Space-charge limited conduction with traps in poly(phenylene vinylene) light emitting diodes

The current-voltage characteristics of poly͑dialkoxy p-phenylene vinylene͒-based hole-only devices are measured as a function of temperature. The hole current is space-charge limited, which provides a direct measurement of the hole mobility p as a function of electric field E and temperature. The hole mobility exhibits a field dependence ln p ϰͱE as has also been observed from time-of-flight experiments in many molecularly doped polymers and amorphous glasses. For the zero-field hole mobility an activation energy of 0.48 eV is obtained. The combination of a field-dependent mobility and space-charge effects provides a consistent description of the hole conduction in conjugated polymer films as a function of voltage, temperature, and layer thickness. ͓S0163-1829͑97͒51602-X͔

The transport properties of a ferromagnet-superconductor (FS) junction are studied in a scattering formulation. Andreev reflection at the FS interface is strongly affected by the exchange interaction in the ferromagnet. The conductance GFS of a ballistic point contact between F and S can be both larger or smaller than the value GFN with the superconductor in the normal state, depending on the ratio of the exchange and Fermi energies. If the ferromagnet contains a tunnel barrier (I), the conductance GFIFS exhibits resonances which do not vanish in linear response -in contrast to the Tomasch oscillations for non-ferromagnetic materials.PACS numbers: 74.80. Fp, 72.10.Bg, 74.50.+r Electrons in a metal can not penetrate into a superconductor if their excitation energy with respect to the Fermi level is below the superconducting gap ∆. Still, a current may flow through a normal-metal-superconductor (NS) junction in response to a small applied voltage V < ∆/e, by means of a scattering process known as Andreev reflection [1]: An electron in the normal metal is retroreflected at the NS interface as a hole and a Cooper pair is carried away in the superconductor. Andreev reflection near the Fermi level conserves energy and momentum but does not conserve spin -in the sense that the incoming electron and the Andreev reflected hole occupy opposite spin bands. This is irrelevant for materials with spin-rotation symmetry, as is the case for normal metals. However, the change in spin band associated with Andreev reflection may cause an anomaly in the conductance of (metallic) ferromagnet-superconductor (FS) junctions, because the spin-up and the spin-down band in the ferromagnet are different. This paper contains a theoretical study of Andreev reflection in FS junctions. We use a scattering approach based on the Bogoliubovde Gennes equation to study the transport properties for zero temperature and small V (eV ∆). We will concentrate on two distinct effects, which we think are experimentally observable. First, due to the change in spin band there is no complete Andreev reflection at the FS interface. This has a clear influence on the conductance and the shot-noise power of clean FS point contacts. Second, the different spin-up and spin-down wavevector at the Fermi level may lead to quantum-interference effects. This shows up in the linear-response conductance of FIFS junctions, where the ferromagnet contains an insulating tunnel barrier (I).In the past, FS junctions with an insulating layer between the ferromagnet and the superconductor have been used in spin-dependent tunneling experiments [2]. There the emphasis was on the voltage scale eV > ∼ ∆ and Andreev reflection did not play a role. Tunneling through S-Fi-S junctions, where Fi is a magnetic insulator, has been studied both experimentally [3] and theoretically [4,5]. In addition, there has been theoretical work on the Josephson effect in SFS junctions [6,7]. An experimental investigation of the boundary resistance of sputtered SFS sandwiches has also been reported [8]....