Research Topics

• Transparent and conducting oxides for photovoltaics and optoelectronics
• Ferroelectric ceramic materials for piezoelectric devices
• Structure and stability of internal boundaries in materials
• Deformation and fracture of materials

Transparent and conducting oxides for photovoltaics and optoelectronics

Optically transparent electrically conducting oxides (TCO) offer a wide variety of applications in photovoltaics and optoelectronics. Oxides like ZnO or TiO₂ are prominent examples for such a two-fold property. Because of their large electronic band gaps they have a high transmission in the spectral range of visible light, and by chemical doping they can be made electrically well conducting. In order to compare various chemical dopant elements with respect to the energetics of incorporation into the host oxide, and to predict the resulting electronic structure, ab-initio calculations based on density functional theory (DFT) are carried out: atomistic supercell models are used to describe grain boundaries or dopant atoms in polycrystalline microstructure, to calculate interface and defect energies, and to analyse electronic band structures and local densities of states.  Altogether, this yields reliable predictions for novel combinations of materials with better properties.

Transparente, leitfähige Oxide für die Photovoltaik und die Optoelektronik. (PDF)

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Ferroelectric ceramic materials for piezoelectric devices

Ferroelectrics are widely used for powerful and precise mechanical actuator or sensor components. Lead zirconate titanate (PZT) is presently the material of choice because of its outstanding electromechanical properties. An attractive and promising lead-free alternative is sodium potassium niobate (KNN). Density functional theory (DFT) and atomistic simulations are employed to investigate influences of atomic defects, domain walls or grain boundaries on the electromechanical behavior of PZT and KNN, in order to offer a better control of bad influences or to enhance good influences in the industrial materials synthesis.  Compared to DFT calculations, atomistic simulations with empirical shell-model potentials are useful for computationally more efficient although physically less accurate studies of larger atomistic systems which form a bridge between the atomic scale and microstructural or even macroscopic scales.  

Kritische Dicke für Ferroelektrizität von ultradünnen PbTiO₃-Funktionsschichten: (PDF)

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Structure and stability of internal boundaries in materials

For the development of novel functional materials, for instance in microelectronic or automotive components, a fundamental atomic-scale understanding of the materials’ properties is essential, since mechanical structure-function relationships of materials, like plasticity and fracture, are controlled by the interatomic bonding at interfaces.  This bonding behavior is studied by computational modelling, which allows simulations of functional properties of novel materials already before synthesis efforts in small laboratory-experiment or large industy-production scales are needed.

Thermodynamische und mechanische Stabilität von dünnen Kupfer-Filmen auf Tantal. (PDF)

Struktur und Stabilität von Metall-Keramik-Grenzflächen auf atomarer Skala. (PDF)

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Deformation and fracture of materials

The plastic deformability of polycrystalline metallic materials is influenced more and more by interactions of lattice dislocations with grain boundaries for microstructures with smaller and smaller grain sizes.  This relationship is the atomic-scale origin on one hand for an extraordinarily high mechanical hardness which can be achieved for nanocrystalline metals, on the other hand however for well known failure problems by fracture under extreme deformation, for instance in the production of very thin metal wires. A widely used empirical interpretation of this failure mechanism is that dislocations, whose high mobility in the crystal lattice lead to good plastic deformation, can get strongly blocked by grain boundaries. Computational atomistic simulations provide an ideal research tool to investigate and validate such empirical notions of mechanics in detail.

Atomistische Simulation der Wechselwirkung einer Versetzung mit einer Korngrenze in Wolfram. (PDF)

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Information

University lectures

Winter semester 2010/2011 and summer semester 2011
Electronic structure of condensed matter I + II
Christian Elsässer, Faculty for Mathematics und Physics, University of Freiburg

Winter semester 2009/2010 and summer semester 2010
Superconductivity I + II
Christian Elsässer, Faculty for Mathematics und Physics, University of Freiburg

Publications

• Janisch, R.; Elsässer, C., Growth and mechanical properties of a MoC precipitate at a Mo grain boundary: an, ab-initio study, Phys. Rev. B 77, 094118 (2008)
• Benedek, N. A.; Elsässer, C.;. Finnis, M. W., First principles investigation of polarisation at interfaces in multilayered Strontium Titanate, J. Phys: Conf. Series 94, 012005 (2008)
• Hashibon, A.; Lozovoi, A. Y.; Mishin, Y.; Elsässer, C.; Gumbsch, P., Interatomic potential for the Cu-Ta system and its application to surface wetting and dewetting, Phys. Rev. B 77, 094131 (2008). Total Text: Fraunhofer Publica
• Benedek, N. A.; Chua, A. L-.S.; Elsässer, C.; Sutton, A. P.;Finnis, M. W., Transferability of interatomic potentials for SrTiO3 from bulk to grain-boundary properties, Phys. Rev. B 78, 064110 (2008)
• Vedmedenko, O.; Rösch, F.; Elsässer, C., First-principles DFT study of phase transformations in NbCr2 and TaCr2, Acta mater. 56, 4984-4992 (2008)
• Umeno, Y.; Elsässer, C.; Meyer, B.; Gumbsch, P.;Weissmüller, J., Reversible relaxation at charged metal surfaces: an ab initio study, Europhys. Lett. 84, 13002 (2008) Total Text: Fraunhofer Publica
• Cheng, Y.; Mrovec, M.; Gumbsch, P., Crack nucleation at the Sigma 9(2(2)over-bar1) symmetrical tilt grain boundary in tungsten, Materials Science and Engineering A, 483-484 (2008) Total Text: Fraunhofer Publica
• Cheng, Y.; Mrovec, M.; Gumbsch, P., Atomistic simulations of interactions between the 1/2(111) edge dislocation and symmetric tilt grain boundaries in tungsten, Philosophical Magazine, 88 (2008) Total Text: Fraunhofer Publica
• Hashibon, A.; Elsässer, C.; Rühle, M., Ab initio study of electronic densities of states at copper-alumina interfaces, Acta mater. 55, 1657-1665 (2007)
• Umeno, Y.; Elsässer, C.; Meyer, B.; Gumbsch, P.; Nothacker, M.; Weissmüller, J.; Evers, F., Ab initio study of surface stress response to charging, Europhys. Lett. 78, 13001 (2007). Total Text: Fraunhofer Publica
• Mrovec, M.; Gröger, R.; Bailey, A. G.; Nguyen-Manh, D.; Elsässer, C.;Vitek, V., Bond-order potential for simulations for extended defects in tungsten, Phys. Rev. B 75, 104119 (2007). Total Text: Fraunhofer Publica
• Albina, J. M.; Mrovec, M.; Meyer, B.;. Elsässer, C.,Structure, stability, and electronic properties of SrTiO3/LaAlO3 and SrTiO3/SrRuO3 interfaces, Phys. Rev. B 76, 165103 (2007). Total Text: Fraunhofer Publica
• Hashibon, A.; Elsässer, C.; Mishin, Y.;Gumbsch, P., First-principles study of thermodynamic and mechanical stability of thin Copper films deposited on Tantalum, Phys. Rev. B 76, 245434 (2007). Total Text: Fraunhofer Publica
• Umeno, Y.; Meyer, B.; Elsässer, C.;Gumbsch, P.,Ab initio study of the critical thickness for ferroelectricity in ultrathin Pt/PbTiO3/Pt films. Phys. Rev. B 74, 060101(R) (2006). Total Text: Fraunhofer Publica
• Mrovec, M.;. Cheng, Y.; Elsässer, C.; Gumbsch, P., Atomistic simulations of dislocation – grain-boundary interactions in tungsten, in P. Gumbsch (Ed.), Proc. Int. Conf. MMM2006, Fraunhofer IRB Verlag (2006), pp. 213-216
• Albina, J. M.; Mrovec, M.; Meyer, B.;. Elsässer, C.,A study of the semi-coherent ceramic SrTiO3/SrZrO3 interface by first-principles DFT calculations, in P. Gumbsch (Ed.), Proc. Int. Conf. MMM2006, Fraunhofer IRB Verlag (2006), pp. 819-822
• Umeno, Y.; Elsässer, C.; Meyer, B.; Gumbsch, P.; Shimada, T.; Kitamura, T., Ab initio DFT study on ferroelectricity on perovskite surfaces and in thin-film capacitors, in P. Gumbsch (Ed.), Proc. Int. Conf. MMM2006, Fraunhofer IRB Verlag (2006), pp. 901-904
• Umeno, Y.; Shimada, T.; Kitamura, T.;Elsässer, C., Ab initio density functional theory study of strain effects on ferroelectricity at PbTiO3 surfaces, Phys. Rev. B 74, 174111 (2006). Total Text: Fraunhofer Publica
• Mrovec, M.;, Moseler, M.; Elsässer, C.; Gumbsch, P., Atomistic modeling of hydrocarbon systems using analytic bond-order potentials. Progr. Mater. Sci. 52, 230-254 (2006). Total Text: Fraunhofer Publica

• Erünal, E.; Eichel, R.-A.; Körbel, S.; Elsässer, C.; Acker, J.; Kungl, H.; Hoffmann, H. J., Defect structure of copper doped potassium niobate ceramics, Functional Materials Letters 3/1 (2010) 19-24
• Hashibon, A.; Elsässer, C., Approaches to atomistic triple-line properties from first-principles, Scripta Materialia 62 (2010) 939-944
• Hashibon, A.; Schravendijk, P.; Elsässer, C.; Gumbsch, P., Atomistic study of structure and stability of thin Ni films on Fe surfaces, Philosophical Magazine 89/34-36 (2009) 3413-3433
• Hirel, P.; Marton, P.; Mrovec, M.; Elsässer, C., Theoretical investigation of {1 1 0} generalized stacking faults and their relation to dislocation behavior in perovskite oxides, Acta Materialia 58 (2010) 6072-6079
• Janisch, R.; Elsässer, C., Interstitial impurities at grain boundaries in metals: insight from atomistic calculations, International Journal of Materials Research 100 (2009) 1488-1493
• Kalivoda, E. M.; Mrovec, M.; Elsässer, C., Electronic structure and transport properties of transition-metal/ SrTiO3(100) heterojunctions, in Proc. of 5th International Conference Multiscale Materials Modeling MMM2010; Gumbsch, P.; Van der Giessen, E. (Hrsg.), Fraunhofer Verlag, Stuttgart (2010) 777-780
• Körbel, S.; Marton, P.; Elsässer, C., Formation of vacancies and copper substitutionals in potassium sodium niobate under various processing conditions, Physical Review B 81 (2010) 174115:1-11
• Körner, W., Grundlangen für transparente und leitfähige Oxide, in OTTI Tagungsband; OTTI, Regensburg (2010) 10
• Körner, W., Elsässer C., First-principles density functional study of dopant elements at grain boundaries in ZnO, Physical Review B 81 (2010) 085324:1-12
• Moras, G.; Colombi Ciacchi, L.; Elsässer, C.; Gumbsch, P.; De Vita, A., Atomically smooth stress-corrosion cleavage of a hydrogen-implanted crystal, Physical Review Letters 105 (2010) 075502:1-4
• Mrovec, M.; Elsässer, C.; Gumbsch, P., Interactions between lattice dislocations and twin boundaries in tungsten: A comparative atomistic simulation study, Philosophical Magazine 89/34-36 (2009) 3179-3194
• Mrovec, M.; Albina, J.-M.; Meyer, B.; Elsässer, Ch., Schottky barriers at transition-metal/SrTiO3(001) interfaces, Physical Review B, 79 (2009) 245121/1-18, Total Text: Fraunhofer Publica
• Szyszka, B; Loebmann, P.; Georg, A.; May, C.; Elsässer C., Development of new transparent conductors and device applications utilizing a multidisciplinary approach, Thin Solid Films 518 (2010) 3109-3114
• Szyszka, B.; Polenzky, C.; Loebmann, P.; Götzendörfer, S.; Elsässer, C.; Körner, W., Pathways towards p-type oxide layers for optoelectronic applications, Vincenzini, P.; Ginley, D. S.; Bruno, G.; Rigamonti, A.; Zheludev, N. (Eds.), Trans Tech Publications, Stafa-Zurich, Schweiz (2010) 16-24
• Umeno, Y.; Albina, J. M.; Meyer, B.; Elsässer, C., Ab initio calculations of ferroelectric instability in PbTiO3 capacitors with symmetric and asymmetric electrode layers, Physical Review B 80 (2009) 205122:1-8
• von Alfthan, S.; Benedek, N. A.; Chen, L.; Chua, A.; Cockayne, D.; Dudeck, K. J.; Elsässer, C.; Finnis, M. W.; Koch, C. T.; Rahmati, B.; Rühle, M.; Shih, S.-J.; Sutton, A. P., The structure of grain boundaries in strontium titanate: theory, simulation and electron microscopy, Annual Reviews of Materials Research 40 (2010) 557-599
• Zimmermann, J.; Colombi Ciacchi, L., Mechanisms of initial oxidation of the Co(0001) and Cr(110) surfaces, Journal of Physical Chemistry C 114/14 (2010) 6014-6023
• Zimmermann, J.; Colombi Ciacchi, L., Origin of the selective Cr oxidation in CoCr alloy surfaces, Journal of Physical Chemistry Letters 1/15 (2010) 2343-2348
• Dudarev, S. L.; Boutard, J.-L.; Lässer, R.; Caturla, M. J.; Derlet, P. M.; Fivel,M.; Fu, C.-C.; Lavrentiev, M. Y.; Malerba, L.; Mrovec, M.; Dguyen-Manh,D.; Nordlund, K.; Perlado, M.; Schäublin, R.; van Swygenhoven, H.; Terentyev, D.; Wallenius, J.; Weygand, D.; Willaime, F.,The EU Programme for modelling radiation effects in fusion reactor materials: An overview of recent advances and future goals,Journal of Nuclear Materials,386-388 (2009) 1-7, Total Text: Fraunhofer Publica
• Zimmermann, J.; Finnis, M. W.; Colombi Ciacchi, L., Vacancy segregation in the initial oxidation stages of the TiN(100) surface, Journal of Chemical Physics 130/13 (2009) 1-11