Resonance-valence-bond superconductivity in CuO2 planes

We find the exact ground state of a system of 4 × 4 unit cells described by an effective model for cuprate superconductors. The model has the form of the t - J one, but includes a three-site term t like that derived from the Hubbard model in the large U limit, but of opposite sign for realistic or large O-O hopping. For t > t_c (where t_c/t 0.1 for realistic J and doping) the system undergoes a transition to a phase in which the ground state has a significant overlap with a very simple superconducting resonance-valence-bond (RVB) wave function. In the RVB phase there is a large increase of the d-wave superconducting susceptibility. We calculate the boundary of the RVB phase.     

Electronic properties of CuO2 planes

We study a 3-band model of CuO₂ with bare bones interactions: hopping from copper ions to nearest-neighbor oxygenst _pd only, a two-body interaction on the copper ionsU _dd only, and an overlap copper-oxygen parameter _pd. In the limitt _pd U _dd ^1/2, t^*=t _pd ²/2U _dd is the unit of energy with 8_pd ² as the only parameter of significance. If the two-body interaction is invariant under particle-hole interchange, the low-lying states (energy O(t^*)) can be described by conserved particles and can all be classified. They are quite distinct from the high-lying states (energies O(U _dd)). The dynamics of the conserved fermion-like elementary particles are well described by a modifiedt-J model with extended hopping and nearest-neighbor superexchange attraction. This is a scenario known to be favorable to high-temperature superconductivity, but it must be noted that both the hopping range and the exchange are functions of 8_pd ². Moreover, if the Hamiltonian isnot invariant under particle-hole interchange the dynamics becomes much more complex and possibly more inimical to high-temperature superconductivity. This may provide an explanation for the deleterious effect on superconductivity of very small concentrations of certain impurities.     

Electronic structure of Nd2CuO4 and its physical properties

In order to clarify the electronic structure of the electron-doped superconducting material Nd_2?xM_xCuO₄, we have performed a first-principle band structure calculation for the matrix material Nd₂CuO₄. We find that doped electrons can be accommodated in both the Cu 4s and 4p _z conduction bands provided that the Cu 3dx²?y ² band splits into two bands, i.e., the upper and lower Hubbard bands by the strong correlation. Based on this electronic structure, we have calculated the Hall coefficient of the Nd system and have shown for the first time that the sign of the Hall coefficient is negative, coinciding with the experimental result in the low-concentration case. In the overdoped region above x=0.18, the dopant electrons occupy not only 4s and 4p _z bands but also the upper Hubbarddx ²?y ² so that the Hall coefficient changes its sign from negative to positive.     

Electronic Raman scattering in CuO2 superconductors

Experimental results for the electronic Raman effect in differently doped cuprate superconductors will be presented. We show that the B_2g-symmetry data are generally closely related to ordinary transport and are therefore most likely originating from the carriers, while the response at B_1g symmetry cannot be assigned to a specific type of excitations. In the superconducting state the B_2g pair breaking peaks scale with the transition temperature over a wide doping range. All results consistently suggest a strong anisotropy of the gap and can be modeled by assuming symmetry for the order parameter.     

Comparison of the Electronic Structures of La2CuO4, Sr2CuO2Cl2, and Sr2CuO2F2

First-principles cluster calculations are reported of the local electronic structure of the three compounds: La₂CuO₄, Sr₂CuO₂Cl₂, and Sr₂CuO₂F₂. The copper ${\text{3d}}_{x^2 - y^2 } $ and the planar oxygen 2p _σ atomic orbitals exhibit a similar degree of covalency. The out-of-plane orbitals, however, are quite different with the ${\text{3}}d_{3z^2 - r^2 } $ atomic orbital lowered significantly in energy for chlorine and fluorine apical positions.     

The model ofd-pairing in CuO2 planes of HTSC and the collective modes

We develop here for the first time a 2D path integral model of d-pairing in HTSC, which is analogous to the 3D model created earlier by Brusov and Brusova. Within this model we calculate the entire collective mode spectrum for all superconductive states of CuO₂ planes arisen from their symmetry classification. There are four collective modes in each phase. We found two high frequency modes in each phase while two remains modes seem to be Goldstone or quasi-Goldstone ones (having vanishing energy at zero momentum). It turns out that the collective mode spectrum in the phases and is the same.     

Electronic properties of electron-doped Nd2−x Ce x CuO4−δ, Sr1−x Nd x CuO2−δ, and Hole-Doped Nd1.4Ce0.2Sr0.4CuO4−δ studied by photoelectron spectroscopy

We present investigations on the electronic structure of the recently discovered, electron-doped infinite-layer compound Sr_1?xNd_xCuO_2?δ (T _c≈40K) by means of photoelectron spectroscopy with synchrotron radiation. This system is compared with the two closely related cuprates Nd_2?xCe_xCuO_4?δ (electron doped) and Nd_1.4Ce_0.2Sr_0.4CuO_4?δ (hole doped). Valence band spectra in the Cu 3p→3d resonance are interpreted in terms of CuO₄ (n-doped compounds) and CuO₅ (p-doped system) cluster models. From our calculations we find a considerably higher Cu3d Coulomb correlation for the electron-doped compounds as well as a very low Cu-O hybridization for the infinite-layer system. Partial Nd spectra of Sr_0.85Nd_0.15CuO_2?δ show the Nd dopants to be trivalent, thus confirming the infinite-layer compound to be electron doped. Analysis of partial Nd valence bands and magnetic susceptibilities of Nd_2?xCe_xCuO_4?δ and Nd_1.4Ce_0.2Sr_0.4CuO_4?δ gives very similar electronic parameters and antiferromagnetic correlations between Nd spins for both cuprates.     

Electronic structure and BOW-CDW states of CuO3 chains

The interplay of structure and bonding is considered for hole-doped anionic cuprate chains and layers. Based on an analysis of available experimental data it is conjectured that the local doping level tends to be close to Σ =0 or Σ = 0.5 or Σ = 1, where Σ is the hole number relative to the Cu3d state, and that Σ ≈ 0.5 is relevant for superconductivity. Estimates for the strongly anisotropic exchange integrals from LDA band structures for Sr₂CuO₃ and Ca₂CuO₃ are reported. Exact diagonalization studies for undistorted and distorted chain clusters indicate a weakly correlated state in CuO₃ chains doped with 0.5 holes.     

Electrical properties of SnO2·CuO·Ta2O5 varistor system

The CuO and Ta₂O₅-doped SnO₂ system was obtained by conventional ceramics processing. The ceramic phases of samples was analyzed by X-ray diffraction (XRD) and the microstructure by means of scanning electron microscopy (SEM). The electrical field of a single phase of CuO and Ta₂O₅-doped SnO₂ system was studied as a function of current density behavior and compared with the CoO and Ta₂O₅-doped SnO₂ system. A high nonlinear coefficient α=37.9 was obtained. To illustrate the grain–boundary barrier formation of CuO and Ta₂O₅-doped SnO₂-based varistors, a modified defect barrier model is introduced.     

Synthesis and electrical properties of Sr4Mn2CuO9 ceramics

The crystal structure of the compound Sr₄Mn₂CuO₉ is closely related to the hexagonal perovskite structure. In this work, Sr_3.95Mn₂CuO₉ was synthesized by solid state methods and the electrical properties of the polycrystalline ceramics were measured. The conductivity was observed to increase four orders of magnitude over the temperature range 100 to 700 K, with a room temperature conductivity of 5×10⁻⁵ S/cm. The conductivity closely followed an Arrhenius equation with an activation energy of approximately 0.14 eV.     

Preparation,characterization and catalytic properties of CuO/CeO2 system

CeO₂ nano-particles and CuO/CeO₂ system were prepared by sol-gel and impregnation methods and characterized using combined spectroscopic techniques of XRD, XPS, TPR, FT–Raman, BET and HRTEM. It was found the CeO₂ was cubic phase with fluorite structure and CuO was highly dispersed on the CeO₂ particles. Temperature-programmed reduction (TPR) showed a two-step reduction for CuO/CeO₂ catalysts. XPS analysis indicated the presence of redox couple Ce⁴⁺/Ce³⁺ and reduced copper species in the CuO/CeO₂ catalysts. The factors, such as calcination temperature, calcination time and CuO loading, influenced on the catalytic properties of CuO/CeO₂ catalysts.     

Optical and magnetic properties of CuO/CuFe2O4 nanocomposites

CuO/c-CuFe₂O₄ nanocomposites have been synthesized via the oxalate precursor route. Effect of synthesis conditions on the crystal structure, microstructure, magnetic and optical properties of the formed powders was studied. The results indicated that pure CuO nanoparticles were obtained from the oxalate precursor annealed at 600 °C for 2 h. However, substitution of Cu²⁺ ion by Fe³⁺ ion (Cu_1?X Fe _X O, where X = 0, 0.05, 0.1 and 0.2) led to form of CuO/CuFe₂O₄ nanocomposites. The microstructures of the powders appeared as a monoclinic like shape. Furthermore, the band gap energy of the obtained CuO nanopowders was 1.41 eV and the value was slightly decreased by Fe³⁺ ion substitution. In addition, the formed CuO particles had weak ferromagnetic characteristics. However, the substitution Cu²⁺ ion by Fe³⁺ ion enhanced the magnetic properties of the formed composite as the result of increasing the CuFe₂O₄ phase formation. Hence, the saturation magnetization was increased from 0.13 to 9.8 emu/g by increasing the Fe³⁺ ion from 0 to 0.2.     

Influence of Lattice Parameter Scaling on Local Electronic and Magnetic Properties in La2CuO4

A large amount of static and dynamic NMR experiments have been carried out to determine the spin properties in high temperature superconductors cuprates (HTSC). In the majority of cuprates the magnetic shift at the planar copper nucleus is independent of the temperature when the field is applied perpendicular to the CuO₂ planes and does not exhibit the expected reduction of the spin susceptibility in the superconducting state. This has been explained by a coincidental cancellation of on-site and transferred hyperfine fields at the copper nucleus, which would lead to a vanishing spin shift. To examine this explanation we determine the hyperfine fields by ab initio cluster calculations for La₂CuO₄ within the framework of spin-polarized density functional theory. In particular we investigate the sensitivity of the hyperfine fields on the lattice constants.     

Room temperature photoluminescence properties of CuO nanowire arrays

Cupric oxide (CuO) nanowire arrays were synthesized by a simple and cost-effective thermal oxidation method. Optical properties of CuO nanowires (NWs) have been investigated through photoluminescence (PL) measurements at room temperature. CuO NWs appear several defects and vacancies related emission bands which usually appear only at low temperatures. Blue–green light emission with peaks at around 403, 474 and 489 nm is obtained from CuO NWs. In addition, the PL studies of CuO at room temperature reveal three emission peaks at around 713, 735 and 758 nm in the red and near infrared region. The study is attributed to understand the optical properties of CuO NWs and the design of CuO-based photo-electronic devices.     

Effect of CuO addition on the microstructure and electrical properties of SnO2-based varistor

The effect of CuO addition on the properties of (Co, Nb, Cr)-doped SnO₂ varistors were investigated. The samples with different CuO concentrations were fabricated by the conventional ceramic method and sintered at 1,200, 1,250, 1,300 and 1,350 °C for 2 h. It is found that the nonlinear coefficient presents a peak value at 0.2 mol % CuO addition. The leakage current density decreases with increasing CuO from 0 to 0.2 mol %, and then increases when the concentration of CuO is above 0.2 mol %. The breakdown electrical field decreases from 356 to 248 V/mm with increasing CuO from 0 to 0.6 mol %. The optimal samples obtained by doping CuO with 0.2 mol % and sintered at 1,300 °C have the highest nonlinear coefficient value of 31 and the lowest leakage current density of 2 μA/cm².     

Electronic properties of WS2 monolayer films

Monolayer films of layered metal dichalcogenides WS₂ were grown on single crystalline graphite substrates by metal-organic van der Waals-epitaxy. An angle-resolved valence band structure of the quasi free-standing films has been determined by photoelectron spectroscopy.     

Electronic specific heat of Tl2Ba2CuO6+δ from 2 K to 300 K for 0≤δ≤0.1

Using a high-precision differential technique with a resolution of 1∶10⁴, we have measured the heat capacity of Tl₂Ba₂CuO_6+δ over a temperature range 2–300 K for 0≤δ≤0.1. Anomalies atT _c are seen for all superconducting compositions measured, and the results are consistent with a temperature- andδ-independent normal-state electronic termγ _n～0.6 mJ/g-at. K². The samples with the largerT _c 's exhibit strong fluctuations in their specific heat, typical of a highly anisotropic 2D superconductor, but there is some evidence that these fluctuations become weaker atT _c falls-consistent with an increase in the coherence length on hole doping. At temperatures below 5 K an upturn in the data is observed which appears to increase in magnitude asT _c falls, correlating with the increase in the Curie term of published susceptibility data.     

Nonlinear optical properties of laser deposited CuO thin films

In this work we investigate the third-order optical nonlinearities in CuO films by Z-scan method using a femtosecond laser (800 nm, 50 fs, 200 Hz). Single-phase CuO thin films have been obtained using pulsed laser deposition technique. The structure properties, surface image, optical transmittance and reflectance of the films were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and UV-vis spectroscopy. The Z-scan results show that laser-deposited CuO films exhibit large nonlinear refractive coefficient, n₂ = − 3.96 × 10^− 17 m²/W, and nonlinear absorption coefficient, β = − 1.69 × 10^− 10 m/W, respectively.