Ballistic effects and intersubband excitations in multiple quantum well structures

We have studied the transport properties of electrons in asymmetric quantum well structures upon far-infrared optical excitation of carriers from the lowest subband into the continuum. Here the photocurrent consists of a coherent component originating from ballistic transport upon excitation, and of an incoherent part associated with asymmetric diffusion and relaxation processes, which occur after the coherence has been lost. The signature of the coherent contribution is provided by a sign reversal of the photocurrent upon changing the excitation energy. This sign reversal arises from the energy-dependent interference between continuum states, which have a twofold degeneracy characterized by positive and negative momenta. The interference effect also allows us to estimate the coherent mean free path ( nm at 77 K). In specifically designed device structures, we use both the coherent and incoherent components in order to achieve a pronounced photovoltaic infrared response for detector applications.     

Experiments with single atoms in cavities and traps

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On a Model of a Currency Exchange Rate – Local Stability and Periodic Solutions

A delay differential equation is presented which models how the behavior of traders influences the short time price movements of an asset. Sensitivity to price changes is measured by a parameter a. There is a single equilibrium solution, which is non-hyperbolic for all a>0. We prove that for a< 1 the equilibrium is asymptotically stable, and that for a>1 a 2-dimensional global center-unstable manifold connects the equilibrium to a periodic orbit. Its birth at a=1 is not of Hopf type and seems part of a Takens–Bogdanov scenario.     

Janus cylinders at liquid-liquid interfaces

We describe the first study on the self-assembly behavior of Janus cylinders at liquid/liquid interfaces. The Janus cylinders are characterized by a phase separation along the major axis into two hemicylinders of different wettability. The pendant drop technique and microscopic imaging were used to characterize the adsorption behavior and self-assembly of Janus cylinders at perfluorinated oil/dioxane and perfluorinated oil/dimethyl sulfoxide interfaces. According to the evolution of the interfacial tension and a series of TEM images taken during the cylinder adsorption, we will specify the characteristics of early to late stages of the Janus cylinder adsorption at a liquid-liquid interface and discuss the effect of Janus cylinder length and their concentration. We also establish that the broken symmetry of the corona leads to significantly higher interfacial activity as compared to homogeneous core-shell cylinders. The adsorption is characterized by three different adsorption stages: first, free diffusion to the interface, followed by continuous adsorption of cylinders including ordering and domain formation and, finally, additional packing with a rearrangement of domains and formation of a loose multilayer system.     

Singlet oxygen's response to protein dynamics

Singlet molecular oxygen, O(2)(a(1)Δ(g)), is an intermediate in a variety of processes pertinent to the function of biological systems, including events that result in cell death. Many of these processes involve a reaction between singlet oxygen and a given protein. It is acknowledged that the behavior of a protein can change upon reaction with singlet oxygen, as a result of a structural alteration and/or a direct chemical modification of an active site. However, the converse, where one considers how the behavior of singlet oxygen can be altered by changes in protein structure, has received little attention. In this report, we use a variety of proteins to demonstrate how the rate constant for singlet oxygen removal by a protein responds to (a) protein denaturation, (b) macromolecular crowding of the protein, (c) ligand binding by the protein, and (d) polymerization of the protein. From one perspective, the data show that the kinetics of singlet oxygen removal can be used to monitor protein dynamics. Most importantly, however, the data indicate that protein structural changes that either reveal or cloak a given amino acid residue can have a measurable effect on the overall rate constant for singlet oxygen removal which, in turn, can have ramifications for singlet-oxygen-mediated intracellular events that perturb cell function.     

On the weak O-H···halogen hydrogen bond: a rotational study of CH3CHClF···H2O

We measured the molecular beam Fourier transform microwave spectra of six isotopologues of the 1?:?1 adduct of CH(3)CHClF with water. Water prefers to form an O-H···F rather than an O-H···Cl hydrogen bond. This is just the contrary of what was observed in the chlorofluoromethane-water adduct, where an O-H···Cl link was formed (W. Caminati, S. Melandri, A. Maris and P. Ottaviani, Angew. Chem., Int. Ed., 2006, 45, 2438). The water molecule is linked with an O-H···F bridge to the fluorine atom, with r(F···H(w)) = 2.14 ?, and with two C-H···O contacts to the alkyl hydrogens with r(C(1)-H(1)···O(w)) = 2.75 ? and r(C(2)-H(2)···O(w)) = 2.84 ?, respectively. Besides the rotational constants, the quadrupole coupling constants of the chlorine atom have been determined. In addition, information on the internal dynamics has been obtained.