Exciton Relaxation Cascade in Two-dimensional Transition-metal dichalcogenides

Monolayers of transition-metal dichalcogenides (TMDs) are characterized by an extraordinarily strong Coulomb interaction giving rise to tightly bound excitons with binding energies of hundreds of meV. Excitons dominate the optical response as well as the ultrafast dynamics in TMDs. As a result, a microscopic understanding of exciton dynamics is the key for technological application of these materials. In spite of this immense importance, elementary processes guiding the formation and relaxation of excitons after optical excitation of an electron-hole plasma has remained unexplored to a large extent. Here, we provide a fully quantum mechanical description of momentum- and energy-resolved exciton dynamics in monolayer molybdenum diselenide (MoSe$_2$) including optical excitation, formation of excitons, radiative recombination as well as phonon-induced cascade-like relaxation down to the excitonic ground state. Based on the gained insights, we reveal experimentally measurable features in pump-probe spectra providing evidence for the exciton relaxation cascade

Molecule signatures in photoluminescence spectra of transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) show an optimal surface-to-volume ratio and are thus promising candidates for novel molecule sensor devices. It was recently predicted that a certain class of molecules exhibiting a large dipole moment can be detected through the activation of optically inaccessible (dark) excitonic states in absorption spectra of tungsten-based TMDs. In this work, we investigate the molecule signatures in photoluminescence spectra in dependence of a number of different experimentally accessible quantities, such as excitation density, temperature as well as molecular characteristics including the dipole moment and its orientation, molecule-TMD distance, molecular coverage and distribution. We show that under certain optimal conditions, even room temperature detection of molecules can be achieved

Determination of the eta-transition form factor in the gamma p -< p eta -< p gamma e(+)e(-) reaction

The Dalitz decay eta -&gt; gamma e(+)e(-) has been measured using the combined Crystal Ball and TAPS photon detector setup at the electron accelerator MAMI-C. Compared to the most recent transition form-factor measurement in the e(+)e(-) channel, statistics have been improved by one order of magnitude. The e(+)e(-)invariant mass distribution shows a deviation from the QED prediction for a point-like particle, which can be described by a form-factor. Using the usual monopole transition form-factor parameterization, F(m(2)) = (1 - m(2)/Lambda(2))(-1), a value of Lambda(-2) = (1.92 +/- 0.35(stat) +/- 0.13(syst)) GeV(-2) has been determined. This value is in good agreement with a recent measurement of the eta Dalitz decay in the mu(+)mu(-) channel and with recent form-factor calculations. An improved value of the branching ratio BR(eta -&gt; gamma e(+)e(-)) = (6.6 +/- 0.4(stat) +/- 0.4(syst)) . 10(-3) has been determined. (C) 2011 Elsevier B.V. All rights reserved

Vergleichende Untersuchungen zwischen der konventionellen PAH- und Inulin-Clearance und der J 131-o-Hippursaeure und der Cr 51-EDTA-Clearance im Slope

SIGLEDEGerman

Multi-crystal analyzer detector system for use as detector in synchrotron for analysis of powders, has beam limiting device, which divides overall beam into individual beams and absorbs scattered radiation

The multi-crystal analyzer detector system has two beam limiting devices, two collimators (2,3) and detectors, which are connected with evaluation devices. The beam limiting device divides an overall beam into individual beams and absorbs the scattered radiation. The beam limiting device comprises a firmly mounted diaphragm with an exchangeable diaphragm (8,11) for each individual beam. The individual beams enter into a collimator path. The former beam limiting device and the former collimator are made of tungsten or tungsten alloys. An independent claim is included for a method for x-ray diffractometry of powders

The Extreme Conditions Beamline at PETRA III, DESY:Possibilities to conduct time resolved monochromatic and pink beam diffraction experiments in laser heated DAC

Powder x-ray diffraction experiments in laser heated diamond anvil cells (DAC) have been a standard experimental technique used at all 3rd generation extreme condition synchrotron facilities over the last decades. However, the combination of single crystal diffraction at simultaneous high-pressure and –temperature using a laser heated DAC has not been realized. This is in part because single crystal diffraction pattern created by monochromatic beam can only be collected on an area detector when the sample within the DAC is rotated, resulting in the obstruction of the laser heating beam. However, rotations of the sample can be eliminated when one uses pink beam Laue diffraction. In this work we describe the design of the “Extreme Conditions Beamline P02.2” at PETRA III, Hamburg, Germany, that will be used to conduct both monochromatic (8-70 keV) and pink beam diffraction experiments. Attention will be drawn to the pink beam capabilities of the station and the alternate use of monochromatic and pink x-ray beams and the possibility to conducted single crystal diffraction in the laser heated DAC. We will discuss the different stages of the beamline development and the high-pressure experimental techniques that we like to offer once commissioning of the beamline is completed. The possibility of conducting time resolved experiments in the dynamic DAC[1] and the pulsed laser heated DAC[2] in conjunction with fast choppers will be discussed as well as the possibility to shade light on the nature of transient phase stages occurring during phase transition at simultaneous high-pressures and -temperatures.References:[1] Evans WJ, Yoo CS, Lee GW, Cynn H., Lipp MJ, Visbeck K. (2007) Dynamic diamond anvil cell (dDAC): A novel device for studying the dynamic-pressure properties of materials. Rev. Sci. Inst., 78, 073904.[2] Goncharov AF, Beck P, Struzhkin VV, Hemley RJ and Crowhurst JC 2008 J. Phys. Chem. Solids 69 (2008) 2217