Archiv 2016

Raum: Seminar Room I, EG.076

Julian Gebhardt: Ab initio modeling of perovskite materials and two-dimensional organic networks

MPSD Seminar
Perovskite materials with stoichiometry ABX3 are a rich family of ionic compounds with many technological interesting properties. They can be traditional inorganic, metal per- ovskites or hybrid perovskites with one organic cation. In this talk I want to discuss some results of a special modification of the latter kind, the so-called layered perovskites. This family of structures is currently rediscovered, and we report on the geometrical and electronic structure of one promising candidate ((C6H5C2H4NH3)2PbI4) with phenethylammonium cations on the A-site and compare the differences towards the benchmark case of three-dimensional (3D) hybrid perovskites, CH3PbI3. The influence of varying the cation as well as changing the dimensionality from 3D to 2D systems is discussed by comparing bulk and monolayer structures of both systems. In addition, insight into the optical behavior and the observed electron-phonon coupling will be given. [mehr]
Working within the Nonequilibrium Green's Function formalism, a formula for the two-time current correlation function is derived for the case of transport through a nanojunction in response to an arbitrary time-dependent bias. The one-particle Hamiltonian and the wide band limit approximation are assumed, enabling us to extract all necessary Green's functions and self-energies for the system, extending the analytic work presented previously [Ridley et al. Phys. Rev. B (2015)]. [mehr]

Transport through strongly correlated systems described by density functional theory

MPSD Seminar
In this talk I will describe the evolution of our understanding on how to describe transport through strongly correlated systems in the framework of density functional theory (DFT).A first indication that DFT might be useful to tackle this situation came with the realization that the Kondo plateau in the zero-bias conductance may already be captured at the level of standard Landauer theory combined with DFT. Later it has been shown how the description of Coulomb blockade in the zero-bias limit can be achieved within DFT. In a more recent development we have proposed a DFT formalism to describe electronic transport in the steady state which uses the density on the junction and the steady current as basic variables. In a finite window around zero bias, a one-to-one map is established between the basic variables and both local potential on as well as bias across the junction. The resulting Kohn-Sham system features two exchange-correlation (xc) potentials, a local xc potential and an xc contribution to the bias. For weakly coupled junctions the xc potentials exhibit steps in the density-current plane which are shown to be crucial to describe the Coulomb blockade diamonds.Finally, I will present a recent parametrization of the xc potentials for the single-impurity Anderson model which correctly incorporates both the Kondo and Coulomb blockade regimes, i.e., both zero and finite temperature. This parametrization allows for calculation of currents and differential conductances at arbitrary bias and temperature at negligible numerical cost but with the accuracy of sophisticated renormalization group methods. [mehr]

Optical Materials Design of Transition-Metal Dichalcogenides and Frustrated Mott Insulators

MPSD Seminar
Spurred by recent progress in melting, enhancement and induction of electronic order out of equilibrium, a tantalizing prospect concerns instead accessing transient Floquet steady states via broad pump pulses, to affect electronic properties. [mehr]

Collective dynamics in quantum emitter systems

MPSD Seminar
Time evolution in ensembles of quantum emitters (atoms, molecules, ions, Rydberg atoms, quantum dots etc) at low vs. high densities is fundamentally different. As particles get closer together, strong environment-mediated interactions start playing an important role both in the coherent (such as dipole-dipole interactions) as well as in the incoherent evolution (super- /subradiance). [mehr]

Time-dependent quantum transport in nanosystems: A nonequilibrium Green's function approach

MPSD Seminar
Quantum transport is often discussed in the steady-state regime where the characteristics of the system are described in terms of the energy-dependent transmission or conductance. There is, however, no guarantee that this description would capture the essential physics in, say, atomic-scale junction operating at high frequencies. Therefore, we look for an accurate theory for describing the full time-dependence. The time-dependence also provides us with ''transient spectroscopy'' which can give detailed information about the nanosystems out of equilibrium. [mehr]

Exotic s-wave superconductivity in alkali-doped fullerides: equilibrium point of view

MPSD Seminar
The alkali-doped fullerides (A₃C₆₀, A = K, Rb, Cs) show a highest superconducting transition temperature (Tc) among molecular solids. In the phase diagram, s-wave superconductivity (SC) lies next to Mott insulating phase. This adjacency is similar to the cuprates (d-wave SC) but is more surprising because s-wave SC is believed to be severely suppressed by strong correlations. [mehr]
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