Discrete optimization in public rail transport

Many problems arising in traffic planning can be modelled and solved using discrete optimization. We will focus on recent developments which were applied to large scale real world instances. Most railroad companies apply a hierarchically structured planning process. Starting with the definition of the underlying network used for transport one has to decide which infrastructural improvements are necessary. Usually, the rail system is periodically scheduled. A fundamental base of the schedule are the lines connecting several stations with a fixed frequency. Possible objectives for the construction of the line plan may be the minimization of the total cost or the maximization of the passengers’s comfort satisfying certain regulations. After the lines of the system are fixed, the train schedule can be determined. A criterion for the quality of a schedule is the total transit time of the passengers including the waiting time which should be minimized satisfying some operational constraints. For each trip of the schedule a train consisting of a locomotive and some carriages is needed for service. The assignment of rolling stock to schedule trips has to satisfy operational requirements. A comprehensible objective is to minimize the total cost. After all strategic and tactical planning the schedule has to be realized. Several external influences, for example delayed trains, force the dispatcher to recompute parts of the schedule on-line. A Web page with examples quoted in this survey can be found at http://www.math.tu-bs.de/mo/ismp.html.     

Transient magnetic reconnection and unstable shear layers

We study three-dimensional magnetic reconnection caused by the Kelvin-Helmholtz (KH) instability and differential rotation in subsonic and sub-Alfvenic flows. The flows, which are modeled by the resistive magnetohydrodynamic equations with constant resistivity, are stable in the direction of the magnetic field but unstable perpendicular to the magnetic field. Localized transient reconnection is observed on the KH time scale, and kinetic energy increases with decreasing resistivity. As in flux-transfer events in the Earth's magnetopause boundary layer, bipolar structures in the normal flux and bidirectional jetting away from reconnection zones are observed.     

Field induced biquadratic exchange in hard/soft ferromagnetic bilayers

The appearance of biquadratic exchange coupling between soft Fe and hard SmCo thin layers is found. The remanent magnetization in the Fe layer reorients from parallel to perpendicular with respect to the SmCo easy axis after application of large enough negative field. To explain such an unexpected behavior in contacting ferromagnetic layers a model is proposed based on Slonczewski's fluctuating exchange mechanism. In our samples a partial remagnetization of the hard layer creates fluctuations of the bilinear interactions. The intralayer exchange averaging of the resulting magnetization fluctuations in the soft layer causes the observed biquadratic coupling.     

Magnetic moment relaxation of a shallow acceptor center in heavily doped silicon

Results of studying the temperature dependence of the residual polarization of negative muons in crystalline silicon with germanium (9×10 ¹⁹ cm ^?3 ) and boron (4.1×10 ¹⁸ , 1.34×10 ¹⁹ , and 4.9×10 ¹⁹ cm ^?3 ) impurities are presented. It is found that, similarly to n-and p-type silicon samples with impurity concentrations up to ～10 ¹⁷ cm ^?3 , the relaxation rate ν of the magnetic moment of a _μ Al acceptor in silicon with a high impurity concentration of germanium (9×10 ¹⁹ cm ^?3 ) depends on temperature as ν～T ^q , q≈3 at T=(5–30) K. An increase in the absolute value of the relaxation rate and a weakening of its temperature dependence are observed in samples of degenerate silicon in the given temperature range. Based on the experimental data obtained, the conclusion is made that the spin-exchange scattering of free charge carriers makes a significant contribution to the magnetic moment relaxation of a shallow acceptor center in degenerate silicon at T?30 K. Estimates are obtained for the effective cross section of the spin-exchange scattering of holes (σ _h ) and electrons (σ _e ) from an Al acceptor center in Si: σ _h ～10^?13 cm² and σ _e ～8×10^?15 cm² at the acceptor (donor) impurity concentration n _a (n _d )～4×10¹⁸ cm^?3.     

Photorefractive properties of Cr-doped Sr0.61Ba0.39Nb2O6 related to crystal purity and doping concentration

The purity and the concentrations of the constituents Sr, Ba, Nb and Cr were determined in single crystals of chromium-doped Sr_0.61Ba_0.39Nb₂O₆ (SBN) by instrumental neutron activation analysis (INAA) and X-ray fluorescence analysis (XRF). Experiments with different Cr concentrations C_Cr reveal constant Sr and Ba concentrations as well as a decreasing Nb concentration with an increasing C_Cr. Therefore, Cr is incorporated at Nb lattice sites. The distribution coefficient of Cr is 1.2 between 100 and about 7000 ppm Cr and 1 above. From measurements of the holographic two-beam coupling gain Γ for different C_Cr, we deduced response times for the buildup of holographic gratings. The saturation value of Γ depends in a nonlinear manner on C_Cr and reaches its maximum at about 6000 ppm Cr. The inverse photorefractive response time of the grating erasure depends linearly on the erasure intensity for all doping concentrations. Thus a one-center model can be assumed for the charge transport in SBN:Cr. Received: 8 December 2000 / Revised version: 23 January 2001 / Published online: 21 March 2001     

Very compact tunable solid-state laser utilizing a thin-film organic semiconductor

Optically pumped organic semiconductor lasers are fabricated by evaporation of a thin film of tris(8-hydroxyquinoline) aluminum (Alq(3)) molecularly doped with a laser dye on top of a polyester substrate with an embossed grating structure. We achieve low-threshold, longitudinally monomode distributed-feedback laser operation. By varying the film thickness of the organic semiconductor film, we can tune the wavelength of the surface-emitting laser over 44 nm. The low laser threshold allows the use of a very compact all-solid-state pump laser ( approximately 10 cm long). This concept opens up a way to obtain inexpensive lasers that are tunable over the whole visible range.     

10 ps High Power Laser System at 248 nm

A simple 10 ps KrF laser system consisting of quenched dye laser oscillator pumped by a XeCl laser, a distributed feedback dye laser (DFDL), a three-stage dye amplifier and a two-stage KrF amplifier are described. The output of the laser system is 140 mJ with 10 ps pulse width by using polarization-multiplexing amplification. A focused output laser intensity of about 4×1015 W/cm2 is obtained. The saturation fluence for KrF laser is obtained with this laser.     

Application of magneto-optical method for real-time visualization of eddy currents with high spatial resolution for nondestructive testing

In this paper we describe a new, laser supported, eddy current microscope for the real-time visualization of eddy current distributions. In the experimental set-up, the induction of eddy currents is conventionally performed by an alternating current excitation coil above the object surface. The magnetic field induced by the eddy currents is detected by the use of the Faraday effect. For that purpose, a laser beam is passed through a suitable thin crystal integrated in the excitation coil. The polarization direction of the laser beam is rotated in this thin crystal depending on the local magnetic field. The area distribution of the rotation angle is transformed into a grey value picture using an optical set-up, which comprises a conventional microscope, a lighting technique, an analyzator, and a CCD sensor. By choosing a suitable optic, a real-time measurement of the magnetic field with a high spatial resolution can be carried out. In this paper, the basic physics and the design of the new microscope are described. The initial results of experimental investigations concerning the resolution power of magneto-optic eddy current sensors are presented and compared with conventional eddy current sensors.     

Monopoles in an antiferromagnetic Bose-Einstein condensate

We show that even in three dimensions an antiferromagnetic spin-1 Bose-Einstein condensate, which can, for instance, be created with (23)Na atoms in an optical trap, has not only singular linelike vortex excitations, but also allows for singular pointlike topological excitations, i.e., monopoles similar to the 't Hooft-Polyakov monopoles. We discuss the static and dynamic properties of these monopoles.     

Search for heavy particles decaying into electron-positron pairs in pp collisions

We present results of searches for technirho (rho(T)), techniomega (omega(T)), and Z' particles, using the decay channels rho(T),omega(T),Z'-->e(+)e(-). The search is based on 124.8 pb(-1) of data collected by the D0 detector at the Fermilab Tevatron during 1992-1996. In the absence of a signal, we set 95% C.L. upper limits on the cross sections for the processes pp-->rho(T),omega(T),Z'-->e(+)e(-) as a function of the mass of the decaying particle. For certain model parameters, we exclude the existence of degenerate rho(T) and omega(T) states with masses below about 200 GeV. We exclude a Z' with mass below 670 GeV, assuming that it has the same couplings to fermions as the Z boson.