O. M. Ivanenko

Negative U centers, percolation, and the insulator-metal transition in high-Tc superconductors

The mechanism of formation of −U centers in high-Tc superconductors (HTSCs) is considered. It is shown that the transition from the insulator to the metallic state on doping passes through a certain range of dopant concentrations in which it becomes possible for local transitions of singlet electron pairs to occur from oxygen ions to two neighboring cations (a −U center), while single-electron transitions are still forbidden. Conduction arises in such systems at a concentration of −U centers exceeding the percolation threshold for the orbitals of singlet hole pairs. A phase diagram constructed on the basis of the proposed model for the HTSC compounds of the Ln-214 class is in complete agreement with experiment. The mechanisms of formation and relaxation of free hole carriers are considered. It is shown that a distinctive feature of the normal state of HTSCs is the dominant contribution of electron-electron scattering to the charge carrier relaxation processes. It is concluded from the analysis presented that HTSCs comprise a special class of solids in which a nonstandard mechanism of superconductivity, different from the BCS mechanism, is realized.

Charge ordering, superconductivity, and stripes in doped La2CuO4

The special features of the phase diagrams of La2−xSrxCuO4 are considered in terms of the high-temperature superconductivity model according to which the mechanism responsible for the anomalous properties of these compounds is the interaction of electrons with diatomic negative U-centers. A microstructural model that assumes the coexistence of domains with different types of dopant ion ordering is suggested for La2−xSrxCuO4. According to this model, the main characteristics of the experimental phase diagrams of La2−xSrxCuO4 only reflect square lattice geometric relations and competition between different dopant ordering types. Close agreement between the calculated and experimental “superconducting” and “magnetic” phase diagrams is an important argument in favor of the suggested high-temperature superconductivity model.

Possible nature of the pseudogap anomalies in HTSC

An HTSC model, in which the interaction of valence-band electrons with diatomic negative U centers is assumed to be responsible for the anomalous properties of HTSC compounds, is proposed and used to explain the nature of the pseudogap and pseudogap anomalies (including the giant Nernst effect, the anomalous diamagnetism above Tc, the “transfer” of the optical spectral weight). For YBa2Cu3O6 + δ, the pseudogap opening temperature T* and Tc are calculated as functions of the degree of doping δ. The calculated dependences agree quantitatively with the experimental dependences without using scale fitting parameters. The good agreement between the calculated and experimental results can serve as an argument for the proposed HTSC model.

Percolation nature of the 60-K to 90-K phase transition in YBa2Cu3O6 + δ

The temperature dependence of the heat capacity of the HTSC YBa2Cu3O6 + δ is measured in the temperature range 1.8–300 K and the doping range 0.70 < δ < 0.86. The results obtained suggest that the transition from the 60-K to the 90-K phase in YBa2Cu3O6 + δ has a percolation nature and that the underdoped 60-K phase represents a heterophase structure in which numerous superconducting clusters are embedded into an insulating matrix.

On the possibility of developing the non-contact delivery system for cryogenic thermonuclear target transport to the IFE reactor

It is proposed to use the HTSC quantum levitation phenomenon in magnetic fields of various configurations to develop the systems of contact-free positioning and transport of cryogenic fuel targets (CFTs) to the focus of a high-power laser installation or the IFE reactor. The results are presented of a large cycle of experimental studies using YBa2Cu3O7−x superconducting ceramics and permanent magnet guideways based on various combinations of permanentmagnets to develop “CFT-MAGLEV” delivery systems.

On the use of the second-generation HTSC tapes in cryogenic transport systems for IFE targets

The results of experimental studies using SuperOx J-PI-12-20Ag-20Cu tape superconductors in developing capsule carriers for cryogenic systems of noncontact transport of targets for IFE are presented.

Effect of radiation defects on the magnetotransport properties of Ba(Fe1 − xCoxAs)2 high-temperature superconductor

The effect of 200-keV He+ ion irradiation on the transport properties of films of the iron-based Ba(Fe1 − xCoxAs)2 superconductor has been studied. Contributions to the resistivity and magnetoresistance of irradiated samples from scattering by magnetic and nonmagnetic defects have been separated. It has been shown that mainly nonmagnetic defects are generated in the sample under the corresponding irradiation conditions. This result is important in view of the use of the radiation technique for the study of the effect of defects on the properties of iron-based superconductors.

On the interpretation of phase diagram of La2−x(Ba,Sr)x;CuO4

It is shown that the features of La2−x(Ba,Sr)xCuO4 around x=0.125 can be. treated as a consequence of alternate-layer doping when doped holes are added to each second CuO2 plane. The model of heterovalence doping of high-Tc superconductors (HTSC) is proposed to account for the evolution of HTSC properties with doping. According to this model the transition from insulator to metal state under doping proceeds through the stage of-U-centers formation, which arise when the definite spatial arrangement of the doping atoms is realized.

Insulator-Conductor Transition in Doped HTS as a Consequence of -U-Centers Generation

It is supposed that the transition from insulator to metal state under doping in high-Tc superconductors (HTS) passes through the stage of -U-centers formation, which arise on the neighbor Cu ions when the definite spatial arrangement of the doping atoms is realized. In this doping region the free hole carriers (just as in hole-, so in electron-doped HTS) are generated when electron pairs transfer from the oxygen band into -U-centers.

Thermoelectric effect in the hollow superconducting cylinder of Pb and Pb-In alloy

The technique for measuring of the thermoelectric effect in superconductors near the critical temperatureTC has been elaborated. The measured magnetic flux was induced by temperature gradient (ΔT∼1K) between joints. The experimental study of the thermoelectric effect has been carried out in the hollow bimetallic superconducting cylinder consisting of the Pb and Pb-In alloy. The observed temperature dependence of the thermoexcited magnetic flux has been well approximated by the logarithmical function. The magnitude order of the thermopower estimated from the experimental data coincides with the known low temperature thermopowers of metals.