CuO2 Planes Research Articles

Crystal plane effect of copper with different oxides/hydroxides nano-composites on catalytic performance and mechanism towards the reduction of 4-nitrophenol

Here, four copper oxide/hydroxide composites Cu/CuO, Cu-Cu2O/Fe2O3, Cu-CuO/Co(OH)2, and Co(OH)2/CuO with CuO(111) or Cu(111) as the central exposed crystal plane were synthesized and used in the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). A study of the dynamic changes of reaction was carried out by real-time HPLC-Q-Orbitrap-HRMS/MS, HPLC, UV–Vis detection, and DFT simulation. It revealed that in copper based catalytic reduction of 4-NP, three different reaction mechanisms are generated over four catalysts. The different adsorption modes and higher adsorption strength of 4-NP on the CuO(111) and Cu(111) plane make it selective and active for both direct hydrogenation and condensation in the reduction of 4-NP. The synergistic effect of different metal crystal faces with CuO(111) can improve the catalytic activity, especially the CuO(111)/Co(OH)(011) crystal planes that have the best synergistic effect on the reduction of 4-NP. This work reports an example where the reaction mechanism was significantly affected by the principal exposed crystal planes of copper oxide/hydroxide composites. It can be very informative for understanding the catalytic processes and providing new ideas for the design of novel catalysts.

Novel Phase YBa2Cu3O5 derived from the High Tc Superconductor YBa2Cu3O7-x

The orthorhombic unit cell of high Tc oxide superconductor YBa2Cu3O7–x is indicated by two-dimensional CuO2 planes spanned in a-b directions. Partial removal of oxygen, down to x > 1, creates an insulating state and randomly distributed oxygen vacancies. An attempt has been made to increase the concentration of vacancies up to some critical value when their ordering could take place, while balance with copper cations will still maintain the original tetragonal structure. Removal of two oxygen atoms per unit cell results in the YBa2Cu3O5 compound and formation of linear Cu1+– O chains. The novel phase is an insulator, and doping in a low (50 –200 mbar) oxygen atmosphere at T < 800 K converts it to a conducting state.

Light-Induced Melting of Competing Stripe Orders without Introducing Superconductivity in La2−xBaxCuO4

The ultrafast manipulation of quantum material has led to many novel and significant discoveries. Among them, the light-induced transient superconductivity in cuprates achieved by melting competing stripe orders represents a highly appealing accomplishment. However, recent investigations have shown that the notion of photoinduced superconductivity remains a topic of controversy, and its elucidation solely through c-axis time-resolved terahertz spectroscopy remains an arduous task. Here, we measure the in-plane and out-of-plane transient terahertz responses simultaneously in the stripe-ordered nonsuperconducting La2−xBaxCuO4 after near-infrared excitations. We find that although a pump-induced reflectivity edge appears in the c-axis reflectance spectrum, the reflectivity along the CuO2 planes decreases simultaneously, indicating an enhancement in the scattering rate of quasiparticles. This in-plane transient response is clearly distinct from the features associated with superconducting condensation. Therefore, we conclude the out-of-plane transient responses cannot be explained by an equivalent of Josephson tunneling. Notably, those pump-induced terahertz responses remain consistent even when we vary the near-infrared optical pump wavelengths and hole concentrations. Our results provide critical evidence that transient three-dimensional superconductivity cannot be induced by melting the competing stripe orders with pump pulses whose photon energy is much higher than the superconducting gap of cuprates. Published by the American Physical Society 2024

Comparative Studies Between Lattice Dynamical Study and Normal Coordinate Analysis of HTSC EuBa2Cu3O6

It is generally believed, that is needed to sign post the way to the development of materials with high critical temperature, even to room temperature and above, is the formulation of a proper theoretical understanding of superconductivity in these materials. Many experimental studies suggest an intimate relationship between phonons, and the superconducting transition in the high Tc oxides.For example, Raman frequencies and line widths, elastic constants and thermal conductivity all show changes around Tc. Nevertheless, there is not a well- developed theory to explain how the phonons are related to Tc.. In conventional superconductors, the role of phonons is well established. However, in high Tc superconductors, the role played by phonons is controversial. Even if they do not mediate the pairing mechanism, they are likely to play an important subsidiary role. Hence there is much interest in the phonon structure of these materials. The normal state and superconducting properties are believed to arise from the strongly correlated motion of the electronic charge carries (electrons or holes) in the CuO2 planes that essentially define the layered cuprate HTSC. The other cations and oxygen atom in the structure provides structural stability and control the number of charge carries in the CuO2 planes. The Raman spectroscopy of HTSC explains that the role played by the phonons in the mechanism of superconductivity in the perovskite family of superconductors is not fully understood although a large series of experimental results has shown considerable coupling of some phonons to electronic excitation in these systems. Optical spectroscopy and especially Raman and Infrared spectroscopy revealed strong direct influences of the changes in the electronic states to the phonon at the center of the Brillouin.

Low-dimensional magnetic centers in HTSC YBCO film on steel-276 induced by gamma-quanta and electron beam

Coated HTSC tapes of 2-generation based on YBa2Cu3O7-x film were irradiated with 5 MeV electron beam at 273 K to generate low-dimensional defects of atomic displacements in Cu and O sublattices for possible enhancing the magnetic flux pinning and thereby increasing the magnetic critical current. We found the optimal electron beam current and fluency, when the neighbouring linear defects are spaced so as to accommodate vortices in the undamaged superconducting area. The following 60Co gamma-irradiation at 77 K generates defect pairs and causes hole localization and increases magnetic induction. In the mixed vortex state the charge carrier pairing energy is higher than kT due to paramagnetic pinning at radiation induced defects in oxygen sublattice both in CuO chains and CuO2 planes. Such a radiation treatment of HTSC tapes is recommended for the electron beam focusing coils.

Synergy between copper and cobalt on 3D Co/CuO complex promotes peroxymonosulfate activation

Activation of peroxymonosulfate (PMS) to degrade refractory antibiotics is an essential branch of advanced oxidation processes (AOPs). We reported in this study that the (111) and (002) crystal planes of CuO with the (311) crystal plane of Cu0.72Co2.28O4 formed surface heterojunctions, which are used as interfaces for PMS adsorption to explore the performance of different heterojunctions. The formation of Cu0.72Co2.28O4/CuO heterojunction makes up for the CuO (002) surface’s shortcomings and improves the adsorption and activation of PMS by the CuO (002) surface. The heterojunction significantly grows the length of the PMS’ O-O bond compared to the single-crystal surface, which leads to lower energy required for PMS’ O-O bond breaking and further verified the increased covalent character. Based on the quality influencing factors values, the influence of four factors on sulfamethoxazole (SMX) deterioration fits as follows: HCO3– has the largest relative relevance among the factors, followed by H2PO4-, NO3–, and Cl-. The Box-Behnken methodology fitted the optimum efficiency with the initial concentration of Cl-, NO3–, H2PO4-, and HCO3– were 8.0, 10.0, 12.5, and 7.5 mM, respectively. The SMX’s atoms (N, and O) have higher electrophilic attack sites, indicating these atoms are electrophilic attack sites.

Charge Correlations in Cuprate Superconductors

High-temperature superconductivity, with transition temperatures up to ≈134 K at ambient pressure, occurs in layered cuprate compounds. The conducting CuO2 planes, which are universally present, are responsible for the superconductivity but also show a disposition to other competing states including spin and charge order. Charge-density-wave (CDW) order appears to be a universal property of cuprate superconductors. It has been studied via a multitude of probes including X-ray and neutron scattering, nuclear magnetic resonance, scanning probe techniques, electronic transport, and quantum oscillations. Here, we review the microscopic properties of the CDW order. We discuss the nature of the ordered state, that is, its symmetry and microscopic structure. Furthermore, we show how the CDW order is related to quenched disorder, host structure, symmetry breaking perturbations, and magnetic fields. We also describe measurements of dynamic collective charge excitations that are closely related to the quasi-static CDW order. Finally, we highlight some of the debated issues in the field, including the origin of the CDW order, the relationship to spin order, and the nature of the spatial CDW correlations.

First principle calculation of electronic structures and hole concentration of YBCO family compounds

Yttrium barium copper oxide (YBCO) family compounds are similar in structure but have different superconducting temperatures. It is well known that superconducting properties arise from the crystal lattice and electronic structure of compounds. This work reveals and discusses the electronic structures of YBCO family compounds (YBa2Cu3O6, YBa2Cu3O6.5, YBa2Cu3O7, and YBa2Cu4O8) based on density functional theory (DFT). The electronic structure results show that Cu–O chains are the trigger components for conduction, and only Cu–O chains and Cu-O2 planes play important roles in electronic and superconducting properties. The results also reveal that additional hole doped will be non-trivially distributed between the Cu–O chains and the Cu-O2 planes, and hence a more comprehensive method is required to calculate the hole concentration in the Cu-O2 plane. This work also demonstrates the calculation of hole concentrations using DFT. By DFT calculation, the Y1236 has hole concentration of 0.5 and the hole doped of Y12365, Y1237 and Y124 are 0.09, 0.16 and 0.10, respectively. The results are consistent with the cuprate-superconductor phase diagram and the bond-valence sum (BVS) calculations.

Spatial migration of lattice oxygen for copper-iron based oxygen carriers in chemical looping combustion

Oxygen carrier (OC) is of great importance in chemical looping combustion (CLC) technology. The migration rate of bulk lattice oxygen during reduction process is detrimental in reaction rate. However, the migration path of lattice oxygen is still not clear. In this study, a thermogravimetric analyzer (TGA) was used to investigate the reaction characteristics and kinetic parameters of the OC. The oxygen migration paths of CuO and CuFe2O4 bulk lattices were studied by density functional theory (DFT) in a five-layer (2 × 1) CuO (1 1 1) and nine-layer (1 × 1) CuFe2O4 (1 0 0) plane model. The kinetic parameters, activation energy changes and relative kinetic model of the OC reduction were explored. According to the DFT results, the deeper bulk phase oxygen requires greater more energy toovercome energy barrier for migration. CuO has a lower oxygen release capacity than CuFe2O4 in the early stages, but higher in the later stage. The reaction kinetics study revealed that the reaction mechanism of the reduction of the copper-iron composite OC is a tertiary chemical reaction control mechanism. During the reaction, the activation energy of the OC changes continuously. The macroscopic reaction kinetics agree with the microscopic simulation, proving the simulation model's validity.

Improvement of critical current properties and irreversibility lines by metal substitution in Ca-free Hg-based double-layered cuprates

Ca-free Hg-based double-layered cuprates with Sr ions separating the two CuO2 planes, Hg(Sr Ba)2SrCu2O y and its metal-substituted analogue of (Hg, M)(Sr Ba)2SrCu2O y (, Re, and Pb), were successfully synthesized under high pressure. These novel materials exhibit bulk superconductivity at a critical temperature above 100 K. The superconducting properties, including critical current density and irreversibility field , were investigated through magnetic hysteresis loop measurements. Our results revealed that the M substitution with and Re improves the J c and B irr properties, which is likely associated with a decrease in the thickness of the blocking layer. Furthermore, a comparison of the irreversibility lines between Ca-free and representative Ca-containing Hg-based compounds highlighted that the replacement of Ca by Sr in the CuO2 planes does not significantly impact the temperature dependence of B irr. Notably, we found that (Hg, Re)Sr2SrCu2O y has an irreversibility line as high as that of the reported triple-layered (Hg, Re)Sr2Ca2Cu3O y cuprate.

Prominent Josephson tunneling between twisted single copper oxide planes of Bi2Sr2-xLaxCuO6+y

Josephson tunneling in twisted cuprate junctions provides a litmus test for the pairing symmetry, which is fundamental for understanding the microscopic mechanism of high temperature superconductivity. This issue is rekindled by experimental advances in van der Waals stacking and the proposal of an emergent d+id-wave. So far, all experiments have been carried out on Bi2Sr2CaCu2O8+x (Bi-2212) with double CuO2 planes but show controversial results. Here, we investigate junctions made of Bi2Sr2-xLaxCuO6+y (Bi-2201) with single CuO2 planes. Our on-site cold stacking technique ensures uncompromised crystalline quality and stoichiometry at the interface. Junctions with carefully calibrated twist angles around 45° show strong Josephson tunneling and conventional temperature dependence. Furthermore, we observe standard Fraunhofer diffraction patterns and integer Fiske steps in a junction with a twist angle of 45.0±0.2°. Together, these results pose strong constraints on the d or d+id-wave pairing and suggest an indispensable isotropic pairing component.

Oxygen isotope effect on the superfluid density within the [formula omitted]-wave and [formula omitted]-wave pairing channels of YBa[formula omitted]Cu[formula omitted]O[formula omitted

We report on measurements of the oxygen isotope (16O/18O) effect (OIE) on the transition temperature Tc and the zero-temperature in-plane magnetic penetration depth λab(0) in the stoichiometric cuprate superconductor YBa2Cu4O8 by means of muon-spin rotation/relaxation. An analysis of the temperature evolution of λab−2 in terms of coexisting isotropic s-wave and anisotropic d-wave order parameters (s+d-wave) reveals that the OIE on the superfluid density ρs(0)∝λab−2(0) stems predominantly from the d-wave component while the contribution of the s-wave one is almost zero. The OIE on the transition temperature Tc is found to be rather small: δTc/Tc=−0.32(7)%, compared to the total OIE on the superfluid density ρs(0): δρs(0)/ρs(0)=−2.8(1.0)%. The partial OIE’s on the corresponding d-wave and s-wave components of ρs(0) are δρs,d(0)/ρs(0)=−3.0(1.2)%, and δρs,s(0)/ρs(0)=0.2(1.2)%, respectively. Our results demonstrate that polaron formation in the CuO2 planes is the origin of the observed OIE in the d-wave channel. In the much weaker s-wave channel, fermionic quasiparticles are present, which do not contribute to the OIE on ρs(0). Our results support the original idea of K. Alex Müller on the polaronic nature of the supercarriers in high-temperature cuprate superconductors.

Growth and Electronic Structure of Copper Oxide Monolayer Epitaxial Films.

Copper-based high-temperature superconductors share a common feature in their crystal structure, which is the presence of a CuO2 plane, where superconductivity takes place. Therefore, important questions arise as to whether superconductivity can exist in a single layer of the CuO2 plane and, if so, how such superconductivity in a single CuO2 plane differs from that in a bulk cuprate system. To answer these questions, studies of the superconductivity in cuprate monolayers are necessary. In this study, we constructed a heterostructure system with a La2-xSrxCuO4 (LSCO) monolayer containing a single CuO2 plane and measured the resulting electronic structures. Monolayer LSCO has metallic and bulk-like electronic structures. The hole doping ratio of the monolayer LSCO is found to depend on the underlying buffer layer due to the interface effect. Our work will provide a platform for research into ideal two-dimensional cuprate systems.

Equilibrium magnetic properties of the Fe-doped HTSC YBaCuO

For the polycrystalline high-temperature superconductor Y1 –xFexBa2Cu3Oy (0 ≤ x ≤ 0.05), where a significant increase in the intragrain critical current density has recently been found, the effect of iron doping on the equilibrium magnetic characteristics is studied. The field dependences of the magnetization are measured in the temperature range of 40–85 K and in the field range up to 60 kOe with increasing and decreasing fields. The temperature dependences of the London penetration depth λab, the superconducting carrier density ns, and the thermodynamic critical field Hc are determined from an analysis of the middle lines of the magnetic hysteresis loop. It is shown that the thermodynamic properties of Fe substituted samples do not change at x ≤ 0.03. At a higher Fe content, a slight decrease in Hc and the superconducting transition temperature is observed. However, changes in the condensation energy turn out to be much smaller than values expected due to an increase in the density of effective pinning centres created by substitutional atoms in both the yttrium and copper sublattices. It is concluded that the pinning regions in chain CuO planes near substituted Cu sites are two-dimensional in nature and do not affect the superconducting pairing in CuO2 planes. Another possible pinning mechanism, which does not affect Hc, is the interaction of vortices with the magnetic moments of Fe3+ ions.

Discovery of charge order in a cuprate Mott insulator

Copper oxide superconductors universally exhibit multiple forms of electronically ordered phases that break the native translational symmetry of the CuO2 planes. In underdoped cuprates with correlated metallic ground states, charge/spin stripes and incommensurate charge density waves (CDWs) have been experimentally observed over the years, while early theoretical studies also predicted the emergence of a Coulomb-frustrated 'charge crystal' phase in the very lightly doped, insulating limit of CuO2 planes. Here, we search for signatures of CDW order in very lightly hole-doped cuprates from the 123 family RBa2Cu3O7 - δ (RBCO; R: Y or rare earth), by using resonant X-ray scattering, electron transport, and muon spin rotation measurements to resolve the electronic and magnetic ground states fully. Specifically, Pr is used to substitute Y at the R-site to systematically suppress the superconductivity and access the extremely low hole-doping regime of the cuprate phase diagram without changing the oxygen stoichiometry. X-ray scattering data taken on Pr-doped YBCO thin films reveal an in-plane CDW order that follows the same linear evolution of wave vector versus hole concentration as oxygen-underdoped YBCO but extends all the way to the insulating and magnetically ordered Mott limit. Combined with the recent observation of charge crystal phase on an insulating surface of Bi2Sr2CaCu2O8 + z, our results in RBCO suggest that this electronic symmetry breaking is universally present in very lightly doped CuO2 planes. These findings bridge the gap between the Mott insulating state and the underdoped metallic state and underscore the prominent role that Coulomb-frustrated electronic phase separation plays among all cuprates.

Unveiling phase diagram of the lightly doped high-Tc cuprate superconductors with disorder removed

The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO2 planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba2Ca5Cu6O12(F,O)2 with inner CuO2 layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates.

Tc Saturation and Possible Electronic Phase Separation in Strongly Overdoped Cuprates

In order to elucidate the unusual superconducting properties of cuprates in the strongly overdoped region, i.e., at hole-doping levels p≳0.4/Cu in the CuO2 plane, we study the structural and superconducting properties of a series of Cu0.75Mo0.25Sr2YCu2O7+x powder samples oxygenated under high pressure using different concentrations of KClO3 up to 35 mol %. The analysis of X-ray diffraction data indicates a high purity ∼90% of all samples and suggests that the concentration, x, of extra oxygen atoms increases with increasing KClO3 concentration. Surprisingly, the Tc values remain nearly constant within the 80–85 K range independent of KClO3 concentration, which suggests a scenario of Tc saturation. In order to account for this unexpected behaviour, we put forward the hypothesis that overdoping enhances the density of unpaired holes, which is supported by the observation of large values of the Sommerfeld coefficient in all samples. We therefore propose a scenario of electronic phase separation between normal and superconducting holes.

Electronic behavior of Ag-doped YBa2Cu3O7-δ using Hubbard-U correction method

The ability to accurately describe the electronic properties in the highly correlated materials YBa2-xAgxCu3O7-δ was queried. Previous calculations revealed that the electronic bandgap was estimated much lower than the established experimental value when standard density functional theory, DFT was used in the computation of pure YBa2Cu3O7-δ. The electronic properties of Ag-doped at Ba-site samples such as the bandgap, total and partial density of states, electron density, and Mulliken population, were computationally calculated using DFT with the Hubbard-U correction method, DFT+U and compared to the pure samples. The calculation was conducted using CASTEP code in Material Studio software applying both LDA and range of GGA exchange-correlation functionals. 2 × 2 × 1 supercell symmetry was applied to enlarge the single cell crystal structure to able the dopant concentration, x = 0.250 at the Ba-site. The calculated bandgap of pure YBa2Cu3O7-δ (Eg = 3.67 eV) shows 8.25% difference to the referred theoretical value using the GGA-PBE+U functional. Upon Ag-dopant, the bandgap was calculated to be Eg = 3.85 eV. The energy of the Ba 6p orbital on the valence band has shifted from − 10.49 eV to − 10.67 eV, away from the Fermi energy level, and the density of state has increased significantly by + 57.52%. The Ag dopant enhanced the electron density of Cu and O atoms along the CuO chain and CuO2 plane. The study enables the prediction of impurity dopant concentration prior to experimental work, hence promoting green research.

The in- and out-of-plane magnetisation of highly underdoped YBa2Cu3O6+x single crystals

Previously we have shown how measurements of static magnetic susceptibility χc(T) of YBa2Cu3O6+x single crystals for magnetic fields applied along the c-axis, and χab(T) for fields in the ab-plane, can give useful information about their thermodynamic properties which are still being hotly debated. SQUID magnetometry above the superconducting (s/c) transition temperature Tc is used for larger crystals, while piezolever torque magnetometry gives χc(T)-χab(T) for tiny crystals. Here we present new data for more heavily under-doped crystals with hole concentrations per CuO2 plane, p =0.058 to 0.073. We again find that the T-dependent anisotropy well above Tc arises from the pseudogap and the g-factor anisotropy, while at lower T there are Gaussian s/c fluctuations with a strong cut-off. This is possibly a different region where neutron scattering studies give evidence for competition between incommensurate magnetic short-range order and superconductivity. We have studied crystals with three values of x, measuring χc(T) and χab(T) immediately after fixing x by quenching on to a copper block and again after allowing sufficient time at room temperature for the Cu-O chains to order. Thus, we report data for three pairs of underdoped (UD) crystals with Tc ranging from 13 to 36 K, namely UD13 and UD20, UD15 and UD30, UD23 and UD36. As found previously for polycrystalline samples, ordering the Cu-O chains increases Tc and substantially reduces the Curie term. These are isotropic, in contrast to the Pauli-like susceptibility from the CuO2 planes where there is g factor anisotropy, and this allows us to make a powerful novel analysis. At higher T, χc(T)- χab(T) varies as a + bT where the bT term arises from the pseudogap. At lower T, deviations from this behaviour are analysed in terms of 2D or 3D Gaussian s/c fluctuations to obtain in-plane coherence lengths and upper critical fields as T→0.

A System of Josephson Vortices in a Layered Electron-Doped Nd2 – xCexCuO4 Superconductor

A brief review of the transport and galvanomagnetic properties of an electron-doped Nd2 – xCexCuO4 layered superconductor with an emphasis on the role of the structure of Josephson vortices formed in the crystalin a magnetic field oriented in the parallel direction to the CuO2 planes is given. The experimental data on the longitudinal and Hall magnetoresistance of the recently synthesized Nd2–xCexCuO4/SrTiO3 epitaxial films are analyzed. The possibility of formation of a system of Josephson vortices in this highly anisotropic system is determined by the unique orientation of the c axis and the conducting CuO2 planes of the Nd2 – xCexCuO4 compound with respect to the substrate plane.