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1.
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 > tc (where tc/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. 相似文献
2.
Daniel C. Mattis 《Journal of Superconductivity》1995,8(4):479-481
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 8pd
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 8pd
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. 相似文献
3.
Shunichi Matsuno Hiroshi Kamimura 《Journal of Superconductivity and Novel Magnetism》1994,7(3):517-519
In order to clarify the electronic structure of the electron-doped superconducting material Nd2?xMxCuO4, 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. 相似文献
4.
R. Hackl M. Opel P. F. Müller G. Krug B. Stadlober R. Nemetschek H. Berger L. Forró 《Journal of Low Temperature Physics》1996,105(3-4):733-742
Experimental results for the electronic Raman effect in differently doped cuprate superconductors will be presented. We show that the B2g-symmetry data are generally closely related to ordinary transport and are therefore most likely originating from the carriers, while the response at B1g symmetry cannot be assigned to a specific type of excitations. In the superconducting state the B2g 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. 相似文献
5.
C. Bersier E. P. Stoll P. F. Meier T. A. Claxton 《Journal of Superconductivity and Novel Magnetism》2002,15(5):399-402
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. 相似文献
6.
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. 相似文献
7.
M. Klauda J. Markl C. Fink P. Lunz G. Saemann-Ischenko F. Rau K. -J. Range 《Journal of Superconductivity and Novel Magnetism》1994,7(2):485-487
We present investigations on the electronic structure of the recently discovered, electron-doped infinite-layer compound Sr1?xNdxCuO2?δ (T c≈40K) by means of photoelectron spectroscopy with synchrotron radiation. This system is compared with the two closely related cuprates Nd2?xCexCuO4?δ (electron doped) and Nd1.4Ce0.2Sr0.4CuO4?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 Sr0.85Nd0.15CuO2?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 Nd2?xCexCuO4?δ and Nd1.4Ce0.2Sr0.4CuO4?δ gives very similar electronic parameters and antiferromagnetic correlations between Nd spins for both cuprates. 相似文献
8.
S. -L. Drechsler J. Malek H. Eschrig H. Rosner R. Hayn 《Journal of Superconductivity and Novel Magnetism》1997,10(4):393-396
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. 相似文献
9.
《Materials Letters》2005,59(2-3):201-204
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. 相似文献
10.
《Materials Letters》2004,58(27-28):3645-3647
The crystal structure of the compound Sr4Mn2CuO9 is closely related to the hexagonal perovskite structure. In this work, Sr3.95Mn2CuO9 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. 相似文献
11.
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. 相似文献
12.
M. M. Rashad D. A. Rayan A. A. Ramadan 《Journal of Materials Science: Materials in Electronics》2013,24(8):2742-2749
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 (Cu1?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. 相似文献
13.
14.
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. 相似文献
15.
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. 相似文献
16.
Guangliang Hu Jianfeng Zhu Haibo Yang Fen Wang 《Journal of Materials Science: Materials in Electronics》2013,24(8):2944-2949
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. 相似文献
17.
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. 相似文献
18.
19.
J. M. Wade J. W. Loram K. A. Mirza J. R. Cooper J. L. Tallon 《Journal of Superconductivity and Novel Magnetism》1994,7(1):261-264
Using a high-precision differential technique with a resolution of 1∶104, we have measured the heat capacity of Tl2Ba2CuO6+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. 相似文献
20.
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. 相似文献