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Search Results: 1 - 10 of 2808 matches for " Torsten Andersen "
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Theoretical Study of Phase Conjugation in Mesoscopic Interaction Volumes
Torsten Andersen
Physics , 1999,
Abstract: In order to study the possible phase conjugation of optical near-fields, it is necessary to go beyond the slowly varying envelope- and electric dipole approximations that are normally applied in phase conjugation studies where spatially non-decaying (or at least slowly decaying) modes are mixed. In the present dissertation, the minimal coupling Hamiltonian is used to create a microscopic theoretical description of degenerate four-wave mixing. It is a semiclassical description where the electromagnetic field is treated as a classical quantity and the active medium is treated quantum mechanically. Numerical results are given for a single-level quantum well (exclusively intraband contributions) and for a two-level quantum well (mainly interband contribution). Focusing of a phase conjugated field is also discussed. (Full-length abstracts in Danish and English included).
Local-field study of phase conjugation in metallic quantum wells with probe fields of both propagating and evanescent character
Torsten Andersen,Ole Keller
Physics , 1999, DOI: 10.1103/PhysRevB.60.17046
Abstract: The phase conjugated response from nonmagnetic multi-level metallic quantum wells is analyzed and an essentially complete analytical solution is presented and discussed. The description is based on a semi-classical local-field theory for degenerate four-wave mixing in mesoscopic interaction volumes of condensed media developed by the present authors [T. Andersen and O. Keller, Phys. Scripta 58, 132 (1998)]. The analytical solution is supplemented by a numerical analysis of the phase conjugated response from a two-level quantum well in the case where one level is below the Fermi level and the other level is above. This is the simplest configuration of a quantum well phase conjugator in which the light-matter interaction can be tuned to resonance. The phase conjugated response is examined in the case where all the scattering takes place in one plane, and linearly polarized light is used in the mixing. In the numerical work we study a two-monolayer thick copper quantum well using the infinite barrier model potential. Our results show that the phase conjugated response from such a quantum-well system is highly dependent on the spatial dispersion of the matter response. The resonances showing up in the numerical results are analytically identified from the expressions for the linear and nonlinear response tensors. In addition to the general discussion of the phase conjugated response with varying frequency and parallel component of the wavevector, we present the phase conjugated response in the special case where the light is in resonance with the interband transition.
Substrate effects in the magneto-optical second-harmonic generation from first principles: Fe/Cu(001)
Torsten Andersen,W. Huebner
Physics , 2002, DOI: 10.1103/PhysRevB.65.174409
Abstract: We compute the nonlinear optical response of an Fe monolayer placed on top of 1 to 4 monolayers of Cu(001). Our calculation is based on ab initio eigenstates of the slab, which are obtained within the full-potential linearized augmented plane-wave method. The ground-state spin-polarized electronic structure is converged self-consistently to an accuracy better than 0.1 mRy. Subsequently, we take the spin-orbit interaction into account within a second variational treatment. The new set of eigenstates allows us to calculate the magneto-optical transition matrix elements. The second-harmonic response is determined in the reflection geometry with magnetization perpendicular to the surface (the so-called polar configuration) using the surface-sheet model. Adding layers of a noble metal (Cu) to the Fe monolayer gives a new degree of freedom for the inclusion of nonmagnetic Cu d bands to the nonlinear magneto-optical response of the slab, and the energy bands show that such an addition converges essentially to an addition of d states and a small broadening of the d band with growing number of Cu layers. The screened nonlinear optical susceptibility is calculated and converges quite well with a growing number of Cu layers. Our first-principles results confirm that the magnetic tensor elements of the nonlinear optical response tensor are roughly of the same order of magnitude as the nonmagnetic ones (in contrast to linear optics, where the magnetic response is only a minor correction).
Substrate effects on surface magetetism of Fe/W(110) from first principles
Torsten Andersen,Wolfgang Hübner
Physics , 2006, DOI: 10.1103/PhysRevB.74.184415
Abstract: Surface magnetic properties of the pseudomorphic Fe(110) monolayer on a W(110) substrate are investigated from first principles as a function of the substrate thickness (up to eight layers). Analyzing the magnetocrystalline anisotropy energies, we find stable (with respect to the number of substrate layers) in-plane easy and hard axes of magnetization along the [1[overline 1]0] and [001] directions, respectively, reaching a value in good agreement with experiment for thick substrates. Additionally, the changes to the magnetic spin moments and the density of the Fe d states are analyzed with respect to the number of substrate layers as well as with respect to the direction of magnetization. With respect to the number of W(110) substrate layers beneath the Fe(110) surface, we find that the first four substrate layers have a large influence on the electronic and magnetic properties of the surface. Beyond the fourth layer, the substrate has only marginal influence on the surface properties.
Two-dimensional H_alpha kinematics of bulgeless disk galaxies
Nadine Neumayer,C. Jakob Walcher,David Andersen,Sebastian F. Sanchez,Torsten Boeker,Hans-Walter Rix
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.18266.x
Abstract: We present two-dimensional H_alpha velocity fields for 20 late-type, disk-dominated spiral galaxies, the largest sample to date with high-resolution H_alpha velocity fields for bulgeless disks. From these data we derive rotation curves and the location of the kinematic centers. The galaxy sample was selected to contain nucleated and non-nucleated galaxies, which allows us to investigate what impact the gas kinematics in the host disk have on the presence (or absence) of a nuclear star cluster. In general, we find that the velocity fields span a broad range of morphologies. While some galaxies show regular rotation, most have some degree of irregular gas motions. There appears to be no systematic difference in the kinematics of nucleated and non-nucleated disks. Due to the large fields of view of the integral field units we use, we are able to observe the flattening of the rotation curve in almost all of our sample galaxies. This makes modeling of the velocity fields relatively straight-forward. Due to the complexities of the velocity fields, we obtain reliable determinations of the kinematic center for only 6 of our 20 sample galaxies. For all of these the locations of the nuclear star cluster/photometric center and the kinematic center agree within the uncertainties. If we disregard all kinematically irregular galaxies, our study concludes that nuclear star clusters truly occupy the nuclei, or dynamical centers, of their hosts. Our results are thus consistent with in-situ formation of nuclear star clusters. Yet, many well-motivated formation scenarios for nuclear clusters invoke off-center cluster formation and subsequent sinking of clusters due to dynamical friction. In that case, our results imply that dynamical friction in the centers of bulgeless galaxies must be very effective in driving massive clusters to the kinematic center. (abridged)
Systems Analysis Unfolds the Relationship between the Phosphoketolase Pathway and Growth in Aspergillus nidulans
Gianni Panagiotou, Mikael R. Andersen, Thomas Grotkj?r, Torsten B. Regueira, Gerald Hofmann, Jens Nielsen, Lisbeth Olsson
PLOS ONE , 2008, DOI: 10.1371/journal.pone.0003847
Abstract: Background Aspergillus nidulans is an important model organism for studies on fundamental eukaryotic cell biology and on industrial processes due to its close relation to A. niger and A. oryzae. Here we identified the gene coding for a novel metabolic pathway in A. nidulans, namely the phosphoketolase pathway, and investigated the role of an increased phosphoketolase activity. Methodology/Principal Findings Over-expression of the phosphoketolase gene (phk) improved the specific growth rate on xylose, glycerol and ethanol. Transcriptome analysis showed that a total of 1,222 genes were significantly affected by over-expression of the phk, while more than half of the affected genes were carbon source specific. During growth on glucose medium, the transcriptome analysis showed that the response to phk over-expression is targeted to neutralize the effect of the over-expression by regulating the acetate metabolism and initiate a growth dampening response. Conclusions/Significance Metabolic flux analysis using 13C-labelled glucose, showed that over-expression of phosphoketolase added flexibility to the central metabolism. Our findings further suggests that A. nidulans is not optimized for growth on xylose, glycerol or ethanol as the sole carbon sources.
Biochemical characterization of bovine plasma thrombin-activatable fibrinolysis inhibitor (TAFI)
Zuzana Valnickova, Morten Thaysen-Andersen, Peter H?jrup, Trine Christensen, Kristian W Sanggaard, Torsten Kristensen, Jan J Enghild
BMC Biochemistry , 2009, DOI: 10.1186/1471-2091-10-13
Abstract: The four N-linked glycosylation sequons within the activation peptide were all occupied in bovine TAFI, similar to human TAFI, while the sequon located within the enzyme moiety of the bovine protein was non-glycosylated. The enzymatic stability and the kinetic constants of TAFIa differed somewhat between the two proteins, as did the isoelectric point of TAFI, but not TAFIa. Equivalent to human TAFI, bovine TAFI was a substrate for transglutaminases and could be proteolytically cleaved by trypsin or thrombin/solulin complex, although small differences in the fragmentation patterns were observed. Furthermore, bovine TAFI exhibited intrinsic activity and TAFIa attenuated tPA-mediated fibrinolysis similar to the human protein.The findings presented here suggest that the properties of these two orthologous proteins are similar and that conclusions reached using the bovine TAFI may be extrapolated to the human protein.Human thrombin-activatable fibrinolysis inhibitor (TAFI) (EC; UniProt, Q96IY4), also known as plasma pro-carboxypeptidase B, R, and U, is a plasma metallocarboxypeptidase that attenuates fibrinolysis [1-10]. TAFI circulates in plasma as a 58 kDa protein with significant intrinsic activity [11,12]. The majority of the sites that undergo transglutaminase-mediated cross-linking to fibrin are primarily located on the heavily glycosylated pro-peptide, suggesting that TAFI becomes incorporated into the fibrin clot during later stages of the coagulation cascade [13]. A variety of trypsin-like proteinases have been shown to remove this peptide, generating the mature protein, TAFIa [4,14-17]. The isoelectric point (pI) of this proteolytically cleaved protein is around pH 8.5, which is significantly more basic than that of TAFI (pI 5.5) [18]. TAFIa remains in circulation by forming complexes with α2-macroglobulin and pregnancy zone protein [19] but is highly unstable, a feature initially attributed to proteolytic cleavage. However, this instability is now th
ANCA-Associated Vasculitides—An Update  [PDF]
Johanna Kegel, Torsten Kirsch
Health (Health) , 2014, DOI: 10.4236/health.2014.614209
Abstract: Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitides are characterized by destruction of small vessels, granulomatous inflammation of the respiratory tract and necrotizing glomerulonephritis. This review describes the clinical diagnosis and therapy as well as the patho-physiology of ANCA-associated vasculitides with a specific focus on the interplay of ANCAs with activated neutrophils and the deleterious pathophysiological consequences of neutrophil-endothelium interaction.
Scientific autonomy and planned research: the case of space science
Torsten Wilholt
Poiesis & Praxis , 2006, DOI: 10.1007/s10202-006-0031-6
Abstract: La recherche scientifique assistée par la navigation spatiale a toujours été soumise à un contr le externe comparativement important, et son ordre du jour doit souvent se conformer à des objectifs politiques changeants. Les scientifiques de l’espace peuvent-ils en appeler à l’une ou l’autre forme du principe de liberté de la recherche largement acquis pour revendiquer davantage d’autonomie? Dans cette contribution, la difficile question de l’autonomie au sein de la recherche planifiée est abordée par l’examen de trois arguments qui étayent le principe de la liberté de la recherche de différentes manières. Chaque argument a ses forces et ses limites. Utilisés ensemble, ils servent à démontrer les avantages particuliers de l’autonomie scientifique, mais dans le cas des sciences liées à la navigation spatiale, leur efficacité demeure en fin de compte restreinte. Néanmoins, comme ces arguments mettent en évidence les interactions entre l’autonomie scientifique, le processus démocratique et l’intérêt commun au savoir scientifique, ils font appara tre que la meilleure garantie d’un agenda cohérent et durable des activités scientifiques spatiales peut être assurée par une transparence accrue des décisions de politique scientifiques en faisant intervenir l’opinion publique démocratique.
Dark Matter Annihilation Signals: The Importance of Radiative Corrections
Bringmann, Torsten
High Energy Physics - Phenomenology , 2008,
Abstract: Being able to safely distinguish astrophysical from potential dark matter (DM) annihilation signals is of utmost importance for indirect DM searches. To this end, one has to rely on distinctive -- and unique -- spectral signatures to look for. Internal bremsstrahlung (IB), unavoidable in the presence of charged annihilation products, provides such a signature. In fact, as it generically dominates the gamma-ray spectrum expected from DM annihilations, at high energies, it may well turn out to be more important for indirect DM searches than the traditionally looked-for line signals. As illustrated in some detail, the observation of IB signatures would even allow to distinguish between different DM candidates or to constrain significantly the parameter space of, e.g., neutralino DM. The gamma-ray contributions reported here are therefore of great interest for the already launched Fermi/GLAST satellite and the upcoming new generation of Air Cherenkov Telescope systems like CTA -- which are most sensitive at the high energies where these effects are particularly important. Finally, radiative corrections may even significantly alter the positron spectrum from DM annihilations; an intriguing positron excess recently found by the PAMELA satellite might turn out to be an indication of the peculiar spectral signature expected in that case.
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