Nonlocal effects on magnetism in the diluted magnetic semiconductor Ga_{1-x}Mn_{x}As

The magnetic properties of the diluted magnetic semiconductor Ga_{1-x}Mn_{x}As are studied within the dynamical cluster approximation. We use the k-dot-p Hamiltonian to describe the electronic structure of GaAs with spin-orbit coupling and strain effects. We show that nonlocal effects are essential for explaining the experimentally observed transition temperature and saturation magnetization. We also demonstrate that the cluster anisotropy is very strong and induces rotational frustration and a cube-edge direction magnetic anisotropy at low temperature. With this, we explain the temperature-driven spin reorientation in this system.Comment: 4 pages, 4 figures; to be published in Phys. Rev. Let

Extensions of numerical methods for strongly correlated electron systems

This work presents extensions of the numerical methods for strongly correlated electron systems. The first part of the thesis discusses extensions and applications of the quantum cluster theories to the systems of classical spins. It is shown that such extensions can provide faster convergence through better estimation of the effects of fluctuations, yet they can also possess shortcomings which limit their application in the studies of the phase transitions. The second part of the thesis is dedicated to the numerical studies of the Hubbard model. Present Quantum Monte Carlo methods are reviewed and relationships among them are elucidated. The final part of the thesis contains the application of the developed numerical methods to investigate the phase diagram of the two-dimensional Hubbard model, especially the evidence of the Quantum Critical Point (QCP) at a finite doping. High accuracy results for thermodynamic quantities are presented in support of the existence of the QCP at a finite doping in two-dimensional Hubbard model. The relation of the QCP to the charge fluctuations is revealed and a mechanism that relates QCP to incipient phase separation is proposed

Weiterbildung - Herausforderung und Chance: Bericht d. Komm. Weiterbildung erstellt im Auftr. d. Landesregierung von Baden-Wuerttemberg

Available from Bibliothek des Instituts fuer Weltwirtschaft, ZBW, Duesternbrook Weg 120, D-24105 Kiel C 142846 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Chlorophyll-Specific Absorption Coefficient of Phytoplankton in World Oceans: Seasonal and Regional Variability

This study investigates the seasonal and regional variability in the chlorophyll-specific absorption coefficient of phytoplankton at 443 nm (aph*(443); unit: m2 mg−1) in surface oceans. It is focused on the time series data derived from the satellite products of chlorophyll-a (Chl-a) concentration and the phytoplankton absorption coefficient. Global estimates of aph*(443) reveal a decreasing gradient from the open ocean toward the coastal environment, with considerable spatial variance. Seasonal variations are prominent over most oceans, resulting in substantial deviations from the climatological means. A sinusoidal model was fitted to the monthly time series data to characterize the annual and semiannual features. The amplitudes and the phases of the monthly data were latitudinally dependent. The occurrence times of the maximum aph*(443) values were six months out of phase between the northern and southern hemispheres. Satellite observations present a global mean relationship between aph*(443) and Chl-a comparable with those obtained via in situ measurements. However, the seasonal/regional aph*(443) and Chl-a relationships can significantly depart from the global mean relationship. We propose a hypothesis that aph*(443) can be predicted as a function of geolocation and time. Preliminary validations with in situ matchup data confirm that the proposed model is a promising alternative to the traditional approaches requiring Chl-a as the input. The present exploration helps understand the phytoplankton biogeography and facilitates future efforts to improve bio-optical modeling, including estimating the primary production

Feshbach modulation spectroscopy of the Fermi-Hubbard model

In the vicinity of a Feshbach resonance, a system of ultracold atoms in an optical lattice undergoes rich physical transformations which involve molecule formation and hopping of molecules on the lattice and thus goes beyond a single-band Hubbard model description. We explore theoretically the response of this system to a harmonic modulation of the magnetic field, and thus of the scattering length, across the Feshbach resonance. In the regime in which the single-band Hubbard model is still valid, we provide results for the doublon production as a function of the various parameters, such as frequency, amplitude, etc., that characterize the field modulation, as well as the lattice depth. The method may uncover a route towards the efficient creation of ultracold molecules and also provide an alternative to conventional lattice-depth-modulation spectroscopy

In Search of Floating Algae and Other Organisms in Global Oceans and Lakes

Surface floating macroalgae, microalgae, and other marine and freshwater organisms have been reported in many specific regions around the globe. However, it is technically challenging to identify similar occurrences, or other types of floating organisms or materials, within the vast global oceans and lakes. In this study, we address this challenge through combining global-scale, 375-m resolution false-colored Red-Green-Blue (FRGB) imagery from the Visible Infrared Imaging Radiometer Suite (VIIRS) in NOAA\u27s online Ocean Color Viewer (OCView) for visual inspection and data from several other satellite sensors for spectral diagnostics. In the FRGB imagery, the near-infrared (NIR) band (862 nm) is used as the green channel, which is sensitive to floating algae and organisms/materials on the water surface. Visual inspection of the daily FRGB VIIRS imagery from January 2018 to October 2019 reveals the appearance of various slicks with different colors in many ocean regions and lakes. Combined with spectral diagnostics of the quasi-concurrent Sentinel-3A/3B Ocean and Land Colour Instrument (OLCI) and other higher spatial resolution satellite data as well as knowledge of local oceanography/limnology, most of these elongated or diffuse image features can be identified as Ulva, Sargassum, Noctiluca, Trichodesmium, Microcystis, oil slicks, or pumice rafts. Some of these identified features are found in regions where such occurrences have never been reported before. Some features are of unknown type as they have not been reported previously from remote sensing. In such cases, contemporaneous scientific literature and news reports as well as spectral diagnostics allow for educated inferences to be made. One example is from surface features in the northern Gulf of Maine, Bay of Fundy, and Southwest of Nova Scotia between early June and early July of 2019. Spectral shapes of the FRGB image features indicate transparent materials lacking pigments. Knowledge of local fisheries oceanography and frequent news reports all suggest that these unknown image features may be aggregations or blooms of sea jellies and, to a lesser extent, salps. Another example is from the Great Salt Lake, where image features in the south arm of the lake are speculated to be caused by aggregations of brine shrimp eggs. Once confirmed from field sightings, these findings may represent a milestone in satellite remote sensing because previously remote sensing could only be used to infer oceanographic environments conducive to sea jellies or shrimps. In addition to the findings presented here, the approach in this study may serve as a template to discover various known and unknown types of floating algae and organisms/materials as well as to routinely monitor and track their distributions and movements