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CSC@Lunch Seminar Series

We share a weekly seminar series with the Warwick Centre for Predictive Modelling (WCPM). Seminars are held from 1-2 pm on Mondays during the university term. Nominations for speakers are welcome. Please contact James Kermode or Peter Brommer with suggestions.

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Mon, Oct 29, '18
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Gautam Anand (Engineering)
WCPM (D2.02)

Electron-spin mediated distortion in metallic systems

The deviation of atoms from their lattice site in crystalline systems can lead to significant variation in their properties. Such distortion can be tuned to generate the range of structural [1] and functional [2] properties in the materials, for example, local distortions have proved instrumental for superconductivity in Iron-based superconductors [3]. Distortion in ionic systems has traditionally been understood in terms of Jahn-Teller distortion [4], while for a one-dimensional chain of metals, the Peierls distortion mechanism has been proposed [5]. In real three dimensional metallic systems, a fundamental description for distortion is missing which is essential to design alloys with attractive properties. The present investigation aims to study the evolution of distortion in three-dimensional metallic systems under the influence of charge disproportion and spin effects. We present that an antiferromagnetic interaction between transition metals along with electronic charge disproportion leads to significant distortion in these metallic systems. We present ab-initio density functional theory calculation showing a spin-flip phenomena occurs on Cr due to presence of other transition metals dopants, which leads to distortion in Cr based systems. Such spin-flip phenomena depicts the deviation from Hund’s rule of maximum multiplicity. Similar deviation from Hund’s rule has been observed in the case of C603- anion, where orbital degeneracy is removed by Jahn-Teller distortion [6]. This work provides understanding of the evolution of distortion in metallic systems due to charge disproportion and spin-flip, which is analogous to the Jahn-Teller effect for ionic systems and Peierls distortion for a one-dimensional chain of metals with subtle differences introduced due to the delocalised electrons. Cr is one of the quintessential alloying elements and has been used in the range of engineering alloys including stainless steels, super-alloys, Ni-based magnetic alloys, high-entropy and medium entropy alloys. This study provides the explanation of the strengthening effect of the Cr due to its unique magnetic properties.

References
1. F. Gao, H. Song, F. Tian, Q. M. Hu, L. Vitos, Y. Wang, J. Shen, N. Chen, Local lattice distortion in high-entropy alloys, Physical Review Materials, 1 (2017) 023404.
2. D. Berardan, A. K. Meena, S. Franger, C. Herrero, N. Dragoe, Journal of Alloys and Compounds, 704 (2017) 693-700.
3. W. Wang, Y. Song, C. Cao, K. F. Tseng, T. Keller, Y. Li, L. W. Herringer, W. Tian, S. Chi, R. Yu, A . H. Nevidomsky, P. Dai, Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe1-xNixAs, Nature Communications, 9 (2018) 3128.
4. H. A. Jahn, E. Teller, Stability of polyatomic molecules in degenerate electronic states – I – Orbital degeneracy, Proceedings of the Royal Society A, 161 (1937) 220-235.
5. R. Hoffmann, How Chemistry and Physics Meet in the Solid State, Angewandte Chemie International Edition in English, 26 (1987) 846-878.
6. C. C. Chancey, M. C. M. O’ Brian, The Jahn Teller Effect of C60 and other Icosahedral Complexes, Princeton University Press, 1997

Mon, Nov 5, '18
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Martin Weigel (Coventry)
Physics (PS0.17)

Monte Carlo methods for massively parallel architectures

Abstract to follow.

Mon, Nov 12, '18
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Chatu Kumarasinghe (Engineering)
WCPM (D2.02)

Bandstructure engineering of half-Heuslers for a high thermoelectric power factor

Abstract to follow.

Mon, Nov 19, '18
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Johannes Lischner (Imperial)
Physics (MAS2.06)

Simulating charged defects in 2d materials: how we taught electronic screening to machines

Abstract to follow.

Mon, Nov 26, '18
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Adam Noël (Engineering)
WCPM (D2.02)

Stochastic Simulation of Microscopic Reaction-Diffusion Systems

Abstract to follow.

Mon, Dec 3, '18
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Andela Saric (UCL)
Physics (PS0.17)

Minimal coarse-grained models of biological assembly

Abstract to follow.