Pb doped crystals was similar to that in Y123.

Study on magnetic vortex pinning process for the Pb doped crystals indicated that the variable range hopping processes defined the magnetic relaxation at the secondary peak, whereas single vortex pinning was obtained at higher field.

4PO7-46 Improved flux pinning and peak effect in heavily overdoped Bi_2-xPb_xSr₂CaCu_2-yCr_yO_8+d single crystals

Y.P. Sun 1, B. Zhao 1, W.H. Song 1, M.H. Pu 1, X.C. Wu 1, W.D. Huang 1, J.J. Du 1, Y.Y. Hsu 2, B.N. Lin 2, Y.H. Lin 2, H.M. Luo 2, and H.C. Ku 2. 1 Institute of Solid State Physics, Academia Sinica, Hefei 230031, China. 2 Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan.

Presenting Author: Y. Sun

Flux pinning and peak effect of heavily overdoped Bi_2-xPb_xSr₂CaCu_2-yCr_yO_8+d single crystals are investigated by means of magnetization measurements with the magnetic field parallel to the c-axis. Compared with undoped Bi_2-xPb_xSr₂CaCu_2-yCr_yO_8+d single crystals, the irreversibility line is pronouncedly shifted to high temperatures. The anomalous magnetization peak H_2p with maximum peak field over 6000 G is observed. The H_2p exhibits strong temperature dependence from 0.2 T_c to 0.9 T_c .The temperature dependence of H_2p can be explained according to pancake vortex decoupling theory. The improved flux pinning may originate from their less anisotropic electronic structures and sufficient number of effective pinning centers due to the random Cr substitution on the Cu site.

4PO7-47 Doping effect on the structure and superconductivity of Bi₂Sr₂Ca(Cu_1-xM_x)₂O_y (M=Mn, Fe, Co and Ni) single crystals

X.F. Sun, X. Zhao, L. Wang, Q.F. Zhou, W.B. Wu, and X.-G. Li, Structure Research Laboratory, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P.R. China

Presenting Author: X.F. Sun

Substitution for Cu by other 3d transition elements in CuO₂ plane is an effective means to probe the normal state properties and superconductivity of high-T_c cuprates. For this, many works have been carried out but mainly on polycrystallinity samples and studies on single crystals are still insufficient. Recently, we have grown Bi₂Sr₂Ca(Cu_1-xM_x)₂O_y (M = Mn, Fe, Co and Ni) (x < 0.05) single crystals by self-flux method. The x-ray diffraction shows that the basic Bi2212 structure does not change for these doped crystals while the c-axis lattice parameter decreases with x. The superconductivity was suppressed rapidly upon doping. Moreover, the relation between transition temperature T_c and x is nearly linear for Fe and Ni doped crystals, which can be described as the magnetic pair-breaking, while it shows a nonlinear behavior for Mn and Co doped crystals. The anomalous upward curvature of T_c - x curve for Mn and Co doped crystals may be related with the complex magnetism of Mn and Co ions as embodied in the doped rare-earth manganites or cobaltates that show the colossal magnetoresistance. In comparison, Mn is the most destructive dopant for the superconductivity of Bi2212.

4PO7-48 Different anisotropy in normal and superconducting state of Bi₂Sr₂Ca(Cu_1-xM_x)₂O_y (M = Mn, Fe, Co and Ni) single crystals

X.F. Sun, X. Zhao, L. Wang, Q.F. Zhou, W.B. Wu, and X.-G. Li, Structure Research Laboratory, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P.R. China

Presenting Author: X.F. Sun

One of the peculiar characteristics of high-T_c superconductors is the large electromagnetic anisotropy, defined as g =(r _c/r _ab)^1/2 in normal state and g _s=l _c/l _ab=(mc*/mab*)^1/2 in superconducting state. The previous works revealed that both g _n and g _s decrease monotonously with increasing carrier concentration and it is usually taken for granted that the anisotropy is a unique parameter in normal and superconducting states. However, the variation law of anisotropy is still far from consensus for substitution of Cu by other 3d transition elements, which hardly changes the carrier concentration but greatly affects the properties of CuO₂ plane. So we study the anisotropic resistivity and the second peak of magnetization curves for Bi₂Sr₂Ca(Cu_1-xM_x)₂O_y (M = Mn, Fe, Co and Ni) single crystals in this paper. The r _c(T) and g _n decrease rapidly with increasing x. While the g _s, obtained from the second peak field by relation B_sp = F ₀/(sg _s)², increases systematically with x. This indicates that the anisotropy in normal state is different from that in superconducting state, inconsistent with the conventional concept. The decrease of r _c(T) and g _n is mainly due to the decrease of barrier potential for the c-axis charge transport, while the increase of g _s is due to the weakening of interlayer Josephson coupling.

4PO7-49 Imaging Single Atom Impurity Scattering in Doped Bi₂Sr₂CaCu₂O_8+d

K.M. Lang 1, E.W. Hudson 1, V. Madhavan 1, S.H. Pan 2, H. Eisaki 3, S. Uchida 3, and J.C. Davis 1. 1 Department of Physics, University of California, Berkeley, CA 94720. 2 Department of Physics, Boston University, Boston, MA, 02215. 3 Department of Superconductivity, University of Tokyo, Tokyo, Japan.

Presenting Author: K.M. Lang

Although the mechanism of high temperature superconductivity in the cuprates is still a mystery, it is believed to originate from strongly interacting electrons in their CuO₂ planes. Substitution of a single impurity atom at a Cu site creates a simple but powerful perturbation to these interactions. Detailed knowledge of the effects of such an impurity atom on the superconducting order parameter and on the quasi-particle local density of states could allow competing theories of high temperature superconductivity to be tested at the atomic scale. Here we describe scanning tunneling microscopy studies of the effects of individual Zn and Ni impurity atoms located at the Cu site in the high-T_c superconductor Bi₂Sr₂CaCu₂O_8+d . These two impurities show vastly different quasi-particle scattering resonances, both spectrally and spatially, thus providing detailed information about the effects of single atom perturbations on superconductivity.