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NTH – School for Contacts in Nanosystems
Logo Leibniz Universität Hannover
NTH – School for Contacts in Nanosystems
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Projects 2010-2013

To tackle the problem of contacts in nanosystems a small number of model systems was chosen. Therefore, the research within the school is divided into 4 key projects. Three of these projects concentrate on certain material systems for the formation of nanostructures, one project is purely theoretical which will give additional support to the other projects in the detailed understanding of the coupling effects of a nanostructure with a complex environment.

Project P1 with the title Energy Conversion Processes in Molecular Systems at Nano Contacts deals with porphyrin molecules as model systems. In project P2 with the title Quantum Impurities in Contact to a Complex Environment different theoretical methods will be applied to study interaction effects between a quantum impurity and the environment. Project P3 with the title Spin and Charge Dynamics at Surfaces and Interfaces in Wide Gap Semiconductor Nanostructures concentrates on quantum wells and nanorods in III/V and ZnO semiconductors. In project P4 with the title Interaction Effects in Contacting Semiconducting Nanostructures quantum dots in AlGaAs/GaAs heterostructures and graphene are taken as model systems.

In this way a small number of model systems covers quite a wide range of different systems ranging from zerodimensional systems which are realized in molecules and quantum dots, over one-dimensional systems which might be realized in nanorods up to two-dimensional systems in graphene and quantum wells. Collaborations will not only exist within the individual projects, but also between groups working in different projects. It is envisioned that the 4 research projects in this school will lay the foundation for future joint applications for one or more larger projects.

P1: Energy Conversion Processes in Molecular Systems at Nano Contacts

We will explore in this project fundamental processes of energy conversion, charge transfer
and structural changes in molecular species with established functionality given by porphyrins in contact with tailored nano-structured metal and insulator systems. We will apply a large range of spectroscopic and microscopic methods to combine molecular specificity, interface sensitivity, high spatial and high time resolution to systems prepared under ultra-high vacuum conditions as well as to electrochemically prepared systems. The research on well-defined model systems for contacts in nanosystems is aimed at a better understanding and control of the above processes in functionalized contacts such as in solar cells, molecular electronics and sensors.

Peter Blöchl (TU Clausthal) Winfried Daum (TU Clausthal), Peter Lemmens (TU Braunschweig), Herbert Pfnür (Leibniz Univ. Hannover), Daniel Schaadt (TU Clausthal)

P2: Quantum impurities in contact to a complex environment

The coupling of quantum impurities, i.e. local scatterers with internal degrees of freedom such as quantum dots or complex molecules, to nanowires or surfaces through nano contacts (NCs) strongly affects their electronic and magnetic properties and thus the physical phenomena appearing on the nanometer scale. The understanding of these phenomena is an indispensable pre-requisite in many emerging branches of nano-science like molecular electronics, molecular spintronics and femtochemistry with promising prospects for applications in information storage and spin-based devices, quantum computing, and nonlinear switches. A typical impurity will contain a few interacting electrons in multiple electronic orbitals possibly coupled to bosonic modes or as for single-molecule magnets (SMMs) comprise a few transition metal ions interacting via exchange through (in)organic ligands. In these realizations a variety of quantum phenomena is displayed, e.g. magnetization tunneling, Berry-phase interference, and long transverse spin-relaxation times.

Wolfram Brenig (TU Braunschweig), Holger Frahm (Leibniz Univ. Hannover), Eric Jeckelmann (Leibniz Univ. Hannover), Patrik Recher (TU Braunschweig), Gertrud Zwicknagl (TU Braunschweig)

P3: Spin and Charge Dynamics at Surfaces and Interfaces in Wide Gap Semiconductor Nanostructures

Wide bandgap semiconductors are among the most intensely studied materials due to their
increasing use both in optoelectronic and in high-frequency high-power electronic devices. However, due to the complexity of wide-gap nanostructures, the dynamics of spins and charge carriers at internal interfaces as well as at surfaces are far from being well understood. As those materials are highly polar, energy and momentum relaxation as well as spin relaxation are expected to be subject to very strong phonon coupling, affecting charge and spin transport across interfaces.

It is the aim of the present project to study the dynamics of spins and charges at interfaces and surfaces of wide bandgap semiconductor nanostructures. The research groups collaborating towards this goal contribute highly complementary expertise ranging from epitaxial growth of III-nitride and II-VI based nanostructures over surface science to polarization, spatial, and timeresolved magneto-optical spectroscopy and transport experiments.

Winfried Daum (TU Clausthal, associated), Andreas Hangleiter (TU Braunschweig), Michael
Oestreich (Leibniz Univ. Hannover), Herbert Pfnür (Leibniz Univ. Hannover, associated), Marc Tornow (TU Braunschweig), Andreas Waag (TU Braunschweig)

P4: Interaction Effects in Contacting Semiconducting Nanostructures

Contacting low-dimensional systems in semiconductors is still a difficult task. In addition, the
physics involved in contacting such systems is strongly influenced by interaction effects. Here,
we will concentrate on two systems, quantum dots in AlGaAs/GaAs heterostructures and lowdimensional systems in the novel semiconducting material graphene. In concentrating on these two systems it will be possible to come to a detailed understanding of the influence of interaction effects in contacting these nanostructures and to advance our understanding of contact effects in more detail.

Rolf Haug (Leibniz Univ. Hannover), Georg Nachtwei (TU Braunschweig), Patrik Recher (TU Braunschweig); Hans Werner Schumacher and Franz Ahlers (PTB Braunschweig, associated)