Spin singlet small bipolarons in Nb-doped BaTiO3

The magnetic susceptibility and electrical resistivity of n-type BaTi{1-x}Nb{x}O3 have been measured over a wide temperature range. It is found that, for 0 < x < 0.2, dopant electrons form immobile spin singlet small bipolarons with binding energy around 110 meV. For x = 0.2, a maximum in the electrical resistivity around 15 K indicates a crossover from band to hopping transport of the charge carriers, a phenomenon expected but rarely observed in real polaronic systems.Comment: 5 pages, 4 figure

Determination of the Predominant Ionization and Loss Mechanisms for the Low‐Voltage Arc Mode in a Neon Plasma Diode

A reaction‐rate analysis is used to determine the relative importance of the predominant ionization and loss mechanisms in a neon low‐voltage arc. Experimental data are derived from various experiments to determine density, cross section, etc. It is found that the resonance and metastable state atoms are primarily generated in a region near the cathode which corresponds to the ``cathode ball‐of‐fire'' region of the low‐voltage arc. The predominant ion‐generation process is found to be a result of collisions between excited atoms which cause the formation of a molecular ion. Direct ionization of ground‐state atoms is of secondary but non‐negligible, importance. Consideration of quasi‐equilibrium multistage ionization shows that, unlike the cesium low‐voltage arc, it is unimportant in the neon low‐voltage arc. The escape of resonance radiation accounts for approximately one‐fifth of the total power loss while ionization accounts for approximately one‐eighth. Most of the remaining power loss appears as power dissipation at the anode.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69978/2/JAPIAU-39-9-4299-1.pd

Heating of refractory cathodes by high-pressure arc plasmas: II

A model of the near-cathode plasma layer in a plasma under a pressure of the order of one or several bars is reconsidered on the basis of recent theoretical results. Physics of the near-cathode layer is analysed in the range of near-cathode voltage drops of up to 50 V, in accord to recent experimental results which have shown that the near-cathode voltage drop in high-pressure arc discharges may be that high. It is found that a non-monotony of the dependence of the energy flux density on the surface temperature at fixed values of the near-cathode voltage drop is caused by one of the three mechanisms: overcoming of the increase of combined ion and plasma electron heating by the increase of thermionic cooling as the plasma approaches full ionization; non-monotony of the dependence of the ion current on the electron temperature which is caused by the deviation of the ion current from the diffusion value; rapid increase of the plasma electron heating which is subsequently overcome by thermionic cooling. A closed description of the plasma–cathode interaction is obtained by numerically solving the nonlinear boundary-value problem for the temperature distribution inside the cathode body. Results of numerical modelling of the diffuse discharge under conditions of a model arc lamp are given and a good agreement with the experimental data is shown. 1

Model for superconductivity in optimally doped electron cuprates

The very anomalous normal state properties and superconductivity of electron-doped supercon-ducting cuprates with the T* structure are discussed. The literature values of high mobilities for electrons and holes are explained by the electrons and holes existing in different layers. The annealing procedures necessary for superconductivity produce oxygen-vacancy negative-U pairing centers. The relative insensitivity of Tc to the annealing condition is attributed to stabilization of the chemical potential near the middle of the narrow oxygen band formed by oxygen vacancies where the electronic entropy is large