Cluster structures within Fermionic Molecular Dynamics

The many-body states in an extended Fermionic Molecular Dynamics approach are flexible enough to allow the description of nuclei with shell model nature as well as nuclei with cluster and halo structures. Different many-body configurations are obtained by minimizing the energy under constraints on collective variables like radius, dipole, quadrupole and octupole deformations. In the sense of the Generator Coordinate Method we perform variation after projection and multiconfiguration calculations. The same effective interaction derived from realistic interactions by means of the Unitary Correlation Operator Method is used for all nuclei. Aspects of the shell model and cluster nature of the ground and excited states of C12 are discussed. To understand energies and radii of neutron-rich He isotopes the soft-dipole mode is found to be important.Comment: 5 pages, proceedings of the 8th International conference on Clustering Aspects of Nuclear Structure and Dynamics, Nov. 2003, Nara, Japan, to be published in Nucl. Phys.

Dynamical phase diagram of Gaussian BEC wave packets in optical lattices

We study the dynamics of self-trapping in Bose-Einstein condensates (BECs) loaded in deep optical lattices with Gaussian initial conditions, when the dynamics is well described by the Discrete Nonlinear Schr\"odinger Equation (DNLS). In the literature an approximate dynamical phase diagram based on a variational approach was introduced to distinguish different dynamical regimes: diffusion, self-trapping and moving breathers. However, we find that the actual DNLS dynamics shows a completely different diagram than the variational prediction. We numerically calculate a detailed dynamical phase diagram accurately describing the different dynamical regimes. It exhibits a complex structure which can readily be tested in current experiments in BECs in optical lattices and in optical waveguide arrays. Moreover, we derive an explicit theoretical estimate for the transition to self-trapping in excellent agreement with our numerical findings, which may be a valuable guide as well for future studies on a quantum dynamical phase diagram based on the Bose-Hubbard Hamiltonian

Nucleon-nucleon potentials in phase-space representation

A phase-space representation of nuclear interactions, which depends on the distance $\vec{r}$ and relative momentum $\vec{p}$ of the nucleons, is presented. A method is developed that permits to extract the interaction $V(\vec{r},\vec{p})$ from antisymmetrized matrix elements given in a spherical basis with angular momentum quantum numbers, either in momentum or coordinate space representation. This representation visualizes in an intuitive way the non-local behavior introduced by cutoffs in momentum space or renormalization procedures that are used to adapt the interaction to low momentum many-body Hilbert spaces, as done in the unitary correlation operator method or with the similarity renormalization group. It allows to develop intuition about the various interactions and illustrates how the softened interactions reduce the short-range repulsion in favor of non-locality or momentum dependence while keeping the scattering phase shifts invariant. It also reveals that these effective interactions can have undesired complicated momentum dependencies at momenta around and above the Fermi momentum. Properties, similarities and differences of the phase-space representations of the Argonne and the N3LO chiral potential, and their UCOM and SRG derivatives are discussed

From nucleon-nucleon interaction matrix elements in momentum space to an operator representation

Starting from the matrix elements of the nucleon-nucleon interaction in momentum space we present a method to derive an operator representation with a minimal set of operators that is required to provide an optimal description of the partial waves with low angular momentum. As a first application we use this method to obtain an operator representation for the Argonne potential transformed by means of the unitary correlation operator method and discuss the necessity of including momentum dependent operators. The resulting operator representation leads to the same results as the original momentum space matrix elements when applied to the two-nucleon system and various light nuclei. For applications in fermionic and antisymmetrized molecular dynamics, where an operator representation of a soft but realistic effective interaction is indispensable, a simplified version using a reduced set of operators is given

Satellite microwave observations of soil moisture variations

The electrically scanning microwave radiometer (ESMR) on the Nimbus 5 satellite was used to observe microwave emissions from vegetated and soil surfaces over an Illinois-Indiana study area, the Mississippi Valley, and the Great Salt Lake Desert in Utah. Analysis of microwave brightness temperatures (T sub B) and antecedent rainfall over these areas provided a way to monitor variations of near-surface soil moisture. Because vegetation absorbs microwave emission from the soil at the 1.55 cm wavelength of ESMR, relative soil moisture measurements can only be obtained over bare or sparsely vegetated soil. In general T sub B increased during rainfree periods as evaporation of water and drying of the surface soil occurs, and drops in T sub B are experienced after significant rainfall events wet the soil. Microwave observations from space are limited to coarse resolutions (10-25 km), but it may be possible in regions with sparse vegetation cover to estimate soil moisture conditions on a watershed or agricultural district basis, particularly since daily observations can be obtained

Nuclear Structure based on Correlated Realistic Nucleon-Nucleon Potentials

We present a novel scheme for nuclear structure calculations based on realistic nucleon-nucleon potentials. The essential ingredient is the explicit treatment of the dominant interaction-induced correlations by means of the Unitary Correlation Operator Method (UCOM). Short-range central and tensor correlations are imprinted into simple, uncorrelated many-body states through a state-independent unitary transformation. Applying the unitary transformation to the realistic Hamiltonian leads to a correlated, low-momentum interaction, well suited for all kinds of many-body models, e.g., Hartree-Fock or shell-model. We employ the correlated interaction, supplemented by a phenomenological correction to account for genuine three-body forces, in the framework of variational calculations with antisymmetrised Gaussian trial states (Fermionic Molecular Dynamics). Ground state properties of nuclei up to mass numbers A<~60 are discussed. Binding energies, charge radii, and charge distributions are in good agreement with experimental data. We perform angular momentum projections of the intrinsically deformed variational states to extract rotational spectra.Comment: 32 pages, 15 figure

Global analysis reveals differential regulation of mRNA decay in human induced pluripotent stem cells

2013 Fall.Includes bibliographical references.Induced Pluripotent Stem (iPS) cells are able to proliferate indefinitely while maintaining the capacity for unlimited differentiation and these properties are reflected by global changes in gene expression required for reprogramming of differentiated cells. Although the rate of transcription is an important regulator of steady-state mRNA levels, mRNA decay also plays a significant role in modulating the expression of cell-specific genes. The contribution of regulated mRNA decay towards establishing and maintaining pluripotency is largely unknown. To address this, we sought to determine global mRNA decay rates in iPS cells and the genetically-matched fibroblasts (HFFs) they were derived from. Using a microarray based approach, we determined half-lives for 5,481 mRNAs in both cell lines and identified three classes of mRNAs whose decay is differentially regulated in iPS cells compared to HFFs. We found that replication-dependent histone mRNAs are more abundant and more stable in iPS cells, resulting in increased histone protein abundances. This up-regulation of histone expression may facilitate the unique chromatin dynamics of pluripotent cells. A large set of C2H2 ZNF mRNAs are also stabilized in iPS cells compared to HFFs, possibly through reduced expression of miRNAs that target their coding regions. As many of these mRNAs encode transcriptional repressors, stabilization of these transcripts may support the overall increased expression of C2H2 ZNF transcription factors in early embryogenesis. Finally, we found that mRNAs containing C-rich elements in their 3'UTR are destabilized in iPS cells compared to HFFs and many of these mRNAs encode factors important for development. Interestingly, we also identified the Poly(C)-Binding Protein (PCBP) family as differentially regulated in iPS cells and investigated their possible involvement in regulation of the mRNAs in our dataset identified as destabilized in iPS cells and having C-rich 3'UTR elements. Thus, we identified several interesting classes of mRNAs whose decay is differentially regulated in iPS cells compared to HFFs and our results highlight the importance of post-transcriptional control in stem cell gene expression. Coordinated control of mRNA decay is evident in pluripotency and characterization of the mechanisms involved would further contribute to our limited understanding of pluripotent gene expression and possibly identify additional targets for reprogramming