Radiation-stimulated processes in CdTe solar cells

The parameters of polycrystalline cadmium telluride (CdTe) solar cells exhibit nonmonotonic variation with increasing dose of γ radiation. It is established that this behavior is related to a nonmonotonic dose dependence of the minority carrier lifetime in the base region. The short circuit current, the fill factor, and the conversion efficiency of γ-irradiated CdTe solar cells can be greater than the analogous parameters of unirradiated devices.     

Radiation-stimulated erosion of field electron emitters

The features of erosion of the surface of point field electron emitters bombarded by helium ions were studied. An analysis of the ion-bombardment-induced atomic-scale surface roughness showed evidence of a nondynamic character of the surface atom displacement for the primary ion energies below a threshold for the stable Frenkel pair formation and cathode sputtering. A quasi-static mechanism of the surface erosion is considered, according to which the process is related to the displacement of metal atoms into low-coordinated positions upon evolution of the energy of formation of interstitial helium atoms.     

Nonequilibrium superconductivity

In general, the quasiparticles, phonons, and pair field in a superconductor driven by an external source are out of equilibrium. Here we review briefly the various quasiparticle and phonon lifetimes in real metals and estimate the strengths necessary to induce a strongly nonequilibrium state. Then we investigate the properties of a thin film driven in a spatially uniform steady state by quasiparticle injection, heater phonon pumping, and microwave radiation. This analysis uses a set of coupled kinetic equations for quasiparticle and phonon distributions and the BCS gap equation with the self-consistently determined quasiparticle distribution. Such films can be used to generate phonons with energy near 2 . In addition, the gap enhancement in a thin film due to microwave irradiation is discussed.Work supported by the Office of Naval Research. Preliminary results have been reported at the Applied Superconductivity Conference, Stanford, August 1976 [IEEE Trans. Magn. MAG-13, 747 (1977)] and at the NATO Advanced Study Institute at Lago di Garda, Italy, September 1976 [NATO Adv. Study Inst. Series (B-Physics) 21, 447–486 (1977)].J. S. Guggenheim Foundation Fellow (1976–77).     

Radiation-stimulated point defects in Li2B4O7 single crystals

Using thermally stimulated luminescence, optical absorption, and x-ray crystallographic analysis methods, it is established that oxygen vacancies appear near Li ions in lithium tetraborate single crystals after the crystals are irradiated with (Sr:Y)-90 β particles with dose 7×10⁵ Gy and that these vacancies are capable of trapping one or two electrons, which are freed at temperatures 565 and 630 K, respectively. It is shown that hole localization on oxygen ions, joining triply and quadruply coordinated boron ions near Li vacancies, and freeing of the holes at 414 K are possible. Pis’ma Zh. Tekh. Fiz. 25, 78–83 (September 12, 1999)     

Magnetic-field-induced superconductivity?

It is shown that a giant negative magnetoresistance effect in doped manganites can be explained using a model developed earlier to explain high-temperature superconductivity, based on allowance for the covalence of the transition metal-oxygen bond. This effect is caused by delocalization of π-electrons along asymmetric π-orbitals belonging to chains of covalently coupled Mn and O ions. Unlike high-temperature superconductors, in manganites this delocalization is induced by a magnetic field which ferromagnetically orients unpaired π-electrons and thereby lifts the restriction against their collective motion along the π-orbital when the manganese ions are ferromagnetically ordered. Pis’ma Zh. Tekh. Fiz. 25, 1–3 (July 12, 1999)     

Radiation-stimulated surface diffusion of adatoms on a tungsten {321} face

Technical Physics Letters - The surface erosion of tungsten bombarded with accelerated 5-keV helium atoms has been studied by the method of field-ion microscopy. Radiation-stimulated appearance of...     

Progress in superconductivity

Superconductors are in a quantum state, extending over macroscopic distances. Consequently, they trap magnetic flux in multiples of the flux quantum. This point of view will be taken for a discussion of some recent developments related mainly to induction phenomena in superconductors. 1) In superconductors of the second kind magnetic flux penetrates at external magnetic field values within a certain interval (mixed state). This state violates the traditional characteristics of superconductivity. Not only does it fail to show the perfect Meissner diamagnetism, but it is also, in principle, a resistive state. The latter feature is correlated with the instability of the magnetic structure in the mixed state when a current is injected, the induced flux flow phenomenon being responsible for a variety of effects recently studied, including the Hall effect and a mode of magnetic coupling which makes a dc transformer feasible. 2) It is well known by now that superconductors of the second kind may obtain excellent current-carrying capacities if the mixed state magnetic pattern is stabilized, or pinned. This process is still not fully understood. The recent manufacture of some interesting and practicable wires and cables and the problems of coil construction and their stabilization will be briefly surveyed. 3) By proper manipulation of magnetic flux in superconducting systems, electromagnetic induction of heavy direct currents may be achieved. Fully superconducting dc generators or dynamos have recently been developed, the qualities of which will be discussed. The results of some of the author's recent investigations will be given, including those on multikiloampere dynamos for energizing supermagnets.     

Mixed valence and superconductivity

Mixed-valence (MV) systems are referred to here as MV-2 and MV-3 depending on whether two or three consecutive valence states are involved. MV-3 systems range from systems with Hubbard U>0, corresponding to a single stable, intermediate valence state, and U<0, corresponding to stable alternating valences differing by two units. Experiments using inelastic neutron scattering or inelastic X-ray scattering show softening of breathing phonon modes in MV systems compared with related systems with a single valence. It is hypothesized that softening is due to coupling between potential energy surfaces, corresponding to differing localizations of the electron. As predicted, softening is larger in the delocalized case. A mechanism for superconductivity is suggested.     

Molecular bond in superconductivity

In the presence of a superconducting current an additional intermolecular bond appears, whose value determines the critical parameters.Belarusian Technological Institute, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 67, Nos. 3–4, pp. 184–189, September–October, 1994.     

Anderson localization and superconductivity

The possibility of superconductivity is considered for a strongly disordered metal approaching the Anderson transition. A microscopic derivation of the coefficients of the Ginzburg-Landau expansion is given for a system in the vicinity of the mobility edge. The localization transition is described within the framework of the self-consistent theory of localization. The superconducting response persists in the localization region. The appropriate change in the behavior of the upper critical fieldH _c2 is considered for the localization region. The Coulomb repulsion grows as the Fermi level approaches the mobility edge, leading to a degradation of the superconductingT _c . However, under rather rigid conditions superconductivity is possible both at the mobility edge and in a narrow region below the mobility edge, i.e., in an Anderson insulator. Finally, experimental data for superconducting molybdenum sulfides irradiated by fast neutrons are discussed.     

Research opportunities in superconductivity

Opportunities for research in the field of superconductivity are identifield in this report of a ‘Workshop on problems in superconductivity’ held at Copper Mountain, Colorado, August 22–23, 1983. Key problems in superconductivity, high payoff areas of research, barriers to progress, and the need for new facilities are outlined in the three areas of basic physics, materials, and devices.     

Photoinduced enhancement of superconductivity

The photoinduced enhancement of superconductivity in RBa₂Cu₃O _x (R=rare earth or yttrium) and Pr _y R_1–y Ba₂Cu₃O _x was explored through temperature-dependent resistivity, Hall coefficient and mobility, and x-ray diffraction measurements. The increases inT _c are enhanced near the metal-insulator transition, although photoinduced changes always exist in oxygendeficient samples. Several explanations, including intergrain Josephson coupling, photoassisted oxygen ordering, and the trapping of photogenerated electrons in oxygen vacancies, are discussed.     

Electron-phonon locking and superconductivity

The electron-phonon vertex part is investigated as a function of real frequencies in a narrow-band electron system coupled with local phonons. Even though the bare electron-phonon coupling is not very strong, is found to diverge at some characteristic frequencies _l. The divergence induces the vanishment of the electron Green's function at _l. This phenomenon can be interpreted as the locking of an electron by such local phonons. By including this effect in the theory of superconductivity, we suggest an anomalous behavior in the tunneling spectroscopy.     

Space applications of superconductivity

This is the first of a seven part series on the potential applications of superconductivity in space. The series considers six classes of superconducting instrumentation for space applications; high field magnets, low frequency sensors, digital electronics, microwave and infrared detectors, instruments for gravitational studies, and high-O cavities. This introductory article provides background information for the study and briefly summarizes the contents of the articles which follow. This series is derived from a study supported by NASA and, as such, contains some speculation about ultimate instrument performance levels and future space missions.     

Large bipolarons and superconductivity

Superconductivity has long been speculated to result from charge carriers paired as mobile charged bosons. Although the pairing of carriers as small (single-site) bipolarons is known, small bipolarons readily localize. By contrast, large (multi-site) bipolarons, in analogy with large polarons, should be mobile. It is shown that large bipolarons can form in solids with very displaceable ions, e.g., many oxides. Large-polaronic (but not small-polaronic) carriers produce absorption spectra like the carrier-induced absorptions observed in cuprates. Redistribution of the self-trapped carriers of large bipolarons among sites of carriers' molecular orbitals in response to atomic motions lowers phonon frequencies. The dependence of the phonon zero-point energy on the spatial distribution of large bipolarons produces a phonon-mediated attraction between them. This dynamic quantum-mechanical attraction fosters the condensation of large bipolarons into a liquid. Superconductivity can result when the large-bipolarons' groundstate remains liquid rather than solidifying.