- Email: [email protected]
One-dimensional electronic order in underdoped surface of YBa2Cu3Oy studied by STM
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 69 (2008) 3014– 3017
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
One-dimensional electronic order in underdoped surface of YBa2Cu3Oy studied by STM Terukazu Nishizaki a,, Makoto Maki b, Norio Kobayashi a a b
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Department of Physics, Saga University, Saga 840-8502, Japan
a r t i c l e in f o
Keywords: A. Superconductors C. Scanning tunnelling microscopy (STM) D. Electronic structure
a b s t r a c t We have performed low-temperature scanning tunneling microscopy (STM) experiments on the coldcleaved surface of YBa2Cu3Oy single crystals to study the nanoscale electronic order in high-Tc superconductors. STM images measured at low-bias voltage below 50 meV show the one-dimensional (1D) electronic modulation along the Cu–O bonds (parallel to the b-axis). The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range 2a–4a depending on the position on the surface, indicating the glassy electronic order in the underdoped CuO2 plane. & 2008 Elsevier Ltd. All rights reserved.
1. Introduction In high-Tc superconductors with low carrier concentration (i.e., the underdoped regime), anomalous transport, magnetic, and optical properties have been observed below a temperature T* larger than the superconducting transition temperature Tc [1]. These unconventional electronic states appear when charge carriers are added to antiferromagnetic Mott insulators. The pseudogap state below T* is characterized by the opening of a partial gap in the low energy density of states and is observed by photoemission, optical, and tunneling spectroscopy experiments. Recent scanning tunneling microscopy/spectroscopy (STM/STS) experiments have revealed that a two-dimensional periodic pattern (i.e., a checkerboard pattern with a periodicity: 4a 4a, where a is the lattice constant) in the local density of states (LDOS, N(r,E)pdI/dV) appears in the superconducting state [2–4], pseudogap state (T4Tc) [5], zero temperature pseudogap (ZTPG) region [6], and vortex core region [7] of Bi2Sr2CaCu2Oy [2–5,7] and Ca2xNaxCuO2Cl2 [6]. According to these results, the checkerboard electronic order was considered to be the appearance of ‘‘a hidden order’’ in the high-Tc superconductivity. However, recent tunneling-asymmetry (TA) imaging technique indicates that spatial arrangements of the electronic structure are a Cu–O–Cu bond-centered electronic glass state with unidirectional domains in underdoped Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2Oy [8]. Since these domains have a 4a width,
Corresponding author. Tel.: +81 22 215 2029; fax: +81 22 215 2026.
E-mail address: [email protected] (T. Nishizaki). 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.06.010
the relation between checkerboard modulations (dI/dV maps) and electronic glass states (TA maps) is an interesting issue. In addition, the universality of these ordered states and the relation between the spatial variation of the novel electronic order and the ground state of the high-Tc superconductivity are a matter of current interest. In this paper, we report on STM studies of cold-cleaved surface of YBa2Cu3Oy single crystals. We find a one-dimensional (1D) electronic modulation along the Cu–O bonds for the low-bias voltage below 50 mV. The 1D modulation does not have longrange order and the periodicity changes depending on the position, contrary to the checkerboard electronic order in Bi2Sr2CaCu2Oy [2–5,7] and Ca2xNaxCuO2Cl2 [6].
2. Experimental High quality single crystals of YBa2Cu3Oy were grown by the self-flux method using a Y2O3 crucible [9,10]. Slightly overdoped YBa2Cu3O6.96 crystals (TcC91.5 K) were prepared by annealing in 1 bar oxygen at 450 1C. For STM measurements of YBa2Cu3Oy single crystals, the fresh surface was prepared by the cleaving method at low temperatures (LT) below 20 K in the ultrahigh vacuum (UHV), P1010 Torr, and the cleaved crystals were immediately inserted into the cold STM head by the transfer rod. The STM measurements were performed at 4.5 K in UHV condition [11,12]. A mechanically sharpened Pt–Ir wire was used as the STM tip, which was approached perpendicular to the CuO2 plane.
ARTICLE IN PRESS T. Nishizaki et al. / Journal of Physics and Chemistry of Solids 69 (2008) 3014–3017
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on the surface; thus the carrier doping into the topmost CuO2 plane is not sufficient to appear the superconductivity, indicating the strongly underdoped condition [11,13,14]. These results suggest that STM/STS experiments through the cleaved BaO surface of YBa2Cu3Oy can provide information on the electronic state of the underdoped CuO2 plane even if the sample is the optimally doped YBa2Cu3O6.96. Fig. 1(c) shows the STM image of the BaO surface measured on the left terrace in Fig. 1(a) under the condition of V ¼ 100 mV. The square lattice structure with a lattice constant 3.9 A˚ corresponds to the arrangement of oxygen atoms on the BaO surface, because the valence charge around oxygen atoms spreads much more than that around barium atoms and oxygen atoms are more remarkable on the BaO surface [11]. Contrary to the CuO chain layer in Fig. 1(b), the atomic lattice structure is clearly visible even in the dark regions in Fig. 1(c); thus the dark region is not due to the missing atoms but due to the spatial variation of the electronic state on the BaO and/or underlying CuO2 layers. Previous STM studies have presented that the atomic arrangement of the BaO lattice has the relatively homogeneous background at high-bias voltage (V4500 mV) [11,13]. Therefore, the weak background modulation in Fig. 1(c), which seems to align with the b-axis, is a characteristic feature of the low-bias STM image. Since the STM image is a map of the integrated LDOS between the Fermi energy EF and EF+eV, the modulated STM image reflects the local electronic state near the Fermi level. In order to examine the characteristic energy scale of electronic modulations, Fig. 2(a) and (b) shows STM images for high-bias (V ¼ 1 V) and low-bias (V ¼ 6 mV) voltage; these STM images were measured at the different position on the same BaO terrace in Fig. 1(a). As shown in Fig. 2(a), the STM image at highbias voltage shows a spatially homogeneous atomic arrangement of the BaO lattice, as presented by previous papers [11,13]. At low-bias voltage (V ¼ 6 mV), on the other hand, the STM image in Fig. 2(b) shows nanoscale modulations: the modulation
3. Results and discussion In YBa2Cu3Oy, the two dimensionality is not so strong as compared with Bi2Sr2CaCu2O8+y, several terraces and step edges are often observed on the cleaved surface. Fig. 1(a) shows the STM topographic image of cold-cleaved surface in YBa2Cu3O6.96. This surface consists of a single step edge between two atomic terraces. It is known that YBa2Cu3Oy single crystals cleave between the BaO and CuO chain layeres, so the ab-surface termination of the cleaved surface is composed of BaO and/or CuO chain layers [11–14]. According to high-resolution STM images on each terraces shown in Fig. 1(b) and (c), lower (right) and upper (left) terraces correspond to the CuO chain and BaO layers, respectively. Fig. 1(b) shows the 1D electronic modulation on the CuO chain layer along the b-axis which has been discussed in terms of the charge density wave (CDW) due to the 2kF instability of the 1D Fermi surface [14–17] and 1D quasiparticle scattering resonances [18]. The dark regions along the b-axis correspond to missing atoms such as oxygen vacancy clusters along the CuO chain [11,19]. Employing the one dimensionality of the CuO chain parallel to the b-axis, a- and b-axes can be estimated on the BaO surface as shown in Fig. 1(c). It is important to note that the step edge in Fig. 1(a) is parallel to the b-axis and there are no twin boundaries in the measured region, because ambiguity remains for the definition of crystal axes if the step edge is parallel to the /11 0S direction (i.e., the twin boundary direction). For cleaved YBa2Cu3Oy single crystals, the carrier concentration on the surface is different from that on the bulk, so the electronic state of the topmost CuO2 plane is modified by the surface termination [11,13,14]. The CuO chain surface shows superconducting tunneling spectra dI/dV with superconducting gap structures. However, the BaO surface (i.e., the CuO2 plane just under the BaO surface) does not show superconducting spectra because the carrier reservoir (i.e., the CuO chain) is not complete
b a
b
BaO
CuO
b a
Fig. 1. Constant-current STM images of cold-cleaved surface of YBa2Cu3O6.96 at T ¼ 4.5 K. (a) 3D display of step edge region between BaO and CuO surfaces. V ¼ 100 mV, I ¼ 20 pA, 1100 A˚ 1100 A˚. (b) CuO chain surface measured on lower (right) terrace in (a), V ¼ 400 mV, I ¼ 50 pA, 110 A˚ 110 A˚. (c) BaO surface measured on upper (left) terrace in (a), V ¼ 100 mV, I ¼ 20 pA, 96 A˚ 96 A˚.
ARTICLE IN PRESS 3016
T. Nishizaki et al. / Journal of Physics and Chemistry of Solids 69 (2008) 3014–3017
b
a
b
a
is very difficult to associate the 1D modulation with the electronic state of the BaO plane. Thus, the natural interpretation of the 1D modulation observed on the BaO surface would be the appearance of the electronic order in the underdoped CuO2 plane which exists just under the BaO surface, rather than the electronic state of the BaO plane or CuO chain layer which is underlying 9.85 A˚ below the surface. The 1D electronic modulation along the Cu–O bonds does not have long-range order and the periodicity along the a-axis varies within the range 2a–4a, depending on the position on the surface. If the short-range 1D modulation is an appearance of ‘‘the hidden electronic order’’ in the underdoped CuO2 plane of YBa2Cu3Oy, this glassy electronic order differs from the 4a 4a checkerboard structure observed in Bi2Sr2CaCu2Oy [2–5,7] and Ca2xNaxCuO2Cl2 [6]. In order to discuss similarities between the glassy 1D-modulation in YBa2Cu3Oy and the electronic glass state with unidirectional domains in underdoped Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2Oy [8], the detailed analysis of the TA map is necessary in YBa2Cu3Oy. Although the origin of the 1D modulation is not clear in YBa2Cu3Oy, the remanent orthorhombicity due to CuO chain layers, which exist deep inside of the crystal, can be a trigger of the one dimensionality. Finally, our finding of the 1D electronic order is consistent with resistivity measurements of the in-plane anisotropy in underdoped YBa2Cu3Oy [20]; in Ref. [20], the enhancement of the in-plane anisotropy with underdoping is interpreted as the self-organization of electrons into (nematic) charge stripes [21]. The relation between the 1D electronic modulation in YBa2Cu3Oy, the static stripe order in (La, Nd)2xSrxCuO4 [22], the checkerboard structure in Bi2Sr2CaCu2Oy [2–5,7] and Ca2xNaxCuO2Cl2 [6], and the electronic glass state in underdoped Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2Oy [8] is a future issue to be studied.
4. Summary LT-STM studies have been performed in the underdoped surface of the cold-cleaved YBa2Cu3Oy single crystals. From the low-bias STM measurements near the step edge region, we find the 1D electronic modulation along the Cu–O bonds (parallel to the b-axis) which can be clearly distinguished from the CDW order on the CuO chain layer. The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range 2a–4a depending on the position, indicating the glassy electronic order in the underdoped CuO2 plane in YBa2Cu3Oy.
Fig. 2. Constant-current STM images of BaO surface of YBa2Cu3O6.96 at T ¼ 4.5 K. (a) V ¼ 1 V, I ¼ 20 pA, 110 A˚ 110 A˚. (b) V ¼ 6 mV, I ¼ 20 pA, 83 A˚ 83 A˚.
is roughly independent of the polarity of the bias voltage and becomes remarkable for low-bias voltage below 50 mV. These results indicate that the modulation is closely related to the electronic structure near the Fermi level. The electronic modulation has 1D character along the Cu–O bonds (parallel to the b-axis). However, the modulation does not originate from CDW on 1D CuO chains, because the characteristic length scale of CDW on the CuO chain layer (Fig. 1(b)) is different from that of the modulation on the BaO layer (Fig. 2(b)). The modulation of CDW is periodic along the b-axis with a wavelength lb13 A˚ and has short-range correlations with a length of 3–4 neighboring chains [16,17]. On the other hand, the characteristic length of the modulation on the BaO surface is roughly 60–70 A˚ along the b-axis without a clear periodicity and correlations. In addition, it
Acknowledgments This study was partly supported by a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
T. Timusk, B. Statt, Rep. Prog. Phys. 62 (1999) 61. C. Howald, et al., Phys. Rev. B 67 (2003) 014533. K. McElroy, et al., Phys. Rev. Lett. 94 (2005) 197005. N. Momono, et al., J. Phys. Soc. Jpn. 74 (2005) 2400. M. Vershinin, et al., Science 303 (2004) 1995. T. Hanaguri, et al., Nature 430 (2004) 1001. J.E. Hoffman, et al., Science 295 (2002) 466. Y. Kohsaka, et al., Science 315 (2007) 1380. T. Naito, et al., in: S. Nakajima, M. Murakami (Eds.), Advances in Superconductivity, vol. IX, Springer, Tokyo, 1997, p. 601. [10] T. Nishizaki, et al., J. Low Temp. Phys. 117 (1999) 1375.
ARTICLE IN PRESS T. Nishizaki et al. / Journal of Physics and Chemistry of Solids 69 (2008) 3014–3017
[11] [12] [13] [14] [15] [16]
M. Maki, et al., J. Phys. Soc. Jpn. 70 (2001) 1877. T. Nishizaki, et al., Physica C 437/438 (2006) 220. H.L. Edwards, et al., J. Vac. Sci. Technol. B 12 (1994) 1886. H.L. Edwards, et al., Phys. Rev. Lett. 69 (1992) 2967. H.L. Edwards, et al., Phys. Rev. Lett. 73 (1994) 1154. M. Maki, et al., Phys. Rev. B 65 (2002) 140511.
[17] [18] [19] [20] [21] [22]
M. Maki, et al., Phys. Rev. B 72 (2005) 024536. D.J. Derro, et al., Phys. Rev. Lett. 88 (2002) 097002. T. Nishizaki, et al., J. Low Temp. Phys. 131 (2003) 931. Y. Ando, et al., Phys. Rev. Lett. 88 (2002) 137005. S.A. Kivelson, et al., Nature 393 (1998) 550. J.M. Tranquada, et al., Nature 375 (1995) 561.
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Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
One-dimensional electronic order in underdoped surface of YBa2Cu3Oy studied by STM Terukazu Nishizaki a,, Makoto Maki b, Norio Kobayashi a a b
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Department of Physics, Saga University, Saga 840-8502, Japan
a r t i c l e in f o
Keywords: A. Superconductors C. Scanning tunnelling microscopy (STM) D. Electronic structure
a b s t r a c t We have performed low-temperature scanning tunneling microscopy (STM) experiments on the coldcleaved surface of YBa2Cu3Oy single crystals to study the nanoscale electronic order in high-Tc superconductors. STM images measured at low-bias voltage below 50 meV show the one-dimensional (1D) electronic modulation along the Cu–O bonds (parallel to the b-axis). The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range 2a–4a depending on the position on the surface, indicating the glassy electronic order in the underdoped CuO2 plane. & 2008 Elsevier Ltd. All rights reserved.
1. Introduction In high-Tc superconductors with low carrier concentration (i.e., the underdoped regime), anomalous transport, magnetic, and optical properties have been observed below a temperature T* larger than the superconducting transition temperature Tc [1]. These unconventional electronic states appear when charge carriers are added to antiferromagnetic Mott insulators. The pseudogap state below T* is characterized by the opening of a partial gap in the low energy density of states and is observed by photoemission, optical, and tunneling spectroscopy experiments. Recent scanning tunneling microscopy/spectroscopy (STM/STS) experiments have revealed that a two-dimensional periodic pattern (i.e., a checkerboard pattern with a periodicity: 4a 4a, where a is the lattice constant) in the local density of states (LDOS, N(r,E)pdI/dV) appears in the superconducting state [2–4], pseudogap state (T4Tc) [5], zero temperature pseudogap (ZTPG) region [6], and vortex core region [7] of Bi2Sr2CaCu2Oy [2–5,7] and Ca2xNaxCuO2Cl2 [6]. According to these results, the checkerboard electronic order was considered to be the appearance of ‘‘a hidden order’’ in the high-Tc superconductivity. However, recent tunneling-asymmetry (TA) imaging technique indicates that spatial arrangements of the electronic structure are a Cu–O–Cu bond-centered electronic glass state with unidirectional domains in underdoped Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2Oy [8]. Since these domains have a 4a width,
Corresponding author. Tel.: +81 22 215 2029; fax: +81 22 215 2026.
E-mail address: [email protected] (T. Nishizaki). 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.06.010
the relation between checkerboard modulations (dI/dV maps) and electronic glass states (TA maps) is an interesting issue. In addition, the universality of these ordered states and the relation between the spatial variation of the novel electronic order and the ground state of the high-Tc superconductivity are a matter of current interest. In this paper, we report on STM studies of cold-cleaved surface of YBa2Cu3Oy single crystals. We find a one-dimensional (1D) electronic modulation along the Cu–O bonds for the low-bias voltage below 50 mV. The 1D modulation does not have longrange order and the periodicity changes depending on the position, contrary to the checkerboard electronic order in Bi2Sr2CaCu2Oy [2–5,7] and Ca2xNaxCuO2Cl2 [6].
2. Experimental High quality single crystals of YBa2Cu3Oy were grown by the self-flux method using a Y2O3 crucible [9,10]. Slightly overdoped YBa2Cu3O6.96 crystals (TcC91.5 K) were prepared by annealing in 1 bar oxygen at 450 1C. For STM measurements of YBa2Cu3Oy single crystals, the fresh surface was prepared by the cleaving method at low temperatures (LT) below 20 K in the ultrahigh vacuum (UHV), P1010 Torr, and the cleaved crystals were immediately inserted into the cold STM head by the transfer rod. The STM measurements were performed at 4.5 K in UHV condition [11,12]. A mechanically sharpened Pt–Ir wire was used as the STM tip, which was approached perpendicular to the CuO2 plane.
ARTICLE IN PRESS T. Nishizaki et al. / Journal of Physics and Chemistry of Solids 69 (2008) 3014–3017
3015
on the surface; thus the carrier doping into the topmost CuO2 plane is not sufficient to appear the superconductivity, indicating the strongly underdoped condition [11,13,14]. These results suggest that STM/STS experiments through the cleaved BaO surface of YBa2Cu3Oy can provide information on the electronic state of the underdoped CuO2 plane even if the sample is the optimally doped YBa2Cu3O6.96. Fig. 1(c) shows the STM image of the BaO surface measured on the left terrace in Fig. 1(a) under the condition of V ¼ 100 mV. The square lattice structure with a lattice constant 3.9 A˚ corresponds to the arrangement of oxygen atoms on the BaO surface, because the valence charge around oxygen atoms spreads much more than that around barium atoms and oxygen atoms are more remarkable on the BaO surface [11]. Contrary to the CuO chain layer in Fig. 1(b), the atomic lattice structure is clearly visible even in the dark regions in Fig. 1(c); thus the dark region is not due to the missing atoms but due to the spatial variation of the electronic state on the BaO and/or underlying CuO2 layers. Previous STM studies have presented that the atomic arrangement of the BaO lattice has the relatively homogeneous background at high-bias voltage (V4500 mV) [11,13]. Therefore, the weak background modulation in Fig. 1(c), which seems to align with the b-axis, is a characteristic feature of the low-bias STM image. Since the STM image is a map of the integrated LDOS between the Fermi energy EF and EF+eV, the modulated STM image reflects the local electronic state near the Fermi level. In order to examine the characteristic energy scale of electronic modulations, Fig. 2(a) and (b) shows STM images for high-bias (V ¼ 1 V) and low-bias (V ¼ 6 mV) voltage; these STM images were measured at the different position on the same BaO terrace in Fig. 1(a). As shown in Fig. 2(a), the STM image at highbias voltage shows a spatially homogeneous atomic arrangement of the BaO lattice, as presented by previous papers [11,13]. At low-bias voltage (V ¼ 6 mV), on the other hand, the STM image in Fig. 2(b) shows nanoscale modulations: the modulation
3. Results and discussion In YBa2Cu3Oy, the two dimensionality is not so strong as compared with Bi2Sr2CaCu2O8+y, several terraces and step edges are often observed on the cleaved surface. Fig. 1(a) shows the STM topographic image of cold-cleaved surface in YBa2Cu3O6.96. This surface consists of a single step edge between two atomic terraces. It is known that YBa2Cu3Oy single crystals cleave between the BaO and CuO chain layeres, so the ab-surface termination of the cleaved surface is composed of BaO and/or CuO chain layers [11–14]. According to high-resolution STM images on each terraces shown in Fig. 1(b) and (c), lower (right) and upper (left) terraces correspond to the CuO chain and BaO layers, respectively. Fig. 1(b) shows the 1D electronic modulation on the CuO chain layer along the b-axis which has been discussed in terms of the charge density wave (CDW) due to the 2kF instability of the 1D Fermi surface [14–17] and 1D quasiparticle scattering resonances [18]. The dark regions along the b-axis correspond to missing atoms such as oxygen vacancy clusters along the CuO chain [11,19]. Employing the one dimensionality of the CuO chain parallel to the b-axis, a- and b-axes can be estimated on the BaO surface as shown in Fig. 1(c). It is important to note that the step edge in Fig. 1(a) is parallel to the b-axis and there are no twin boundaries in the measured region, because ambiguity remains for the definition of crystal axes if the step edge is parallel to the /11 0S direction (i.e., the twin boundary direction). For cleaved YBa2Cu3Oy single crystals, the carrier concentration on the surface is different from that on the bulk, so the electronic state of the topmost CuO2 plane is modified by the surface termination [11,13,14]. The CuO chain surface shows superconducting tunneling spectra dI/dV with superconducting gap structures. However, the BaO surface (i.e., the CuO2 plane just under the BaO surface) does not show superconducting spectra because the carrier reservoir (i.e., the CuO chain) is not complete
b a
b
BaO
CuO
b a
Fig. 1. Constant-current STM images of cold-cleaved surface of YBa2Cu3O6.96 at T ¼ 4.5 K. (a) 3D display of step edge region between BaO and CuO surfaces. V ¼ 100 mV, I ¼ 20 pA, 1100 A˚ 1100 A˚. (b) CuO chain surface measured on lower (right) terrace in (a), V ¼ 400 mV, I ¼ 50 pA, 110 A˚ 110 A˚. (c) BaO surface measured on upper (left) terrace in (a), V ¼ 100 mV, I ¼ 20 pA, 96 A˚ 96 A˚.
ARTICLE IN PRESS 3016
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b
a
b
a
is very difficult to associate the 1D modulation with the electronic state of the BaO plane. Thus, the natural interpretation of the 1D modulation observed on the BaO surface would be the appearance of the electronic order in the underdoped CuO2 plane which exists just under the BaO surface, rather than the electronic state of the BaO plane or CuO chain layer which is underlying 9.85 A˚ below the surface. The 1D electronic modulation along the Cu–O bonds does not have long-range order and the periodicity along the a-axis varies within the range 2a–4a, depending on the position on the surface. If the short-range 1D modulation is an appearance of ‘‘the hidden electronic order’’ in the underdoped CuO2 plane of YBa2Cu3Oy, this glassy electronic order differs from the 4a 4a checkerboard structure observed in Bi2Sr2CaCu2Oy [2–5,7] and Ca2xNaxCuO2Cl2 [6]. In order to discuss similarities between the glassy 1D-modulation in YBa2Cu3Oy and the electronic glass state with unidirectional domains in underdoped Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2Oy [8], the detailed analysis of the TA map is necessary in YBa2Cu3Oy. Although the origin of the 1D modulation is not clear in YBa2Cu3Oy, the remanent orthorhombicity due to CuO chain layers, which exist deep inside of the crystal, can be a trigger of the one dimensionality. Finally, our finding of the 1D electronic order is consistent with resistivity measurements of the in-plane anisotropy in underdoped YBa2Cu3Oy [20]; in Ref. [20], the enhancement of the in-plane anisotropy with underdoping is interpreted as the self-organization of electrons into (nematic) charge stripes [21]. The relation between the 1D electronic modulation in YBa2Cu3Oy, the static stripe order in (La, Nd)2xSrxCuO4 [22], the checkerboard structure in Bi2Sr2CaCu2Oy [2–5,7] and Ca2xNaxCuO2Cl2 [6], and the electronic glass state in underdoped Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2Oy [8] is a future issue to be studied.
4. Summary LT-STM studies have been performed in the underdoped surface of the cold-cleaved YBa2Cu3Oy single crystals. From the low-bias STM measurements near the step edge region, we find the 1D electronic modulation along the Cu–O bonds (parallel to the b-axis) which can be clearly distinguished from the CDW order on the CuO chain layer. The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range 2a–4a depending on the position, indicating the glassy electronic order in the underdoped CuO2 plane in YBa2Cu3Oy.
Fig. 2. Constant-current STM images of BaO surface of YBa2Cu3O6.96 at T ¼ 4.5 K. (a) V ¼ 1 V, I ¼ 20 pA, 110 A˚ 110 A˚. (b) V ¼ 6 mV, I ¼ 20 pA, 83 A˚ 83 A˚.
is roughly independent of the polarity of the bias voltage and becomes remarkable for low-bias voltage below 50 mV. These results indicate that the modulation is closely related to the electronic structure near the Fermi level. The electronic modulation has 1D character along the Cu–O bonds (parallel to the b-axis). However, the modulation does not originate from CDW on 1D CuO chains, because the characteristic length scale of CDW on the CuO chain layer (Fig. 1(b)) is different from that of the modulation on the BaO layer (Fig. 2(b)). The modulation of CDW is periodic along the b-axis with a wavelength lb13 A˚ and has short-range correlations with a length of 3–4 neighboring chains [16,17]. On the other hand, the characteristic length of the modulation on the BaO surface is roughly 60–70 A˚ along the b-axis without a clear periodicity and correlations. In addition, it
Acknowledgments This study was partly supported by a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
T. Timusk, B. Statt, Rep. Prog. Phys. 62 (1999) 61. C. Howald, et al., Phys. Rev. B 67 (2003) 014533. K. McElroy, et al., Phys. Rev. Lett. 94 (2005) 197005. N. Momono, et al., J. Phys. Soc. Jpn. 74 (2005) 2400. M. Vershinin, et al., Science 303 (2004) 1995. T. Hanaguri, et al., Nature 430 (2004) 1001. J.E. Hoffman, et al., Science 295 (2002) 466. Y. Kohsaka, et al., Science 315 (2007) 1380. T. Naito, et al., in: S. Nakajima, M. Murakami (Eds.), Advances in Superconductivity, vol. IX, Springer, Tokyo, 1997, p. 601. [10] T. Nishizaki, et al., J. Low Temp. Phys. 117 (1999) 1375.
ARTICLE IN PRESS T. Nishizaki et al. / Journal of Physics and Chemistry of Solids 69 (2008) 3014–3017
[11] [12] [13] [14] [15] [16]
M. Maki, et al., J. Phys. Soc. Jpn. 70 (2001) 1877. T. Nishizaki, et al., Physica C 437/438 (2006) 220. H.L. Edwards, et al., J. Vac. Sci. Technol. B 12 (1994) 1886. H.L. Edwards, et al., Phys. Rev. Lett. 69 (1992) 2967. H.L. Edwards, et al., Phys. Rev. Lett. 73 (1994) 1154. M. Maki, et al., Phys. Rev. B 65 (2002) 140511.
[17] [18] [19] [20] [21] [22]
M. Maki, et al., Phys. Rev. B 72 (2005) 024536. D.J. Derro, et al., Phys. Rev. Lett. 88 (2002) 097002. T. Nishizaki, et al., J. Low Temp. Phys. 131 (2003) 931. Y. Ando, et al., Phys. Rev. Lett. 88 (2002) 137005. S.A. Kivelson, et al., Nature 393 (1998) 550. J.M. Tranquada, et al., Nature 375 (1995) 561.
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