O.M. Ivanenko

Low temperature measurements of Hc2 in HTSC using megagauss magnetic fields

Abstract Measurements of the upper critical fields in YBCO and BSCCO polycrystalline samples and YBCO epitaxial films were carried out at low temperatures in a pulsed ultrahigh magnetic field obtained in a magnetocumulative generator MC-1. The values of the critical fields at 4.2 K were found to be 168±26 T and 200±25 T for YBCO and BSCCO polycrystalline samples, respectively, and 110±10 T for YBCO films.

Destruction of superconductivity in YBa2Cu3O7−x ceramics by ultrahigh magnetic field

Upper critical fields of high-T/sub c/ superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ ceramics were measured in a pulsed magnetic field H/spl ap/300 T generated by magneto-cumulative plant. The presence of a superconducting phase in the cylindrical sample was detected by the appearance of S-like peculiarities on the dM/dt curve at the moment of short-time slowing of the increasing of applied magnetic field. Upper critical fields H/sub C2/ at T=4.2 and 77 K, defined in such a way, are equal to 180/spl plusmn/36 T and 48/spl plusmn/10 T accordingly.

Low temperature direct measurements of Hc2 in HTSC using megagauss magnetic fields

Abstract The measurements of upper critical fields in YBCO and BSCCO polycrystalline samples and YBCO epitaxial films were carried out at low temperatures in ultrahigh pulsed magnetic field obtained in magnetocumulative generator MC-1. The values of critical fields at 4.2 K defined by different methods were found to be 168±26 T and 200±25 T for YBCO and BSCCO polycrystalline samples, respectively, and 110±10 T for YBCO films.

HTSC — the undegenerated impurity semiconductor?

Abstract The model of electron spectrum of HTSC — a narrow local pair level placed near the top of the two-dimensional (2D) filled electron band — is discussed. Holes in this band is result of thermal activation of electrons from the band to the pair level. The temperature dependences of resistivity and Hall constant of HTSC in this model are in agreement with experiment of YBCO. The possibility of the first-order phase-transition in such system is considered.

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

Abstract 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 CuO 2 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.

Local structure, percolation and insulator-conductor transition in doped HTS

Abstract In this work we conclude that the transition from insulator to conductor (and superconductor) state in HTS under doping is the consequence of the transition through the threshold of two-dimensional -U-center percolation. As proposed -U-centers are formed on the neighbour cations when the definite spatial arrangement of the localized doped carriers is realized. These -U-centers are the pair acceptors and the transfer of electron pairs from oxygen ions to -U-centers gives rise the free hole carriers. The feutures of phase diagrams of doped HTS are connected with the different patterns of localized doped carrier distribution.

Measurement of the de Haas-van Alphen oscillations in YBCO using pulsed ultra-high magnetic fields

Abstract The measurement of magnetization for YBCO samples with the c -axis oriented along the magnetic field direction have been carried out at liquid helium temperature in ultrahigh pulsed magnetic fields up to 300 T. The magnetic fields were obtained with magnetocumulative generator MC-1 by compression of the magnetic flux by cylindrical detonation wave. The Fourier analysis of the magnetization signal has revealed the existence of oscillation with four well-defined frequencies at 3.8, 10, 13 and 20 kT. The frequencies 3.8 and 10 kT are consistent with calculated frequencies of the de Haas-van Alphen effect in YBCO. The appearance of two other frequencies calls for further investigation.

Eigenvalue problem for permittivity operator of conductors with the spatial dispersion in a microwave field

Conducting media with spatial dispersion may be described formally by the single operator – operator of dielectric permittivity, which, as it is well known, completely defines the microwave response of conductors with spatial dispersion. So the eigenvalue problem for permittivity operator of conductors and superconductors possessing strong spatial dispersion at low temperatures is of a great importance since the corresponding solutions are the stable waves for constitutive equation in a self-consistent microwave field. Here a wave problem is formulated to search the solutions, which correspond to the eigenvalues of permittivity operator, similar to the problem of wave propagation in hollow waveguides and resonators, but non-self conjugated. Dispersion relations and general solutions are obtained. Significant role of the spatial-type force resonances is considered. Conditions for the spatial resonances are derived. The obtained resonances includes particular solutions corresponding to traditional surface impedance for anomalous skin effect, surface impedance of superconductor, as well as four novel solutions, obviously related to polarization, two of which correspond to the waves with amplitude increasing into the depth of conductor, and two else describe solutions with unusual properties.

La214 phase diagram features as a consequence of percolation over negative-U centers

The features of La214 phase diagram (including superconducting and stripe phases) are explained in terms of formation of negative-U center (NUC), ordering of localized doped carriers and percolation over NUC.

Mechanism of hole carrier generation and nature of pseudogap in doped La214

Abstract The generation of hole carriers in HTS is considered in the framework of model [JETP 91 (2000) 579]. According to this model the hole conductivity starts up at the dopant concentration exceeding the percolation threshold for chain of negative-U centers (NUC). The conductivity (and superconductivity) takes place in the band formed by the oxygen pπ-orbitals hybridized with the pair orbitals of NUC. It is shown that this two-particle hybridization results in the temperature dependences of hole carrier concentration n ( T )∝ T and resistivity R ( T )∝ T . As far as the mechanism of superconducting gap suppression in HTS is the occupation of pair level with electrons, the superconducting gap (pseudogap) in small nonpercolative clusters containing short chains of NUC has to be opened at T above T c because of the large relative fluctuations of the number of particles between pπ-band and pair level in small clusters.