Modern Physics Letters B

AC resistivity measurements in fields up to 8 Tesla in fully oxygenated and oxygen deficient single crystals of YBa₂Cu₃O_7−δ are presented. Fully oxygenated crystals show the characteristic ‘fantail’ broadening in magnetic field where the onset temperature remains virtually independent of the applied magnetic field and the zero resistance temperature decreases dramatically. However, in oxygen deficient crystals, the temperature dependence of the resistivity undergoes a change in shape at high magnetic fields. These results are compared with the recently discovered highest T_c=11.6 K organic superconductor κ-(ET)₂Cu[N(CN)₂]Br where the shape of the resistive transition for H||b undergoes a change from a ‘fantail’ resistive broadening in magnetic field to that of a more conventional low T_c superconductor.

The finite-size scaling of the correlation length for directed and undirected systems is reviewed with emphasis on the asymptotic eigenvalue degeneracy associated with long-range order or long-range connectivity. The standard scaling mechanism for matching asymptotic behaviors on approach to criticality applies for Ising and other models. However, numerical evidence suggests that for directed percolation in 2D a modified formulation is appropriate, incorporating irrelevant-variable corrections in a new pattern of asymptotic scaling.

In the interaction of particles approaching a surface energy dissipation to the surface and charge exchange processes between the incoming ion or atom and the surface are two features which have found continuous interest over decades. Only few workers tried to resolve the interrelation of the change of the charge state of the incoming particle and the energy lost during the interaction. Here a brief review of earlier work in this field is given and some recently obtained new experimental and theoretical results are presented concerning the interaction of slow (2–5 keV) incoming He ions with a Ni(110) surface.

We calculate the ground state energy and the magnetization of spinless fermions on a two-dimensional lattice in an external magnetic field. We prove that the absolute minimum of the energy corresponds to a flux value equal to the filling, i.e. the “commensurate flux phase” state is preferable. The magnetization of these fermions has a paramagnetic character of special orbital type.

We have performed an X-ray photoelectron spectroscopy investigation on single-phase samples of Sn-doped YBCO system, together with structure analysis, oxygen content analysis, and superconductivity measurements. The experiment gave evidence that there is a strong correlation between the electronic states of copper and oxygen. When the sample was heated to 600°C for 20 minutes in vacuum chamber, the oxygen escaped from the sample, the binding energy of Cu 2p was decreased, and the two indistinct components of O 1s became clear. Keeping the sample in ultra-high vacuum for 24 hours, a similar result was obtained.

The concept is introduced of squeezed excitons that appear during the process of their interaction with squeezed photons. Situations are considered where the excitons initially are in the Fock or coherent states. For the excitons to become squeezed it is more favourable to prepare them beforehand in a coherent state than in a Fock one. The squeezing degree of excitons depends on that of the initial photons and on detuning.

The mass gap for the two-dimensional O(3) non-linear sigma model has been recently extracted from the Bethe Ansatz solution. Combining this result with the Renormalization Group treatment of the Quantum Heisenberg Ferromagnet as well as the Quantum non-linear sigma model we present new formulas for the correlation length of the square lattice quantum ferro and antiferromagnet.

Two mechanisms of solid state reaction from 2212 to 2223 in Bi-Sr-Ca-Cu-O have been investigated and are elucidated in this paper. The first mechanism is related to nucleation of 2223 phase in a liquid matrix with the composition of Bi₂SrCaCu₂O +Ca₂CuO₃+CuO and subsequent growth. The second mechanism is related to intragrain reaction in which only two layers of atoms (a Ca layer and a CuO layer) are required to move into the 2212 structure forming the 2223 structure. To study the mechanisms of such a solid state reaction may be very important for the preparation of pure 2223 phase in Bi-Sr-Ca-Cu-O.

Strong differential resistance peaks have been observed in the point-contact characteristics of normal state copper-oxide ceramics. A possible interpretation of the spikes that have been found previously for a variety of high-T_c superconducting polycrystalline samples is discussed.