Carrier and polarization dynamics in monolayer MoS2

In monolayer MoS2 optical transitions across the direct bandgap are governed by chiral selection rules, allowing optical valley initialization. In time resolved photoluminescence (PL) experiments we find that both the polarization and emission dynamics do not change from 4K to 300K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature induced change in bandgap energy with the excitation laser energy an emission polarization of 40% is recovered at 300K, close to the maximum emission polarization for this sample at 4K.Comment: 7 pages, 7 figures including supplementary materia

Performance of concrete-filled stainless steel tubular (CFSST) columns after exposure to fire

The post-fire performance of concrete-filled stainless steel tubular (CFSST) columns subjected to an entire loading–fire history, including four characteristic phases: (i) ambient temperature loading, (ii) heating, (iii) cooling with constant external loads, and (iv) post-fire loading, is investigated in this paper. Sequentially coupled thermal-stress analyses are performed using ABAQUS to establish the temperature field and structural response of CFSST columns. To improve the precision of the finite element analysis (FEA) models, the influence of moisture on the thermal conductivity and specific heat of the concrete in the heating and cooling phases is considered by using subroutines. Existing fire and post-fire test data on CFSST columns are used to validate the FEA modelling. Comparisons between FEA and test results indicate that the accuracy of the model is acceptable; the FEA model is then extended to simulate CFSST columns subjected to the four characteristic phases. The behaviour of the CFSST columns during the four characteristic phases is explained by analysis of the temperature distribution, load versus axial deformation relations, failure modes and internal force redistribution. The excellent post-fire performance of CFSST columns is examined in comparison with traditional concrete-filled carbon steel tubular (CFST) columns with the same total cross-sectional area. The residual strength index is studied with respect to a series of parametric analyses. It is found that the residual strength of CFSST columns is higher than that of CFST columns after the same fire exposure, and that the diameter of the stainless steel tube, slenderness, heating time ratio and load ratio have a significant influence on the residual strength index

EPIC 219217635: A Doubly Eclipsing Quadruple System Containing an Evolved Binary

We have discovered a doubly eclipsing, bound, quadruple star system in the field of K2 Campaign 7. EPIC 219217635 is a stellar image with $Kp = 12.7$ that contains an eclipsing binary (`EB') with $P_A = 3.59470$ d and a second EB with $P_B = 0.61825$ d. We have obtained followup radial-velocity (`RV') spectroscopy observations, adaptive optics imaging, as well as ground-based photometric observations. From our analysis of all the observations, we derive good estimates for a number of the system parameters. We conclude that (1) both binaries are bound in a quadruple star system; (2) a linear trend to the RV curve of binary A is found over a 2-year interval, corresponding to an acceleration, $\dot \gamma = 0.0024 \pm 0.0007$ cm s$^{-2}$; (3) small irregular variations are seen in the eclipse-timing variations (`ETVs') detected over the same interval; (4) the orbital separation of the quadruple system is probably in the range of 8-25 AU; and (5) the orbital planes of the two binaries must be inclined with respect to each other by at least 25$^\circ$. In addition, we find that binary B is evolved, and the cooler and currently less massive star has transferred much of its envelope to the currently more massive star. We have also demonstrated that the system is sufficiently bright that the eclipses can be followed using small ground-based telescopes, and that this system may be profitably studied over the next decade when the outer orbit of the quadruple is expected to manifest itself in the ETV and/or RV curves.Comment: Accepted for publication in MNRA

Semi-classical States in Homogeneous Loop Quantum Cosmology

Semi-classical states in homogeneous loop quantum cosmology (LQC) are constructed by two different ways. In the first approach, we firstly construct an exponentiated annihilation operator. Then a kind of semi-classical (coherent) state is obtained by solving the eigen-equation of that operator. Moreover, we use these coherent states to analyze the semi-classical limit of the quantum dynamics. It turns out that the Hamiltonian constraint operator employed currently in homogeneous LQC has correct classical limit with respect to the coherent states. In the second approach, the other kind of semi-classical state is derived from the mathematical construction of coherent states for compact Lie groups due to Hall.Comment: 13 pages, submitted to CQ

Construction of a high-efficiency multi-site-directed mutagenesis

Although site-directed mutagenesis has been used in many fields, it still has low rate of success and high cost because of low-yield target products. A modified method for multi-site-directed mutagenesis was developed with shifted primer design and cold-start polymerase chain reaction (PCR). The developed method was successfully applied to hexapeptide gene synthesis and recombinant enterokinase gene modification in the plasmids pET41a and pET24b-EK. The efficiency was pronounced at a 1:10 molar ratio of 7-base mutant products to 705-bp fragment products as control. Even in a 10-base substitution mutagenic PCR, a 1:50 molar ratio of mutant  products to 705-bp fragment products was reached. Meanwhile, the quality of mutants was proved through the transformation efficiency and  sequencing. This method was beneficial to prepare high-quality multibase mutagenesis and also implied that large-scale multibase mutagenesis was feasible, efficient, economical, and productive.Key words: Site-directed multibase mutagenesis, shift primer, hexapeptide gene, enterokinase gene

Heated Motorized Stage for Nanoscale Thin Film Deposition

ME450 Capstone Design and Manufacturing Experience: Fall 2015Thin film deposition is a technology of applying a very thin film of material onto a substrate surface to be coated, or onto a previously deposited coating to form layers. As a modified Atomic Layer Deposition (ALD) method, Spatial ALD greatly improves the efficiency of the process by separating the half-reactions spatially instead of through the use of purge steps in convectional ALD. This paper proposes the use of heated motorized stage as a research prototype to further study the effect of deposition conditions such as gap size, gap alignment and substrate temperature on the SALD coating results. This strategy utilized three stepper motors and high precision non-contact gap sensors to control gap alignment and gap size between the substrate and the depositor. The details of the involved methodology, engineering analysis, validation of the final design and the design critiques are discussed in this paper.http://deepblue.lib.umich.edu/bitstream/2027.42/117338/1/ME450-F15-Project19-FinalReport.pd