University of Stuttgart

About: University of Stuttgart is a education organization based out in Stuttgart, Germany. It is known for research contribution in the topics: Laser & Finite element method. The organization has 27715 authors who have published 56370 publications receiving 1363382 citations. The organization is also known as: Universität Stuttgart.

Energy-consistent pseudopotentials for group 11 and 12 atoms: adjustment to multi-configuration Dirac–Hartree–Fock data

Abstract: Two-component relativistic pseudopotentials (i.e., scalar-relativistic and spin–orbit (SO) potentials) of the energy-consistent variety have been adjusted for the group 11 and 12 atoms Cu, Zn; Ag, Cd; Au, Hg, replacing the 1s–2p; 1s–3d; and 1s–4f cores, respectively. The adjustment has been done for the valence-energy spectrum of (near-)neutral atoms, to reference data from numerical all-electron four-component multi-configuration Dirac–Hartree–Fock (MCDHF) calculations, including the two-electron Breit interaction. For use in molecular calculations, the potentials have been supplemented by energy-optimized (12s12p9d3f2g)/[6s6p4d3f2g] valence basis sets. First benchmark applications of the potentials and basis sets are presented for atomic excitation energies and SO splittings at a correlated level, and for ground and excited state spectroscopic properties of group 11 monohalides and group 12 dimers.

Cooperative phenomena in systems far from thermal equilibrium and in nonphysical systems

Abstract: This article consists of two parts. The first part presents a tutorial approach to cooperative phenomena in systems far from thermal equilibrium and in nonphysical systems. Particular emphasis is placed on the question of how order and self-organization may arise. The following example is treated among others: the ordered phase of the laser giving rise to both coherently oscillating atomic dipole moments and coherent light emission. A complete analogy of the laser light distribution function to that of the Ginzburg-Landau theory of superconductivity is found mathematically which allows us to interpret the laser threshold as a quasi-second-order phase transition with soft modes, critical slowing down, etc. Similar phenomena, again closely resembling phase transitions, are found in tunnel diodes and in the nonlinear wave interaction which occurs, for example, in nonlinear optics. Remarkable analogies between the instability of the laser and those in hydro-dynamics are elaborated. While these phenomena show pronounced analogies to phase transitions in thermal equilibrium, there are further classes of instabilities and new phases which rather resemble hard excitations known in electrical engineering. Chemical oscillations are particularly important examples. In addition, the first part of this article contains the example of the cooperative behavior of neuron networks and shows the applicability of simple physical concepts, e.g., the Ising model, to the problem of the dynamics of social groups. All these above-mentioned examples demonstrate clearly that rather complex phenomena brought about by the cooperation of many subsystems can be understood and described by a few simple concepts. One of the main concepts is the order parameter; another is the adiabatic elimination of the variables of the subsystems, which is based upon a hierarchy of time constants present in most systems. The second part of this article gives a systematic account of the mathematical tools which allow us to deal with fluctuations. It contains the master equation, the Fokker-Planck equation, the generalized Fokker-Planck equation, and the Langevin equations, and gives several general methods for deriving the stationary and, in certain cases, the nonstationary solutions of master equations and the Fokker-Planck equations. Such general classes comprise those in which detailed balance is present or in which the coupling to the reservoirs is weak. In the quantum mechanical domain, the density matrix and the projection formalism for its reduction are presented. Finally, it is shown how the principle of quantum-classical correspondence allows us to translate quantum statistical problems completely into the classical domain.

Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films

Abstract: Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.

Three-dimensional metamaterials at optical frequencies

Abstract: We experimentally demonstrate the implementation of three-dimensional optical metamaterials. We investigate the interaction between adjacent stacked layers using the method of plasmon hybridization and analyze the optical properties of stacked metamaterials with increasing layer numbers.

Low-voltage organic transistors with an amorphous molecular gate dielectric

Abstract: Organic thin film transistors (TFTs) are of interest for a variety of large-area electronic applications, such as displays, sensors and electronic barcodes. One of the key problems with existing organic TFTs is their large operating voltage, which often exceeds 20 V. This is due to poor capacitive coupling through relatively thick gate dielectric layers: these dielectrics are usually either inorganic oxides or nitrides, or insulating polymers, and are often thicker than 100 nm to minimize gate leakage currents. Here we demonstrate a manufacturing process for TFTs with a 2.5-nm-thick molecular self-assembled monolayer (SAM) gate dielectric and a high-mobility organic semiconductor (pentacene). These TFTs operate with supply voltages of less than 2 V, yet have gate currents that are lower than those of advanced silicon field-effect transistors with SiO2 dielectrics. These results should therefore increase the prospects of using organic TFTs in low-power applications (such as portable devices). Moreover, molecular SAMs may even be of interest for advanced silicon transistors where the continued reduction in dielectric thickness leads to ever greater gate leakage and power dissipation.