Probing carrier dynamics in photo-excited graphene with time-resolved ARPES

The dynamics of photo-generated electron-hole pairs in solids are dictated by many-body interactions such as electron-electron and electron-phonon scattering. Hence, understanding and controlling these scattering channels is crucial for many optoelectronic applications, ranging from light harvesting to optical amplification. Here we measure the formation and relaxation of the photo-generated non-thermal carrier distribution in monolayer graphene with time- and angle-resolved photoemission spectroscopy. Using sub 10fs pulses we identify impact ionization as the primary scattering channel, which dominates the dynamics for the first 25fs after photo-excitation. Auger recombination is found to set in once the carriers have accumulated at the Dirac point with time scales between 100 and 250fs, depending on the number of non-thermal carriers. Our observations help in gauging graphene's potential as a solar cell and TeraHertz lasing material.Comment: 10 pages, 4 figure

An Investigation of the Last Interglacial's Climate Characteristics: Insights from a Stable Water Isotope Equipped Climate Model

The Last Interglacial (LIG), spanning from approximately 130,000 to 115,000 years ago, is the warm period immediately preceding the last ice age, and represents one of the most recent intervals in Eartha s history that was significantly warmer than the pre-industrial. As such, it is an excellent test bed for understanding the controlling dynamics of warm climate periods. By performing simulations of this period using a fully coupled, stable water isotope enhanced climate model (COSMOS-WISO), new insights into the strength of stable water isotopes as temperature proxies could be uncovered. The utility of the isotopic composition of rainfall, I 8OP, as a paleothermometer is examined. It was found that the changes in I 18OP do not always correspond to changes in temperature, particularly when only small magnitude temperature changes are considered. A second set of studies examined the match between simulated responses to LIG climate boundary conditions to measurements from various different paleoclimate archives. Of particular interest is the ability to reconstruct the North Atlantic temperature changes during the LIG, as these are closely tied to changes in the Atlantic Meridional Overturning Circulation (AMOC), which in turn redistributes large amounts of heat from the equatorial latitudes to the mid and high latitudes. Proxy evidence points to a cooling in this region during the early LIG, which may be indicative of a relatively weaker overturning circulation. However, the extent of this weakening is difficult to gauge based solely on temperature differences. When compared to model simulations, feasible temperature differences could be simulated with AMOC possibilities ranging from only a slightly weaker overturning circulation, to a stronger collapse possibly triggered by ice sheet melting. In order to eliminate one of these possibilities, additional comparisons with simulated isotopic signature in calcite were performed. When comparing to measurements from planktic foraminifera, a strong AMOC collapse triggered by ice melting could be ruled out, as the resulting simulated I 18OC values do not match with the observations. Comparing against Italian speleothem records allowed for the discovery of possible rapid climate change events during the LIG. Simultaneous excursions of I 18OC enrichment and I 13C enrichment indicate rapid drying and cooling, a typical response of an AMOC collapse. When comparing against model simulations of a hypothetical freshwater perturbation due to ice sheet melting, it was found that COSMOS-WISO was able to qualitatively reproduce the cooling and drying signals, yet quantitative comparisons of the I 18OC failed to produce the response seen in the records, and instead of an enrichment, a depletion is simulated. This led to several hypotheses; either the triggering mechanism used in the simulation is incorrect, and that the AMOC collapse is caused in a different way. Alternatively, the model and real-world have different dominant effects acting on the speleothem record. While in the real-world, the isotopic signature is dominated by the precipitation amount effect, in the model, changes to the source region play a more important role. Should this source region change also occur in the real world, this would indicate that the drying needed to achieve the I 18OC enrichment might be larger than previously thought. Finally, when examining possible triggering mechanisms for this overturning collapse, instabilities in the West Antarctic Ice Sheet (WAIS) were found. With the aid of a dynamic Ice Sheet Model, it could be determined that the WAIS collapses if ocean temperatures increase above a certain threshold, between 2-3E C. Applications to the future also demonstrated that Greenland Ice sheet melting plays an important role on the AMOC strength in the next several hundred years. Coupled climate-ice sheet modeling revealed that if Greenland melting is not included in the simulations, the AMOC strength is overestimated by as much as 2 Sv, which consequently also leads to an overestimation of the amount of future warming. Collectively, it could be found that the AMOC is a key player in the climate system, as changes to the overturning circulation induce feedbacks in other subcomponents of the climate as well. The isotopic signature of precipitation, I 18O is a useful simulation addition that allows for more direct model data comparisons, but it is still prone to the same limitations of model resolution as is also seen in the more traditional simulation/proxy comparisons

Inference and Estimation in Change Point Models for Censored Data

In general, the change point problem considers inference of a change in distribution for a set of time-ordered observations. This has applications in a large variety of fields and can also apply to survival data. With improvements to medical diagnoses and treatments, incidences and mortality rates have changed. However, the most commonly used analysis methods do not account for such distributional changes. In survival analysis, change point problems can concern a shift in a distribution for a set of time-ordered observations, potentially under censoring or truncation. In this dissertation, we first propose a sequential testing approach for detecting multiple change points in the Weibull accelerated failure time model, since this is sufficiently flexible to accommodate increasing, decreasing, or constant hazard rates and is also the only continuous distribution for which the accelerated failure time model can be reparametrized as a proportional hazards model. Our sequential testing procedure does not require the number of change points to be known; this information is instead inferred from the data. We conduct a simulation study to show that the method accurately detects change points and estimates the model. The numerical results along with a real data application demonstrate that our proposed method can detect change points in the hazard rate. In survival analysis, most existing methods compare two treatment groups for the entirety of the study period. Some treatments may take a length of time to show effects in subjects. This has been called the time-lag effect in the literature, and in cases where time-lag effect is considerable, such methods may not be appropriate to detect significant differences between two groups. In the second part of this dissertation, we propose a novel non-parametric approach for estimating the point of treatment time-lag effect by using an empirical divergence measure. Theoretical properties of the estimator are studied. The results from the simulated data and real data example support our proposed method

Illuminating the dark corridor in graphene: polarization dependence of angle-resolved photoemission spectroscopy on graphene

We have used s- and p-polarized synchrotron radiation to image the electronic structure of epitaxial graphene near the K-point by angular resolved photoemission spectroscopy (ARPES). Part of the experimental Fermi surface is suppressed due to the interference of photoelectrons emitted from the two equivalent carbon atoms per unit cell of graphene's honeycomb lattice. Here we show that by rotating the polarization vector, we are able to illuminate this 'dark corridor' indicating that the present theoretical understanding is oversimplified. Our measurements are supported by first-principles photoemission calculations, which reveal that the observed effect persists in the low photon energy regime.Comment: 5 pages, 4 figure

NUMERICAL AND EXPERIMENTAL ANALYSIS OF A CENTRIFUGAL PUMP WITH DIFFERENT ROTOR GEOMETRIES

The paper presents a comparative analysis of the operation of two variants of centrifugal pump rotors, a description of the main parameters, and the influence of the blade geometry on the performance characteristics obtained. Rotors have been designed using the arc and point method. Based on the developed 3D CAD models, the rotors were printed using the rapid prototyping method on a 3D printer in FFF (Fused Filament Fabrication) technology, in order to experimentally verify the performance, by placing them on the Armfield FM50 test stand. The analysis part of the CFD includes a fluid flow in Ansys Fluent. The process of creating a flow domain and generating a structural mesh was described, along with the definition of boundary conditions, the definition of physical conditions and the turbulence model. The distribution of pressures and velocities in the meridional sections is shown graphically. The chapter with the experimental analysis contains a description of the measuring stand and the methodology used. The results obtained made it possible to generate the characteristics, making it possible to compare the results received. The results allowed to note the influence of geometry on the behavior of the rotors during operation in the system and to indicate that the arc rotor gets a 7% higher head and 2% higher efficiency than the point method rotor, which gives the basis for its commercial use in industry

An Error in the Copower Considerations

Accompanies October 13, 1976 letter from Karl Heinrich Hofmann to Jimmie D. Lawson

Electronic-structural dynamics in graphene

We review our recent time- and angle-resolved photoemission spectroscopy experiments, which measure the transient electronic structure of optically driven graphene. For pump photon energies in the near infrared (ℏωpump = 950 meV), we have discovered the formation of a population-inverted state near the Dirac point, which may be of interest for the design of THz lasing devices and optical amplifiers. At lower pump photon energies (ℏωpump pump = 200 meV), a transient enhancement of the electron-phonon coupling constant is observed, providing interesting perspective for experiments that report light-enhanced superconductivity in doped fullerites in which a similar lattice mode was excited. All the studies reviewed here have important implications for applications of graphene in optoelectronic devices and for the dynamical engineering of electronic properties with light