Band structure of YBa2Cu3Ox in relation with the oxygen vacancy distribution (original) (raw)
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CNDO studies of Y-Ba-Cu-O superconductors II. Simple models of YBa2Cu3Ox
Czechoslovak Journal of Physics, 1992
The electronic structure of YBa2Cu3Oz (x = 6 and 7) is investigated using the CNDO molecular orbital method. Electronic structures of model clusters [Cu3010] -15, [BasCu3010] +1, [Y8Cu3010] +9 of the non-superconducting (x ----6) and [Cu3012]-17,[Ba8Cu3012] -1, [Y8Cu3012] +7 of the superconducting (x = 7) phases are compared. Y and Ba layers cause a considerable electron density transfer from the central Cu(1) region.
Structural behavior and superconductivity of YBa2Cu3Ox
Solid State Communications, 1987
The compound YBa&+Ox was given various oxidation treatments over a range of oxygen pressures from 0.02 to 200 MPa and temperatures up to 1223K. A tetragonal to orthorhombic phase transformation was observed, leading to superconducting behavior below 94K. The orthorhombic unit cell parameters (i.e. volume, bond lengths, and axial distortions) were found to correlate with the superconducting transition temperatures. These correlations are rationalized in terms of the interrelationship of oxygen stoichiometry with the lattice parameters of the orthorhombic phase. The nature of superconductivity in YBa$u,O, and LazCuO, is related to the Cu-0 coordination polyhedra and an explanation is proposed for the observed multiple superconducting transitions in YBa$u,O,.
Physical Review B, 1990
A detailed analysis of the effect that oxygen disorder has on the electronic structure ol the highly correlated high-T, superconductors is presented. The system is modeled by a Hubbard-type Hamiltonian in which the on-site and intersite correlations between oxygen and copper atoms are included. The Green-function formalism is used to calculate the local electronic density of states at the Cu and 0 sites characteristic of YBa2Cu306+, as a function of oxygen concentration x and of the cation charge Q. Due to the large electronic interactions, a Mott-Anderson metal-nonmetal transition is obtained, in accordance with the experimental findings. The hole concentration n is calculated for disordered and ordered systems. A plateau in the concentration dependence of n is obtained for 0.45&x~0.75, only in the ordered system. The correlation functions of the various charge states in Cu and 0 are also calculated and discussed.
Effect of Zn and Ni substitution on the local electronic structure of the YBa2Cu3O7 superconductor
Physical Review B, 2002
Embedded cluster calculations at the MP2 electron correlation level of pure and Zn-and Ni-doped YBa 2 Cu 3 O 7 (Y123) ceramics reveal great changes of the local charge distribution and crystal bonding in the vicinity of impurities. In the Zn-doped ceramics the obtained changes are stronger and more extended than in the Ni-doped ceramics. This can be one of explanations of the stronger depression of T c due to Zn. The calculated increase of positive charge for both impurities, comparing with substituted Cu, indicates that the impurity atoms lose more negative charge than Cu. As a result, all nearest CuO 2 units around impurity lose their holes: completely in the Zn case and partly in the Ni case. This leads to the breaking of the hole pairs in the vicinity of impurity and correlated with an increase of the coherence length in Zn-and Ni-doped Y123 crystals measured by Timomoto et al. ͓Phys. Rev. B 60, 114 ͑1999͔͒. Both Zn and Ni impurities, the latter to a less extent, act as effective pair breakers due to the elimination of holes on adjacent CuO 2 groups.
Solid State Communications, 1987
Possible mechanisms for the oxygen intercalation in the ternary mixed-Cu-valence YBa.Cu.O.. oxide are considered, which account for the observed cell symmetry transi~orn{atl~+~tfiven by the oxygen concentration. These mechanisms are consistent with the formation of both single-and double-cell superstructures of ordered oxygen vacancies that have been encountered in crystallographic investigations of the high-To superconductor YBa2Cu30~7. Several now well-established factors appear to be important to obtain and control the superconducting properties of YBa~CuaO~7 materials usually prepared by high-temperature r~aetions of stoichiometric proportions of BaCO 3, CuO or Cu~20, and Y203, followed by appropriate controlled-atmosph~'e and heat treatments. All intermediate compositions can be described by the general formula YBa-.Cu~O.. in which x varies continuously from about
CNDO studies of Y-Ba-Cu-O superconductors IV. The oxygen content effect
Czechoslovak Journal of Physics, 1994
The dependence of the electronic structure of superconducting YBa2Cu3Oy is investigated using a quasi-relativistic CNDO/1 molecular orbital method. Electronic structures of model clusters [BasCu30,~] q with total charges q = 23 -2n for n = 11 to 14 in various Cu(1) coordination geometries are compared. The increasing oxygen content causes the electron density transfer from the central Cu(1)O(1,5)~ area to the lateral Cu(2)O(3, 4)2 sheets. The isomerization effects are important for the structures with n --12.
Energy loss study of the electronic structure of YBa2Cu3O7−δ high Tc superconductor
Solid State Communications, 1988
An electron energg loss investigation up to 40 eV of YBa2Cu307_ s (5=0.2) is presented . All structures are identified and the E 2 spectrum in the region 0-10 eV is accounted for bg a simple convolution of occupied and emptg densities of states. A sizeable temperature dependence of E 2, below and above T c, involving Cu 3d-O 2p valence states is observed.
Zeitschrift f�r Physik B Condensed Matter, 1987
The x-ray absorption spectra of the single phase high temperature superconducting YBa2Cu30_ 7 with T~ ..... t=97 K have been measured between 30 K and 300 K by using synchrotron radiation. The copper K-edge x-ray absorption near edge structure (XANES) spectrum gives experimental evidence of the lack of delocalized Cup states close to the Fermi level, in fact a spectral distribution of local (at the Cu site) partial (/= 1) density of states as in the insulating CuO has been found. The EXAFS (extended x-ray absorption fine structure) spectrum shows evidence of the formation of layers formed by Cu ions coordinated by oxygen square planes sharing corners. The average Cu-O distance at 30 K in the layers has been found to be 1.93 i.e. 0.02~ shorter than in CuO and the average distance between Cu ions at the centers of the square planes is 3.85~. The present local structure suggests that superconductivity should occur in the crystalline planes formed by CuO4 square plane units. The lack of delocalized Cu states is in agreement with the hypothesis that conductivity is due to itinerant holes L in the oxygen derived valence band.