In the present work we report the band structure calculations for the high temperature superconductor Nd$_{2-x}$Ce$_x$CuO$_4$ in the regime of strong electronic correlations within an LDA+GTB method, which combines the local density approximation (LDA) and the generalized tight-binding method (GTB). The two mechanisms of band structure doping dependence were taken into account. Namely, the one-electron mechanism provided by the doping dependence of the crystal structure, and the many-body mechanism provided by the strong renormalization of the fermionic quasiparticles due to the large on-site Coulomb repulsion. We have shown that in the antiferromagnetic and in the strongly correlated paramagnetic phases of the underdoped cuprates the main contribution to the doping evolution of band structure and Fermi surface comes from the many-body mechanism.
Slightly underdoped high-Tc system La(1.86)Sr(0.14)CuO(4) (LSCO) is studied by means of high energy high resolution angular resolved photoemission spectroscopy (ARPES) and combined computational scheme LDA+DMFT+Sk. Corresponding one band Hubbard model is solved via dynamical mean-field theory (DMFT), while model parameters needed are obtained from first principles within local density approximation (LDA). An "external" k-dependent self-energy Sk describes interaction of correlated electrons with antiferromagnetic (AFM) pseudogap fluctuations. Experimental and theoretical data clearly show "des…
A novel hybrid scheme is proposed. The {\it ab initio} LDA calculation is used to construct the Wannier functions and obtain single electron and Coulomb parameters of the multiband Hubbard-type model. In strong correlation regime the electronic structure within multiband Hubbard model is calculated by the Generalized Tight-Binding (GTB) method, that combines the exact diagonalization of the model Hamiltonian for a small cluster (unit cell) with perturbation treatment of the intercluster hopping and interactions. For undoped La$_2$CuO$_4$ and Nd$_2$CuO$_4$ this scheme results in charge transfer…
Using LDA+GTB (local density approximation+generalized tight-binding) hybrid scheme we investigate the band structure of the electron-doped high-$T_c$ material Sm$_{2-x}$Ce$_{x}$CuO$_4$. Parameters of the minimal tight-binding model for this system (the so-called 3-band Emery model) were obtained within the NMTO ($N$-th order Muffin-Tin orbital) method. Doping evolution of the dispersion and Fermi surface in the presence of electronic correlations was investigated in two regimes of magnetic order: short-range (spin-liquid) and long-range (antiferromagnetic metal). Each regime is characterized …
Material specific electronic band structure of the electron-doped high-Tc cuprate Nd(1.85)Ce(0.15)CuO(4) (NCCO) is calculated within the pseudo gap regime, using the recently developed generalized LDA+DMFT+Sigma_k scheme. LDA/DFT (density functional theory within local density approximation) provides model parameters (hopping integral values, local Coulomb interaction strength) for the one-band Hubbard model, which is solved by DMFT (dynamical mean-field theory). To take into account pseudogap fluctuations LDA+DMFT is supplied with "external" k-dependent self-energy Sigma_k, which describes in…
While the traditional local-density approximation (LDA) cannot describe Mott insulators, {\it ab-initio} determination of the Hubbard $U$, for example, limits LDA-plus dynamical mean field theory (DMFT) approaches. Here, we attempt to overcome these bottlenecks by achieving fusion of the quantum chemistry (QC) approach with DMFT. QC+DMFT supplants the LDA bandstructure by its QC counterpart as an input to DMFT. Using QC+DMFT, we show that undoped $La_{2}CuO_{4}$ is a $d$-Mott insulator, and qualitatively discuss the circulating current- and incoherent metal phase, at small but finite hole dopi…
The multielectron LDA+GTB approach has been developed to calculate electronic structure of strongly correlated cuprates. At low energies the effective Hamiltonian of the $t - t' - t" - {t_ \bot } - {J^ * } - {J_ \bot }$-model has been derived with parameters coming from the ab initio calculation for LSCO. The electronic structure of LSCO has been calculated self-consistently with the short-range antiferromagnetic order for various doping level. Two Lifshitz-type quantum phase transitions with Fermi surface topology changes have been found at dopings $x_{c1}=0.15$ and $x_{c2}=0.24$. Its effect …
Typical density-functional theory calculations that wrongly predict undoped cuprates to be metallic also predict Cu-O half- and full-breathing phonon energies that are significantly softer than observed, presumably because of weak on-site Coulomb repulsion on the Cu 3d orbitals. We used DFT+U calculations with antiferromagnetic supercells of La$_2$CuO$_4$ to establish correlation between the on-site repulsion strength, tuned via adjusting the value of U, and phonon dispersions. We find that breathing and half-breathing phonons reach experimental values when U is tuned to obtain the correct opt…
Pseudogap regime for the prototype high-Tc compounds hole doped Bi2Sr2CaCu2O8-x (Bi2212) and electron doped Nd2-xCexCuO4 (NCCO) is described by means of novel generalized LDA+DMFT+Sk approach. Here conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Sk. In the present case Sk describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material specific model parameters of two neighboring CuO2 layers of Bi2212 and single CuO2 layer of NCCO were obtained with…
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 throug…
High-$T_c$ superconductors with CuO$_2$ layers, manganites La$_{1-x}$Sr$_x$MnO$_3$, and cobaltites LaCoO$_3$ present several mysteries in their physical properties. Most of them are believed to come from the strongly-correlated nature of these materials. From the theoretical viewpoint, there are many hidden rocks in making the consistent description of the band structure and low-energy physics starting from the Fermi-liquid approach. Here we discuss the alternative method -- multielectron approach to the electronic structure calculations for the Mott insulators -- called LDA+GTB (local density…
We study the doping evolution of the electronic structure in the normal phase of high-$T_c$ cuprates. Electronic structure and Fermi surface of cuprates with single CuO$_2$ layer in the unit cell like La$_{2-x}$Sr$_x$CuO$_4$ have been calculated by the LDA+GTB method in the regime of strong electron correlations (SEC) and compared to ARPES and quantum oscillations data. We have found two critical concentrations, $x_{c1}$ and $x_{c2}$, where the Fermi surface topology changes. Following I.M. Lifshitz ideas of the quantum phase transitions (QPT) of the 2.5-order we discuss the concentration depe…
By means of density functional theory, we investigate the magnetic ground state of edge-sharing CuO_2 spin-chains, as found in the (La,Ca,Sr)_14Cu_24O_41system, for instance. Our data rely on spin-polarized electronic structure calculations including onsite interaction (LDA+U) and an effective model for the interchain coupling. Strong doping dependence of the magnetic order is characteristic for edge-sharing CuO_2 spin-chains. We determine the ground state magnetic structure as function of the spin-chain filling and quantify the competing exchange interactions.
We discuss the effects of interlayer hopping and the resulting $k_z$-dispersion in the cuprates within the framework of the one-band tight binding (TB) model Hamiltonian. Specific forms of the dispersion relations in terms of the in-plane hopping parameters $t$, $t'$, $t''$ and $t'''$ and the effective interlayer hopping $t_z$ in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO) and the added intracell hopping $t_{bi}$ between the CuO$_2$ bilayers in Bi$_2$Sr$_2$CaCu$_2$O$_{8}$ (Bi2212) are presented. The values of the `bare' parameters are obtained via fits with the first prin…
We study the electronic structure of the ladder compounds (SrCa)CuO 14-24-41 and SrCuO 123. LDA calculations for both give similar Cu 3d-bands near the Fermi energy. The hopping parameters estimated by fitting LDA energy bands show a strong anisotropy between the t_perp t_par intra-ladder hopping and small inter-ladder hopping. A downfolding method shows that this anisotropy arises from the ladder structure.The conductivity perpendicular to the ladders is computed assuming incoherent tunneling giving a value close to experiment.
For many years a spin-state transition at $T \approx 100 K$ and insulator - metal transition (IMT) at $T_{IMT} \approx 600 K$ in LaCoO$_3 $ remains a mystery. Small low-spin - high-spin spin gap $Δ_S = E\left( {HS} \right) - E\left( {LS} \right) \sim 100 K$ results in the spin-state transition. The large charge gap $2E_a \approx 2300 K$ ($E_a $ is the activation energy) vs. $Δ_S $ and $T_{IMT} $ implies that LaCoO$_3 $ is not a simple narrow-gap semiconductor. Here we explain both the spin-state and IMT on the same footing. We obtain strong temperature dependent band structure in LaCoO$_3 $ by…
The electron-doped Pr(2-x)Ce(x)CuO(4) (PCCO) compound in the pseudogap regime (x~0.15) was investigated using angle-resolved photoemission spectroscopy (ARPES) and the generalized dynamical mean-field theory (DMFT) with the k-dependent self-energy (LDA+DMFT+Σ_k). Model parameters (hopping integral values and local Coulomb interaction strength) for the effective one-band Hubbard model were calculated by the local density approximation (LDA) with numerical renormalization group method (NRG) employed as an "impurity solver" in DMFT computations. An "external" k-dependent self-energy Σ_k was used …
Using LDA+GTB multi-band approach, we studied the compression dependence of the electronic structure and in-plane superexchange interaction J(P) in the antiferromagnetic La214 at the 0% and 3% - hydrostatic and unaxial (along c axial) compression. We obtained the superexchange interaction J(P=0)~0.15eV is enhanced by ~20% under the 3% - hydrostatic compression and vice versa the J(P) is decreased slightly by ~5,7% under the 3% - uniaxial compression. In both cases the J(P) correlates with the in-plane hopping parameters and dd-excitation energy δ_s=e(^3B_{1})-e(A_{1})$ involving the the two-ho…
Recently we presented a new band structure for La(2-x)Sr(x)CuO(4) and other high temperature superconductors in which a second narrow band was seen to cross the primary band at the Fermi level. The existence of this second Fermi level band is in complete disagreement with the commonly accepted LDA band structure. Yet it provided a crucial piece of physics which led to an explanation for superconductivity and other unusual phenomena in these materials. In this work we present details as to the nature of the failure of conventional methods in deriving the band structure of the cuprates. In parti…
To date, angle-resolved photoemission spectroscopy has been successful in identifying energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy. Here, at higher energy scale, we present improved experimental data from four families of high-Tc superconductors over a wide doping range that reveal a hierarchy of many-body interaction scales focused on: the low energy anomaly ("kink") of 0.03-0.09eV, a high energy anomaly of 0.3-0.5eV, and an anomalous enhancement of the width of the LDA-based CuO2 band extend…
