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Doping and strain dependences of the electronic structure of the CuO6-octahedra layer within LDA+GTB method in the frameworks of six-band p-d model are calculated. Band structure and Fermi surface of the quasiparticle excitations in the effective Hubbard model are characterized by inhomogeneous distribution of the k -dependent spectral weight. Doping results in reconstruction of the band structure, redistribution of the spectral weight over dispersion surface, and reconstruction of Fermi surface from small hole pockets in the underdoped region to large hole contour in the overdoped region through two quantum phase transitions (QPT). Strain increasing leads to displacement of the valence and conductivity bands, bandwidths decreasing, and shift of the concentrations corresponding to QPTs. Strain dependences of the antiferromagnetic exchange coupling and DOS at the Fermi level determining superconducting temperature T c are obtained. Effective exchange coupling in the equation for T c demonstrates monotonic strain dependence. [ABSTRACT FROM AUTHOR]

The electronic structure in the two-dimensional t-J* model with canted antiferromagnetic order in an external magnetic field has been calculated within the cluster perturbation theory. In zero external field, the evolution of the Fermi surface with n-type doping has been obtained in good agreement with experimental data on cuprate superconductors. It has been shown that the inclusion of short-range correlations can result in a nonmonotonic dependence of the spectral weight distribution at the Fermi level on the external magnetic field. In contrast to the case of electron doping, such changes in the case of hole doping can be expected at experimentally achievable fields. [ABSTRACT FROM AUTHOR]

The pressure effects on the normal state electronic structure, the superexchange interaction, and the critical temperature of d-type superconductivity mediated by magnetic pairing have been studied within the multielectron hybrid scheme LDA+GTB that takes into account electron correlations in planes. We have found the changes of the multiband p- d model parameters at 3% compression of different symmetry: (i) hydrostatic, (ii) along the c-axis, and (iii) in a- b plane. We have studied the changes of the Fermi surface under external pressure for different hole doping concentration x. In general, this effect is too small except two critical concentrations and where the Lifshitz transitions occur with the change of the Fermi surface topology. In the vicinity of the critical concentration, we have found the giant change of the Fermi surface area up to 100% related to the pressure-induced Lifshitz transition. The effects of pressure on the antiferromagnetic coupling J and the mean-field value of are obtained in a good agreement to experimental data. [ABSTRACT FROM AUTHOR]

We investigate the electronic structure of the two-dimensional t-J model in a transverse external static magnetic field with canted long-range magnetic order using cluster perturbation theory. The distribution of the spectral weight in the whole range of fields from zero to ferromagnetic saturation is explored. We demonstrate the possibility of a sharp change in a distribution of spectral weight at the Fermi level associated with the magnetic correlations when varying magnetic field. [ABSTRACT FROM AUTHOR]

Theoretical investigations of the properties of high- T superconducting cuprates within the LDA + GTB method taking into account the magnetic and phonon pairing mechanisms have been reviewed. These properties are the concentration-dependent electronic structure, quantum phase transitions with a change in the topology of the Fermi surface, and the superconducting phase of the $$d_{x^2 - y^2 }$$ symmetry. [ABSTRACT FROM AUTHOR]

The concentration dependences of the band structure, spectral weight, density of states, and Fermi surface in the paramagnetic state are studied in the Hubbard model within cluster pertubation theory with 2 × 2 clusters. Representation of the Hubbard X operators makes it possible to control conservation of the spectral weight in constructing cluster perturbation theory. The calculated value of the ground-state energy is in good agreement with the results obtained using nonperturbative methods such as the quantum Monte Carlo method, exact diagonalization of a 4 × 4 cluster, and the variational Monte Carlo method. It is shown that in the case of hole doping, the states in the band gap (in-gap states) lie near the top of the lower Hubbard band for large values of U and near the bottom of the upper band for small U. The concentration dependence of the Fermi surface strongly depends on hopping to second ( t′) and third ( t″) neighbors. For parameter values typical of HTSC cuprates, the existence of three concentration regions with different Fermi surfaces is demonstrated. It is shown that broadening of the spectral electron density with an energy resolution typical of contemporary ARPES leads to a pattern of arcs with a length depending on the concentration. Only an order-of-magnitude decrease in the linewidth makes it possible to obtain the true Fermi surface from the spectral density. The kinks associated with strong electron correlations are detected in the dispersion relation below the Fermi level. [ABSTRACT FROM AUTHOR]

The results of recently developed norm-conserving cluster perturbation theory for doping dependent electronic structure of the Hubbard model are reported. We have found that the momentum distribution of the spectral weight strongly depends on the broadening value δ. At δ = 0.1 t, we reproduce the angle-resolved photoemission spectroscopy data, while at δ = 0.01 t we obtain two quantum phase transitions. [ABSTRACT FROM AUTHOR]

We report on the results of measurements of anisotropic resistivity of RBaCuO (R = Tm, Lu) high-temperature superconducting single crystals in a wide range of doping levels, indicating a nontrivial effect of magnetic order on the electronic properties of cuprates. In particular, our results visually demonstrate the crossover from the state with moderate anisotropy of resistivity ρ/ρ ∼ 30 to a strongly anisotropic state with ρ/ρ ∼ 7 × 10 upon cooling as well as upon a decrease in the hole concentration in the CuO planes. It is also shown that anisotropy is sensitive to the magnetic state of CuO planes and attains its maximum value after the establishment of the long-range antiferromagnetic order. The results are discussed in the framework of the theory based on the t- t′- t″- J model of CuO layers taking into account strong electron correlations and short-range magnetic order. In this theory, anomalies of spin correlators and Fermi surface topology for a critical hole concentration of p* ≈ 0.24 are demonstrated. The concentration dependence of the charge carrier energy indicates partial suppression of energy due to the emergence of a pseudogap at p < p*. This theory explains both the experimentally observed sensitivity of anisotropy in conductivity to the establishment of the antiferromagnetic order and the absence of anomalies in the temperature dependence of resistivity ρ( T) in the vicinity of the Néel temperature. [ABSTRACT FROM AUTHOR]

Changes in the electronic structure in the normal phase of high- T c superconductors (HTSCs), viz., layered cuprates, are considered. The results of LDA + GTB calculations of the electron structure and the Fermi surface of La2 − xSr xCuO4 one-layer cuprates with allowance for strong correlations are compared with ARPES and quantum oscillations data. Two critical points x c1 and x c2 are discovered at which the rear-rangement of the Fermi surface takes place. In the vicinity of these points, changes in the thermodynamic properties at low temperatures are determined using the Lifshits ideology concerning 2.5-order quantum phase transitions. A singularity δ( C/ T) ∝ ( x − x e)1/2 in the electron heat capacity agrees well with the available experimental data in the vicinity of x c1 ≈ 0.15. Sign reversal of the Hall constant upon doping is also considered qualitatively. [ABSTRACT FROM AUTHOR]

In this paper a mean-field theory for the spin-liquid paramagnetic non-superconducting phase of the p- and n-type high-Tc cuprates is developed. This theory applied to the effective t-t'-t′′-J* model with the ab initio calculated parameters and with the three-site correlated hoppings. The static spin-spin and kinematic correlation functions beyond Hubbard-I approximation are calculated self-consistently. The evolution of the Fermi surface and band dispersion is obtained for the wide range of doping concentrations x. For p-type systems the three different types of behavior are found and the transitions between these types are accompanied by the changes in the Fermi surface topology. Thus a quantum phase transitions take place at x = 0.15 and at x = 0.23.Due to the different Fermi surface topology we found for n-type cuprates only one quantum critical concentration, x = 0.2. The calculated doping dependence of the nodal Fermi velocity and the effective mass are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The spectral density, dispersion relations, and the position of the Fermi level for n-doped compositions based on NCO and LCO were calculated within the framework of the generalized tight binding method. As distinguished from LCO, the dielectric gap in NCO is nonlinear in character. We observe a virtual level both at the bottom of the conduction band and at the top of the valence band in both compounds. However, its position corresponds to the extreme bottom of the conduction band in LCO and is 0.1–0.2 eV above the bottom in NCO. This explains why we observe Fermi level pinning in n-LCO as the concentration of the doping component grows and reproduce its absence in NCCO at low doping values. We also found both compositions to be unstable in a narrow concentration range with respect to a nonuniform charge density distribution. The relation between the phase diagram for NCCO and the calculated electronic structure is discussed. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

Quasiparticle band structure in hole doped CuO[SUB2] layer is calculated with account for strong electron correlations in the framework of multiband p-d model. For undoped layer we obtain the charge-transfer antiferromagnetic insulator. With doping unusual impurity-like quasiparticle appears at the top of the valence band with spectral weight proportional to doping concentration. In the overdoped regime the band structure in the paramagnetic phase results in the doping dependent Fermi surface in agreement to ARPES data. [ABSTRACT FROM AUTHOR]

We consider the doping dependence of the normal and superconducting properties of LaSrCuO and YBaCuO in the low energy effective model based on the ab initio LDA+GTB calculations. With doping we have found a concentration region with electronic instability of the uniform state. We have shown that two quantum phase transitions (QPT) of the Lifshitz type correspond well to the experimental phase diagram. For superconducting state we have considered both magnetic and phonon mechanisms of pairing. [ABSTRACT FROM AUTHOR]