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STS observations of Co impurities in Bi2.1Sr1.8Ca(Cu1−xCox)2O8+y single crystals
Physica B 284}288 (2000) 1065}1066
STM/STS observations of Co impurities in Bi Sr Ca(Cu Co ) O single crystals \V V >W
Xiaoru Zhao *, Shoichiro Nakao , Kazunori Ueno , Go Kinoda, Takeshi Endo , Tetsuo Hanaguri , Koichi Kitazawa , Tetsuya Hasegawa Department of Superconductivity, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan CREST, Japan Science and Technology Corporation, Japan Materials and Structures Laboratory, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8503, Japan
Abstract Cleaved surfaces of Bi Sr Ca(Cu Co ) O (x"0.04) have been investigated by STM/STS at cryogenic \V V >W temperatures. The obtained conductance image, which maps the local density of states (LDOS) near the Fermi level, clearly showed Co impurities present in the CuO planes, indicating that the low-energy excitation spectrum is substantially a!ected by the impurity doping. The site-speci"ed tunneling spectra on both the impurity site and host superconductor region at 66 K revealed a smeared gap-like structure. On the impurity sites, we have found nearly linear background extended up to a sample bias voltage of 1 V, while the background in the host superconductor region was rather #at. The present STM/STS results suggest that the scattering of electrons by magnetic impurities modi"es the LDOS in an energy region scaled by J. 2000 Elsevier Science B.V. All rights reserved. Keywords: Impurity doping; Bi-2212 substituted; Local density of states; STM/STS
1. Introduction Recently, local density of states (LDOS) around impurities doped in high-¹ cuprates has been extensively studied as a direct probe for their pairing mechanism and normal state properties [1]. In particular, the e!ects of magnetic impurities are interesting, because they may re#ect the magnetic contribution in high-¹ supercon ductivity. In this report, we present a cryogenic scanning tunneling microscopy/spectroscopy (STM/STS) study of Bi Sr Ca(Cu Co ) O (x"0.04) single crys \V V >W tals. Our spatially resolved STS data showed that Co impurities substantially modify the LDOS around them up to an energy region of &1 eV, which cannot be
interpreted by the quasi-classical theory governed by the energy scale D.
2. Experimental Single crystals of Co-doped Bi2212 were grown by the #oating-zone method and were post-annealed in air at 8003C for 3 d. All STM/STS measurements were performed by a commercial UHV-STM (Unisoku, USM602). Surface cleaving was done in UHV (2;10\ Torr). Mechanically sharpened Pt/Ir wires were used as STM tips in this study.
3. Results and discussion * Corresponding author. Department of Superconductivity, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel./Fax: #81-045-924-5363. E-mail address: [email protected] (X. Zhao)
The obtained conductance image at 66 K for a Codoped Bi2212 crystal was shown in Fig. 1. The strong contrast in brightness re#ects impurity Co sites (dark spots) doped into Cu}O plane and host superconductor
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 4 1 4 - X
1066
X. Zhao et al. / Physica B 284}288 (2000) 1065}1066
Fig. 2. Typical tunneling spectra observed on Co site (marked A) and host superconductor region (marked B or C) at 66 K.
Fig. 1. Conductance map of 100 mV normalized at 900 mV. The image size is 14.4 nm;14.4 nm.
(bright region). It is noted that dark spots are slightly less than nominally Co-doped concentration. We, for the "rst time, successfully observed the changes of LDOS near an impurity, indicating that the low-energy excitation spectrum is a!ected by the impurity doping. The site-speci"ed tunneling spectra on sites A}C revealed a smeared gap-like structure, in common, as shown in Fig. 2. On sites B and C, a superconducting gap with conductance peaks at the gap edge was well developed, while at the cores of Co impurities, a pseudogap without overshooting peaks was reproducibly observed. On the impurity sites, moreover, we have found nearly linear background extended up to a sample bias voltage of 1 V, in clear contrast to that in the host superconductor regions with rather #at background [2}4]. According to the quasi-classical approaches, the LDOS around an impurity are modi"ed in an energy region scaled by D&50 meV [1]. Thus, the present STS
results suggest that a magnetic interaction of the order of J must be taken into consideration for describing the LDOS near the magnetic impurities in high-¹ cuprates. 4. Summary In summary, STM/STS observations of a Co-doped Bi2212 crystal were performed at cryogenic temperatures. We successfully imaged the Co impurities, clearly indicating that the low-energy excitation spectrum is a!ected by Co ions doped into the Cu}O planes. The present STM/STS results suggest that the scattering of electrons by magnetic impurities modi"es LDOS in an energy region scaled by J. References [1] [2] [3] [4]
M.I. Salkola et al., Phys. Rev. Lett. 77 (1996) 1841. C. Renner et al., Phys. Rev. B 51 (1995) 9208. S. Matsuura et al., Physica C 300 (1998) 26. S. Kaneko et al., Physica C 298 (1998) 105.
STM/STS observations of Co impurities in Bi Sr Ca(Cu Co ) O single crystals \V V >W
Xiaoru Zhao *, Shoichiro Nakao , Kazunori Ueno , Go Kinoda, Takeshi Endo , Tetsuo Hanaguri , Koichi Kitazawa , Tetsuya Hasegawa Department of Superconductivity, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan CREST, Japan Science and Technology Corporation, Japan Materials and Structures Laboratory, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8503, Japan
Abstract Cleaved surfaces of Bi Sr Ca(Cu Co ) O (x"0.04) have been investigated by STM/STS at cryogenic \V V >W temperatures. The obtained conductance image, which maps the local density of states (LDOS) near the Fermi level, clearly showed Co impurities present in the CuO planes, indicating that the low-energy excitation spectrum is substantially a!ected by the impurity doping. The site-speci"ed tunneling spectra on both the impurity site and host superconductor region at 66 K revealed a smeared gap-like structure. On the impurity sites, we have found nearly linear background extended up to a sample bias voltage of 1 V, while the background in the host superconductor region was rather #at. The present STM/STS results suggest that the scattering of electrons by magnetic impurities modi"es the LDOS in an energy region scaled by J. 2000 Elsevier Science B.V. All rights reserved. Keywords: Impurity doping; Bi-2212 substituted; Local density of states; STM/STS
1. Introduction Recently, local density of states (LDOS) around impurities doped in high-¹ cuprates has been extensively studied as a direct probe for their pairing mechanism and normal state properties [1]. In particular, the e!ects of magnetic impurities are interesting, because they may re#ect the magnetic contribution in high-¹ supercon ductivity. In this report, we present a cryogenic scanning tunneling microscopy/spectroscopy (STM/STS) study of Bi Sr Ca(Cu Co ) O (x"0.04) single crys \V V >W tals. Our spatially resolved STS data showed that Co impurities substantially modify the LDOS around them up to an energy region of &1 eV, which cannot be
interpreted by the quasi-classical theory governed by the energy scale D.
2. Experimental Single crystals of Co-doped Bi2212 were grown by the #oating-zone method and were post-annealed in air at 8003C for 3 d. All STM/STS measurements were performed by a commercial UHV-STM (Unisoku, USM602). Surface cleaving was done in UHV (2;10\ Torr). Mechanically sharpened Pt/Ir wires were used as STM tips in this study.
3. Results and discussion * Corresponding author. Department of Superconductivity, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel./Fax: #81-045-924-5363. E-mail address: [email protected] (X. Zhao)
The obtained conductance image at 66 K for a Codoped Bi2212 crystal was shown in Fig. 1. The strong contrast in brightness re#ects impurity Co sites (dark spots) doped into Cu}O plane and host superconductor
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 4 1 4 - X
1066
X. Zhao et al. / Physica B 284}288 (2000) 1065}1066
Fig. 2. Typical tunneling spectra observed on Co site (marked A) and host superconductor region (marked B or C) at 66 K.
Fig. 1. Conductance map of 100 mV normalized at 900 mV. The image size is 14.4 nm;14.4 nm.
(bright region). It is noted that dark spots are slightly less than nominally Co-doped concentration. We, for the "rst time, successfully observed the changes of LDOS near an impurity, indicating that the low-energy excitation spectrum is a!ected by the impurity doping. The site-speci"ed tunneling spectra on sites A}C revealed a smeared gap-like structure, in common, as shown in Fig. 2. On sites B and C, a superconducting gap with conductance peaks at the gap edge was well developed, while at the cores of Co impurities, a pseudogap without overshooting peaks was reproducibly observed. On the impurity sites, moreover, we have found nearly linear background extended up to a sample bias voltage of 1 V, in clear contrast to that in the host superconductor regions with rather #at background [2}4]. According to the quasi-classical approaches, the LDOS around an impurity are modi"ed in an energy region scaled by D&50 meV [1]. Thus, the present STS
results suggest that a magnetic interaction of the order of J must be taken into consideration for describing the LDOS near the magnetic impurities in high-¹ cuprates. 4. Summary In summary, STM/STS observations of a Co-doped Bi2212 crystal were performed at cryogenic temperatures. We successfully imaged the Co impurities, clearly indicating that the low-energy excitation spectrum is a!ected by Co ions doped into the Cu}O planes. The present STM/STS results suggest that the scattering of electrons by magnetic impurities modi"es LDOS in an energy region scaled by J. References [1] [2] [3] [4]
M.I. Salkola et al., Phys. Rev. Lett. 77 (1996) 1841. C. Renner et al., Phys. Rev. B 51 (1995) 9208. S. Matsuura et al., Physica C 300 (1998) 26. S. Kaneko et al., Physica C 298 (1998) 105.