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Phase diagram of high-Tc superconductor: Cu-NMR studies on multi-layered cuprates
ARTICLE IN PRESS
Physica B 403 (2008) 1059–1061 www.elsevier.com/locate/physb
Phase diagram of high-T c superconductor: Cu-NMR studies on multi-layered cuprates H. Mukudaa,, M. Abea, S. Shimizua, Y. Kitaokaa, A. Iyob, Y. Kodamab, Y. Tanakab, K. Tokiwac, T. Watanabec a Department of Materials Science, Osaka University, Toyonaka, Osaka 560-8531, Japan National Institute of Advanced Industrial Science and Technology (AIST), Umezono, Tsukuba 305-8568, Japan c Department of Applied Electronics, Science University of Tokyo, Noda, Chiba 278-8510, Japan
b
Abstract A new phase diagram is presented for an ideally flat CuO2 plane through the Cu-NMR studies on five-layered cuprates MBa2 Ca4 Cu5 Oy (M-1245) ðM ¼ Hg; Tl; CuÞ, which includes an antiferromagnetic (AFM) metal phase and a uniform mixing phase of AFM metal and high-T c superconductivity (HTSC) in an under-doped region. It has been found that a disorder causes a quantum-phase transition from an AFM metal to an insulating state in an under-doped regime in the Cu-1245 where a disorder is introduced via an oxygen-reduced process. This finding reinforces that an AFM metallic phase exists between the AFM insulating phase and the SC phase for the ideally flat CuO2 plane provided that a disorder is absent. r 2007 Elsevier B.V. All rights reserved. PACS: 74.72.Jt; 74.25.Ha; 74.25.Nf Keywords: Multilayer; High-T c cuprate; Antiferromagnetism; Superconductivity
A possible coexistence of antiferromagnetism and highT c superconductivity (HTSC) remains as one of the most interesting problems in high-T c cuprates. Recently, an anomalous superconducting (SC) and antiferromagnetic (AFM) characteristics have been revealed for five-layered cuprates MBa2 Ca4 Cu5 Oy (M-1245) ðM ¼ Hg; Tl; CuÞ [1–3]. In the multi-layered high-T c cuprates including more than three CuO2 planes in a unit cell, there are two types of CuO2 planes, an outer CuO2 plane (OP) in a fivefold pyramidal coordination and an inner plane (IP) in a fourfold square one. Since a distance from charge reservoir layers is far for IPs than for OPs, a doping level at IPs is smaller than that at OPs [4,5]. As a result, in the fivelayered optimally doped Hg-1245(OPT), the three underdoped IPs show an AFM metallic behavior below T N ¼ 60 K, whereas the optimally doped two OPs are predominantly SC below T c ¼ 108 K, realizing the coexCorresponding author. Tel./fax: +81 6 6850 6437.
E-mail address: [email protected] (H. Mukuda). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.263
istence of the AFM metal phase and the SC phase in a unit cell [1]. The first microscopic evidence for the uniform mixing of AFM metal and HTSC on a single CuO2 plane was revealed for the OP of the very under-doped Hg-1245(UD) with T N ¼ 290 K and T c ¼ 72 K [2], whereas the IPs are possibly AFM insulators with nearly non-doping. Through the Cu-NMR studies in these fivelayered cuprates, a new phase diagram have been experimentally established for the ideally flat CuO2 plane, including the AFM metal phase and the uniform mixing phase of AFM metal and HTSC in an UD region (see Fig. 2(a)). This result originates from the ideal flatness of CuO2 planes which was homogeneously doped. Here we report on a disorder effect in the UD IPs in a five-layered cuprate Cu-1245(OPT) [3]. The Cu-NMR study on oxygen-reduced Cu-1245(OPT) has revealed that the carrier density at both OP and IP is smaller than in the as-prepared heavily over-doped Cu-1245(OVD). As a result of the reduction of the carrier density, the SC for Cu-1245(OPT) takes place at the nearly optimally doped
ARTICLE IN PRESS H. Mukuda et al. / Physica B 403 (2008) 1059–1061
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OP with T c ¼ 98 K. We demonstrate that the disorder effect makes carriers localize and eventually drives a quantum-phase transition from an AFM metal to an insulating state.
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Fig. 1. (color online) Cu-NMR spectra at 1.5 K and zero magnetic field for (b) Cu-1245(OPT), together with those in (a) Hg-1245(UD) [2] and (c) Hg-1245(OPT) [1]. Each discrete peak in Cu-1245(OPT) is qualitatively explained by a carrier-localization model. The solid curve is a simulated one for a case that the carrier of N h 6% distributes randomly. (d) Illustration of possible distribution patterns of localized carriers at low temperatures.
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As shown in Fig. 1(b), the zero-field-Cu-NMR spectra for the IPs in Cu-1245(OPT) are distributed over a wide frequency range, which contrast with the respective Figs. 1(a) and (c) for the non-doped IPs of Hg-1245(UD) and the metallic IPs of Hg-1245(OPT). We explain the discrete NMR spectra of IPs in terms of a carrier-localization model, which is described by the discrete values of internal field H int . These observed resonance frequencies are given ðiÞ ðiÞ by f ðiÞ res ¼ ðgN =2pÞH int ¼ ðgN =2pÞjAhf jM AF , where gN is a nuclear gyromagnetic ratio, AðiÞ hf is a hyperfine coupling constant, and M AF is an AFM moment. We consider several possible distribution patterns of the localized carriers that are shown in Fig. 1(d). Each H int is calculated as follows; ð4BÞM AF , ðA 4BÞM AF , ð3BÞM AF , ðA 3BÞM AF , ð2BÞM AF , and ðA 2BÞM AF . Here A and B are the on-site hyperfine field and the supertransferred hyperfine field from the four nearest neighboring Cu site, respectively, which are assumed to be A37 kOe=mB and B61 kOe=mB [1,2].M AF is assumed to be 0:66mB that is almost the same as in nearly non-doped CuO2 plane [2]. Actually, each resonance peak is qualitatively reproduced by this simple model, as indicated by the arrows in Fig. 1(b). The curve in the figure is a simulated one for the case that doped holes with N h 6% distribute randomly, assuming the same carrier density as in Hg-1245(OPT). Thus the magnetic ground state at the IPs in Cu1245(OPT) is suggested to be an AFM insulating state or spin-glass (SG) state with short range AFM order, as illustrated in Fig. 2(b) [6], which resembles the SG state in La2x Srx CuO4 (LSCO). It should be noted that the result contrasts with the AFM metal with reduced AFM moments 0:3mB in Hg-1245(OPT) in spite of the similar carrier density [2]. We consider that the disorder mapped on the IPs in Cu-1245(OPT) from either the charge reservoir layers or the OP makes carriers localize at low temperatures. It was suggested that the carriers are
T*
Fig. 2. (color online) The phase diagram for (a) ideally flat CuO2 plane and (b) disordered CuO2 plane are illustrated. The disorder-driven localization at low temperature seems to be analogous with the spin-glass phase established for LSCO [6]. The carrier density is based on the tentative estimation in Ref. [2].
ARTICLE IN PRESS H. Mukuda et al. / Physica B 403 (2008) 1059–1061
localized owing to the Anderson-localization mechanism with strong electron correlation, since a small amount of carriers are doped into two-dimensional IPs near the metal–insulator transition. This finding reinforces that an AFM metallic phase exists between the AFM insulating phase and the SC phase for the ideally flat CuO2 plane provided that the disorder is absent. It is likely that the SG phase emerging in the under-doped region of LSCO is caused by the disorder, since the disorder effect is inevitable because of the chemical substitution introduced into the out-of-planes. In conclusion, we have investigated the magnetic characteristics for the oxygen-reduced Cu-based fivelayered high-T c cuprates Cu-1245(OPT), and compared with those in the previously reported five-layered cuprates. It has been revealed that the bulk SC takes place at the nearly optimally doped OP with T c ¼ 98 K. This contrasts with the SC for the heavily overdoped Cu-1245(OVD) that emerges at the optimally doped IP with T c ¼ 65 K [5]. This gives evidence that optimally doping carriers into either IP
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or OP determines its bulk T c in the multi-layered cuprates. The ZF-NMR experiments have revealed that the static AFM order at IPs is caused by the localization of holes due to the disorder effect, although the SC transition takes place at OPs below T c ¼ 98 K. This is in remarkable contrast with the AFM metallic phase in Hg-1245(OPT) with the similar doping level as in Cu-1245(OVD). This finding reinforces that the AFM metallic phase exists between the AFM insulating state and the SC state for ideally flat CuO2 plane without disorder. References [1] [2] [3] [4] [5] [6]
H. Kotegawa, et al., Phys. Rev. B 69 (2004) 014501. H. Mukuda, et al., Phys. Rev. Lett. 96 (2006) 087001. H. Mukuda, et al., J. Phys. Soc. Japan 75 (2006) 123702. Y. Tokunaga, et al., Phys. Rev. B 66 (2000) 9707. H. Kotegawa, et al., Phys. Rev. B 64 (2001) 064515. The carrier localization occurs at least below 1.5 K for Cu-1245(OPT). The under-doped region of phase diagram in Fig. 2(b) is the illustration deduced from the one of LSCO.
Physica B 403 (2008) 1059–1061 www.elsevier.com/locate/physb
Phase diagram of high-T c superconductor: Cu-NMR studies on multi-layered cuprates H. Mukudaa,, M. Abea, S. Shimizua, Y. Kitaokaa, A. Iyob, Y. Kodamab, Y. Tanakab, K. Tokiwac, T. Watanabec a Department of Materials Science, Osaka University, Toyonaka, Osaka 560-8531, Japan National Institute of Advanced Industrial Science and Technology (AIST), Umezono, Tsukuba 305-8568, Japan c Department of Applied Electronics, Science University of Tokyo, Noda, Chiba 278-8510, Japan
b
Abstract A new phase diagram is presented for an ideally flat CuO2 plane through the Cu-NMR studies on five-layered cuprates MBa2 Ca4 Cu5 Oy (M-1245) ðM ¼ Hg; Tl; CuÞ, which includes an antiferromagnetic (AFM) metal phase and a uniform mixing phase of AFM metal and high-T c superconductivity (HTSC) in an under-doped region. It has been found that a disorder causes a quantum-phase transition from an AFM metal to an insulating state in an under-doped regime in the Cu-1245 where a disorder is introduced via an oxygen-reduced process. This finding reinforces that an AFM metallic phase exists between the AFM insulating phase and the SC phase for the ideally flat CuO2 plane provided that a disorder is absent. r 2007 Elsevier B.V. All rights reserved. PACS: 74.72.Jt; 74.25.Ha; 74.25.Nf Keywords: Multilayer; High-T c cuprate; Antiferromagnetism; Superconductivity
A possible coexistence of antiferromagnetism and highT c superconductivity (HTSC) remains as one of the most interesting problems in high-T c cuprates. Recently, an anomalous superconducting (SC) and antiferromagnetic (AFM) characteristics have been revealed for five-layered cuprates MBa2 Ca4 Cu5 Oy (M-1245) ðM ¼ Hg; Tl; CuÞ [1–3]. In the multi-layered high-T c cuprates including more than three CuO2 planes in a unit cell, there are two types of CuO2 planes, an outer CuO2 plane (OP) in a fivefold pyramidal coordination and an inner plane (IP) in a fourfold square one. Since a distance from charge reservoir layers is far for IPs than for OPs, a doping level at IPs is smaller than that at OPs [4,5]. As a result, in the fivelayered optimally doped Hg-1245(OPT), the three underdoped IPs show an AFM metallic behavior below T N ¼ 60 K, whereas the optimally doped two OPs are predominantly SC below T c ¼ 108 K, realizing the coexCorresponding author. Tel./fax: +81 6 6850 6437.
E-mail address: [email protected] (H. Mukuda). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.263
istence of the AFM metal phase and the SC phase in a unit cell [1]. The first microscopic evidence for the uniform mixing of AFM metal and HTSC on a single CuO2 plane was revealed for the OP of the very under-doped Hg-1245(UD) with T N ¼ 290 K and T c ¼ 72 K [2], whereas the IPs are possibly AFM insulators with nearly non-doping. Through the Cu-NMR studies in these fivelayered cuprates, a new phase diagram have been experimentally established for the ideally flat CuO2 plane, including the AFM metal phase and the uniform mixing phase of AFM metal and HTSC in an UD region (see Fig. 2(a)). This result originates from the ideal flatness of CuO2 planes which was homogeneously doped. Here we report on a disorder effect in the UD IPs in a five-layered cuprate Cu-1245(OPT) [3]. The Cu-NMR study on oxygen-reduced Cu-1245(OPT) has revealed that the carrier density at both OP and IP is smaller than in the as-prepared heavily over-doped Cu-1245(OVD). As a result of the reduction of the carrier density, the SC for Cu-1245(OPT) takes place at the nearly optimally doped
ARTICLE IN PRESS H. Mukuda et al. / Physica B 403 (2008) 1059–1061
1060
OP with T c ¼ 98 K. We demonstrate that the disorder effect makes carriers localize and eventually drives a quantum-phase transition from an AFM metal to an insulating state.
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Fig. 1. (color online) Cu-NMR spectra at 1.5 K and zero magnetic field for (b) Cu-1245(OPT), together with those in (a) Hg-1245(UD) [2] and (c) Hg-1245(OPT) [1]. Each discrete peak in Cu-1245(OPT) is qualitatively explained by a carrier-localization model. The solid curve is a simulated one for a case that the carrier of N h 6% distributes randomly. (d) Illustration of possible distribution patterns of localized carriers at low temperatures.
T*
As shown in Fig. 1(b), the zero-field-Cu-NMR spectra for the IPs in Cu-1245(OPT) are distributed over a wide frequency range, which contrast with the respective Figs. 1(a) and (c) for the non-doped IPs of Hg-1245(UD) and the metallic IPs of Hg-1245(OPT). We explain the discrete NMR spectra of IPs in terms of a carrier-localization model, which is described by the discrete values of internal field H int . These observed resonance frequencies are given ðiÞ ðiÞ by f ðiÞ res ¼ ðgN =2pÞH int ¼ ðgN =2pÞjAhf jM AF , where gN is a nuclear gyromagnetic ratio, AðiÞ hf is a hyperfine coupling constant, and M AF is an AFM moment. We consider several possible distribution patterns of the localized carriers that are shown in Fig. 1(d). Each H int is calculated as follows; ð4BÞM AF , ðA 4BÞM AF , ð3BÞM AF , ðA 3BÞM AF , ð2BÞM AF , and ðA 2BÞM AF . Here A and B are the on-site hyperfine field and the supertransferred hyperfine field from the four nearest neighboring Cu site, respectively, which are assumed to be A37 kOe=mB and B61 kOe=mB [1,2].M AF is assumed to be 0:66mB that is almost the same as in nearly non-doped CuO2 plane [2]. Actually, each resonance peak is qualitatively reproduced by this simple model, as indicated by the arrows in Fig. 1(b). The curve in the figure is a simulated one for the case that doped holes with N h 6% distribute randomly, assuming the same carrier density as in Hg-1245(OPT). Thus the magnetic ground state at the IPs in Cu1245(OPT) is suggested to be an AFM insulating state or spin-glass (SG) state with short range AFM order, as illustrated in Fig. 2(b) [6], which resembles the SG state in La2x Srx CuO4 (LSCO). It should be noted that the result contrasts with the AFM metal with reduced AFM moments 0:3mB in Hg-1245(OPT) in spite of the similar carrier density [2]. We consider that the disorder mapped on the IPs in Cu-1245(OPT) from either the charge reservoir layers or the OP makes carriers localize at low temperatures. It was suggested that the carriers are
T*
Fig. 2. (color online) The phase diagram for (a) ideally flat CuO2 plane and (b) disordered CuO2 plane are illustrated. The disorder-driven localization at low temperature seems to be analogous with the spin-glass phase established for LSCO [6]. The carrier density is based on the tentative estimation in Ref. [2].
ARTICLE IN PRESS H. Mukuda et al. / Physica B 403 (2008) 1059–1061
localized owing to the Anderson-localization mechanism with strong electron correlation, since a small amount of carriers are doped into two-dimensional IPs near the metal–insulator transition. This finding reinforces that an AFM metallic phase exists between the AFM insulating phase and the SC phase for the ideally flat CuO2 plane provided that the disorder is absent. It is likely that the SG phase emerging in the under-doped region of LSCO is caused by the disorder, since the disorder effect is inevitable because of the chemical substitution introduced into the out-of-planes. In conclusion, we have investigated the magnetic characteristics for the oxygen-reduced Cu-based fivelayered high-T c cuprates Cu-1245(OPT), and compared with those in the previously reported five-layered cuprates. It has been revealed that the bulk SC takes place at the nearly optimally doped OP with T c ¼ 98 K. This contrasts with the SC for the heavily overdoped Cu-1245(OVD) that emerges at the optimally doped IP with T c ¼ 65 K [5]. This gives evidence that optimally doping carriers into either IP
1061
or OP determines its bulk T c in the multi-layered cuprates. The ZF-NMR experiments have revealed that the static AFM order at IPs is caused by the localization of holes due to the disorder effect, although the SC transition takes place at OPs below T c ¼ 98 K. This is in remarkable contrast with the AFM metallic phase in Hg-1245(OPT) with the similar doping level as in Cu-1245(OVD). This finding reinforces that the AFM metallic phase exists between the AFM insulating state and the SC state for ideally flat CuO2 plane without disorder. References [1] [2] [3] [4] [5] [6]
H. Kotegawa, et al., Phys. Rev. B 69 (2004) 014501. H. Mukuda, et al., Phys. Rev. Lett. 96 (2006) 087001. H. Mukuda, et al., J. Phys. Soc. Japan 75 (2006) 123702. Y. Tokunaga, et al., Phys. Rev. B 66 (2000) 9707. H. Kotegawa, et al., Phys. Rev. B 64 (2001) 064515. The carrier localization occurs at least below 1.5 K for Cu-1245(OPT). The under-doped region of phase diagram in Fig. 2(b) is the illustration deduced from the one of LSCO.