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News - The muon site: DFT and MuSR 2.0

The Muon Site: MuSR 2.0

MuSR, muon spin spectroscopy, is an important experimental technique, unique in providing very fast turnout information on the properties of magnetic materials, hydrogen reactions, slow local motions (e.g. ionic diffusion). It is extremely easy to implant muons in virtually any material and to detect the muon spin evolution in time, in conceptual analogy with NMR, but with a much faster, broadband experiment. For magnetic materials MuSR, neutron scattering and NMR are the techniques of election.

The main drawback of the muon has been until now our a-priori ignorance of where the muon sits when implanted in the lattice. We know that it is most frequently an interstitial site, but we do not know where in the cell. Part of the information remains therefore quantitatively relative, not absolute.

We have recently learnt how to predict the muon site by Density Functional Theory. DFT is an established theory, building on the Nobel Prize discovery of Walter Kohn, and built into a number of codes (e.g. Quantum Expresso) available on our computers. This is the beginning of MuSR 2.0, since it allows direct quantitative measurements of important physical quantities, previously available only when extensive single crystal experiments were possible. For instance the muon site assignment allows the precise determination of the ordered magnetic moment in a known magnetic structure, normally obtained with neutron diffraction, but prone to pitfalls in many non trivial cases. Our new strategy 1 has been shown to work in a steadily growing number of cases 2,3, sufficient to establish the method, but far from having exploited its potentials yet.

The ideal perspective Master/PhD student (4) is not afraid of theory, has a passion for problem solving, some skills in computing, love and respect for experimental data, all qualities that may develop during the journey. The only pre-requisite is a generic background in solid state physics at the Master level.

The supervisor is a MuSR expert, who contributed to the development of the international muon facilities. Support of DFT theorists is provided locally (including an introductory course), and through established national and international collaborations.

 

Fig. 1:Candidate muon sites at the center of the yellow energy isosurfaces, in LaCoPO, a relative of the Fe superconductors.

 

The task will be to build on what we did until know, by participating in MuSR experiments, mostly on magnets and superconductors, and by running the DFT search for the muon site. A number of internationally established muon groups, working on cutting edge problems, is interested in collaborating on this subject, offering co-authorship in influential papers. Experience in DFT is an important asset for a PhD student.

Ask any questions to roberto.derenzi@unipr.it, impressions and group life reference to the present PhD students pietro.bonfa@studenti.unipr.it (DFT, MuSR, NMR) and sara.bordignon@studenti.unipr.it (NMR, MuSR).

 

References

1 J. S. Moeller, P. Bonfà, D. Ceresoli, F. Bernardini, S. J. Blundell, T. Lancaster, R. De Renzi, N. Marzari Playing quantum hide-and-seek with the muon: localizing muon stopping sites stopping Phys. Scr. 88 068510 (2013)

2 F. Bernardini, P. Bonfa, S. Massidda and R. De Renzi, Phys. Rev. B 87, 115148 (2013)

3 G. Prando, P. Bonfà, G. Profeta, R. Khasanov, F. Bernardini, M. Mazzani, E. M. Brüning, A. Pal, V. P. S. Awana, H.-J. Grafe, B. Büchner, R. De Renzi, P. Carretta, and S. Sanna Phys. Rev. B, 87, 064401 (2013)

4 The Parma Doctorate in Physics has at least six payed positions for the XXX Cycle, applications July-August 2014, admission October 2014; one of them is very likely to be available on this subject; a Master thesis on this subject is also available.

5 Download the flyer PhD-DFT-MuSR2.0.pdf of this PhD position description

Pubblicato Venerdì, 15 Giugno, 2018 - 11:48 | ultima modifica Venerdì, 15 Giugno, 2018 - 12:27