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A consistent device model to describe current-voltage characteristics of metal/insulator/metal systems is developed. In this model the insulator and the metal electrodes are described within the same theoretical framework by using density of states distributions. This approach leads to differential equations for the electric field which have to be solved in a self consistent manner by considering the continuity of the electric displacement and the electrochemical potential in the complete system. The model is capable of describing the current-voltage characteristics of the metal/insulator/metal system in forward and reverse bias for arbitrary values of the metal/ insulator injection barriers. In the case of high injection barriers, approximations are provided offering a tool for comparison with experiments. Numerical calculations are performed exemplary using a simplified model of an organic semiconductor., Comment: 21 pages, 8 figures

We present a closed description of the charge carrier injection process from a conductor into an insulator. Common injection models are based on single electron descriptions, being problematic especially once the amount of charge-carriers injected is large. Accordingly, we developed a model, which incorporates space charge effects in the description of the injection process. The challenge of this task is the problem of self-consistency. The amount of charge-carriers injected per unit time strongly depends on the energy barrier emerging at the contact, while at the same time the electrostatic potential generated by the injected charge- carriers modifies the height of this injection barrier itself. In our model, self-consistency is obtained by assuming continuity of the electric displacement and the electrochemical potential all over the conductor/insulator system. The conductor and the insulator are properly taken into account by means of their respective density of state distributions. The electric field distributions are obtained in a closed analytical form and the resulting current-voltage characteristics show that the theory embraces injection-limited as well as bulk-limited charge-carrier transport. Analytical approximations of these limits are given, revealing physical mechanisms responsible for the particular current-voltage behavior. In addition, the model exhibits the crossover between the two limiting cases and determines the validity of respective approximations. The consequences resulting from our exactly solvable model are discussed on the basis of a simplified indium tin oxide/organic semiconductor system., Comment: 23 pages, 6 figures, accepted to Phys.Rev. B

We perform a study of the possible existence of hybrid stars with color superconducting quark cores using a specific hadronic model in a combination with an NJL-type quark model. It is shown that the constituent mass of the non-strange quarks in vacuum is a very important parameter that controls the beginning of the hadron-quark phase transition. At relatively small values of the mass, the first quark phase that appears is the two-flavor color superconducting (2SC) phase which, at larger densities, is replaced by the color-flavor locked (CFL) phase. At large values of the mass, on the other hand, the phase transition goes from the hadronic phase directly into the CFL phase avoiding the 2SC phase. It appears, however, that the only stable hybrid stars obtained are those with the 2SC quark cores., Comment: 12 pages, 7 eps figures; v2: figures and table modified after correction of a minor numerical mistake, discussion clarified, references added, conclusions unchanged; version to appear in PLB

We explore the relevance of color superconductivity inside a possible quark matter core for the bulk properties of neutron stars. For the quark phase we use an Nambu--Jona-Lasinio (NJL) type model, extended to include diquark condensates. For the hadronic phase, a microscopic many-body model is adopted, with and without strangeness content. In our calculations, a sharp boundary is assumed between the hadronic and the quark phases. For NJL model parameters fitted to vacuum properties we find that no star with a pure quark core does exist. Nevertheless the presence of color superconducting phases can lower the neutron star maximum mass substantially. In some cases, the transition to quark matter occurs only if color superconductivity is present. Once the quark phase is introduced, the value of the maximum mass stays in any case below the value of two solar masses., Comment: 11 pages, 3 figures, v2: minor corrections in the text, layout of the figures improved, references added, v3: transition densities from hadronic to quark matter added, version accepted for publication in PLB

We examine electrically and color neutral quark matter in beta-equilibrium focusing on the possibility of mixed phases between different color superconducting phases. To that end we apply the Gibbs criterion to ensure phase equilibrium and discuss the external conditions under which these mixed phases can occur. Neglecting surface and Coulomb effects we find a rich structure of different mixed phases with up to four components, including 2SC and CFL matter as well as more ``exotic'' components, like a phase with us- and ds-pairing but without ud-pairing. Preliminary estimates indicate, however, that the mixed phases become unstable if surface and Coulomb effects are included., Comment: 22 pages, 9 figures, v2: minor changes in the text, version to appear in Nucl. Phys. A