Investigating the ecology of a threatened ecosystem: Alpine snowbank communities of Mt. Washington, NH

In northeastern North America, alpine snowbank (or snowbed) communities are rare plant assemblages that form in sheltered sites above treeline where late-lying snow provides insulation from late-season frosts and a longer-lasting source of water. These communities are highly diverse and may provide many beneficial ecosystem services. Though work has been done to document their location and community composition, little is known about the relationships between plants and abiotic conditions in alpine snowbanks of the Northeast. We studied the relative effects of snowmelt date and temperature on the phenological responses of seven alpine snowbank plants and examined plant traits and community metrics (diversity and richness) across the snowmelt gradient at alpine snowbank sites on Mt. Washington, NH. Peak of observed phenophases was positively correlated with snowmelt date, but lag time (time between snowmelt date and peak phenophase) was negatively correlated with snowmelt date. Higher temperature was an important factor in the quickened phenological response of plants at later-melting sites. There was a clear transition in both community composition and traits across the snowmelt gradient; moving outward from snowbank cores, vascular plant diversity decreased and lichen diversity increased, with no trend evident in bryophytes. This corresponded to a transition in observed traits both within species and at the community-level, with snowbank core habitats having lower leaf dry matter content and greater height, leaf area, and specific leaf area than edge habitats. A similar difference in plant traits was observed among conspecifics between lowland and alpine habitats, though we were unable to conclude whether alpine ecotypes of those species exist. The change in environmental conditions across the snowmelt gradient, mediated by snow persistence, is important in determining plant phenological responses and growing conditions on Mt. Washington in ways as found elsewhere at similar sites worldwide. Due to prevalence of leafy species and reliance on specific environmental conditions, alpine snowbank communities are considered particularly sensitive to environmental change, and may be indicators of climatic trends occurring in northeastern North America

Simulation based parameterization for process monitoring of machining operations

Process monitoring can prevent machine and tool failure in metal-cutting. A successful process monitoring of cutting processes depends on reliable monitoring limits for the process. In industrial applications these limits have to be generated in a learning phase during a ramp-up process. In order to enable process monitoring for single batch production without a learning phase, this paper describes a simulation based approach for generating reference data to set process limits. As a foundation for calculation of monitoring limits a position-based process simulation has to be established. In a first step an approach of modeling material removal is evaluated to check whether it fits the application for parameterizing the process monitoring. In this context the potentials of a process simulation for calculating process limits are clarified. Additionally the quality of data generated by this kind of simulation is discussed. In a second step a method is described to implement machine properties by a virtual machine tool within a simulation of material removal. For that purpose a method to use actual data of axis position and tool within the simulation of material removal is necessary. With these data a way-based simulation of material removal can generate reference parameters for monitoring limits instead of using data from a learning phase during the ramp-up process. By using position data of a virtual machine tool a reliable source for the actual position of all axes enables the position-based simulation to perform material removal in a more accurate way

A- and B-Exciton Photoluminescence Intensity Ratio as a Measure of Sample Quality for Transition Metal Dichalcogenide Monolayers

The photoluminescence (PL) in monolayer transition metal dichalcogenides (TMDs) is dominated by recombination of electrons in the conduction band with holes in the spin-orbit split valence bands, and there are two distinct emission features referred to as the A-peak (ground state exciton) and B-peak (higher spin-orbit split state). The intensity ratio of these two features varies widely and several contradictory interpretations have been reported. We analyze the room temperature PL from MoS2, MoSe2, WS2, and WSe2 monolayers and show that these variations arise from differences in the non-radiative recombination associated with defect densities. Hence, the relative intensities of the A- and B-emission features can be used to qualitatively asses the non-radiative recombination, and thus the quality of the sample. A low B/A ratio is indicative of low defect density and high sample quality. Emission from TMD monolayers is governed by unique optical selection rules which make them promising materials for valleytronic operations. We observe a notably higher valley polarization in the B-exciton relative to the A-exciton. The high polarization is a consequence of the shorter B-exciton lifetime resulting from rapid relaxation of excitons from the B-exciton to the A-exciton of the valence band.Comment: Final version is published online at APL Material

Understanding Variations in Circularly Polarized Photoluminescence in Monolayer Transition Metal Dichalcogenides

Monolayer transition metal dichalcogenides are promising materials for valleytronic operations. They exhibit two inequivalent valleys in the Brillouin zone, and the valley populations can be directly controlled and determined using circularly polarized optical excitation and emission. The photoluminescence polarization reflects the ratio of the two valley populations. A wide range of values for the degree of circularly polarized emission, Pcirc, has been reported for monolayer WS2, although the reasons for the disparity are unclear. Here we optically populate one valley, and measure Pcirc to explore the valley population dynamics at room temperature in a large number of monolayer WS2 samples synthesized via chemical vapor deposition. Under resonant excitation, Pcirc ranges from 2% to 32%, and we observe a pronounced inverse relationship between photoluminescence (PL) intensity and Pcirc. High quality samples exhibiting strong PL and long exciton relaxation time exhibit a low degree of valley polarization, and vice versa. This behavior is also demonstrated in monolayer WSe2 samples and transferred WS2, indicating that this correlation may be more generally observed and account for the wide variations reported for Pcirc. Time resolved PL provides insight into the role of radiative and non-radiative contributions to the observed polarization. Short non-radiative lifetimes result in a higher measured polarization by limiting opportunity for depolarizing scattering events

Exciton Diamagnetic Shifts and Valley Zeeman Effects in Monolayer WS2_2 and MoS2_2 to 65 Tesla

We report circularly-polarized optical reflection spectroscopy of monolayer WS$_2$ and MoS$_2$ at low temperatures (4~K) and in high magnetic fields to 65~T. Both the A and the B exciton transitions exhibit a clear and very similar Zeeman splitting of approximately $-$230~$\mu$eV/T ($g\simeq -4$), providing the first measurements of the valley Zeeman effect and associated $g$-factors in monolayer transition-metal disulphides. These results complement and are compared with recent low-field photoluminescence measurements of valley degeneracy breaking in the monolayer diselenides MoSe$_2$ and WSe$_2$. Further, the very large magnetic fields used in our studies allows us to observe the small quadratic diamagnetic shifts of the A and B excitons in monolayer WS$_2$ (0.32 and 0.11~$\mu$eV/T$^2$, respectively), from which we calculate exciton radii of 1.53~nm and 1.16~nm. When analyzed within a model of non-local dielectric screening in monolayer semiconductors, these diamagnetic shifts also constrain and provide estimates of the exciton binding energies (410~meV and 470~meV for the A and B excitons, respectively), further highlighting the utility of high magnetic fields for understanding new 2D materials.Comment: 9 pages, 5 figure

Magneto-reflection spectroscopy of monolayer transition-metal dichalcogenide semiconductors in pulsed magnetic fields

We describe recent experimental efforts to perform polarization-resolved optical spectroscopy of monolayer transition-metal dichalcogenide semiconductors in very large pulsed magnetic fields to 65 tesla. The experimental setup and technical challenges are discussed in detail, and temperature-dependent magneto-reflection spectra from atomically thin tungsten disulphide (WS$_2$) are presented. The data clearly reveal not only the valley Zeeman effect in these 2D semiconductors, but also the small quadratic exciton diamagnetic shift from which the very small exciton size can be directly inferred. Finally, we present model calculations that demonstrate how the measured diamagnetic shifts can be used to constrain estimates of the exciton binding energy in this new family of monolayer semiconductors.Comment: PCSI-43 conference (Jan. 2016; Palm Springs, CA

Innovative drive concept for machining robots

In this paper an innovative drive concept for robots to improve their machining capability is presented. As a test bed a two-axis robot is designed and equipped with torque motors with load-sided high resolution encoders in addition to conventional gear motors with harmonic drive gearboxes. The gear motors are used for positioning tasks while the torque motors in particular compensate static and dynamic load-sided angle errors. The model-based control algorithm is decoupled and separately actuates both the servo gear and torque motors. It is shown that a considerable increase of performance is possible when adding the torque motors especially regarding the compensation of dynamic angle errors. The paper will present the design and details of the new drive concept, the modeling basics and first simulation results

A Correlation-Based Fingerprint Verification System

In this paper, a correlation-based fingerprint verification system is presented. Unlike the traditional minutiae-based systems, this system directly uses the richer gray-scale information of the fingerprints. The correlation-based fingerprint verification system first selects appropriate templates in the primary fingerprint, uses template matching to locate them in the secondary print, and compares the template positions of both fingerprints. Unlike minutiae-based systems, the correlation-based fingerprint verification system is capable of dealing with bad-quality images from which no minutiae can be extracted reliably and with fingerprints that suffer from non-uniform shape distortions. Experiments have shown that the performance of this system at the moment is comparable to the performance of many other fingerprint verification systems