Electron Transport through Nanosystems Driven by Coulomb Scattering

Electron transmission through nanosystems is blocked if there are no states connecting the left and the right reservoir. Electron-electron scattering can lift this blockade and we show that this feature can be conveniently implemented by considering a transport model based on many-particle states. We discuss typical signatures of this phenomena, such as the presence of a current signal for a finite bias window.Comment: final version, to appear in Physical Beview B (6 pages and 6 figures included in text, simulation details added and discussion clarified in comparison to first version

Quantum transport: The link between standard approaches in superlattices

Theories describing electrical transport in semiconductor superlattices can essentially be divided in three disjoint categories: i) transport in a miniband; ii) hopping between Wannier-Stark ladders; and iii) sequential tunneling. We present a quantum transport model, based on nonequilibrium Green functions, which, in the appropriate limits, reproduces the three conventional theories, and describes the transport in the previously unaccessible region of the parameter space.Comment: 4 Page

Theoretical analysis of spectral gain in a THz quantum cascade laser: prospects for gain at 1 THz

In a recent Letter [Appl. Phys. Lett. 82, 1015 (2003)], Williams et al. reported the development of a terahertz quantum cascade laser operating at 3.4 THz or 14.2 meV. We have calculated and analyzed the gain spectra of the quantum cascade structure described in their work, and in addition to gain at the reported lasing energy of ~= 14 meV, we have discovered substantial gain at a much lower energy of around 5 meV or just over 1 THz. This suggests an avenue for the development of a terahertz laser at this lower energy, or of a two-color terahertz laser.Comment: in press APL, tentative publication date 29 Sep 200

The Prohibition of the Proposed Springer-ProSiebenSat.1-Merger: How much Economics in German Merger Control?

We review the Bundeskartellamt (Federal Cartel Office Germany) decision on the proposed merger between Springer and ProSiebenSat.1 from an economic point of view. In doing so, it is not our goal to analyse whether the controversial decision by the Bundeskartellamt has been correct or flawed from a legal point of view. Instead, we analyse whether the economic reasoning in the decision document reflects state-of-the-art economic theory on conglomerate mergers. Regarding such types of mergers, anticompetitive effects either do not occur regularly or are more often than not overcompensated by efficiency gains, so that a standard welfare perspective demands reluctance concerning antitrust interventions. This is particularly true if two-sided markets, like media markets, are involved. However, anticompetitive conglomerate mergers are not impossible, in particular in neighbouring markets where there is some relationship between the products of the merging companies. In line with the more-economic approach in European merger control, a particular thorough line of argumentation, backed with particularly convincing economic evidence, is necessary to justify a prohibition of a conglomerate merger from an economic point of view. Against this background, we do not find the reasoning of the Bundeskartellamt entirely convincing and sufficiently strong to justify a prohibition of the proposed combination from an economic perspective. The reasons are that (i) the Bundeskartellamt fails to continuously consider consumer and customer welfare as the relevant standards, (ii) positive efficiency and welfare effects of cross-media strategies are neglected, (iii) in contrast, the competition agency sometimes appears to view profitability of post-merger strategy options to be per se anticompetitive (efficiency offence), (iv) the incontestability of the relevant markets is not sufficiently substantiated, (v) inconsistencies occur regarding the symmetry of the TV advertising market duopoly versus the unique role of the BILD-Zeitung and (vi) the employment of modern economic instruments appears to be underdeveloped. Thus, we conclude that the Bundeskartellamt has not embraced the European more-economic approach in the analysed decision. However, one can discuss whether economic effects are overcompensated in this case by concerns about a reduction in diversity of opinion and threats to free speech. Similar to the Bundeskartellamt, we do not consider these concerns in our analysis.merger control, media markets, more-economic approach, conglomerate mergers, cross-promotion