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Dr. Philipp Hansmann
Head of Research Group
Phone:+49 711 689-1758Fax:+49 711 689-1702

Max Planck Institute for Solid State Research, Stuttgart

Electronic Structure of Correlated Materials

The development of novel functional materials in experimental labs combined with computer-based compound simulation brings the vision of materials design on a microscopic scale continuously closer to reality. For many applications interface and surface phenomena rather than bulk properties are key. One of the most fundamental qualities of a material-vacuum interface is the energy required to transfer an electron across this boundary, i.e., the work function. It is a crucial parameter for numerous applications, including organic electronics, field electron emitters, and thermionic energy converters. Being generally very resistant to degradation at high temperatures, transition metal oxides present a promising materials class for such devices. We have performed a systematic study for perovskite oxides that provides reference values and, equally important, reports on materials trends and the tunability of work functions. Our results identify and classify dependencies of the work function on several parameters including specific surface termination, surface reconstructions, oxygen vacancies, and heterostructuring.

Tuning the work function in transition metal oxides and their heterostructures

The development of novel functional materials in experimental labs combined with computer-based compound simulation brings the vision of materials design on a microscopic scale continuously closer to reality. For many applications interface and surface phenomena rather than bulk properties are key. One of the most fundamental qualities of a material-vacuum interface is the energy required to transfer an electron across this boundary, i.e., the work function. It is a crucial parameter for numerous applications, including organic electronics, field electron emitters, and thermionic energy converters. Being generally very resistant to degradation at high temperatures, transition metal oxides present a promising materials class for such devices. We have performed a systematic study for perovskite oxides that provides reference values and, equally important, reports on materials trends and the tunability of work functions. Our results identify and classify dependencies of the work function on several parameters including specific surface termination, surface reconstructions, oxygen vacancies, and heterostructuring.
The workshop intends to bring together a group of leading theoretical and computational physicists in the field of correlated materials to discuss the main challenges of today's state-of-the-art ab-initio calculations, the strengths and weaknesses of different methodologies and to explore the possibilities to integrate different numerical approaches into efficient and reliable computational tools.

CECAM Workshop Lausanne, Switzerland (October 24, 2016 to October 26, 2016)

The workshop intends to bring together a group of leading theoretical and computational physicists in the field of correlated materials to discuss the main challenges of today's state-of-the-art ab-initio calculations, the strengths and weaknesses of different methodologies and to explore the possibilities to integrate different numerical approaches into efficient and reliable computational tools.

Explaining properties of solids from a first-principle, atomistic perspective is a tremendously difficult problem due to the need of solving the many-particle Schrodinger equation. Only with the conception of density functional theory (DFT) and its local density approximation (LDA) numerical computations for real materials became feasible. The successes of DFT in explaining the electronic properties of many elements and compounds, and its applicability to many quantum-chemical problems, are extraordinarily impressive. Nonetheless, the effective single particle language of Kohn-Sham DFT, is not sufficient for an analysis of the excitation spectrum when many-body effects exceed a certain strength. Hence, strongly correlated electron systems turned out to be beyond the possibilities of the DFT description. As a consequence, the treatment of such systems has been limited to model-type approaches. Experimentally, however, it is seen that specifically such correlated systems show a great potential for unusual phenomena. Mott insulators, heavy-fermion rare-earth compounds with unconventional superconductivity, colossal magneto-resistance in manganites and, probably the most intriguing phenomenon of all, high temperature superconductivity in cuprates. The list continues with more recent discoveries like iron based superconductors (discovered in 2008) and the ever growing effort to find new materials continues in all thinkable directions. Motivated by experimental discoveries also on the theory side new concepts have been developed within the past twenty years to handle the challenge of treating electronic correlation in real materials. Among others, the merger of density functional theory in its local density approximation with the so called Dynamical Mean Field Theory [1-3](LDA+DMFT) turned out to be particularly successful. Due to the non-perturbative nature of the theory it is capable to capture the itinerant, i.e., weakly correlated and the localized, i.e. strongly correlated limit on equal footing. The method has evolved to maturity and is nowadays supplemented by other techniques which allow for systematic extension beyond the intrinsic DMFT limits, i.e. the local nature of its self-energy.

We employ standard DFT tools like Wien2k[5] and VASP[6] as well as the latest releases of the TRIQS[7] libraries to tackle the main target of our group: The hunt for novel functional materials.

[1] W. Metzner and D. Vollhardt, Phys. Rev. Lett. 62, 324 (1989).
[3] A. Georges and G. Kotliar, Phys. Rev. B 45, 6479 (1992).
[4] M. Jarrell, Phys. Rev. Lett. 69, 168 (1992).
[5] K. Schwarz, P. Blaha, and G. K. H. Madsen, Computer Physics Communications, 147, 71 (2002).
[6] G. Kresse and J. Furthmüller, Phys. Rev. B, 54, 11169 (1996).
[7] O. Parcollet, M. Ferrero, T. Ayral, H. Hafermann, I. Krivenko, L. Messio, and P. Seth, Comp. Phys. Comm. 196, 398-415 (2015).

 
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