Delmenhorst, Germany
Delmenhorst, Germany

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Sengupta A.,Hanse Wissenschaftskolleg
International Conference on Simulation of Semiconductor Processes and Devices, SISPAD | Year: 2016

In this work, we study the carrier transport properties of three non-graphitic armchair nanotubes (of Silicene, Germanene and Phosphorene) under an empirical tight binding (ETB)-non-equilibrium Green's function (NEGF) approach. The electronic properties are studied with extended Hückel theory, while phonon calculations are carried out with Stillinger-Weber classical potentials in ATK. The impact of Stone-Wales (SW) defects in electron and phonon transport properties of such tubes is also investigated. Our simulations show Silicene and Germanene nanotubes to offer much better electrical conduction than phosphorene NTs. The carrier transport and change charge density around the SW defect site is found to be affected more significantly in phosphorene nanotubes. Suppression of phonon transmission with introduction of defect is observed for all the cases. The overall results show a good possibility of defect engineered tailoring of electrical and thermal properties of these nanotubes. © 2016 IEEE.

Winter C.,University of Bremen | Bartholoma A.,Senckenberg Institute | Capperucci R.,Senckenberg Institute | Callies U.,Helmholtz Center Geesthacht | And 7 more authors.
Wasser und Abfall | Year: 2014

Statements on the state and the development of coastal waters can only be made through long-term observations and model approaches. In the joint project WIMO (Scientific Monitoring Concepts for the German Bight), concepts and methods of measurement are developed and assessed that deal with basic scientific issues as well as meet the requirements of regulatory monitoring in the framework of the European legislation.

Eilbeck J.C.,Heriot - Watt University | Enolski V.Z.,Hanse Wissenschaftskolleg | Enolski V.Z.,Ukrainian Academy of Sciences | Gibbons J.,Imperial College London
Journal of Physics A: Mathematical and Theoretical | Year: 2010

We compare and contrast three different methods for the construction of the differential relations satisfied by the fundamental Abelian functions associated with an algebraic curve. We realize these Abelian functions as logarithmic derivatives of the associated sigma function. In two of the methods, the use of the tau function, expressed in terms of the sigma function, is central to the construction of differential relations between the Abelian functions. © 2010 IOP Publishing Ltd.

Nikolic B.,Free University of Berlin | Nikolic B.,University of Belgrade | Balaz A.,University of Belgrade | Pelster A.,Hanse Wissenschaftskolleg | Pelster A.,University of Kaiserslautern
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

Here we study properties of a homogeneous dipolar Bose-Einstein condensate in a weak anisotropic random potential with Lorentzian correlation at zero temperature. To this end we solve perturbatively the Gross-Pitaevskii equation to second order in the random potential strength and obtain analytic results for the disorder ensemble averages of both the condensate and the superfluid depletion, the equation of state, and the sound velocity. For a pure contact interaction and a vanishing correlation length, we reproduce the seminal results of Huang and Meng, which were originally derived within a Bogoliubov theory around a disorder-averaged background field. For dipolar interaction and isotropic Lorentzian-correlated disorder, we obtain results which are qualitatively similar to the case of an isotropic Gaussian-correlated disorder. In the case of an anisotropic disorder, the physical observables show characteristic anisotropies which arise from the formation of fragmented dipolar condensates in the local minima of the disorder potential. © 2013 American Physical Society.

Al-Jibbouri H.,Free University of Berlin | Vidanovic I.,University of Belgrade | Vidanovic I.,Goethe University Frankfurt | Balaz A.,University of Belgrade | And 2 more authors.
Journal of Physics B: Atomic, Molecular and Optical Physics | Year: 2013

We investigate geometric resonances in Bose-Einstein condensates by solving the underlying time-dependent Gross-Pitaevskii equation for systems with two- and three-body interactions in an axially symmetric harmonic trap. To this end, we use a recently developed analytical method (Vidanović et al 2011 Phys. Rev. A 84 013618), based on both a perturbative expansion and a Poincaré-Lindstedt analysis of a Gaussian variational approach, as well as a detailed numerical study of a set of ordinary differential equations for variational parameters. By changing the anisotropy of the confining potential, we numerically observe and analytically describe strong nonlinear effects: shifts in the frequencies and mode coupling of collective modes, as well as resonances. Furthermore, we discuss in detail the stability of a Bose-Einstein condensate in the presence of an attractive two-body interaction and a repulsive three-body interaction. In particular, we show that a small repulsive three-body interaction is able to significantly extend the stability region of the condensate. © 2013 IOP Publishing Ltd.

Moore W.S.,Hanse Wissenschaftskolleg | Moore W.S.,University of South Carolina | Beck M.,University of Oldenburg | Riedel T.,University of Oldenburg | And 6 more authors.
Geochimica et Cosmochimica Acta | Year: 2011

A decade of studies of metal and nutrient inputs to the back-barrier area of Spiekeroog Island, NW German Wadden Sea, have concluded that pore water discharge provides a significant source of the enrichments of many components measured in the tidal channels during low tide. In this paper we add studies of radium isotopes to help quantify fluxes into and out of this system. Activities of radium isotopes in surface water from tidal channels in the back-barrier area exhibit pronounced changes in concert with the tide, with highest activities occurring near low tide. Other dissolved components: silica, total alkalinity (TA), manganese, and dissolved organic carbon (DOC) exhibit similar changes, with patterns matching the Ra isotopes. Uranium follows a reverse pattern with highest concentrations at high tide. Here we use radium isotope measurements in water column and pore water samples to estimate the fluxes of pore waters that enter the tidal channels during low tide. Using a flushing time of 4 days and the average activities of 224Ra, 223Ra, and 228Ra measured in the back-barrier surface and pore waters, we construct a balance of these isotopes, which is sustained by a deep pore water flux of (2-4)×108L per tidal cycle. This flux transports Ra and the other enriched components to the tidal channels and causes the observed low tide enrichments. An independent estimate of pore water recharge is based on the depletion of U in the tidal channels. The U-based recharge is about two times greater than the Ra-based discharge; however, other sinks of U could reduce the recharge estimate. The pore waters have wide ranges of enrichment in silica, alkalinity, manganese, DOC, and depletion of U with depth. We estimate concentrations of these components in pore water from the depth expected to contribute the majority of the pore water flux, 3.5m, to determine fluxes of these components to the tidal channels. Samples from this depth have minimum concentrations of silica, alkalinity, manganese, and DOC. We also estimate the exports of these components (and import of U) due to mixing based on average measured concentrations in the tidal creeks and the 4-day flushing time. A comparison of these estimates reveals that the exports (negative in the case of U) equal or exceed the pore water fluxes. By using values slightly higher than the minimum concentrations at 3.5m to calculate inputs, the two estimates could be forced to match. We conclude that pore water drainage is the major factor regulating fluxes of Ra isotopes, silica, alkalinity, manganese, DOC, and uranium in this system. © 2011 Elsevier Ltd.

Hinrichs D.,Carl von Ossietzky University | Pelster A.,Hanse Wissenschaftskolleg | Pelster A.,University of Kaiserslautern | Holthaus M.,Carl von Ossietzky University
Applied Physics B: Lasers and Optics | Year: 2013

We develop a strategy for calculating critical exponents for the Mott insulator-to-superfluid transition shown by the Bose-Hubbard model. Our approach is based on the field-theoretic concept of the effective potential, which provides a natural extension of the Landau theory of phase transitions to quantum critical phenomena. The coefficients of the Landau expansion of that effective potential are obtained by high-order perturbation theory. We counteract the divergency of the weak-coupling perturbation series by including the seldom considered Landau coefficient a 6 into our analysis. Our preliminary results indicate that the critical exponents for both the condensate density and the superfluid density, as derived from the two-dimensional Bose-Hubbard model, deviate by less than 1 % from the best known estimates computed so far for the three-dimensional XY universality class. © 2013 Springer-Verlag Berlin Heidelberg.

Lima A.R.P.,Free University of Berlin | Pelster A.,Hanse Wissenschaftskolleg | Pelster A.,University of Kaiserslautern
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2012

We theoretically investigate various beyond mean-field effects on Bose gases at zero temperature featuring the anisotropic and long-range dipole-dipole interaction in addition to the isotropic and short-range contact interaction. Within the realm of the Bogoliubov-de Gennes theory, we consider static properties and low-lying excitations of both homogeneous and harmonically trapped dipolar bosonic gases. For the homogeneous system, the condensate depletion, the ground-state energy, the equation of state, and the speed of sound are discussed in detail. Making use of the local density approximation, we extend these results in order to study the properties of a dipolar Bose gas in a harmonic trap and in the regime of large particle numbers. After deriving the equations of motion for the general case of a triaxial trap, we analyze the influence of quantum fluctuations on important properties of the gas, such as the equilibrium configuration and the low-lying excitations in the case of a cylinder-symmetric trap. In addition to the monopole and quadrupole oscillation modes, we also discuss the radial quadrupole mode. We find that the latter acquires a quantum correction exclusively due to the dipole-dipole interaction. As a result, we identify the radial quadrupole as a reasonably accessible source for the signature of dipolar many-body effects and stress the enhancing character that dipolar interactions have for quantum fluctuations in the other oscillation modes. © 2012 American Physical Society.

Nietner C.,TU Berlin | Pelster A.,Hanse Wissenschaftskolleg | Pelster A.,University of Kaiserslautern
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2012

We develop a Ginzburg-Landau theory for the Jaynes-Cummings-Hubbard model which effectively describes both static and dynamic properties of photons evolving in a cubic lattice of cavities, each filled with a two-level atom. To this end we calculate the effective action to first order in the hopping parameter. Within a Landau description of a spatially and temporally constant order parameter we calculate the finite-temperature mean-field quantum phase boundary between a Mott insulating and a superfluid phase of polaritons. Furthermore, within the Ginzburg-Landau description of a spatiotemporal varying order parameter we determine the excitation spectra in both phases and, in particular, the sound velocity of light in the superfluid phase. © 2012 American Physical Society.

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