Pleistocene crees in Cyrenaica (Libya)

Nord-Cyrenaika besteht aus einem Gebiet von Kalksteinhügeln, welche von tiefen und schmalen Tälern durchsetzt sind. Auf den Abhängen dieser Täler finden sich Schuttdecken zweierlei Alters, wovon die älteren festverkittet und mit terrassiertem Kies wechsellagern, während die jüngeren nicht verfestigt sind und keine zugeordneten Terrassen haben. Beide Schichten können archäologisch mit der Schichtfolge in der Haua Fteah-Höhle korreliert werden, wo Higgs (1961) eine klimatische Folge nachgewiesen hat. Es wurde gefunden, daß die beiden Schichten zwei sukzessiven kalten Zeiten des Spätpleistozäns entsprechen. Die Gerölle werden dem Spaltenfrost zugeschrieben, welch letzterer daher mittelbar für die terrassierten Kiese verantwortlich wäre. Es wird versucht, diese Resultate als abhängig von Temperatur und Niederschlag zu erklären.researc

Experimental evidence of delocalized states in random dimer superlattices

We study the electronic properties of GaAs-AlGaAs superlattices with intentional correlated disorder by means of photoluminescence and vertical dc resistance. The results are compared to those obtained in ordered and uncorrelated disordered superlattices. We report the first experimental evidence that spatial correlations inhibit localization of states in disordered low-dimensional systems, as our previous theoretical calculations suggested, in contrast to the earlier belief that all eigenstates are localized.Comment: 4 pages, 5 figures. Physical Review Letters (in press

Mechanism of non-classical light emission from acoustically populated (311)A GaAs quantum wires

We employ surface acoustic waves (SAWs) to control the transfer of photo-generated carriers between interconnected quantum wells and quantum wires (QWRs) grown on pre-patterned (311)A GaAs substrates. Optical studies, carried out under remote acoustic excitation of a single QWR, have shown sharp photoluminescence lines and antibunched photons with tunable emission energy. These features are attributed to recombination of acoustically transported carriers in potential inhomogeneities within the wire. The origin of the photon antibunching is discussed in terms of a 'bottleneck' in the number of carriers trapped in the QWR, which restricts the number of recombination events per SAW cycle. We propose a model for antibunching based on the trapping of carriers induced by the SAW piezoelectric field in states at the interface between the GaAs QWR and the AlGaAs barriers. Non-classical light is emitted during the subsequent release of the trapped carriers into the recombination centers within the wire. The spatial distribution of the emitting recombination centers is estimated using time-resolved measurements

Quantum effects in linear and non-linear transport of T-shaped ballistic junction

We report low-temperature transport measurements of three-terminal T-shaped device patterned from GaAs/AlGaAs heterostructure. We demonstrate the mode branching and bend resistance effects predicted by numerical modeling for linear conductance data. We show also that the backscattering at the junction area depends on the wave function parity. We find evidence that in a non-linear transport regime the voltage of floating electrode always increases as a function of push-pull polarization. Such anomalous effect occurs for the symmetric device, provided the applied voltage is less than the Fermi energy in equilibrium

Gauge Theories in Particle Physics: A Practical Introduction, Volume 2: Non-Abelian Gauge Theories

Volume 2 of this revised and updated edition provides an accessible and practical introduction to the two non-Abelian quantum gauge field theories of the Standard Model of particle physics: quantum chromodynamics (QCD) and the Glashow-Salam-Weinberg (GSW) electroweak theory.This volume covers much of the experimental progress made in the last ten