CuO2 Bilayers Research Articles

Moiré-like Superlattice Generated van Hove Singularities in a Strained CuO2 Double Layer

While it is known that the double-layer Bi2Sr2CaCu2O8+y (BSCCO) cuprate superconductor exhibits a one-dimensional (1D) incommensurate superlattice (IS), the effect of IS on the electronic structure remains elusive. Following the recent shift of interest from an underdoped phase to optimum and overdoped phases in BSCCO by increasing the hole doping x, controlled by the oxygen interstitials concentration y, here we focus on the multiple splitting of the density of states (DOS) peaks and emergence of higher order van Hove singularities (VHS) due to the 1D incommensurate superlattice. It is known that the 1D incommensurate wave vector q=ϵb (where b is the reciprocal lattice vector of the orthorhombic lattice) is controlled by the misfit strain between different atomic layers in the range 0.209–0.215 in BSCCO and in the range 0.209–0.25 in Bi2Sr2Ca1−xYxCu2O8+y (BSCYCO). This work reports the theoretical calculation of a complex pattern of VHS due to the 1D incommensurate superlattice with large 1D quasi-commensurate supercells with the wave vector ϵ=9/η in the range 36>η>43. The similarity of the complex VHS splitting and appearing of higher order VHS in a mismatched CuO2 bilayer with VHS due to the moiré lattice in strained twisted bilayer graphene is discussed. This makes a mismatched CuO2 bilayer quite promising for constructing quantum devices with tuned physical characteristics.

Coexistence of magnetism and superconductivity in the iron containing FeSr2YCu2O7.57 cuprate as studied by μSR

Substitution of copper by iron in the charge reservoir block of the YSr2Cu3O6+δ (YSCO) cuprate superconductor brings out an appealing insight on the interplay between superconductivity and magnetism. In the resulting FeSr2YCu2O7+δ (Fe-1212) materials, FeO1+δ layers and CuO2 bi-layers are alternated along the stacking direction, in close analogy to the RuSr2GdCu2O8 (Ru-1212) ferromagnetic superconductor. For the FeSr2YCu2O7.57 compound, both BVS calculations and spectroscopic data reflect a high doping level in the superconducting planes of pCuO2 ∼ 0.30, placing this cuprate on the overdoped region of the conventional phase diagram for cuprate superconductors. Nonetheless, iron cations in the charge reservoir block (CRB) are quite oxidized, reaching a high formal oxidation state of Fe3.6+. This highly doped FeSr2YCu2O7.57 cuprate is superconducting below Tc = 30 K and, from our muon spin relaxation spectroscopy (μSR) study, the presence of magnetic interactions has been substantiated. Most interestingly, the coincidence of the onset of the magnetic interactions with the superconducting critical temperature suggests a complex interplay between magnetism and superconductivity in this overdoped cuprate.

Spatially inhomogeneous competition between superconductivity and the charge density wave in YBa2Cu3O6.67

The charge density wave in the high-temperature superconductor YBa2Cu3O7−x (YBCO) has two different ordering tendencies differentiated by their c-axis correlations. These correspond to ferro- (F-CDW) and antiferro- (AF-CDW) couplings between CDWs in neighbouring CuO2 bilayers. This discovery has prompted several fundamental questions: how does superconductivity adjust to two competing orders and are either of these orders responsible for the electronic reconstruction? Here we use x-ray diffraction to study YBa2Cu3O6.67 as a function of magnetic field and temperature. We show that regions with F-CDW correlations suppress superconductivity more strongly than those with AF-CDW correlations. This implies that an inhomogeneous superconducting state exists, in which some regions show a fragile form of superconductivity. By comparison of F-CDW and AF-CDW correlation lengths, it is concluded that F-CDW ordering is sufficiently long-range to modify the electronic structure. Our study thus suggests that F-CDW correlations impact both the superconducting and normal state properties of YBCO.

Stabilization of three-dimensional charge order in YBa2Cu3O6+x via epitaxial growth

Incommensurate charge order (CO) has been identified as the leading competitor of high-temperature superconductivity in all major families of layered copper oxides, but the perplexing variety of CO states in different cuprates has confounded investigations of its impact on the transport and thermodynamic properties. The three-dimensional (3D) CO observed in YBa2Cu3O6+x in high magnetic fields is of particular interest, because quantum transport measurements have revealed detailed information about the corresponding Fermi surface. Here we use resonant X-ray scattering to demonstrate 3D-CO in underdoped YBa2Cu3O6+x films grown epitaxially on SrTiO3 in the absence of magnetic fields. The resonance profiles indicate that Cu sites in the charge-reservoir layers participate in the CO state, and thus efficiently transmit CO correlations between adjacent CuO2 bilayer units. The results offer fresh perspectives for experiments elucidating the influence of 3D-CO on the electronic properties of cuprates without the need to apply high magnetic fields.

Local Noncentrosymmetric Structure of Bi2Sr2CaCu2O8+y by X-ray Magnetic Circular Dichroism at Cu K-Edge XANES

The two-dimensional Bi2Sr2CaCu2O8+y (Bi2212), the most studied prototype cuprate superconductor, is a lamellar system made of a stack of two-dimensional corrugated CuO2 bilayers separated by Bi2O2+ySr2O2 layers. While the large majority of theories, proposed to interpret unconventional high Tc superconductivity in Bi2Sr2CaCu2O8+y, assume a centrosymmetric tetragonal CuO2 lattice for the [CuO2]Ca[CuO2] bilayer here we report new compelling results providing evidence for local noncentrosymmetric structure at the Cu site. We have measured polarized Cu K-edge XANES (x-ray absorption near edge structure) and the K-edge X-ray magnetic circular dichroism (XMCD) of a Bi2212 single crystal near optimum doping. The Cu K edge XMCD signal was measured at ID12 beamline of ESRF with the k-vector of x-ray beam parallel to c-axis i.e. with the electric field of x-ray beam E//ab, using a 17 T magnetic field parallel to the c-axis of a Bi2212 single crystal. Numerical simulations of the XMCD signal of Bi2212 by multiple scattering theory have shown agreement with the experimental XMCD signal only for the local structure with noncentrosymmetric Bb2b space group of Bi2Sr2CaCu2O8+y.

Magnetic field controlled charge density wave coupling in underdoped YBa2Cu3O6+x

The application of magnetic fields to layered cuprates suppresses their high-temperature superconducting behaviour and reveals competing ground states. In widely studied underdoped YBa2Cu3O6+x (YBCO), the microscopic nature of field-induced electronic and structural changes at low temperatures remains unclear. Here we report an X-ray study of the high-field charge density wave (CDW) in YBCO. For hole dopings ∼0.123, we find that a field (B∼10 T) induces additional CDW correlations along the CuO chain (b-direction) only, leading to a three-dimensional (3D) ordered state along this direction at B∼15 T. The CDW signal along the a-direction is also enhanced by field, but does not develop an additional pattern of correlations. Magnetic field modifies the coupling between the CuO2 bilayers in the YBCO structure, and causes the sudden appearance of the 3D CDW order. The mirror symmetry of individual bilayers is broken by the CDW at low and high fields, allowing Fermi surface reconstruction, as recently suggested.

The microscopic structure of charge density waves in underdoped YBa2Cu3O6.54 revealed by X-ray diffraction.

Charge density wave (CDW) order appears throughout the underdoped high-temperature cuprate superconductors, but the underlying symmetry breaking and the origin of the CDW remain unclear. We use X-ray diffraction to determine the microscopic structure of the CDWs in an archetypical cuprate YBa2Cu3O6.54 at its superconducting transition temperature ∼60 K. We find that the CDWs in this material break the mirror symmetry of the CuO2 bilayers. The ionic displacements in the CDWs have two components, which are perpendicular and parallel to the CuO2 planes, and are out of phase with each other. The planar oxygen atoms have the largest displacements, perpendicular to the CuO2 planes. Our results allow many electronic properties of the underdoped cuprates to be understood. For instance, the CDWs will lead to local variations in the electronic structure, giving an explicit explanation of density-wave states with broken symmetry observed in scanning tunnelling microscopy and soft X-ray measurements.

Characteristics of Plasma Sprayed BSCCO Superconductor Coatings with Annealing Time After Partial Melt Process

<TEX>$Bi_2Sr_2CaCu_2O_x$</TEX>(Bi-2212) and <TEX>$Bi_2Sr_2Ca_2Cu_3O_y$</TEX>(Bi-2223) high-Tc superconductors(HTS) have been manufactured by plasma spraying, partial melt process(PMP) and annealing treatment(AT). A Bi-2212/2223 HTS coating layer was synthesized through the peritectic reaction between a 0212 oxide coating layer and 2001 oxide coating layer by the PMP-AT process. The 2212 HTS layer consists of whiskers grown in the diffusion direction. The Bi-2223 phase and secondary phase in the Bi-2212 layer were observed. The secondary phase was distributed uniformly over the whole layer. As annealing time goes on, the Bi-2212 phase decreases with mis-orientation and irregular shape, but the Bi-2223 phase increases because a new Bi-2223 phase is formed inside the pre-existing Bi-2212 crystals, and because of the nucleation of a Bi-2223 phase at the edge of Bi-2212 crystals by diffusion of Ca and Cu-O bilayers. In this study the spray coated layer showed superconducting transitions with an onset Tc of about both 115 K, and 50 K. There were two steps. Step 1 at 115 K is due to the diamagnetism of the Bi-2223 phase and step 2 at 50 K is due to the diamagnetism of the Bi-2212 phase.

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.

Coherent generation of phonon-polaritons in Bi2Sr2CaCu2O8+xintrinsic Josephson junctions

The Bi2Sr2CaCu2O8+x high-temperature superconductor represents a natural metamaterial, composed of metallic CuO bilayers sandwiched between ionic BiO planes. Each pair of CuO bilayers forms an atomic-scale Josephson junction. Here we employ the intrinsic Josephson effect for in situ generation and detection of electromagnetic waves in Bi2Sr2CaCu2O8+x single crystals. We observe that electromagnetic waves form polaritons with several transverse optical phonons. This indicates the presence of unscreened polar response in cuprates, which may lead to strong electronphonon interaction. Our technique can provide intense local sources of coherent, monochromatic phonon-polaritons with kW/cm2 power densities.

Multiple quantum oscillation frequencies in YBa2Cu3O6+δ and bilayer splitting

Experiments have revealed multiple quantum oscillation frequencies in underdoped high-temperature superconductor YBa2Cu3O6+δ, corresponding to approximately 10% doping, which contains CuO bilayers in the unit cell. These unit cells are further coupled along the c-axis by a tunneling matrix element. A model of the energy dispersion that has its roots in the previously determined electronic structure, combined with twofold commensurate density waves, reveals multiple electron and hole pockets. To the extent that quasiparticles of the reconstructed Fermi surface have finite residues, however small, the formation of Landau levels is the cause of these oscillations, and the bilayer splitting and warping of the electronic dispersion along the direction perpendicular to the CuO-planes are firm consequences. The goal here is to explore this possibility from various directions and provide a better understanding of the rapidly developing experimental situation involving multiple frequencies. An important conclusion is that bilayer splitting is considerably renormalized from the value obtained from band structure calculations. It would be extremely interesting to perform these experiments for higher values of doping. We roughly expect the splitting of the frequencies to increase with doping, but the full picture may be more complex because the density wave order parameter is also expected to decrease with doping, vanishing around the middle of the superconducting dome.

Proximity-Induced Pseudogap: Evidence for Preformed Pairs

The temperature evolution of the proximity effect in Au/La(2-x)Sr(x)CuO(4) and La(1.55)Sr(0.45)CuO(4)/La(2-x)Sr(x)CuO(4) bilayers was investigated using scanning tunneling microscopy. Proximity-induced gaps, centered at the chemical potential, were found to persist above the superconducting transition temperature, T(c), and up to nearly the pseudogap crossover temperature in both systems. Such independence of the spectra on the details of the normal-metal cap layer is incompatible with a density-wave order origin. However, our results can be accounted for by a penetration of incoherent Cooper pairs into the normal metal above T(c).

Relation between inhomogeneous structure and transport properties for superconductor-insulator transitions

In the study of superconductor-insulator transitions of high-Tc cuprates complicated behaviors in transport properties were observed for example quasi-reentrant behaviors. In this study we report that the relation between microscopic structure and transport properties for Bi2Sr2CaCu2O8+y crystal with substitution of Ca by Y. From the energy dispersive X-ray spectroscopy it was estimated that samples contain almost 45% Y when the starting composition was only 10% Y. Also the values of c-axis length were almost constant and shorter than those of Y free samples. However there were various ρ(T) behaviors dependent on sample: localized, quasi-reentrant, broad superconducting transition etc. All sheet resistances defined per CuO2 bilayer in the normal state were much larger than the quantum resistance h/4e2 or 6450 Ω. We were able to observe the inhomogeneous distribution of Y using a combination of electron energy loss spectroscopy and a high-angle annular dark-field technique in a scanning transmission electron microscope. The Y rich region formed the belt with about 20 nm width. This inhomogeneous structure seems to be the origin of various transport properties.

Spontaneous vortex state in the weak-ferromagnetic superconductor RuSr2EuCu2O8

Similar to the 56K weak-ferromagnetic superconductor RuSr2GdCu2O8, a spontaneous vortex state (SVS) between 16K and 36K was observed for the weak-ferromagnetic superconductor RuSr2EuCu2O8 with the ferromagnetic Curie temperature TCurie=133K and the superconducting transition temperature Tsc=36K. The low-field (±20G) superconducting hysteresis loop indicates a weak and narrow Meissner state region of the average lower critical field Bc1ave(T)=Bc1ave(0)[1-(T/TSVS)2], with Bc1ave(0)=7G and TSVS=16K. Full Meissner shielding signal for bulk sample in a very low applied field indicates an ab-plane lower critical field Bc1ab(0)∼2G with an estimated anisotropic parameter γ∼8.5 for this layered system. The existence of a spontaneous vortex state between 16K and 36K is the result of weak-ferromagnetic (canted antiferromagnetic) order with a very small net spontaneous magnetic moment of ∼0.1μB/Ru, which generates a weak magnetic dipole field around 9G in the CuO2 bilayers. The vortex melting transition temperature Tmelting=21K<Tsc=36K from the broad resistivity transition and the onset of diamagnetic signal indicate a very broad vortex liquid region due to the coexistence and the interplay between superconductivity and weak-ferromagnetic order.

Critical fields and the spontaneous vortex state in the weakly ferromagnetic superconductorRuSr2GdCu2O8

A spontaneous vortex state (SVS) between 30 K and 56 K was observed for the weak-ferromagnetic superconductor RuSr2GdCu2O8 with ferromagnetic Curie temperature TC = 131 K and superconducting transition temperature Tc = 56 K. The low field (20 G) Superconducting hysteresis loop indicates a narrow Meissner state region within average lower critical field Bc1(T)= Bc1(0)[1 - (T/T0)2], with average Bave c1 (0) = 12 G and T0 = 30 K. Full Meissner shielding signal in very low applied field indicates an ab-plane Bab c1(0) ~ 4 G with an estimated anisotropic parameter g ~ 7 for this layered system. The existence of a spontaneous vortex state between 30 K and 56 K is the result of weak-ferromagnetic order with a net spontaneous magnetic moment of ~ 0.1 muB/Ru, which generates a weak magnetic dipole field around 10 G in the CuO2 bi-layers. The upper critical field Bc2 varies linearly as (1 - T/Tc) up to 7-T field. The vortex melting line Bm varies as (1 - T/Tm)3.5 with melting transition temperature Tm = 39 K and a very broad vortex liquid region due to the coexistence and the interplay between superconductivity and weak-ferromagnetic order.

Anomalous Pr Ordering, Pr L3-Edge and Cu K-Edge XANES Studies for the Insulating PrBa2Cu3O7−y System

X-ray absorption near-edge structure (XANES) data for the PrBa2Cu3O7−y (6.21≤7−y≤6.95) insulating system with anomalous Pr antiferromagnetic order are reported. Pr L3-edge XANES indicates that Pr formal valence decreases from 3.17–3.18 for oxygenated samples to 3.09 for oxygen-depleted PrBa2Cu3O6.43, 3.03 for PrBa2Cu3O6.38 and 3. 01 for PrBa2Cu3O6.21. The variation of Pr valence indicates that the doped holes initially stay in the CuO1−y plane and contribute to Cu(I)–Cu(II) transformation in this plane, but gradually move into the CuO2 bi-layers with PrO8 cage and result in the appearance of Pr(IV) localized-hole state with, increasing oxygen content. Cu K-edge XANES indicates that the average Cu formal valence increases from less than 2 in oxygen-depleted region to a value larger than 2 for oxygenated samples. with the estimated Cu valence of 2.24 for PrBa2Cu3O6.95. The small average Cu formal valence or large Pr formal valence is closely related to the absence of superconductivity.

Direct correlation betweenTcandCuO2bilayer spacing inYBa2Cu3O7−x

We report the effects of epitaxial strain and deoxygenation on high quality [YBa2Cu3O7-x (YBCO)_N / PrBa2Cu3O7 (PBCO)_5] (1000 A thick) superlattices, with 1 <= N <= 12 unit cells. High spatial resolution electron energy loss spectroscopy (EELS) shows that strained, fully oxygenated YBCO layers are underdoped. Irrespective of whether underdoping is induced by strain or deoxygenation, X-ray diffraction analysis shows that Tc correlates directly with the separation of the CuO2 bilayers.

Characteristic transport properties observed in underdoped YBa2Cu3O6+x

By annealing under the equilibrium oxygen pressure, we have succeeded in obtaining YBa2Cu3O6+x with any doping level. Both in bulk and film samples, the doping driven superconductor-to-insulator (S–I) transition has been observed at ρab≃0.8mΩcm which corresponds to the universal resistance h/4e2 per CuO2 bi-layer. The observed critical carrier density is nHSI≃3×1020cm−3. Within the critical region of the S–I transition, using a thin film sample, we have succeeded in observing the resistivity scaling as a function of doping. The carrier density obeys the relation nH∝x−0.2 in the underdoped region (0.2⩽x<0.5), where the conductivity is substantially small and transport properties show weaker temperature dependence. The quantum effects are considered to be more important e.g., the quantum phase fluctuation in superconductors with small superfluid density.

Observation of the Andreev reflection in thec-axis transport ofBi2Sr2CaCu2O8+xsingle crystals nearTcand the search for the preformed-pair state

We observed an enhancement of the c-axis differential conductance around the zero bias in $\mathrm{Au}/{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ (Bi2212) junctions near the superconducting transition temperature ${T}_{c}.$ We attribute the conductance enhancement to the Andreev reflection between the surface Cu-O bilayer with suppressed superconductivity and the neighboring superconducting inner bilayer. The continuous evolution from depression to an enhancement of the zero-bias differential conductance, as the temperature approaches ${T}_{c}$ from below, points to weakening of the barrier strength of the nonsuperconducting layer between adjacent Cu-O bilayers. We observed that the conductance enhancement persisted up to a few degrees above ${T}_{c}$ in junctions prepared on slightly overdoped Bi2212 crystals. However, no conductance enhancement was observed above ${T}_{c}$ in underdoped crystals, although a recently proposed theoretical consideration suggests an even wider temperature range of enhanced zero-bias conductance. This seems to provide a negative perspective to the existence of the phase-incoherent preformed pairs in the pseudogap state.

Superconductor-to-insulator transition and transport properties of underdoped YBa2Cu3O(y) crystals.

The carrier-concentration-driven superconductor-to-insulator (SI) transition as well as transport properties in underdoped YBa2Cu3O(y) twinned crystals is studied. The SI transition takes place at y approximately 6.3, carrier concentration n(SI)H approximately 3x10(20) cm(-3), anisotropy rho(c)/rho(ab) approximately 10(3), and the threshold resistivity rho(SI)ab approximately 0.8 mOmega cm which corresponds to a critical sheet resistance h/4e2 approximately 6.5 kOmega per CuO2 bilayer. The evolution of a carrier, nH infiniti y - 6.2, is clearly observed in the underdoped region. The resistivity and Hall coefficient abruptly acquire strong temperature dependence at y approximately 6.5 indicating a radical change in the electronic state.