Research Topics
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Asymmetric Bethe-Salpeter equation for pairing and condensation
P. Lipavsky (Charles University Prague) | ||||
| The Martin-Schwinger hierarchy of correlations are reexamined and the three-part icle correlations are investigated under various partial summations. Besides the known approximations of screened, ladder and maximally crossed diagrams the pair-pair correlations are considered. It is shown that the recently proposed asymmetric Bethe-Salpeter equation to avoid unphysical repeated collisions is derived as a result of the hierarchical dependencies of correlations. Exceeding the parquet approximation we show that an asymmetry appears in the selfconsistent propagators. This form is superior over the symmetric selfconsistent one since it provides the Nambu-Gorkov equations and gap equation for fermions and the Beliaev equations for bosons while from the symmetric form no gap equation results. The selfenergy diagrams which account for the subtraction of unphysical repeated collisions are derived from the pair-pair correlation in the three-particle Greenfunction. It is suggested to di stinguish between two types of selfconsistency, the channel-dressed propagators and the completely dressed propagators, with the help of which the asymmetric ex pansion completes the Ward identity and is phi-derivable. The resulting T-matrix theory provides a consistent approach to superconductivity above and below the critical temprature. |
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Birth of quasiparticles - Femtosecond formation of collective modes
P. Lipavsky (Charles University Prague) | |
| Starting from a quantum kinetic equation including the mean field and a conserving relaxation-time approximation we derive an analytic formula which describes the time dependence of the dielectric function in a plasma created by a short intense laser pulse. This formula reproduces universal features of the formation of collective modes seen in recent experimental data of femtosecond spectroscopy. The presented formula offers a tremendous simplification for the description of the formation of quasiparticle features in interacting systems. Numerical demanding treatments can now be focused on effects beyond these gross features found here to be describable analytically. |
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Spin dependent transport in quantum dot systems Fabricio Macedo de Souza (Univeristy Uberlandia) | |
| Spin-dependent transport in quantum dots is a subject of intense study nowdays due to its relevance to the new generation of proposed spintronic devices that encompasses, for instance, the Datta-Das transistor, memory devices and as an ultimate goal quantum computers. In particular, the recent progress in the coherent control of electron spins in quantum dots has stimulated even further the research in this field, for possible applications in quantum computation and quantum information processing. In addition to these fascinating technological applications, quantum dots constitute a unique well-controllable system to study fundamental physical aspects of transport in the strong Coulomb-correlated regime, and its interplay with spin-dependent effects. Our group deals with both ac and dc transport of charge and spin in quantum dots coupled to nonmagnetic of ferromagnetic leads. |
| Spin and density response with magentic fields and spin-orbit coupling J. Kailasvuori (MPIPKS Dresden), H. Schmidt (HZDR Dresden) | ||
| The main motivation is to derive in an unambiguous way the linear response including magnetic field for an interacting polarized system where the many-body interactions are described by a spin-depenent mean field and spin-orbit coupling present. That this is expected to describe interesting effect can be seen e.q. by the recently observed anomalous spin segregation in weakly interacting Li in a trap, the anomalous spin-Hall effect as well as . |
| Formation
of brine channels in sea ice Bernd Kutschan (FH Münster), Silke Thoms (AWI Bremerhafen), Sibylle Gemming (HFZD Dresden-Rossendorf) | ||
| Sea ice is a very variable biotope with respect to extension,thickness, porosity or texture. Therefore the basic understanding of brine channel formation in sea ice is important for the interplay between the microbial colonization and their natural habitat. The early phase of brine channel formation in sea ice is considered. The first structures emerging during sea-ice formation are determined by the phase instability of the ice-water system in the presence of salt. We apply a Ginzburg-Landau type approach to describe the phase separation in the two-component system (ice, salt). The free energy density involves two order parameters: one for the hexagonal ice phase with low salinity, and one for the liquid water with high salinity. A gradient dynamics minimizes the free energy with respect to the conservation of the salinity. The resulting model equations are solved numerically in one and two dimensions. The numerical solution shows a short-time behavior of structure formation where the freezing is assumed and a large-time broadening of the structure. A stability analysis provides the phase diagram where brine channels can be formed. In thermodynamics the parameters determine the supercooling or superheating region and the specific heat respectively. The size of the brine channels depends on the salinity and the temperature. With the help of realistic parameters the brine channel distribution is calculated and found in agreement with the measured samples. |
Turing vs phase field structure
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Transport and noise in organic field effect devices L. M. Eng (IaPP Dresden), S. Gemming (HFZD Dresden-Rossendorf), T. Kunze (HFZD Dresden Rossendorf), P. Lipavsky (Charles University Prague), R. Luschtinetz (TU Dresden), P. Milde (IaPP Dresden), G. Seifert (TU Dresden) | ||
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The goal to develop low cost storage and microelectronic devices has triggered an enormous activity in the research of organic field effect transistors (OFETs) based on different polymers. It is desireable that the molecular material possesses a high structural ordering to reach high charge carrier mobilities and low resistive losses. One of the most promising materials are Oligothiophenes and their derivatives. In general, the charge transport is largest in the direction perpendicular to the plane of the thiophene rings. This suggest to construct devices with parallely ordered molecular rings. Just recently an OFET structure has been built from ultra-thin self-assembly films made up from
oligothiophenes, which are arranged in a high-order lamellar stacking perpendicular to the substrate surface. The transport properties are calculated with the help of surface Green functions.
movies: shuttling transport of molecules(red +, green -,grey 0 charged) |
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Metal-insulator transition in heterojunctions
E. Nakhmedov (TU Chemnitz), C. Olbricht (TU Chemnitz) | |
| The conductivity in quasi two-dimensional systems is calculated using the quantu m kinetic equation as well as molecular dynamical simulations. The system of qua si two-dimensional electrons in hetero-junctions which interact with charged and neutral impurities and the low temperature correction to the conductivity is ca lculated analytically. It turns out that the dynamical screening due to charged impurities leads to a linear temperature dependence, while the scattering from n eutral impurities leads to the usual Fermi-liquid behavior. The experimental met al-insulator transition at low temperatures are reproduced. The effective ma ss of quasiparticle excitations in quasi two-dimensional systems is calculated analytically. It is shown that the effective mass increases sharply when the density approaches the critical one of metal-insulator transition. This suggests a Mott-type of transition rather than an Anderson-like transition. |
| Anomalous 0.7 conductance and analogs in 1D systems Marcelo Rosenau da Costa | ||
| We analyze the fractional quantization of the ballistic conductance associated with the light and heavy hole bands in Si, Ge, and GaAs systems. It is shown that the formation of a localized hole state in the region of the quantum point contact connecting two quasi-one-dimensional hole leads drastically modifies the conductance pattern. The exchange interaction between localized and propagating holes results in fractional quantization of the ballistic conductance unlike that in electronic systems. The value of the conductance at the additional plateaus depends on the offset between the bands of the light and heavy holes and the sign of the exchange interaction constant. |
| Equation of state for interacting Bose/Fermi systems and the BCS-BEC transition
P. Lipavsky (Charles University Prague), M. Männel (FH Münster) | ||
| Near Feshbach resonances it is possible to tune the interaction in ultra-cold Fermi and Bose gases. One can therefore vary the interaction in a Bose-Einstein condensate (BEC) or drive a Fermi gas through a transition from Cooper pairs in a BCS state to two-particle bound states which can also form a BEC. The main interest lies on the conditions for the formation of bound states, Cooper pairs or a BEC. Especially the dependence of these conditions on the interaction is investigated. Calculations are done mainly analytically with a many-body Green functions technique for idealized potentials. Therefore also medium effects such as Bose enhancement and Pauli repulsion can be included. |
| Enhancement of pairing in correlated electron systems with nanostructures P. Lipavsky (Charles University Prague), M. Männel (TU Chemnitz), M. Schreiber (TU Chemnitz) | ||
| A correlated fermion system is considered surrounding a finite cavity with virtual levels. The pairing properties are calculated and the influence of the cavity is demonstrated. To this end the Gell-Mann and Goldberger formula is generalized to many-body systems. We find a possible enhancement of pairing temperature if the Fermi momentum times the cavity radius fulfills a certain resonance condition which suggests an experimental realization. |
| Bernoulli potential at superconducting surfaces and deformation E. H. Brandt (MPI Stuttgart), J. Kolacek (Charles University Prague), P. Lipavsky (Charles University Prague) | ||
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In superconductors penetrated by Abrikosov vortices the magnetic
pressure and the inhomogeneous condensate density induce a
deformation of the ionic lattice. Forces evoked
by vortices create a tension which modifies the total volume of
the sample and which was observed as magnetostriction.
The force which holds
electrons inward naturally pulls the oppositely charged ions
outwards and also corrugates the surface. The general relation
between the surface dipole and the surface tension with the
contribution of the superconducting condensate has been worked out for
anisotropic materials.
movie: direction dependent effective vortex mass of YBCO The NMR and NQR spectra of 63Cu in the CuO2 plane of YBa2Cu3O7 in the superconducting state are discussed in terms of a phenomenological theory of Ginzburg-Landau type extended to lower temperatures. We show that the observed spectra, can be explained by a standard theory of the Bernoulli potential with the charge transfer between CuO2 planes and CuO chains assumed. Book: P. Lipavský, J. Kolacek, K. Morawetz, E. H. Brandt, T. Yang; Bernoulli potential in superconductors - how electric fields help to understand superconductivity Springer, Lecture Notes in Physics, Vol. 733, Berlin (2008) |
| Nonlocal kinetic theory and simulation of heavy ion collisions P. Lipavsky (Charles University Prague) | ||
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With the non-relativistic Green's-function approach to the kinetic
equations for Fermi liquids far from equilibrium a consistent treatment is reached
of the off-shell motion between collisions
and on the non-instant and non-local picture of binary collisions.
The resulting kinetic equation is of the Boltzmann type, and it
represents
an interpolation between the theory of transport in metals and the
theory of moderately dense gases. The free motion of particles is
renormalised by various mean field and mass corrections in the spirit
of Landau's quasiparticles in metals. The collisions are non-local in
the spirit of Enskog's theory of non-ideal gases. The collisions are
moreover non-instant, a feature which is absent in the theory of gases,
but which is
shown to be important for dense Fermi systems.
In spite of its formal complexity, the presented theory has a simple
implementation within the Monte-Carlo simulation schemes. Applications
in nuclear physics are given for heavy-ion reactions and the results are
compared with the former theory and recent experimental data.
movie: Ta +Au at 33 MeV movie: central collisions movie: Au+Au (40-150 MeV, 0-12 fm impact) Book: P. Lipavský, V. Spicka, K. Morawetz; Kinetic equation for strongly interacting dense Fermi systems Annales de Physique, Vol. 26 No. 01 (February 2001), ISBN 2-86883-541-4 |
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