K. V. Mitsen

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.

Specific heat of CexLa1 − xB6 in the low cerium concentration limit (x ≤ 0.03)

The specific heat of high-quality CexLa1 − xB6 (x = 0, 0.01, 0.03) single crystals is studied in the temperature range 0.4–300 K. LaB6 samples with various boron isotope compositions (10B, 11B, natB) are analyzed to estimate the effect of boron vacancies. The experimental data are used to take into account the electron component correctly under the renormalization of the density of states at T < 8 K, the contribution of the quasi-local vibrational mode of a rare-earth ion with the Einstein temperature ΘE ≈ 152 K, the Debye contribution from the rigid cage of boron atoms with the Debye temperature ΘD ≈ 1160 K, and the low-temperature Schottky contribution related to the presence of 1.5−2.3% boron vacancies in the rare-earth hexaborides. The detected low-temperature anomalies in the specific heat are shown to be interpreted in terms of the formation of two-level systems with an energy ΔE = 92–98 K caused by the displacement of rare-earth ions from their centrosymmetric positions. A scenario of heavy fermion formation that is alternative to the Kondo mechanism is proposed for the systems with a magnetic impurity.

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.

10B–11B isotope substitution and superconductivity in ZrB12

The specific heat of the ZrB12 compound in the normal and superconducting states (TC ≈ 6 K) has been studied in the 1.9–7 K temperature range for high-quality single crystals with different relative contents of boron isotopes. For Zr10B12, ZrnatB12, and Zr11B12 dodecaborides, the electron density of states and the electronphonon coupling constant, λe-ph ∼ 0.4, are found. The dependence of the thermodynamic and upper critical fields, as well as of the Ginzburg-Landau parameter (κ = 0.8–1.14) on temperature and isotope composition is determined. The results suggest the existence of the magnetic field induced phase transition at T* = 4–5 K, which is not related to the transition from type-I to type-II superconductivity. The possibilities of the existence of two-gap superconductivity and a structural phase transition at T* in zirconium dodecaboride are discussed.

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

We propose a model of heterovalent doping that enables determination of the superconductivity dome position on phase diagram of a particular HTSC compound proceeding from its crystal structure and dopant location. As it follows from the model, the observed similarity of the phase diagrams of cuprates and pnictides is due to the mechanism of superconductivity common for both HTSC families, the basis of which is the pairing of carriers on certain two-centre atomic complexes - Heitler-London (HL) centres. Each HL centre is formed in the basal plane between two dopant projections being at a certain distance one from the other. For a superconducting domain to be formed, HL centres should make up a percolation cluster. We calculated the concentration ranges corresponding to the formation of a percolation cluster of HL centres for a number of well investigated cuprates and pnictides and found an excellent coincidence of these ranges with the positions of the superconductivity domes on the corresponding phase diagrams. We consider this coincidence to be a confirmation of the model, which assumes the realization of the mechanism of superconducting pairing at HL centres.

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.