CuO2 Planes Research Articles

Synthesis, characterization, and Vickers microhardness for (YIG)x/(Bi,Pb)-2223 superconducting phase

The current study investigated the effect of adding yttrium iron garnet (Y3Fe5O12; YIG) nanoparticles to the (Bi1.8Pb0.4)Sr2Ca2.1Cu3.2Oy ((Bi,Pb)-2223) superconductor. The classical solid-state reaction technique was used to synthesize the nano-(YIG)x/(Bi,Pb)-2223 composites (0.00 ≤ x ≤ 2.00 wt.%). X-ray diffraction (XRD) confirmed the formation of tetragonal (Bi,Pb)-2223 as the major phase. The morphology and elemental composition of the synthesized samples were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. Iodometric titration analysis proved that the addition of nano-YIG increased the oxygen content in the CuO2 planes. The critical transition temperature (Tc) and the critical current density (Jc) increased with the nano-YIG addition up to x = 0.25 wt.% with values of 119.25 K and 638.37 A/cm2, respectively. X-ray photoelectron spectroscopy (XPS) revealed the elemental composition and oxidation state of all elements in the prepared samples. Room temperature Vickers microhardness (Hv) measurements were performed at different applied loads (0.49–9.80 N) and a duration time of 20 s. The best enhancement in Vickers microhardness (Hv) was achieved at x = 2.00 wt.%. The modified proportional sample resistance (MPSR) model provided the best theoretical analysis at the plateau limit region based on Vickers microhardness (Hv) observations. Moreover, nano-YIG addition into the (Bi,Pb)-2223 superconductor adversely affected several mechanical parameters such as the elastic modulus (E), yield strength (Y), and fracture toughness (K).

Interrelations among zero-field critical current density, irreversibility field and pseudogap in hole doped high-Tc cuprates

Critical current density and the irreversibility magnetic fields are two parameters which control most of the applications of high-temperature superconductors. In this brief communications we have investigated the intrinsic effects that determine the magnitudes of these two parameters. The roles of hole concentration in the CuO2 plane (p) and oxygen deficiency (δ) in the CuO1-δ chains on the zero-field low-temperature critical current density, Jc0, were studied for a series of Y1-xCaxBa2Cu3O7-δ (x = 0.00, 0.05, 0.10, and 0.20) thin films. Jc0 above the optimum hole concentration were found to be high. The magnitude of Jc0 were largely determined by the hole concentration, maximizing at p ∼ 0.185 ± 0.005, regardless of the values of x and δ. This implied that oxygen deficiency/disorder plays a secondary role and the intrinsic Jc0 is governed largely by the number of doped holes in the CuO2 planes of Y1-xCaxBa2Cu3O7-δ. Further support in favor of this finding was garnered from the analysis of the in-plane resistive transitions of thin films of YBa2Cu3O7-δ (YBCO) with magnetic fields (H) applied along the crystallographic c-direction. The characteristic magnetic field (H0), linked to the vortex activation energy and the irreversibility field, exhibits similar p-dependence as shown by Jc0(p). We have explained these observations in terms of the doping dependent pseudogap (PG) energy/temperature scale in the low-energy electronic energy density of states observed in high-Tc cuprates. Both the intrinsic critical current density and the irreversibility field depend directly on the superconducting condensation energy, which in turn is primarily determined by the magnitude of the hole concentration dependent PG in the quasiparticle spectral density.

Strain effect on the high T c superconductor YBa2Cu3O7: an ab initio study comparing bulk and monolayer models

In this work, the effect of strain on the vibrational and electronic properties of the YBa2Cu3O7 compound was studied through ab initio calculations. For this, two structural models were used: a bulk model and a surface model (a monolayer with CuO2 and BaO as the terminating layers). The phonon spectra was calculated for both structures under different levels of c axis strain. The most appreciable change occurs in the vibrational properties, and in the surface case. From the simulation of the Raman spectra, we were able to quantify the Raman shift ratio as a function of the applied strain, and analyzed its behavior in terms of the overlap population of the different bonds and the reduced mass of selected phonons. The effect of the level of deformation on the band structure and the electronic density of states is small for both structures, although more noticeable in the case of the surface model. In both cases, tendencies are observed when the fine features of the band structure are analyzed by means of the tight binding model. Due to the lower symmetry, the surface model also shows modifications of the bands related to the CuO2 planes.

Two types of superconducting pairs in stripe-ordered La2−xBaxCuO4 (x=1/8) : Evidence from resistivity measurements

Recent angle-resolved $c$-axis resistivity measurements of the stripe-ordered ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ (LBCO) with $x=1/8$ revealed an unexpected dependence on the direction of the in-plane magnetic field. We argue that these and other available data for the $c$-axis transport point to the existence of superconducting pairs of two different types in the $x=1/8$ LBCO below the stripe ordering temperature. The pairs of one type carry finite momentum and are confined to the Cu-O planes; the pairs of other type (probably the conventional $d$ wave with zero momentum) propagate along narrow conducting channels traversing the sample in the $c$-axis direction. The evidence for this comes from the observed exponential temperature dependence of the $c$-axis resistivity ${\ensuremath{\rho}}_{c}(T)$ which we attribute to the thermally excited slips of the superconducting phase and flux flows. We present a simple theory to fit the observed $\ensuremath{\pi}/2$-periodic dependence of ${\ensuremath{\rho}}_{c}$ on the direction of the in-plane magnetic field and the other data.

The relationship between orbital hybridization and superconductivity of sm-doped YBa2Cu3O7–δ studied by x-ray spectroscopy

Sm-doped YBa2Cu3O7–δ samples were prepared by conventional solid state reaction method in air. The studies of X-ray photoemission spectroscopy and X-ray absorption spectroscopy indicate that the extra electrons fill the holes in CuO2 planes, so that the hole carrier concentration decrease under Sm substitution. The X-ray absorption spectroscopy of Cu L-edge and O K-edge demonstrates that the hybridization strength between Cu 3d and O 2p decreases with increasing content of Sm, but the hybridization strength between Y 4d, Ba 5d, Sm 4f and O 2p increases with the increasing of Sm content, while the superconductivity of the Sm-doped YBa2Cu3O7–δ samples is suppressed, i.e., the superconducting transition temperature of the samples was lowered. Thus, our experimental results suggest a close relationship between the superconductivity and orbital hybridization.

How pressure enhances the critical temperature of superconductivity in YBa2Cu3O6+y

High-temperature superconducting cuprates respond to doping with a dome-like dependence of their critical temperature (Tc). But the family-specific maximum Tc can be surpassed by application of pressure, a compelling observation known for decades. We investigate the phenomenon with high-pressure anvil cell NMR and measure the charge content at planar Cu and O, and with it the doping of the ubiquitous CuO2 plane with atomic-scale resolution. We find that pressure increases the overall hole doping, as widely assumed, but when it enhances Tc above what can be achieved by doping, pressure leads to a hole redistribution favoring planar O. This is similar to the observation that the family-specific maximum Tc is higher for materials where the hole content at planar O is higher at the expense of that at planar Cu. The latter reflects dependence of the maximum Tc on the Cu-O bond covalence and the charge-transfer gap. The results presented here indicate that the pressure-induced enhancement of the maximum Tc points to the same mechanism.

First-principles calculation of the optical properties of the YBa2Cu3O7-δ oxygen vacancies model.

We used first-principles methods to investigate how oxygen vacancy defects affect the optical properties of YBa2Cu3O7-δ (0 < δ < 1), a high-temperature superconductor with potential applications in optical detectors. We calculated the electronic structure of YBa2Cu3O7-δ with different amounts of oxygen vacancies at three different sites: Cu-O chains, CuO2 planes, and apical oxygens. The formation energy calculations support the formation of oxygen vacancies in the Cu-O chain at higher concentrations of vacancy defects, with a preference for alignment in the same chain. The presence of oxygen vacancies affects the optical absorption peak of YBa2Cu3O7-δ in different ways depending on their location and concentration. The optical absorption peaks in the visible range (1.6-3.2 eV) decrease in intensity and shift towards the infrared spectrum as oxygen vacancies increase. We demonstrate that oxygen vacancies can be used as a powerful tool to manipulate the optical response of YBa2Cu3O7-δ to different wavelengths in optical detector devices.

Superconductivity Experiments Confirm an Important Mechanism for Increasing the Maximum Critical Temperature in Cuprates

Some cuprate superconductors exhibit high critical temperatures at atmospheric pressure. To understand the underlying processes, researchers recently carried out high-pressure nuclear magnetic resonance experiments on the cuprate YBa2Cu3O6+y to determine the charge densities at the Cu and O sites of its CuO2 plane with high precision. Their results revealed that the more generous the in-plane Cu atoms are in transferring their hole content to the O atoms, the higher the critical temperatures that can be achieved.

Key factor for low anisotropy and high irreversibility field in (Cu,C)Ba2Ca3Cu4O11+δ

In cuprate superconductors, the Hg-, Tl- and Bi-based systems with multilayers of CuO2 planes within one unit cell usually have critical temperatures higher than 100 K. However, the applications of these materials under high magnetic fields in liquid nitrogen temperature region are strongly limited by the very high anisotropy and strong vortex motion. The observation of an unprecedented high irreversibility field and low anisotropy in (Cu,C)Ba2Ca3Cu4O11+δ with Tc = 116 K was an exception, and the reason remains elusive. Here, we report the investigations on (Cu,C)Ba2Ca3Cu4O11+δ single crystals. The angle-dependent resistivity under different temperatures and magnetic fields reveals clear evidence of lower anisotropy and higher irreversibility fields. The high resolution scanning transmission electron microscopy allows to clearly visualize that CuO chains and CO chains are formed alternatively at the places where the heavy metal oxides would locate. First-principle calculations confirm that the newly formed CuO chains in the charge reservoir blocks significantly enhance the c-axis coherent electronic conduction, lower the anisotropy and strengthen the irreversibility field. This discovery may be extended in a general sense to other layered cuprates and stimulate high power applications of cuprate superconductors in liquid nitrogen temperature region.

Stabilization of three-dimensional charge order through interplanar orbital hybridization in PrxY1−xBa2Cu3O6+δ

The shape of 3d-orbitals often governs the electronic and magnetic properties of correlated transition metal oxides. In the superconducting cuprates, the planar confinement of the {d}_{{x}^{2}-{y}^{2}} orbital dictates the two-dimensional nature of the unconventional superconductivity and a competing charge order. Achieving orbital-specific control of the electronic structure to allow coupling pathways across adjacent planes would enable direct assessment of the role of dimensionality in the intertwined orders. Using Cu L3 and Pr M5 resonant x-ray scattering and first-principles calculations, we report a highly correlated three-dimensional charge order in Pr-substituted YBa2Cu3O7, where the Pr f-electrons create a direct orbital bridge between CuO2 planes. With this we demonstrate that interplanar orbital engineering can be used to surgically control electronic phases in correlated oxides and other layered materials.

Superfluid density and two-component conductivity in hole-doped cuprates

While the pseudogap dominates the phase diagram of hole-doped cuprates, connecting the antiferromagnetic parent insulator at low doping to the strange metal at higher doping, its origin and relation to superconductivity remains unknown. In order to proceed, a complete understanding of how the single hole–initially localized in the Mott state–becomes mobile and ultimately evolves into a coherent quasiparticle at the end of the superconducting dome is required. In order to affect this development, we examine recent transport and spectroscopic studies of hole-doped cuprates across their phase diagram. In the process, we highlight a set of empirical correlations between the superfluid density and certain normal state properties of hole-doped cuprates that offer fresh insights into the emergence of metallicity within the CuO2 plane and its influence on the robustness of the superconducting state. We conclude by arguing that the overall behavior is best understood in terms of two distinct current-carrying fluids, only one of which dominates the superconducting condensate and is gapped out below the pseudogap endpoint at a critical hole concentration p∗.

Scale-Free Distribution of Oxygen Interstitial Wires in Optimum-Doped HgBa2CuO4+y

Novel nanoscale probes are opening new venues for understanding unconventional electronic and magnetic functionalities driven by multiscale lattice complexity in doped high-temperature superconducting perovskites. In this work, we focus on the multiscale texture at supramolecular level of oxygen interstitial (O-i) atomic stripes in HgBa2CuO4+y at optimal doping for the highest superconducting critical temperature (TC) of 94 K. We report compelling evidence for the nematic phase of oxygen interstitial O-i atomic wires with fractal-like spatial distribution over multiple scales using scanning micro- and nano-X-ray diffraction. The scale-free distribution of O-i atomic wires at optimum doping extending from the micron down to the nanoscale has been associated with the intricate filamentary network of hole-rich metallic wires in the CuO2 plane. The observed critical opalescence provides evidence for the proximity to a critical point that controls the emergence of high-temperature superconductivity at optimum doping.

First-principles electronic structure investigation of HgBa2Can−1CunO2n+2+x with the SCAN density functional

We perform first-principles calculations to study the electronic structure of HgBa2Can−1CunO2n+2+x copper oxides up to n = 6 for the undoped parent compound (x = 0) and up to n = 3 for the doped compound (x &amp;gt; 0) by means of the strongly constrained and appropriately normed (SCAN) density functional. Our calculations predict an antiferromagnetic insulator ground state for the parent compounds with an energy gap that decreases with the number of CuO2 planes. We report structural, electronic, and magnetic order evolution with x, which agrees with the experiments. We find an enhanced density of states at the Fermi level at x ≈ 0.25 for the single-layered compound. This manifests in a peak of the Sommerfeld parameter of electronic specific heat, which has recently been discussed as a possible signature of quantum criticality generic to all cuprates.

On the electron pairing mechanism of copper-oxide high temperature superconductivity

The elementary CuO2 plane sustaining cuprate high-temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO5 pyramids. Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate t/ℏ and across the charge-transfer energy gap [Formula: see text], generate "superexchange" spin-spin interactions of energy [Formula: see text] in an antiferromagnetic correlated-insulator state. However, hole doping this CuO2 plane converts this into a very-high-temperature superconducting state whose electron pairing is exceptional. A leading proposal for the mechanism of this intense electron pairing is that, while hole doping destroys magnetic order, it preserves pair-forming superexchange interactions governed by the charge-transfer energy scale [Formula: see text]. To explore this hypothesis directly at atomic scale, we combine single-electron and electron-pair (Josephson) scanning tunneling microscopy to visualize the interplay of [Formula: see text] and the electron-pair density nP in Bi2Sr2CaCu2O8+x. The responses of both [Formula: see text] and nP to alterations in the distance δ between planar Cu and apical O atoms are then determined. These data reveal the empirical crux of strongly correlated superconductivity in CuO2, the response of the electron-pair condensate to varying the charge-transfer energy. Concurrence of predictions from strong-correlation theory for hole-doped charge-transfer insulators with these observations indicates that charge-transfer superexchange is the electron-pairing mechanism of superconductive Bi2Sr2CaCu2O8+x.

Tuning of anisotropy in Bi-based high temperature superconducting thin films

• Highly anisotropic Bi-based superconducting thin films. • Development of planar defects in thin films enhance vortex pinning. • Pb substitution strengthen 2D pancake arrangement of vortices in CuO planes. • Origin of anisotropy influence the axial dependence of pinning potential. The origin of the anisotropy behavior in the thin films of Bi 1.75 Pb 0.25 Sr 2 Ca 2 Cu 3 O 10±δ grown using pulsed laser deposition technique on three single crystalline substrates was investigated using magnetotransport measurements. Based on the evolution of the activation energy deduced from the magnetotransport data, it is surmised that Pb substitution in the present system strengthens the two-dimensional pancake arrangement of the vortices in the superconducting slab, which enhances the pinning of the vortices to the defect sites. The development of these planar defects with the decrease in the dimensionality of the sample was found to be the reason for the enhanced value of the pinning potential obtained in the thin film samples compared to the bulk. The interplay of Pb-assisted cation chemistry was found to have an influence on the anisotropy behavior in these films leading to an axial dependence of the pinning potential. Film grown on SrTiO 3 substrate with an anisotropy factor of ∼ 26 and higher superconducting parameters is the most anisotropic among the series of samples investigated.

Pronounced interplay between intrinsic phase-coexistence and octahedral tilt magnitude in hole-doped lanthanum cuprates

Definitive understanding of superconductivity and its interplay with structural symmetry in the hole-doped lanthanum cuprates remains elusive. The suppression of superconductivity around 1/8th doping maintains particular focus, often attributed to charge-density waves (CDWs) ordering in the low-temperature tetragonal (LTT) phase. Central to many investigations into this interplay is the thesis that La1.875Ba0.125CuO4 and particularly La1.675Eu0.2Sr0.125CuO4 present model systems of purely LTT structure at low temperature. However, combining single-crystal and high-resolution powder X-ray diffraction, we find these to exhibit significant, intrinsic coexistence of LTT and low-temperature orthorhombic domains, typically associated with superconductivity, even at 10 K. Our two-phase models reveal substantially greater tilting of CuO6 octahedra in the LTT phase, markedly buckling the CuO2 planes. This would couple significantly to band narrowing, potentially indicating a picture of electronically driven phase segregation, reminiscent of optimally doped manganites. These results call for reassessment of many experiments seeking to elucidate structural and electronic interplay at 1/8 doping.

Anomalous Ferromagnetism of quasiparticle doped holes in cuprate heterostructures revealed using resonant soft X-ray magnetic scattering

We report strong ferromagnetism of quasiparticle doped holes both within the ab-plane and along the c-axis of Cu-O planes in low-dimensional Au/d-La1.8Ba0.2CuO4/LaAlO3(001) heterostructures (d = 4, 8 and 12 unit-cells) using resonant soft X-ray and magnetic scattering together with X-ray magnetic circular dichroism. Interestingly, ferromagnetism is stronger at a hole doped peak and at an upper Hubbard band of O with spin-polarization degree as high as 40%, revealing strong ferromagnetism of Mottness. For in-ab-plane spin-polarizations, the spin of doped holes in O2p–Cu3d–O2p is a triplet state yielding strong ferromagnetism. For out-of-ab-plane spin-polarization, while the spins of doped holes in both O2p–O2p and Cu3d–Cu3d are triplet states, the spin of doped holes in Cu3d–O2p is a singlet state yielding ferrimagnetism. A ferromagnetic-(002) Bragg-peak of the doped holes is observed and enhanced as a function of d revealing strong ferromagnetism coupling between Cu-O layers along the c-axis.

Pseudogap in underdoped cuprate seen in longitudinal magnetoresistance

We report the results of in-plane magnetotransport study of slightly underdoped cuprate La1.85Sr0.15CuO4 (LSCO15) with Ni impurity. Increasing Ni content y causes a sharp drop in longitudinal magnetoresistance (LMR) in LSCO15 to broaden and move towards higher temperatures. Temperature of this local maximum in LMR coincides with temperature , below which ideal resistivity from the parallel-resistor model deviates from its T 2-dependence. A direct comparison with the hole doping evolution of pseudogap (PG) in LaSr x CuO4 (LSCO), possible through the mobile-carrier concentration extracted from the thermopower measurements, allows to equate both characteristic temperatures with PG opening temperature . The rate of PG closing by magnetic field parallel to the CuO2 plane, in measurements up to 9 T, is consistent with spin-paramagnetic effect in this configuration and yields PG closing field B pc close to the second critical field predicted for superconducting gap with the help of Werthamer–Helfand–Hohenberg theory. The field anisotropy of B pc suggests that orbital degrees of freedom also play a role in PG formation.

Probing effect of strain field on Ca diffusion in BaZrO3-doped YBa2Cu3O7-x /Ca0.3Y0.7Ba2Cu3O7-x multilayer nanocomposite films

Recently, we reported that incorporating thin Ca0.3Y0.7Ba2Cu3O7-x spacer layers in BaZrO3/ YBa2Cu3O7-x (BZO/YBCO) films in multilayers (ML) could lead to a coherent BZO/YBCO interface due to reduced lattice mismatch of ∼1.4% and hence improved pinning by the BZO 1D artificial pinning centers (BZO 1D-APCs). It was hypothesized that the highly strained YBCO columns around the BZO 1D-APCs due to the larger lattice constant of BZO than YBCO's in c-axis by 7.7% provide channels to facilitate Ca diffusion more preferably near the BZO/YBCO interface, leading to an enlarged YBCO's c-axis lattice constant as a consequence of the Ca/Cu substitution on the YBCO's Cu-O planes. Confirmation of this hypothesis is hence important to understand the mechanism of Ca diffusion in BZO/YBCO ML films for further enhancement of pinning efficiency of the BZO 1D-APCs. Motivated by this, this work presents a comparative study of two sets of BZO/YBCO ML films: singly-doped (SD-ML, BZO only) and the doubly-doped (DD-ML, BZO + Y2O3). The modulated strain field in the SD-ML films is in contrast to the non-modulated strain in the DD-ML samples as confirmed by transmission electron microscopy. Interestingly, improved pinning by >four times of the reference SD single-layer (SD-SL) film’ was observed in the SD-ML samples even when the constituent BZO/YBCO layer thickness was varied between 50-100 nm, indicating that the Ca diffusion along the strained BZO/YBCO interface and the Ca/Cu substitution on YBCO's Cu-O planes indeed correlate with the modulated strain field. In contrast, pinning degrades by >twice in DD-ML samples with respect to their DD-SL counterpart's. This result therefore suggests that strained BZO/YBCO interfaces serve as channels for Ca diffusion in SD-ML, which leads to the coherent BZO/YBCO interfaces and hence improved pinning of BZO 1D-APCs. This effect diminishes when such a modulated strain field is removed in the DD-ML samples.

Atomic origin of the coexistence of high critical current density and high Tc in CuBa2Ca3Cu4O10+δ superconductors

For cuprate superconductors, a high critical transition temperature (Tc) can be realized in compounds containing multiple CuO2 layers in the unit cell, while a high critical current density (Jc) is rarely sustained above liquid nitrogen temperature. The CuBa2Ca3Cu4O10+δ (Cu-1234) superconductors synthesized under high oxygen pressure incredibly exhibit high Tc (~117 K) and high Jc (>104 A/cm2, 100 K) values. Here, the “double high” traits of Cu-1234 were investigated with advanced scanning transmission electron microscopy. It was revealed that ordering vacancies and plate-like 90° microdomains induced efficient microstructure pinning centers that suppressed vortex flux flow and enhanced Jc. Furthermore, metallic charge-reservoir blocks [Ba2CuO3+δ] were composed of unique compressed [CuO6] octahedra, which induced many holes with 2pz symmetry that significantly decreased the superconducting anisotropy and dramatically enhanced the interlayer coupling that guaranteed a high Jc. On the other hand, optimally doped CuO2 planes inside the thick superconducting blocks [Ca3Cu4O8] maintained a high Tc. Our results are applicable to design and synthesis of new superconductors with “double high” traits.