Extraction-Controlled Quantum Cascade Lasers

A simple two-well design for terahertz quantum cascade lasers is proposed which is based on scattering injection and the efficient extraction of electrons from the lower laser level by resonant tunneling. In contrast to existing designs this extraction also controls the positive differential conductivity. The device is analyzed by calculations based on nonequilibrium Green functions, which predict lasing operation well above 200 K at a frequency of 2.8 THz.Comment: 3 pages, 3 figures included in tex

Gain and Loss in Quantum Cascade Lasers

We report gain calculations for a quantum cascade laser using a fully self-consistent quantum mechanical approach based on the theory of nonequilibrium Green functions. Both the absolute value of the gain as well as the spectral position at threshold are in excellent agreement with experimental findings for T=77 K. The gain strongly decreases with temperature.Comment: 7 pages, 3 figures directly include

The Influence of Trade with the EU-15 on Wages in the Czech Republic, Hungary, Poland, and Slovakia between 1997 and 2005

I use the STAN database of the OECD and different econometric methods to investigate the effects of exports towards the EU-15 on wages in the Visegrad countries (CEEC-4; Czech Republic, Hungary, Poland, and Slovakia). The results do not allow to draw any definite statements about this effect. While the impact of exports towards the EU-15 on wages in the countries investigated is likely to be negative in the short run (1-2 years), it seems to be positive in the medium and long run, at least for Hungary and Poland. Nevertheless, it is clear that the pattern of the CEEC-4 exports towards the EU-15 does not correspond with the predictions of the Heckscher-Ohlin model. Therefore, also the theorems of Stolper and Samuelson (1941) and concerning the equalization of factor prices, which are based on the Heckscher-Ohlin model, do not seem accurate to describe the underlying forces linking trade with factor prices. I argue that missing regional and related inter-sectoral labor mobility might be a potential factor preventing employees from taking advantage of trade liberalization. To substantiate this suspicion, however, analysis of more disaggregated data is necessary.panel data, EU enlargement, trade, wages, factor prices, income distribution, Central and Eastern Europe, Visegrad Countries

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

Phase Space Invertible Asynchronous Cellular Automata

While for synchronous deterministic cellular automata there is an accepted definition of reversibility, the situation is less clear for asynchronous cellular automata. We first discuss a few possibilities and then investigate what we call phase space invertible asynchronous cellular automata in more detail. We will show that for each Turing machine there is such a cellular automaton simulating it, and that it is decidable whether an asynchronous cellular automaton has this property or not, even in higher dimensions.Comment: In Proceedings AUTOMATA&JAC 2012, arXiv:1208.249

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