arXiv subjects

V. A. Gavrichkov

Publications and source records attributed to V. A. Gavrichkov.

12 recordsLinked to original sources

arXiv2025

Diagonal superexchange in a simple square CuO$_2$ lattice

Many microscopic models with the interaction between the next-nearest neighbours as a key parameter for cuprate physics have inspired us to study the diagonal superexchange interaction in a CuO$_2$ layer. Our investigation shows that models with extended hopping provide a correct representation of magnetic interactions only in a hypothetical square CuO$_2$ layer, where the diagonal superexchange interaction with the next-nearest neighbors always has the AFM nature. The conclusions are based on the symmetry prohibition on FM contribution to the diagonal superexchange between the next-nearest neighbors for a simple square CuO$_2$ layer rather than for a real CuO$_2$ layer, where diagonal AFM superexchange may be overestimated. We also discuss the reasons for magnetic frustration effects and high sensitivity of spin nanoinhomogeneity to square symmetry breaking.

cond-mat.str-el↗

arXiv2022

Magnetic Interaction in Doped 2D Perovskite Cuprates with Nanoscale Inhomogeneity: Lattice Nonlocal Effects vs Superexchange

We have studied the superexchange interaction $J_{ij}$ in doped $2D$ cuprates. The AFM interaction strongly depends on the state of the lattice of a CuO$_2$ layer surrounded by two LaO rock salt layers. In a static $U$ and $D$ stripe nanostructure, the homogeneous AFM interaction is impossible due to the $U/D/U...$ periodic stripe sequence and $T_N=0$. In a dynamic stripe nanostructure, the ideal CuO$_2$ layer with nonlocal effects and the homogeneous AFM interaction are restored. However the interaction $J_{ij}$ decreases by the exponential factor due to partial dynamic quenching. The meaning of the transition from the dynamic to the static cases lies into the spontaneous $θ$-symmetry breaking with respect to the rotation of all the tilted CuO$_6$ octahedra by an orientation angle $δθ= n \cdot \left( {45^\circ } \right)$ (where $n=1\div4$) in the $U$ and $D$ stripe nanostructure of the CuO$_2$ layer. Moreover, the structural features help to study various experimental data on the charge inhomogeneity, Fermi level pinning in the p type cuprates only and a time reversal symmetry breaking from a unified point of view.

cond-mat.str-el↗

arXiv2015

Magnetic mechanisms of pairing in strongly correlated electron system of copper oxides

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 on normal and superconducting properties has been calculated. The interatomic exchange parameter and its pressure dependence has been calculated within LDA+GTB scheme. The magnetic mechanisms of d-wave pairing induced by static and dynamical spin correlations are discussed. Simultaneous treatment of magnetic and phonon pairing results in the conclusion that both contributions are of the same order. For two layer cuprates like YBCO the interlayer hopping and exchange effects on the electronic structure and doping dependence of $T_c$ is discussed as well as the Coulomb interaction induced mechanism of pairing.

cond-mat.str-el↗

arXiv2014

A simple metal-insulator criterion for the doped Mott-Hubbard materials

We derived a simple metal-insulator criterion in analytical form for the doped Mott-Hubbard materials. Its readings closely related to the orbital and spin nature of the ground states of the unit cell. The available criterion readings (metal or insulator) in the paramagnetic phase points to the possibility of the insulator state of doped materials with the forbidden first removal electron states. According to its physical meaning the result is similar to Wilsons criterion in the itinerant electron systems. An application of the criterion to the high-Tc cuprates discussed.

cond-mat.str-el↗

arXiv2013

Density Functional Theory and Generalized Tight-Binding combined method for Hubbard fermion-phonon coupling study in strongly correlated LSCO-system

We present ab initio results for the electron-phonon interaction of the Γ-point phonons in the tetragonal high-temperature phase of La2CuO4. Eigenfrequencies and eigenvectors for the symmetry-allowed phonon modes are calculated with the full-potential augmented plane wave+local orbitals method using the frozen phonon approach. It is found that the Γ-point phonons with the strongest electron-phonon interaction are the A{2u} modes with 236 cm^{-1}, 131 cm^{-1} and 476 cm^{-1}. To take effect of strong electron on-site interaction into account we use generalized tight-binding method that results in the interaction of phonons with Hubbard fermions forming quasiparticle's band structure. Finally, the matrix elements of Hubbard fermion-phonon interaction and their reduction due to strong electron correlation are obtained.

cond-mat.str-el↗

arXiv2012

Cuprates, Manganites, and Cobaltites: Multielectron Approach to the Band Structure

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 approximation + generalized tight-binding) method. Its origin is a straightforward generalization of the Hubbard perturbation theory in the atomic limit and the multiband $p-d$ Hamiltonian with the parameters calculated within LDA. We briefly discuss the method and focus on its applications to cuprates, manganites, and cobaltites.

cond-mat.str-el↗

arXiv2007

Dominance of many-body effects over one-electron mechanism for band structure doping dependence in Nd$_{2-x}$Ce$_x$CuO$_4$: LDA+GTB approach

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.

cond-mat.str-el↗

arXiv2005

A new hybrid LDA and Generalized Tight-Binding method for the electronic structure calculations of strongly correlated electron systems

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 insulators with correct values of gaps and dispersions of bands in agreement to the ARPES data.

cond-mat.str-el↗

arXiv2004

Strong spin triplet contribution of the first removal state in the insulating regime of Bi2Sr2Ca1-xYxCu2O8+delta

The experimental dispersion of the first removal state in the insulating regime of Bi2Sr2Ca1-xYxCu2O8+delta is found to differ significantly from that of other parent materials: oxyclorides and La2CuO4 . For Y-contents of 0.92 > x > 0.55 due to nonstoichiometric effects in the Bi-O layers, the hole concentration in the CuO2 -layers is almost constant and on the contrary the crystal lattice parameters a,b,c change very strongly. This (a,b) parameter increase and c parameter decrease results in an unconventional three peak structure at (0,0);(pi/2, pi/2);(pi,pi) for x=0.92. We can describe the experimental data only beyond the framework of the 3-band pd-model involving the representations of a new triplet counterpart for the Zhang-Rice singlet state.

cond-mat.supr-con↗

arXiv2004

Parameters of the Effective Singlet-Triplet Model for Band Structure of High-$T_c$ Cuprates by Different Approaches

The present paper covers the problem of parameters determination for High-$T_c$ superconductive copper oxides. Different approaches, {\it ab initio} LDA and LDA+U calculations and Generalized Tight-Binding (GTB) method for strongly correlated electron systems, are used to calculate hopping and exchange parameters of the effective singlet-triplet model for $CuO_2$-layer. The resulting parameters are in remarkably good agreement with each other and with parameters extracted from experiment. This set of parameters is proposed for proper quantitative description of physics of hole doped High-$T_c$ cuprates in the framework of effective models.

cond-mat.supr-con↗

arXiv2003

Prediction of the in-Gap States Above the Top of the Valence Band in the Undoped Insulating Cuprates due to Spin-Polaron Effect

In the framework of the generalized tight binding method we have calculated the quasiparticle band structure and the spectral functions of the undoped cuprates like $La_2 CuO_4$, $Sr_2CuO_2Cl_2$ etc. Due to spin fluctuations the in-gap state appears above the top of the valence band in the undoped antiferromagnetic insulator similar to in-gap states induced by hole doping. In the ARPES experiments the in-gap states can be detected as weak low energy satellites.

cond-mat.str-el↗

arXiv2002

Effective dimension of quasiparticle states and remnant Fermi surface in oxychlorides Ca_2CuO_2Cl_2 and Sr_2CuO_2Cl_2

Formal similarity between k- area for the contribution from ZR-states and the remnant Fermi surface has been found. Presence of the contribution from 3B_{1g}- states at other k- area means the two-channel processes of scattering. As consequence, the method of integration with a constant energy interval reproduces not only remnant Fermi surface at least, but one of the contributions of above states to the total spectral intensity

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