We investigate the manifestation of stripes in the in-plane resistivity anisotropy in untwinned single crystals of La2−xSrxCuO4 (x = 0.02-0.04) and YBa2Cu3Oy (y = 6.35-7.0). It is found that both systems show strongly temperature-dependent in-plane anisotropy in the lightly hole-doped region and that the anisotropy in YBa2Cu3Oy grows with decreasing y below ∼6.60 despite the decreasing orthorhombicity, which gives most direct evidence that electrons self-organize into a macroscopically anisotropic state. The transport is found to be easier along the direction of the spin stripes already reported, demonstrating that the stripes are intrinsically conducting in cuprates.PACS numbers: 74.25. Fy, 74.25.Dw, 74.20.Mn, 74.72.Dn, 74.72.Bk The mechanism of the high-temperature superconductivity is not settled 15 years after its discovery, mostly because it is unclear how best to describe the stronglycorrelated electrons in the high-T c cuprates. It has recently been discussed [1][2][3][4][5][6][7][8][9][10][11][12][13][14] that the electrons in cuprates self-organize into quasi-one-dimensional stripes, which might bring a paradigm shift in our way of understanding the two-dimensional (2D) electronic system in the cuprates. Though the self-organization is interesting in itself, the conventional wisdom suggests that the stripes are destructive to the superconductivity, because charge ordering would normally lead to an insulating state. However, there are intriguing theoretical proposals [10][11][12] that stripes can instead be responsible for the occurrence of the high-temperature superconductivity if they are conducting and meandering -properties that have never been clearly demonstrated before.In this Letter, we report novel in-plane transport anisotropy in the cuprates, which gives direct evidence for the conducting charge stripes in these materials; the temperature dependence and the magnitude of the anisotropy strongly suggest the stripes to be meandering and forming an electronic liquid crystal [11]. The evidence is shown for two representative materials of the high-temperature superconductors, La 2−x Sr x CuO 4 (LSCO) and YBa 2 Cu 3 O y (YBCO), highlighting the universality of the charge-stripe phenomenon in the cuprates. Most notably, the data for YBCO indicate that the charge stripes govern the transport in samples with T c of as high as 50 K, demonstrating that theories of high-temperature superconductivity should inevitably consider the self-organization of the electrons as an integral part.It is fair to say that the majority of the researchers today believe that the stripes are irrelevant to the superconductivity. This general belief comes not only from the conventional wisdom mentioned above but also from the current experimental situation, which can be summarized as follows: (i) strong evidence for the charge stripe order has been reported [1] only for Nd-doped LSCO, where the superconductivity is strongly suppressed; (ii) in other superconducting cuprates, evidence [2][3][4][5][6][7] is reasonably strong f...
Ando, Lavrov, and Segawa Reply: In the preceding Comment [1], Jánossy, Simon, and Fehér (JSF) offered an alternative picture to explain our recent findings [2] and argued that it is unlikely that the charged stripes exist in hole-doped antiferromagnetic YBa 2 Cu 3 O 61x (YBCO). They proposed that the anomalous behavior of the magnetoresistance (MR) is essentially due to an inplane anisotropy in the resistivity of the bulk. Here we show that it is very unlikely that the charge transport through the bulk is taking place in the antiferromagnetic YBCO, and thus the formation of charged stripes in this system is an inevitable conclusion.The staggered magnetization of the antiferromagnetic (AF) YBCO lies in the ab plane and there should be two equivalent easy axes for the spins. When a small number of holes are doped into the CuO 2 planes that have the Néel order, there are four possible phases for the system: (a) the system is a single domain AF (all the spins lie along only one of the easy axes) and the doped holes are distributed over the system, (b) an AF domain structure is formed (with alternating direction of the easy axis) and the doped holes are distributed in the domains, (c) an AF domain structure is formed and the doped holes are confined in the domain walls (in other words, charged stripes constitute the domain walls), and (d) a macroscopic phase separation takes place into the AF regions and doped nonmagnetic ones.The ESR experiment by JSF demonstrates [1] that there is a magnetic domain structure, which gives evidence against case (a). The equivalence of the a and b axes follows also from the symmetry of the MR [2]. Case (d) can easily be eliminated, because in this case the system must contain an admixture of the superconducting phase (remember that YBCO is either an antiferromagnet or a superconductor); experimentally we have never observed such a macroscopic phase separation. We can also eliminate case (b), because the domain walls in this case have a very large energy cost (of the order of NJ, where N is the number of broken spin bonds along the domain walls and J is the antiferromagnetic interaction energy). Such a large energy cost of the domain walls would cause a transition into a phase without domain walls [case (a)] at low temperature. Therefore, case (c) is the only possible phase in antiferromagnetic YBCO given the AF domain structure is observed down to low T [1]. In case (c), the energy cost of the domain walls is grossly smaller than that in case (b) due to the spinless stripes. The above discussion makes it clear that the charge transport through the bulk is unlikely to be taking place in the antiferromagnetic YBCO.JSF argued that an array of charged stripes cannot respond to the magnetic field because the Coulomb repulsion would make it extremely rigid. This is their central reasoning to reject the stripe scenario. However, the Coulomb repulsion does not necessarily make the stripes rigid; actu-FIG. 1. Magnetization of YBa 2 Cu 3 O 6.3 for Hkab.
Single crystals of the layered perovskite GdBaCo2O5+x (GBCO) have been grown by the floatingzone method, and their transport, magnetic, and structural properties have been studied in detail over a wide range of oxygen contents, 0 ≤ x ≤ 0.77. The obtained data are used to establish a rich phase diagram centered at the "parent" compound GdBaCo2O5.5 -an insulator with Co ions in the 3+ state. An attractive feature of GdBaCo2O5+x is that it allows a precise and continuous doping of CoO2 planes with either electrons or holes, spanning a wide range from the charge-ordered insulator at 50% electron doping (x = 0) to the undoped band insulator (x = 0.5), and further towards the heavily hole-doped metallic state. This continuous doping is clearly manifested in the behavior of thermoelectric power which exhibits a spectacular divergence with approaching x=0.5, where it reaches large absolute values (±800 µV/K) and abruptly changes its sign. At low temperatures, the homogeneous distribution of doped carriers in GBCO becomes unstable, as is often the case with strongly correlated systems, and both the magnetic and transport properties point to an intriguing nanoscopic phase separation into two insulating phases (for electron-doped region) or an insulating and a metallic phases (for hole-doped region). We also find that throughout the composition range the magnetic behavior in GBCO is governed by a delicate balance between ferromagnetic (FM) and antiferromagnetic (AF) interactions, which can be easily affected by temperature, doping, or magnetic field, bringing about FM-AF transitions and a giant magnetoresistance (MR) phenomenon. What distinguishes GBCO from the colossal-MR manganites is an exceptionally strong uniaxial anisotropy of the Co spins, which dramatically simplifies the possible spin arrangements. This spin anisotropy together with the possibility of continuous ambipolar doping turn GdBaCo2O5+x into a model system for studying the competing magnetic interactions, nanoscopic phase separation and accompanying magnetoresistance phenomena.
The oxygen-exchange behavior has been studied in half-doped manganese and cobalt perovskite oxides. We have found that the oxygen diffusivity in Gd0.5Ba0.5MnO 3−δ can be enhanced by orders of magnitude by inducing crystallographic ordering among lanthanide and alkali-earth ions in the A-site sublattice. Transformation of a simple cubic perovskite, with randomly occupied A-sites, into a layered crystal GdBaMn2O5+x (or isostructural GdBaCo2O5+x for cobalt oxide) with alternating lanthanide and alkali-earth planes reduces the oxygen bonding strength and provides disorder-free channels for ion motion, pointing to an efficient way to design new ionic conductors.PACS numbers: 66.30. Hs, 82.47.Ed Oxygen ion conductors -solids exhibiting very fast oxygen diffusion -constitute the basis for such emerging technologies as the membrane oxygen separation or the solid-oxide fuel cell (SOFC) power generation.1,2 . These technologies offer enormous economical and ecological benefits provided high performance materials can be developed: The scientific challenge is to design materials demonstrating high oxygen diffusivity at low enough temperature.In general, a crystal must meet two fundamental requirements to be a good oxygen-ion conductor: (i) it must contain a lot of vacancies in the oxygen sublattice, and (ii) the energy barrier for oxygen migration from one site to another must be fairly small, typically less than ∼ 1 eV. Only a few types of oxides, and perovskites ABO 3 (A is a rare-earth or an alkali-earth element and B is typically a transition metal) among them, have been found to possess these features. 1,2,3,4,5,6,7,8 Doped perovskites, which possess a high electronic conductivity in addition to the ionic one, are considered for using as electrodes in SOFC and as oxygen-selective membranes; for example, strontium-doped lanthanum manganese oxide, La 1−y Sr y MnO 3−δ , is a standard cathode material for SOFC applications operating at temperatures around 1000• C. 6 Recently, serious efforts are made to reduce the operation temperature of SOFC, and for the operation at 700 -800• C, strontium-doped lanthanum cobalt oxide, La 1−y Sr y CoO 3−δ , is considered to be the most promising cathode material.7,8 The performance of perovskite oxides has been already optimized as much as possible mostly by means of various ion substitutions in both A and B sublattices, 6,7,8,9 but they still fail to operate at low enough temperatures ∼ 500• C required for successful commercialization of the fuel cell technology.In this Letter, we show that the oxygen-ion diffusion in a doped perovskite can be enhanced by orders of magnitude if a simple cubic crystal [schematically shown in Fig.1(a)] transforms into a layered compound with ordered lanthanide and alkali-earth ions [ Fig. 1(b)], which reduces the oxygen bonding strength and provides disorder-free channels for ion motion.Recently, the A-site-ordered manganese and cobalt perovskite oxides have been synthesized by several , which demonstrate the doubling of the unit cell along the c axis ...
The emergence and the evolution of the metallic charge transport in the La2-xSrxCuO4 system from lightly to optimally doped samples (x = 0.01-0.17) are studied. We demonstrate that in high-quality single crystals the in-plane resistivity shows a metallic behavior for all values of x at moderate temperatures and that the hole mobility at 300 K changes only by a factor of 3 from x = 0.01 to 0.17, where its x dependence is found to be intriguingly similar to that of the inverse antiferromagnetic correlation length. We discuss an incoherent-metal picture and a charged-stripe scenario as candidates to account for these peculiar features.
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