arXiv subjects

O. M. Ivanenko

Publications and source records attributed to O. M. Ivanenko.

12 recordsLinked to original sources

arXiv2022

Cluster structure of superconducting phase and the nature of peaks in the doping dependences of the London penetration depth in iron pnictides

The mechanism of formation of dome-like phase diagrams and the features of magnetic field penetration in iron pnictides with hetero- and isovalent doping are considered within the framework of the previously proposed model, which assumes the local character of doping and the cluster structure of resulted superconducting phase. It is shown that the proposed model, despite its simplicity and neglect of the features of the electronic structure, makes it possible not only to accurately calculate the positions of superconducting domes on the phase diagrams of specific HTSC compounds, but also to explain the nature and position of sharp peaks in the London penetration depth depending on the doping level.

cond-mat.supr-con↗

arXiv2019

Towards the issue of the origin of Fermi surface, pseudogaps and Fermi arcs in cuprate HTSCs

Earlier we have proposed a new approach to the analysis of superconducting phase diagrams for cuprates and pnictides and have shown that the positions of superconducting domes on the diagrams can be predicted with high accuracy proceeding from only the crystal structure of a particular compound. The proposed approach uses the concept of the self-localization of doped carriers due to their formation of trion complexes that represent a bound state of the doped carrier and charge transfer excitons emerging under its influence. Here, as exemplified by cuprates, we show that the use of the proposed approach to the analysis of the transformation of an electronic structure with doping enables an explanation to a range of their anomalies: Fermi arcs, large and small pseudogaps etc. The basic conclusion is that the role of the Fermi surface in cuprates is played by an isoenergetic contour that emerges at the sectioning of the surface of a band dispersion by a dispersionless "biexciton" pair level. This level additionally plays the role of an acceptor to lead to the emergence of hole carriers on the isoenergetic contour and to a jump of the chemical potential. Based on the conducted consideration, we propose a possible mechanism of superconducting pairing genetically inherent in such a system.

cond-mat.supr-con↗

arXiv2018

Superconducting phase diagrams of cuprates and pnictides as a key to the HTSC mechanism

This paper reviews experimental phase diagrams of cuprates and pnictides to demonstrate that specific features of the superconducting phase diagrams in bothHTSC families can be understood within the framework of the proposed approach,which assumes the formation, under heterovalent doping, of localized trion complexes consisting of a doped carrier and charge transfer (CT) excitons. The geometry of such cells containing CT excitons (CT plaquettes) in the basal plane of the crystal is determined by its crystal structure and the type of dopant, so that the dopant concentration range corresponding to the existence of a percolation cluster of CT plaquettes can be readily determined for each particular compound. These dopant concentration ranges coincide with good accuracy with the experimental ranges of superconducting domes in the phase diagrams of the HTSC compounds considered. The generation of free carriers and the mechanism of superconducting pairing in this pattern is related to biexciton complexes (Heitler-London centers) emerging in neighboring CT plaquettes.

cond-mat.supr-con↗

arXiv2012

On the possible common nature of the ground state in Cu- and Fe-based HTSCs

A qualitative model describing the ground state and the mechanism of superconducting pairing in Cu- and Fe-based high-temperature superconductors (HTSCs) is suggested. In this model, doping by localized charges (as well as physical or chemical pressure) is supposed to be responsible for transition of Cu- and Fe-based HTSC to new ground state common for both HTSC classes where specific mechanism of superconductivity takes place. The resulting HTSC ground state is strongly correlated insulator with not fully filled exciton-electronic band, where the incoherent electron transport is impossible but coherent superconducting transport is possible because the band is not fully occupied. It is shown also that such electronic system is inherently predisposed to superconductive pairing because each pair of nearest cations acts as a two-atom negative-U center. The nature of Fermi arcs and mechanism of pseudogap are considered. It is shown that both of these features result from d-wave pairing and therefore have to be observed only in cuprates. We believe that the considered ground state is common for various families of HTSCs including cuprates, pnictides, selenides, bismutates and probably some other.

cond-mat.supr-con↗

arXiv2011

On the possible mechanism of superconductivity in cuprates and pnictides

The nature of the normal state and the mechanism of superconductivity in two families of high-temperature superconductors, cuprates and pnictides, remain a matter of intense discussions. According to band-structure calculations, confirmed by ARPES data, the electronic structure of pnictides differs considerably from that of cuprates. However, in spite of these differences, it looks like in both cases there exists some general and fairly "coarse" mechanism independent of fine details of the band structure and responsible for superconductive pairing in these materials. Here, we suggest a qualitative model describing the ground state and the mechanism of superconductive pairing in cuprates and pnictides that, in our opinion, can explain many of their unusual properties.

cond-mat.supr-con↗

arXiv2011

Model for the electronic structure, Fermi arcs, and pseudogap in cuprate superconductors

We suggest a model for electronic structure of cuprate superconductors that makes it possible to describe evolution of this structure with the doping and provides a new explanation for a number of typical features of cuprates, including the pseudogap and the Fermi arcs. According to this model, the unique electronic structure of cuprates is favorable for the formation of two-atomic negative-U centers (NUCs) and realization of a peculiar mechanism of the electron-electron interaction.

cond-mat.supr-con↗

arXiv2009

Fermi arcs as a visible manifestation of pair level of negative-U centers

Here we consider the possibility that the Fermi arcs observed in ARPES experiments result from interaction of valence band electrons with negative-U centers (NUCs) which are formed under doping on the pairs of neighboring Cu cations in CuO2 plane. This interaction results in two-particle hybridization and vanishing of the gap over the part of Fermi contour. Furthermore the transitions of electrons to NUCs result in the generation of free hole carriers. Just these carriers rather than doping induced charges provide conduction in the normal state.

cond-mat.supr-con↗

arXiv2009

Percolative nature of the transition from 60 K to 90 K phase in YBa2Cu3O6+d

We have measured the heat capacity of YBa2Cu3O6+d for 0.7<d<0.8 between 1.8 and 300K. It was found that doping dependences of specific heat jump and temperature of heat capacity jump contradict to the assumption of spatially homogeneous electronic density. The results suggest that the transition from 60K to 90K phase has a percolative nature and the structure of underdoped 60K phase can be considered as array of superconducting nanoclusters embedded in the insulating matrix.

cond-mat.supr-con↗

arXiv2005

Phase diagrams of La2-xSrxCuO4 and YBa2Cu3O{6+/delta} as the keys to understanding the nature of high-Tc superconductors

The model that explains many of the details of superconducting and magnetic phase diagrams of YBa2Cu3O{6+/delta} and La2-xSrxCuO4 is presented. The model is based on the assumption of rigid localization of doped charges in the close vicinity of doped ion. This localization results in local variation of electronic structure of the parent charge-transfer insulator that depends on the local mutual arrangement of the doped charges. It is shown that in such system the negative-U centers (NUCs) may form under certain conditions on the pairs of neighboring Cu cations in CuO2 plane. We consider the mechanism of hole generation. The calculated dependences of hole concentration in YBa2Cu3O{6+/delta} on doping and temperature are found to be in a perfect quantitative agreement with experimental data. In the framework of the model the phase diagrams of YBa2Cu3O{6+/delta} is considered and the interpretation of pseudogap and 60 K-phases in YBa2Cu3O{6+/delta} is offered. The pseudogap has superconducting nature and arises at temperature T*>Tc/inf>Tc in small clusters uniting a number of NUCs due to large fluctuations of NUC occupation. (Tc/inf and Tc are the temperatures of superconducting transition for infinite and finite clusters, accordingly). The calculated T*(/delta) and Tc(/delta) dependences are in accordance with experiment. The range between T*(/delta) and Tc(/delta) corresponds to the range of fluctuations where small clusters fluctuate between superconducting and normal states owing to fluctuations of NUC occupation. The phase diagram of La2-xSrxCuO4 is calculated. It is shown the features of the superconducting phase diagram and the characteristics of stripe textures of La2-xSrxCuO4 only reflect the geometrical relations existing in a square lattice and the competition of different types of dopant ordering.

cond-mat.supr-con↗

arXiv2005

The mechanism of hole carrier generation and the nature of pseudogap- and 60K-phases in YBCO

In the framework of the model assuming the formation of NUC on the pairs of Cu ions in CuO$_{2}$ plane the mechanism of hole carrier generation is considered and the interpretation of pseudogap and 60 K-phases in $YBa_{2}Cu_{3}O_{6+δ}$. is offered. The calculated dependences of hole concentration in $YBa_{2}Cu_{3}O_{6+δ}$ on doping $δ$ and temperature are found to be in a perfect quantitative agreement with experimental data. As follows from the model the pseudogap has superconducting nature and arises at temperature $T^{*}>T_{c\infty}>T_{c}$ in small clusters uniting a number of NUC's due to large fluctuations of NUC occupation. Here $T_{c\infty}$ and $T_{c}$ are the superconducting transition temperatures of infinite and finite clusters of NUC's, correspondingly. The calculated $T^{*}(δ)$ and $T_{n}(δ)$ dependences are in accordance with experiment. The area between $T^{*}(δ)$ and $T_{n}(δ)$ corresponds to the area of fluctuations where small clusters fluctuate between superconducting and normal states owing to fluctuations of NUC occupation. The results may serve as important arguments in favor of the proposed model of HTSC.

cond-mat.supr-con↗

arXiv2005

Phase diagram of La2-xSrxCuO4 and stripes

An elementary model of La2-xSrxCuO4 explaining the features of its superconducting phase diagram and the characteristics of spin textures without any assumption of stripe formation is proposed. The foundations of this model are the mechanism of negative-U center formation proposed earlier, the supposition of rigid localization of doped holes, and the concept of specific ordering of doped ions. It is shown that within the framework of the proposed model the features of stripe textures of La2-xSrxCuO4 reflect exclusively the geometrical relations existing in a square lattice and the competition between different types of doped hole ordering. The detailed agreement between the calculated and experimental phase diagrams is the decisive factor favoring the mechanism of high-Tc superconductivity in which superconducting pairing arises due to scattering processes with intermediate virtual bound states of negative-U centers.

cond-mat.supr-con↗

arXiv2000

Insulator-metal transition in high-T$_c$ superconductors as result of percolation over -U centers

The mechanism of -U center formation in high-$T_{c}$ superconductors (HTS) with doping is considered. It is shown that the transition of HTS from insulator to metal passes through the particular dopant concentration range where the local transfer of singlet electron pairs from oxygen ions to pairs of neighboring cations (-U centers) are allowed while the single-electron transitions are still forbidden. We believe it is this concentration range that corresponds to the region of high-Tc superconductivity and the interelectron attraction results from the interaction of electron pairs with -U centers. Additional hole carriers are generated as the result of singlet electron pair transitions from oxygen ions to -U-centers. The orbitals of the arising singlet hole pairs are localized in the nearest vicinity of -U center. In such a system the hole conductivity starts up at the dopant concentration exceeding the classical 2D-percolation threshold for singlet hole pair orbitals. In the framework of the proposed model the phase diagram Ln-214 HTS compounds is constructed. The remarkable accord between calculated and experimental phase diagrams may be considered as the confirmation of the supposed model. The main features of hole carriers in HTS normal state are found to be the nondegenerate distribution and the dominant contribution of electron-electron scattering to the hole carrier relaxation processes. Various experimentally observed anomalies of HTS properties are shown to be the consequences of the above-mentioned features. The conclusion is made that HTS compounds are the special class of solids where the unusual mechanism of superconductivity different from BCS is realized.

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