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1.
《Solid State Communications》1987,63(6):451-456
Ab-initio electronic band structure calculations are presented for the perovskite La2CuO4 and for this material doped with Sr for a supercell of composition La3SrCu2O8. This material is close to the high Tc superconductor La2−xSrxCuO4 discovered recently. The Sr doping gives rise to strong valence fluctuations. We discuss the effect of the valence fluctuations on the stability of the lattice, indicating a small value of U and enhancing the electron-phonon coupling λ, mainly by a mechanism of incipient peroxide formation. 相似文献
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The electron paramagnetic resonance (EPR) g factors and local structures for Ni3+ in LiAlyCo1–yO2 as the cathode materials for lithium-ion batteries are theoretically investigated at various Al concentrations y (= 0.1, 0.5, 0.8 and 1.0). The [NiO6]9– clusters are subject to the tetragonal elongation distortions along the C4 axis owing to the Jahn–Teller effect and the different degrees of distortions are characterised by the slightly increasing relative elongation ratio ρ (and also crystal-field strength and covalency) with y. In pure LiAlO2 (y = 1.0), the [NiO6]9– cluster exhibits the largest ρ (≈1.4%) and an additional relative variation ratio τ (≈1.4%) of the planar bond lengths, responsible for the perpendicular g anisotropy δg = (gx – gy). The number of impurity Ni3+ centres increases from one at y = 0 and 1.0 to two at y = 0.1 and 0.8 and reaches the maximum four at y = 0.5, in accordance with the increasing degree of disorder of the systems. The distinct impurity centres for the same y may be accompanied by the comparable energies of the ground and near excited states, similar to ‘resonant states’ with slightly different local elongation distortions and tetragonal level splittings. The present studies will be helpful to understand the relationships between local structures and performances of LiAlyCo1–yO2 type cathode materials. 相似文献
5.
《Journal of Physics and Chemistry of Solids》2004,65(8-9):1381-1390
6.
《Solid State Communications》1987,64(10):1267-1271
The phonon dispersion curves, the one-phonon density of states and the specific heat of the high-Tc superconductor La2−xMxCuO4 have been calculated in the framework of a shell model. The zone center frequencies are classified according to their vibrational character and compared with the available IR and Raman data. A slight adjustment of the parameters obtained from the known interaction potentials of oxides with perovskite and rock salt structure yields reasonable agreement with the IR, Raman, specific heat, and inelastic neutron scattering data of the polycrystalline material. 相似文献
7.
Ashok Kumar Rastogi Alok Banerjee Manju Tewari Dhirendra Bahadur 《Phase Transitions》2013,86(1-2):123-130
Abstract Besides investigating a number of oxides of the series La2?xMxCuO4 (M=Ba, Pb, Sr), we have, in this investigation emphasised on the studies of the effect of annealing conditions on superconducting properties of composition La1.8Sr0.2CuO4. The effect is remarkable and is di'scuss'ed in the light of X-ray diffraction results. These samples, studied after a gap of two months, showed interesting changes which has been discussed in the present paper. 相似文献
8.
Abstract Abstract Infrared spectra of La2?xSrxCuO4 and YBa2Cu3O7?δ are discussed, specially with respect to the disappearance of the high-frequency (~650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K eventhough the materials are fairly conducting. YBa2Cu3O7?δ does not show evidence in the far IR spectrum for the presence of an optical gap. 相似文献
9.
Tadashi Kobayashi Hiratoshi Katsuda Yukio Sakabe Kiyoshi Iwai Seiji Kanba 《Phase Transitions》2013,86(3):215-223
EPR spectra of superconducting YBa2Cu3O7-δ and insulating Y2BaCuO5 have been examined. The intensity of Cu2+ ions in YBa2Cu3O7-δ is about 1/3000 of that in Y2BaCuO5 at room temperature. Owing to the Meissner effect in the superconducting state, the EPR signal of Cu2+ ions in YBa2Cu3O7-δ disappears; moreover the EPR signal of DPPH deposited on the YBa2Cu3O7-δ specimen is shifted. An additional EPR line is found for YBa2(Cu2.94Cr0.06)O7-δ at temperatures below 113K; this can be related to the phenomenon of superconductivity. 相似文献
10.
C. Michel J. Provost F. Deslandes B. Raveau J. Beille R. Cabanel P. Lejay A. Sulpice J. L. Tholence R. Tournier B. Chevallier G. Demazeau J. Etourneau 《Zeitschrift für Physik B Condensed Matter》1987,68(4):417-419
Novel high-T
c
superconductors La2–x
Bi
x
Sr
x–x
CuO4–y
have been isolated and studied. The influence of the experimental conditions of synthesis on the superconductivity has been especially studied for the composition La1.7Bi0.1Sr0.2CuO4–y
; it is shown thatT
c
decreases as oxygen is introduced into the matrix, likely due to the partial oxidation of Bi(III) to Bi(V). The highT
c
value observed for one sample (T
c
42 K), as compared withT
c
38 K of La1.8Sr0.2CuO4–y
suggests that bismuth is also a potential candidate for improving the superconducting properties in these oxides. 相似文献
11.
We propose a new magnetic phase diagram of La2?xSrxCuO4 around a quantum critical point x=1/9 based on field-cooled magnetization measurements and critical fittings. A new phase boundary Tm2(H) is discovered which buries deeply below the first order vortex melting line in the vortex solid phase. The coupling between superconductivity and antiferromagnetism is found to be attractive below Tm2(H) while repulsive above. The attractive coupling between superconducting order and static antiferromagnetic order provides compelling experimental evidence that the antiferromagnetism microscopically coexists and collaborates with the high temperature superconductivity in cuprates. 相似文献
12.
《Solid State Communications》1987,63(7):653-656
We synthesized (La1−xSrx)2CuO4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature Tc ≲ 38 K and narrow transition width ΔTc. For the sample with the maximum Tc among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔVs(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏωop/kB ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high Tc of this material. 相似文献
13.
Calculations of the fine structure of Cr3+ energy levels in Cs2NaYF6 accompanied by estimations of the Jahn–Teller (JT) stabilization energy in the first excited 4T2g state of Cr3+ ion are presented. Two independent approaches—effective second-order spin–orbit Hamiltonian and analysis of the potential energy surfaces—are used. The JT energy was estimated to be 216 and 257 cm–1 in the first and the second models, respectively. It is shown that the octahedral [CrF6]3? complex undergoes an equatorial expansion by 0.09 Å and an axial elongation by 0.02 Å due to the combined effect of the a1g and eg normal modes. 相似文献
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E. Z. Kuchinskii I. A. Nekrasov N. S. Pavlov 《Journal of Experimental and Theoretical Physics》2013,117(2):327-337
We propose a generalization of the LDA + DMFT + Σ k approach to the multiband case, in which correlated and uncorrelated states are present in the model simultaneously. Using the multiband version of the LDA + DMFT + Σ k approach, we calculate the density of states and spectral functions for the Emery model in a wide energy interval around the Fermi level. We also obtain the Fermi surfaces for the electron-doped high-temperature superconductor Nd2 ? x Ce x CuO4 in the pseudogap phase. The self-energy part Σ k introduced additionally to take into account pseudogap fluctuations describes the nonlocal interaction of correlated electrons with collective Heisenberg short-range spin fluctuations. To solve the effective impurity model, the numerical renorm-group (NRG) method is used for the DMFT equations. Good qualitative agreement of the Fermi surfaces calculated using the LDA + DMFT + Σ k approach and experimental angle-resolved photoemission spectroscopic data is attained. The stability of the dielectric solution with charge transfer in the Emery model with correction for double counting is analyzed in the Appendix. 相似文献
16.
《Journal of Molecular Liquids》1988,38(1):35-53
A method for the theoretical study of chemical reaction mechanisms in aqueous solutions has been developed. Approximate reaction pathways are determined with the aid of a model Hamiltonian in which medium is described by a set of Langevin point dipoles. Solvation of stationary points on the potential energy surface so obtained is performed by a more exact calculation of the medium contribution using the method of minimization of the solvation shell potential energy. Usefulness of our approach is exemplified by the calculation of the reaction CO2 + OH− ⇄ HCO−3. Method MIND0/13 has been employed in constructing the model Hamiltonian. A good agreement with experiment has been achieved. 相似文献
17.
T. B. Charikova A. I. Ponomarev G. I. Kharus N. G. Shelushinina A. O. Tashlykov A. V. Tkach A. A. Ivanov 《Journal of Experimental and Theoretical Physics》2007,105(3):626-635
The temperature dependences of resistivities ρab in the ab plane and ρc along the c axis have been studied for single-crystal Nd2 ? x CexCuO4 + δ (x = 0.12, 0.15, 0.17, 0.20) films with (001) and (1 $\bar 1$ 0) orientations. The superconducting transition temperature and anisotropy coefficient are shown to be maximal in optimally annealed samples (the oxygen content is close to the stoichiometric content, δ → 0). A combination of the metallic behavior of the ρab(T) dependence and the nonmetallic behavior of the ρc(T) dependence for the optimally annealed samples is an intrinsic property of the substance and an indication of the fact that the system is quasi-two-dimensional. This layered quasi-two-dimensional system is an Anderson dielectric with a strongly anisotropic localization radius (R loc ab ? R loc c ). An increase in the oxygen content and, hence, in the degree of disorder in the Nd2 ? x CexCuO4 + δ system is found to decrease the resistivity anisotropy coefficient. Thus, a disorder-induced Anderson transition takes place in this quasi-two-dimensional system. 相似文献
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In order to obtain insight into the role played by the phonons in the superconducting phase transition, we have performed infrared reflectivity and inelastic neutron scattering experiments on single phase YBa2Cu3O7–(0.1,T
c
=94 K, T
10–90=1 K). These experimental results, together with published Raman scattering and specific heat data are used to determine the parameters of a rigid-ion model; the model is able to reproduce qualitatively the experimental data. On the basis of our model we derive and discuss the phonon density of states and the pattern of atomic displacements of some important vibrational modes. In particular, the model reveals the existence of high-frequency oxygen breathing modes and of low-frequency copper dimerization modes in both the CuO3-chains and the CuO2-planes. Furthermore, Jahn-Teller like oxygen modes exist in the chains. The possible role of these modes with respect to the pairing mechanism is discussed. In addition there exist low-frequency chain-degrading modes which favour jumps of oxygen atoms out of the CuO3-chains into neighbouring vacant sites in the basal plane. 相似文献
20.
K. Kriŝtiaková O. Ŝauŝa J. Kriŝtiak Ŝ. Jánoŝ 《Zeitschrift für Physik B Condensed Matter》1989,75(2):197-199
We have measured the ac susceptibility of a wire with a Nb core (1.27 mm diam.) and a Cu cladding (0.37 mm thickness) atT50 K andB0.1 mG. Due to its proximity to Nb, the Cu becomes fully superconducting. From the data we find a breakdown fieldH
b
=1.2 (mG) and a coherence length =2.2T
–1/2 (m) for the Cu, as well as a field penetration depth -34T
1/2 (m) at the Cu/Nb interface. 相似文献