Carrier lifetime and threshold of Pb1-xSnxTe lasers in a magnetic field

It is well known that a magnetic field modifies not only the photon energy, but also the threshold current required by semiconductor lasers. However, the extent to which a given field modifies this threshold current depends very strongly upon temperature and on the lifetimes of the carriers in the valence and conduction bands. Theoretical calculations that predict values of threshold current as a function of magnetic field in mixed-crystal Pb1-xSnxTe lasers are presented. The carrier lifetimes in these materials have recently been studied in some detail and turn out to be related to the composition of the material and to its preparation history. Assuming that the carrier Lifetimes are largely determined by the acoustical phonon density or by random distribution of short-range scattering centers, the Kubo formalism can be used to express the individual Landau-level densities of states. From these, the quasi-Fermi levels at any pumping rate can be directly calculated. Fork-conserving transitions, the optical density of states is then determined by a convolution integral method recently reported elsewhere, and the threshold current is computed numerically by the method of Lasher and Stern. Using some of the measured values of carrier mobilities given by Calawa et al., for compositions withx = 0.07andx = 0.20, and using the measured values of Butler and Harman for zero magnetic-field threshold to adjust the gain requirement parameter, threshold currents at 77°K are computed for magnetic field strengths up to 250 kG. Curves are obtained that predict a threshold current for the higher mobility material that decreases by a factor of about 3 with increasing magnetic field out to the vicinity of 10 kG, flattens out, and then slowly increases. For the lower mobility material, an overall higher input current is required; however the curve continues to decrease to 25 kG before flattening out and then beginning to rise.     

Editorial Paper of the Special Issue on “Variational Inequalities,Nash Equilibrium Problems and Applications”

Networks and Spatial Economics -     

Implementing disposable credit card numbers by mobile phones

Disposable credit card numbers are a recent approach to tackling the severe problem of credit card fraud, nowadays constantly growing, especially in the context of e-commerce payments. Whenever we cannot rely on a secure communication channel between cardholder and issuer, a possibility is to generate new numbers on the basis of some common scheme, starting from a shared secret information. However, in order to make the approach meaningful from a practical point of view, the solution should guarantee backward compatibility with the current system, absence of new investments in dedicated hardware, wide-spectrum usability, and adequate security level. In this paper, we propose a solution based on the use of standard mobile phones, fully meeting the above desiderata. Importantly, our solution does not require any cryptographic support and, as a consequence, the use of PADs or smart phones, opening then its usability to a wider potential market.     

Far-infrared raman laser with high intensity laser pumping

Theoretical and experimental results are presented for a pulsed far-infrared (FIR) molecular gas laser with high intensity laser pumping. In these FIR lasers, high intensity pumping is found to produce stimulated Raman emission at very large offsets (up to 30 GHz) from resonance with the intermediate state. A theoretical, density matrix model is developed for these lasers to account for simultaneous Raman emission on rotational levels in the ground and excited vibrational states (double Raman resonance). This theoretical approach is necessary in the case of off-resonant, high intensity pumping. Theory predicts the FIR emission frequency, the FIR laser gain, and the pump threshold intensity as a function of pump laser frequency. Experimental results are obtained onP-,Q-, andR-branch transitions in¹²CH3F and¹³CH3F using a single-mode, grating tuned CO2TEA pump laser with an intensity of up to 40 MW/cm². Good agreement is obtained between theory and experiment for the observed values of FIR emission frequency and pump threshold intensity. These results indicate that a widely tunable (150-1200 mum), pulsed FIR CH3F laser could be constructed with a tunable, multiatmospheric CO2pump laser of modest power (about 2-5 MW).     

Structure of a heparin-linked biologically active dimer of fibroblast growth factor

The fibroblast growth factors (FGFs) form a large family of structurally related, multifunctional proteins that regulate various biological responses. They mediate cellular functions by binding to transmembrane FGF receptors, which are protein tyrosine kinases. FGF receptors are activated by oligomerization, and both this activation and FGF-stimulated biological responses require heparin-like molecules as well as FGF. Heparins are linear anionic polysaccharide chains; they are typically heterogeneously sulphated on alternating L-iduronic and D-glucosamino sugars, and are nearly ubiquitous in animal tissues as heparan sulphate proteoglycans on cell surfaces and in the extracellular matrix. Although several crystal structures have been described for FGF molecules in complexes with heparin-like sugars, the nature of a biologically active complex has been unknown until now. Here we describe the X-ray crystal structure, at 2.9 A resolution, of a biologically active dimer of human acidic FGF in a complex with a fully sulphated, homogeneous heparin decassacharide. The dimerization of heparin-linked acidic FGF observed here is an elegant mechanism for the modulation of signalling through combinatorial homodimerization and heterodimerization of the 12 known members of the FGF family.     

Possible high-frequency cavity and waveguide applications of high temperature superconductors

Conventional superconducting materials used at liquid helium temperatures have been employed to greatly reduce ohmic power loss in microwave cavities. If suitable high temperature superconductor surfaces can be developed, it could be possible to alleviate constraints due to relatively low energy gaps and limits due to critical fields, and operate at more convenient temperatures with larger thermal margins. These features could be used to improve performance of present superconducting microwave cavity devices. They may also facilitate new applications and device designs at millimeter-wave and far infrared frequencies. Possible cavity and waveguide applications are described. Sensitivity of RF surface resistance to boundaries and nonaligned grains in the material and to small amounts of nonsuperconducting material may be an obstacle to development. Moreover, critical RF fields can be limiting for high power applications. An additional problem is the adverse effect of D.C. magnetic fields.     

Disulfide bond structure of human epidermal growth factor receptor

The extracellular domain of the human epidermal growth factor receptor (sEGFR) consists of 621 amino acid residues, including 50 cysteines. The connections of the 25 disulfide bonds in the recombinant sEGFR protein, obtained from Chinese hamster ovary cells, have been determined using N-terminal sequencing and matrix-assisted laser desorption/ionization mass spectroscopy. We identified a basic repeat of eight cysteines with a 1-3, 2-4, 5-6, and 7-8 disulfide pairing pattern in the two cysteine-rich regions of sEGFR. By comparison to other cysteine-rich motifs, it was concluded that the cysteine-rich repeat of sEGFR belongs to the laminin-type EGR-like (LE) structural motif. Three-dimensional structure models of the two cysteine-rich regions have been built, based on the three-dimensional structures of the LE domains from the laminin gamma1 chain and secondary structure predictions for the EGF receptor.