CuO Chain Layer Research Articles

Atomically-resolved interlayer charge ordering and its interplay with superconductivity in YBa2Cu3O6.81

High-temperature superconductive (SC) cuprates exhibit not only a SC phase, but also competing orders, suppressing superconductivity. Charge order (CO) has been recognized as an important competing order, but its microscopic spatial interplay with SC phase as well as the interlayer coupling in CO and SC phases remain elusive, despite being essential for understanding the physical mechanisms of competing orders and hence superconductivity. Here we report the achievement of direct real-space imaging with atomic-scale resolution of cryogenically cleaved YBa2Cu3O6.81 using cross-sectional scanning tunneling microscopy/spectroscopy. CO nanodomains are found embedded in the SC phase with a proximity-like boundary region characterized by mutual suppression of CO and superconductivity. Furthermore, SC coherence as well as CO occur on both CuO chain and plane layers, revealing carrier transport and density of states mixing between layers. The CO antiphase correlation along the c direction suggests a dominance of Coulomb repulsion over Josephson tunneling between adjacent layers.

Prediction of f−wave pairing symmetry in YBa2Cu3O6+x cuprates

We perform a numerical simulation of a three-band Hubbard model with two CuO$_2$ planes and a single CuO chain layer for YBCO cuprates. The spin-fluctuation mediated pairing interaction is computed within the multiband random-phase approximation, and its pairing eigenvalues and eigenfunctions are solved as a function of chain state filling factor $n_c$. We find that for the intrinsic value of $n_c$ in YBCO samples, one obtains the usual $d$-wave pairing symmetry. However, if we dope the chain layers with holes, while keeping the plane states doping fixed, the leading pairing symmetry solution becomes an unconventional $f$-wave symmetry. The mechanism behind the $f$-wave pairing is the competition between the plane states antiferromagnetic nesting and chain states' uniaxial nesting. We also find that the pairing strength is strongly augmented when the flat band bottom of the chain state passes the Fermi level for a fixed plane states doping. The $f$-wave pairing symmetry can be realized in YBCO cuprates in future experiments where the self-doping mechanism between the chain and plane states can be minimized so that only chain state can be selectively hole doped.

Atomic scale real-space mapping of holes in YBa2Cu3O(6+δ).

The high-temperature superconductor YBa2Cu3O(6+δ) consists of two main structural units--a bilayer of CuO2 planes that are central to superconductivity and a CuO(2+δ) chain layer. Although the functional role of the planes and chains has long been established, most probes integrate over both, which makes it difficult to distinguish the contribution of each. Here we use electron energy loss spectroscopy to directly resolve the plane and chain contributions to the electronic structure in YBa2Cu3O6 and YBa2Cu3O7. We directly probe the charge transfer of holes from the chains to the planes as a function of oxygen content, and show that the change in orbital occupation of Cu is large in the chain layer but modest in CuO2 planes, with holes in the planes doped primarily into the O 2p states. These results provide direct insight into the local electronic structure and charge transfers in this important high-temperature superconductor.

Electron-like Fermi surface and in-plane anisotropy due to chain states in YBa2Cu3O7−δsuperconductors

We present magneto-transport calculations for YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) materials to show that the electron-like metallic chain state gives both the negative Hall effect and in-plane anisotropic large Nernst signal. We show that the inevitable presence of the metallic 1D CuO chain layer lying between the CuO2 bilayers in YBCO renders an electron-like Fermi surface in the doping range as wide as p=0.05 to overdoping. With underdoping, a pseudogap opening in the CuO2 state reduces its hole-carrier contribution, and therefore the net electron-like quasiparticles dominate the transport properties, and a negative Hall resistance commences. We also show that the observation of in-plane anisotropy in the Nernst signal - which was taken as a definite evidence of the electronic `nematic' pseudogap phase - is naturally explained by including the `quasi-uniaxial' metallic chain state. Finally, we comment on how the chain state can also lead to electron-like quantum oscillations.

In-plane anisotropy in spin-excitation spectra originating from chain states in YBa2Cu3O6+y

We present a random-phase-approximation-based multilayer spin-susceptibility calculation for the trilayer YBa${}_{2}$Cu${}_{3}$O${}_{6+y}$ (YBCO) system (including bilayer CuO${}_{2}$ planes and uniaxial CuO chain layer) in the superconducting state. We show that the observed in-plane anisotropy in the spin-excitation spectrum of YBCO---which is often interpreted as evidence for the electron nematic phase---can alternatively be explained via incorporating the uniaxial CuO chain's contribution. We demonstrate that the neutron spectra in YBCO is dominated by the contribution from the fourfold symmetric CuO${}_{2}$ plane state as in other cuprates; however, it acquires an in-plane anisotropy via finite interlayer coupling with the chain state. The result rules out the claim that an electronic nematic phase is responsible for the pseudogap state in YBCO.

Pulsed laser deposition growth of heteroepitaxial YBa2Cu3O7/La0.67Ca0.33MnO3superlattices on NdGaO3and Sr0.7La0.3Al0.65Ta0.35O3substrates

Heteroepitaxial superlattices of [YBa2Cu3O7(n)/ La0.67Ca0.33MnO3(m)]x, where n and m are the number of YBCO and LCMO monolayers and x the number of bilayer repetitions, have been grown with pulsed laser deposition on NdGaO3 (110) and Sr0.7La0.3Al0.65Ta0.35O3 (LSAT) (001). These substrates are well lattice matched with YBCO and LCMO and, unlike the commonly used SrTiO3, they do not give rise to complex and uncontrolled strain effects due to structural transitions at low temperature. The growth dynamics and the structure have been studied in-situ with reflection high energy electron diffraction (RHEED) and ex-situ with scanning transmission electron microscopy (STEM), x-ray diffraction, and neutron reflectometry. The individual layers are found to be flat and continuous over long lateral distances with sharp and coherent interfaces and with a well-defined thickness of the individual layer. The only visible defects are antiphase boundaries in the YBCO layers that originate from perovskite unit cell height steps at the interfaces with the LCMO layers. We also find that the first YBCO monolayer at the interface with LCMO has an unusual growth dynamics and is lacking the CuO chain layer while the subsequent YBCO layers have the regular Y-123 structure. Accordingly, the CuO2 bilayers at both the LCMO/YBCO and the YBCO/LCMO interfaces are lacking one of their neighboring CuO chain layers and thus half of their hole doping reservoir. Nevertheless, from electric transport measurements on asuperlattice with n=2 we obtain evidence that the interfacial CuO2 bilayers remain conducting and even exhibit the onset of a superconducting transition at very low temperature. Finally, we show from dc magnetization and neutron reflectometry measurements that the LCMO layers are strongly ferromagnetic.

Carbonate effects in single-crystallineYBa2Cu3O7−δ

Physical properties in the normal and superconducting states for ${\text{CO}}_{3}$ substituted ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals are reported. From resistivity measurements in the normal state, it is obtained that the samples of the similar carrier doping, which is estimated by the $c$-axis resistivity, show a different behavior in the $ab$-plane resistivity if they have different concentrations of the carbon. The peak effects of the magnetization, which correspond to the critical current density in field, decrease with the substitution. These results suggest that the substituted carbonate affects the metallicity and superconductivity of the CuO chain layer. The substitution also suppresses the carrier doping and shortens the $c$-axis length as seen in polycrystalline samples. Some attention to purity of ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ will be required because ${T}_{\text{c}}$ can exceed 90 K for even relatively highly substituted samples, ${\text{YBa}}_{2}{\text{Cu}}_{3\ensuremath{-}x}{({\text{CO}}_{3})}_{0.04}{\text{O}}_{7\ensuremath{-}y}$.

Surface of underdoped YBa2Cu3O7- δ as revealed by STM/STS

We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa2Cu3O7- δ at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance.

The effect of CuO chains on the local density of states in the vortex phase ofYBa2Cu3O7

We examine the effects of the CuO chains on the density of states in the vortex phase inYBa2Cu3O7, via a calculation based on the tight-binding proximity model. In thismodel, chain superconductivity results from single-electron hoppingbetween the intrinsically normal chains and intrinsically superconductingCuO2 planes. The calculations are based on self-consistent solutions of theBogolyubov–de Gennes equations for a bilayer consisting of a singleCuO2 layer and a single CuO chain layer. We find that, in addition to the dispersing resonancesfound in single-layer models, there are a second set of dispersing resonances, associatedwith the induced gap in the chain layer. These new resonances are highly anisotropic anddistort the vortex core shape.

Superconductivity in zigzag CuO chains

Superconductivity has recently been discovered in ${\mathrm{Pr}}_{2}{\mathrm{Ba}}_{4}{\mathrm{Cu}}_{7}{\mathrm{O}}_{15\ensuremath{-}\ensuremath{\delta}}$ with a maximum ${T}_{c}$ of about $15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Since the CuO planes in this material are believed to be insulating, it has been proposed that the superconductivity occurs in the double (or zigzag) CuO chain layer. On phenomenological grounds, we propose a theoretical interpretation of the experimental results in terms of a phase for the zigzag chain, labeled by ${\mathrm{C}}_{1}{\mathrm{S}}_{3∕2}$. This phase has a gap in the relative charge mode and a partial gap in the relative spin mode. It has gapless uniform charge and spin excitations and can have a divergent superconducting susceptibility, even for repulsive interactions. A microscopic model for the zigzag CuO chain is proposed, and on the basis of density matrix renormalization group and bosonization studies, we adduce evidence that supports our proposal.

Phenomenology ofâ-axis andb̂-axis charge dynamics from microwave spectroscopy of highly orderedYBa2Cu3O6.50andYBa2Cu3O6.993

Extensive measurements of the microwave conductivity of highly pure and oxygen-ordered \YBCO single crystals have been performed as a means of exploring the intrinsic charge dynamics of a d-wave superconductor. Broadband and fixed-frequency microwave apparatus together provide a very clear picture of the electrodynamics of the superconducting condensate and its thermally excited nodal quasiparticles. The measurements reveal the existence of very long-lived excitations deep in the superconducting state, as evidenced by sharp cusp-like conductivity spectra with widths that fall well within our experimental bandwidth. We present a phenomenological model of the microwave conductivity that captures the physics of energy-dependent quasiparticle dynamics in a d-wave superconductor which, in turn, allows us to examine the scattering rate and oscillator strength of the thermally excited quasiparticles as functions of temperature. Our results are in close agreement with the Ferrell-Glover-Tinkham sum rule, giving confidence in both our experiments and the phenomenological model. Separate experiments for currents along the $\hat a$ and $\hat b$ directions of detwinned crystals allow us to isolate the role of the CuO chain layers in \YBCO, and a model is presented that incorporates both one-dimensional conduction from the chain electrons and two-dimensional transport associated with the \cuplane plane layers.

Universal intrinsic scale of the hole concentration in high-Tccuprates

We have measured thermoelectric power (TEP) as a function of hole concentration per CuO2 layer, Ppl, in Y1-xCaxBa2Cu3O6 (Ppl = x/2) with no oxygen in the Cu-O chain layer. The room-temperature TEP as a function of Ppl, S290(Ppl), of Y1-xCaxBa2Cu3O6 behaves identically to that of La2-zSrzCuO4 (Ppl = z). We argue that S290(Ppl) represents a measure of the intrinsic equilibrium electronic states of doped holes and, therefore, can be used as a common scale for the carrier concentrations of layered cuprates. We shows that the Ppl determined by this new universal scale is consistent with both hole concentration microscopically determined by NQR and the hole concentration macroscopically determined by the Cu valency. We find two characteristic scaling temperatures, TS* and TS2*, in the TEP vs. temperature curves that change systematically with doping. Based on the universal scale, we uncover a universal phase diagram in which almost all the experimentally determined pseudogap temperatures as a function of Ppl fall on two common curves; upper pseudogap temperature defined by the TS* versus Ppl curve and lower pseudogap temperature defined by the TS2* versus Ppl curve. We find that while pseudogaps are intrinsic properties of doped holes of a single CuO2 layer for all high-Tc cuprates, Tc depends on the number of layers, therefore the inter-layer coupling, in each individual system.

Interfacial structures of Y123 and Nd123 films formed on MgO(001) substrates by liquid phase epitaxy

Interfacial structures of c-axis-oriented YBa2Cu3O7–y (Y123) and Nd1+xBa2–xCu3O7–y (Nd123) films were investigated by high-resolution transmission electron microscopy (HRTEM) in conjunction with geometrical lattice matching and molecular orbital calculations. These films were formed on MgO(001) substrates by liquid-phase epitaxy. Despite the similarity in lattice constants between Y123 and Nd123, the in-plane orientation relationship (OR) to the substrates is different: [100]film//[100]substrate(I) for Y123 and [110]film//[100]substrate(II) for Nd123. From the results of HRTEM observations and image simulations, it was found that the Y123 and Nd123 films are terminated by BaO and CuO-chain layers at the interfaces, respectively. For both the Y123/MgO and Nd123/MgO systems, the OR(I) is assessed to be the most favorable in point of geometrical matching and the OR(II) is the second among the rotational misorientations on the [001]film and [001]MgO. The molecular orbital calculations reveal that the interface with the OR(II) and the CuO-chain layer termination is preferable in terms of covalent bonding for both the systems. Consequently, we suggest that the preferential interfacial structures are delicately determined by a balance of the geometrical and chemical factors at the interfaces, resulting in making the lowest interfacial free energies.

LT-STM observation of YBa2(Cu1−xZnx)3O7−δ single crystals

Low-temperature scanning tunneling microscopy on the BaO layer of cold-cleaved YBa2(Cu1−xZnx)3O7−δ single crystals reveals the one-dimensional charge modulations at lower bias voltage. These charge modulations have various lengths and are distributed randomly in space. The spatially inhomogeneous electronic state is regarded to be induced by Zn impurities in the CuO2 plane just under the BaO layer, because there are no charge modulations on Zn-free samples. The one-dimensional character possibly reflects the interaction between the CuO2 and the CuO-chain layers.

Zn-induced one-dimensional electronic modulation inYBa2Cu3O7−δ

Low-temperature scanning tunneling microscopy measurements have been performed on cold-cleaved ${\mathrm{YBa}}_{2}({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{)}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals. We find one-dimensional charge-density modulations on the BaO layer at lower bias voltage, whereas such modulations were never observed on Zn-free samples. These modulations are closely related to the electronic local density of states in the ${\mathrm{CuO}}_{2}$ plane just under the BaO layer and are attributed to the spatial electronic inhomogeneity induced by Zn substitution. The one-dimensional character of the charge-density modulations suggests that the electronic state of the ${\mathrm{CuO}}_{2}$ plane is affected by the CuO-chain layer.

Vortex Phase Transition and Oxygen Vacancy in YBa2Cu3Oy Single Crystals

The atomic images of YBa 2 Cu 3 O y (YBCO, y ≃ 6.94) single crystals are measured by low-temperature scanning tunneling microscopy (LT-STM). We find that the oxygen vacancies form clusters along the b-axis in the CuO chain layer. Near the clusters, the strong atomic distortion is observed along the a-axis. The vortex phase diagram is very sensitive to the amount of oxygen vacancies in the region between optimal and overdoped regions. For optimally doped untwinned YBCO (y ≃ 6.92), we find a first-order transition T L (H) in the vortex liquid above the terminal point H mcp (≃ 7 T) of both the vortex glass line T g (H) and the field-driven disordering transition line H * (T). The obtained small entropy change (∼ 0.02 k B /vortex/layer) and the critical endpoint H cep (≃ 11 T) of T L (H) indicate that the vortex liquid undergoes the vortex slush regime before the solidification into the vortex glass phase. Below H mcp , the vortex liquid phase shows the first-order melting transition into the Bragg glass phase.

Low-temperature scanning tunneling microscopy of YBa 2Cu 3O 7− δ

Low-temperature scanning tunneling microscopy studies have been performed on YBa 2Cu 3O 7− δ single crystals. We present two types of atomic images corresponding to BaO- and CuO-chain layers, which vary with applied bias voltage and temperature. At low bias voltage, the electronic modulation images of the CuO-chain layer tend to be emphasized. Warming the sample up to about 45 K alters the atomic images of the BaO layers but does not alter those of the CuO-chain layers. These results suggest that the electronic modulation images are closely related to the electronic states near the Fermi level and are attributed to the charge density wave.

Electronic structure of the CuO-chain layer inYBa2Cu3O7−δstudied by scanning tunneling microscopy

Low-temperature scanning tunneling microscopy measurements have been performed on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals with various oxygen contents $(6.65l~7\ensuremath{-}\ensuremath{\delta}l~6.98).$ In the CuO-chain layer, we observe the doping-dependent charge modulation images that are attributed to the charge-density wave (CDW) due to the ${2k}_{F}$ instability of the one-dimensional Fermi surface. Even in the so-called 60-K ${T}_{c}$ plateau region, the CDW correlation in the CuO-chain layer survives partially. Our scanning tunneling microscopy results indicate that the CuO chain without oxygen vacancies is 1/4 filled and prefers to form a CDW state.

Low-temperature STM/STS of high- Tc superconductors

We have carried out low-temperature scanning tunneling microscopy and spectroscopy studies on high- T c superconductors, Bi 2Sr 2CaCu 2O 8+ δ and YBa 2Cu 3O 7− δ single crystals. We present an experimental method and STM/STS results of these materials. Samples were cleaved below 20 K in order to obtain clean surfaces. For YBa 2Cu 3O 7− δ , we observe an atomic image of BaO and charge density waves image on the CuO chain layer. Differential tunneling conductance spectrum of YBa 2Cu 3O 7− δ has a clear superconducting energy gap along 100 nm path.

Identification of Atomic Layers of YBa2Cu3O7-δby Scanning Tunneling Microscopy and Spectroscopy

Low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) measurements have been performed on YBa 2 Cu 3 O 7-δ single crystals cleaved below 20 K in order to obtain clean surfaces. The observed STM images are classified into two types of atomic images corresponding to BaO and CuO-chain layers, and the superconducting STS spectra are observed only on the latter layer. In the BaO layer, barium atomic images are clearly observed for the first time in addition to oxygen atomic images. In the CuO-chain layer, on the other hand, we observe the oxygen atomic images, in which a number of oxygen vacancies form clusters along the b -axis. Moreover, the CuO-chain layer shows elliptically modulated electronic images which are attributed to the charge density wave in the CuO chains.