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Search Results: 1 - 10 of 113843 matches for " O. Steffen "
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Structure, Scaling and Phase Transition in the Optimal Transport Network
Steffen Bohn,Marcelo O. Magnasco
Physics , 2006, DOI: 10.1103/PhysRevLett.98.088702
Abstract: We minimize the dissipation rate of an electrical network under a global constraint on the sum of powers of the conductances. We construct the explicit scaling relation between currents and conductances, and show equivalence to a a previous model [J. R. Banavar {\it et al} Phys. Rev. Lett. {\bf 84}, 004745 (2000)] optimizing a power-law cost function in an abstract network. We show the currents derive from a potential, and the scaling of the conductances depends only locally on the currents. A numerical study reveals that the transition in the topology of the optimal network corresponds to a discontinuity in the slope of the power dissipation.
Finite temperature and pressure molecular dynamics for BaFe2As2
Steffen Backes,Harald O. Jeschke
Physics , 2013, DOI: 10.1103/PhysRevB.88.075111
Abstract: We study the temperature and pressure dependence of the structural and electronic properties of the iron pnictide superconductor BaFe2As2. We use density functional theory based Born-Oppenheimer molecular dynamics simulations to investigate the system at temperatures from T=5 K to 150 K and pressures from P=0 GPa to 30 GPa. When increasing the pressure at low temperature, we find the two transitions from an orthorhombic to a tetragonal and to a collapsed tetragonal structure that are also observed in zero temperature structure relaxations and in experiment. However, these transitions are considerably smeared out at finite temperature, whereas the critical pressure for the first transition increases with temperature. We also analyze the electronic structure of BaFe2As2 at finite temperature and work out differences between the time averaged band structure and Fermi surface at finite temperature compared to the known zero temperature results. Our results should be helpful for resolving some open issues in experimental reports for BaFe2As2 under high pressure.
The Anthropocene, global change and sleeping giants: where on Earth are we going?
Will Steffen
Carbon Balance and Management , 2006, DOI: 10.1186/1750-0680-1-3
Abstract: Although a very important exercise, this approach to defining dangerous climate change can itself be dangerous, in particular because it often ignores the systemic nature of the global environment. Feedbacks and nonlinearities are the rule, not the exception, in the functioning of the Earth System [2], and in this Anthropocene era, where human activities have become a global geophysical force in their own right, there is no doubt that surprises await those who apply linear logic to the climate problem. The carbon cycle is centrally involved in many of these feedbacks and nonlinearities.Here we briefly review several of the more important so-called "sleeping giants" in the carbon cycle, processes that have the potential to accelerate the rate of warming beyond that attributed to human emissions of greenhouse gases [3]. The first of these is based on the impact on soil respiration of rising temperature and changing soil moisture, an example of a response of ecosystem physiology to climate change. Although there is still debate about the magnitude of the increase in soil respiration with temperature, and whether there are compensating effects of enhanced plant growth due to mobilisation of nitrogen in the process, the general consensus is that increasing temperature will cause an increase in the emission of CO2 from soil carbon [4].A second "sleeping giant" is the increase in disturbance in terrestrial ecosystems, often associated with pulses of carbon to the atmosphere. The most notable of these are wildfires and pest outbreaks, both sensitive to both warming and changes in the moisture regime. Although these are natural phenomena in the dynamics of terrestrial ecosystems, an increase in the frequency or extent of these disturbances results in a net loss of carbon to the atmosphere. Observations of the large areas of boreal forest in the northern high latitudes suggest that over the past couple of decades, these forests have experienced enhanced rates and/or areas of
Near-surface stellar magneto-convection: simulations for the Sun and a metal-poor solar analog
M. Steffen,H. -G. Ludwig,O. Steiner
Physics , 2009, DOI: 10.1017/S174392130903052X
Abstract: We present 2D local box simulations of near-surface radiative magneto-convection with prescribed magnetic flux, carried out with the MHD version of the CO5BOLD code for the Sun and a solar-like star with a metal-poor chemical composition (metal abundances reduced by a factor 100, [M/H]=-2). The resulting magneto-hydrodynamical models can be used to study the influence of the metallicity on the properties of magnetized stellar atmospheres. A preliminary analysis indicates that the horizontal magnetic field component tends to be significantly stronger in the optically thin layers of metal-poor stellar atmospheres.
Microscopic nature of correlations in multi-orbital AFe2As2 (A=K, Rb, Cs): Hund's coupling versus Coulomb repulsion
Steffen Backes,Harald O. Jeschke,Roser Valenti
Physics , 2015, DOI: 10.1103/PhysRevB.92.195128
Abstract: We investigate via LDA+DMFT (local density approximation combined with dynamical mean field theory) the manifestation of correlation effects in a wide range of binding energies in the hole-doped family of Fe-pnictides $A$Fe$_2$As$_2$ ($A={\rm K}$, Rb, Cs) as well as the fictitious FrFe$_2$As$_2$ and $a$-axis stretched CsFe$_2$As$_2$. This choice of systems allows for a systematic analysis of the interplay of Hund's coupling $J_H$ and on-site Coulomb repulsion $U$ in multi-orbital Fe-pnictides under negative pressure. With increasing ionic size of the alkali metal, we observe a non-trivial change in the iron $3d$ hoppings, an increase of orbitally-selective correlations and the presence of incoherent weight at high-binding energies that do not show the typical lower Hubbard-band behavior but rather characteristic features of a Hund's metal. This is especially prominent in $a$-stretched CsFe$_2$As$_2$. We also find that the coherent/incoherent electronic behavior of the systems is, apart from temperature, strongly dependent on $J_H$ and we provide estimates of the coherence scale $T^*$. We discuss these results in the framework of reported experimental observations.
Resonant sets, diophantine conditions, and Hausdorff measures for translation surfaces
Luca Marchese,Rodrigo Trevi?o,Steffen Weil
Mathematics , 2015,
Abstract: We consider Teichm\"uller geodesics in strata of translation surfaces. We prove a Jarn\'ik-type inequality for geodesics bounded in some compact part of the stratum and we establish generalized logarithmic laws for geodesics admitting excursions to infinity at a given prescribed rate. Our main tool are planar resonant sets arising from a given translation surface, that is the countable sets of directions of its saddle connections or of its closed geodesics, filtered according to length. We study approximations of a general direction by the elements of a resonant set. In an abstract setting, and assuming specific metric properties for resonant sets, we prove a dichotomy for the Hausdorff measure of well approximable directions and an estimate on the dimension of badly approximable directions. Then we prove that resonant sets arising from a translation surface satisfies the required metric properties. Our techniques also give estimates on the Hausdorff dimension of the set of directions in a rational billiard having fast recurrence.
Preservation of the shoulder joint by the use of a hybrid-spacer after septic loosening of a reversed total shoulder joint arthroplasty: a case report
Steffen Brodt,Gunther O. Hofmann,Andreas H. Tiemann
Strategies in Trauma and Limb Reconstruction , 2010, DOI: 10.1007/s11751-010-0086-8
Abstract: Infections of a total joint replacement (TJR) of the shoulder are rare complications. After revision surgery, the incidence rises dramatically. If infection occurs, it leads to a loss of function and may be devastating to the joint. Treatment options range from single- to multiple-staged revision programs, permanent resection arthroplasty or exarticulation. In this case, a reversed shoulder endoprosthesis, which was implanted after multiple revisions of a TJR due to a posttraumatic omarthrosis and rotator cuff insufficiency, got infected. A hybrid-spacer, made of a humeral nail and a custom-made PMMA spacer forming the humeral head, was used during the revision program. After two operations, clinical and paraclinical signs turned back to normal. The patient felt well and was satisfied with the result of the therapy. The hybrid-spacer was then left in situ as a definitive solution with a satisfying range of motion. This case report shows that a hybrid-spacer can be helpful in the treatment of an infected shoulder TJR.
Comparative studies on storage cells in tardigrades during starvation and anhydrobiosis
Andy REUNER, Steffen HENGHERR, Franz Brümmer, Ralph O. SCHILL
Current Zoology , 2010,
Abstract: The impact of starvation and anhydrobiosis on the number and size of the storage cells in the tardigrade species Milnesium tardigradum, Paramacrobiotus tonollii and Macrobiotus sapiens was investigated to gain more insight on the energetic side of anhydrobiosis. Storage cells are free floating cells within the body cavity of tardigrades and are presumed to store and release energy in form of glycogen, protein and fat to maintain a constant nutrient regime for the other tissues. The body size of the animals was not correlated with the size of the storage cells, however, M. tardigradum the largest species analysed also had the largest storage cells. A reduction in the size of the storage cells is apparent in all three species after seven days of starvation. A seven-day period of anhydrobiosis leads to a decrease in cell size in M. tardigradum but not in P. tonollii and M. sapiens. Although M. sapiens was raised on green algae, and M. tardigradum and P. tonollii were fed with rotifers and nematodes this difference in nourishment was not reflected in the response of the storage cells to anhydrobiosis [Current Zoology 56 (2): 259–263, 2010].
Origin of the superconducting state in the collapsed tetragonal phase of KFe2As2
Daniel Guterding,Steffen Backes,Harald O. Jeschke,Roser Valenti
Physics , 2015, DOI: 10.1103/PhysRevB.91.140503
Abstract: Recently, KFe$_2$As$_2$ was shown to exhibit a structural phase transition from a tetragonal to a collapsed tetragonal phase under applied pressure of about $15~\mathrm{GPa}$. Surprisingly, the collapsed tetragonal phase hosts a superconducting state with $T_c \sim 12~\mathrm{K}$, while the tetragonal phase is a $T_c \leq 3.4~\mathrm{K}$ superconductor. We show that the key difference between the previously known non-superconducting collapsed tetragonal phase in AFe$_2$As$_2$ (A= Ba, Ca, Eu, Sr) and the superconducting collapsed tetragonal phase in KFe$_2$As$_2$ is the qualitatively distinct electronic structure. While the collapsed phase in the former compounds features only electron pockets at the Brillouin zone boundary and no hole pockets are present in the Brillouin zone center, the collapsed phase in KFe$_2$As$_2$ has almost nested electron and hole pockets. Within a random phase approximation spin fluctuation approach we calculate the superconducting order parameter in the collapsed tetragonal phase. We propose that a Lifshitz transition associated with the structural collapse changes the pairing symmetry from $d$-wave (tetragonal) to $s_\pm$ (collapsed tetragonal). Our DFT+DMFT calculations show that effects of correlations on the electronic structure of the collapsed tetragonal phase are minimal. Finally, we argue that our results are compatible with a change of sign of the Hall coefficient with pressure as observed experimentally.
Electronic structure and de Haas-van Alphen frequencies in KFe2As2 within LDA+DMFT
Steffen Backes,Daniel Guterding,Harald O. Jeschke,Roser Valenti
Physics , 2014, DOI: 10.1088/1367-2630/16/8/083025
Abstract: Recent density functional theory (DFT) calculations for KFe2As2 have shown to be insufficient to satisfactorily describe angle-resolved photoemission (ARPES) measurements as well as observed de Haas van Alphen (dHvA) frequencies. In the present work, we extend DFT calculations based on the full-potential linear augmented plane-wave method by dynamical mean field theory (DFT+DMFT) to include correlation effects beyond the local density approximation. Our results indicate that KFe2As2 is a moderately correlated metal with a mass renormalization factor of the Fe 3d orbitals between 1.6 and 2.7. Also, the obtained shape and size of the Fermi surface are in good agreement with ARPES measurements and we observe some topological changes with respect to DFT calculations like the opening of an inner hole cylinder at the Z point. As a result, our calculated dHvA frequencies differ greatly from existing DFT results and qualitatively agree with experimental data. On this basis, we argue that correlation effects are important to understand the -presently under debate- nature of superconducting state in KFe2As2.
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