Atomic (nano) wires which can experimentally be created by scanning-tunneling microscope or break-junction techniques, have turned out to be a unique playground to test basic concepts of electronic transport at the atomic scale. But also the effect of quantum mechanics onto macroscopic laws like the Wiedemann-Franz law for the electronic contribution to the thermal conductance, can be studied.
In this context we have studied different materials like Al, Au, Mg, Pt, Fe, Ni and Co. Therefore we combined classical molecular dynamics (MD) simulations with a (non-orthogonal) Tight-Binding based Non-equilibrium Green's function formalism (NEGF) to have simultaneous access to geometric effects like atomic reorganisations due to an external force at finite temperatures and the resulting electronic effects due to the modifications of the geometries like the conductance, the channel transmissions, electronic noise (fano factor / shot noise) and thermopower, but also the electronic contribution to the thermal conductance.
Elongation of an atomic Au wire at T=4K which creates a one atomic chain before the end. The geometries during the process are shown on the right. On the left side are the corresponding conductance (black line, top and middle), the electronic contribution to the thermal conductance (squares, top and middle), the channel transmissions (colored lines, middle), fano factor (red line, bottom) and the thermopower (green line, bottom). The x-axes represents the elongation of the wire.
Similar example for Pt at room temperature, but without the last line of results.
Most studies create many structures and calculate and or measure their transport properties. The direction of these project is the other way: How electronic transport can change the structure of contacts and which structures are preferred in transport experiments and most likely to appear. The basic aim is the microscopic description of a current-dependent nano-switch, including the switching process. The best known example of electromigration - the movement of atoms in response to electronic transport - is the fusing of contacts. Despite this phenomenologically very accessible and well investigated phenomenon, the microscopic mechanism is largely unknown.
Methods include Molecular Dynamics for the contact structure, Tight Binding and DFT for the electronic structure and NEGF for the transport calculations.
a) New Publications
b) Old publications
- M. Dreher, F. Pauly, J. Heurich, J.C. Cuevas, E. Scheer, P. Nielaba, PRB 72, 075435 (2005).
- F. Pauly, M. Dreher, J.K. Viljas, M. Häfner, J.C. Cuevas, and P. Nielaba, PRB 74, 235106 (2006).
- C. Schieback, F. Bürzle, K. Franzrahe, J. Neder, M. Dreher, P. Henseler, D. Mutter, N. Schwierz, P. Nielaba, in: High Performance Computing in Science and Engineering '08, edited by W.E. Nagel, D. Kröner, M.M. Resch, Springer Verlag, pp. 41 (2009).
- M. Häfner, J.K. Viljas, D. Frustaglia, F. Pauly, M. Dreher, P. Nielaba, J.C. Cuevas, PRB 77, 104409 (2008).
- M. Dreher et al., Phase Transitions 78, 751 (2005).
c) Completed work
- M. Dreher, diploma thesis: Computersimulationen zum elektronischen Ladungstransport in Nanostrukturen (2002).
- M. Dreher, dissertation: Untersuchung elektronischer Eigenschaften komplexer Materialien mittels Computer-Simulationen (2008).
- M. Matt, dissertation: Theoretical study of the charge and energy transport in metallic atomic-size contacts (2017).
- N. Agraït, A. Levy Yeyati, and J.M. van Ruitenbeek, Phys. Rep. 377, 81 (2003).
e) External links (programs, potentials ...)
- LAMMPS. S. Plimpton, J. Comput. Phys. 117, 1 (1995). (http://lammps.sandia.gov)
- General potentials LAMMPS http://www.ctcms.nist.gov/potentials/
- EAM-potentials of FCC-metals H. W. Sheng, M. J. Kramer, A. Cadien, T. Fujita, and M. W.
Chen, Phys. Rev. B 83, 134118 (2011). (https://sites.google.com/site/eampotentials/)
- M. J. Mehl and D. A. Papaconstantopoulos, Computational
Materials Science, edited by C. Fong (World Scientific,
J. C. Cuevas and E. Scheer, Molecular Electronics: An Introduction to Theory and Experiment (World Scientific,Singapore, 2010).