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NN9.06 - Length Matters: Keeping Atomic Wires in Check 
December 4, 2014   10:30am - 10:45am

The strive towards smaller electronic devices will result in damage and failure due to processes such as electromigration becoming a central impediment. As the size of the devices reach the nanoscale, where quantum effects become important, the current densities flowing will be huge and this will give rise to large current-induced forces acting on the constituent atoms in the device. The current-induced force consists of fluctuating forces (due to the corpuscular nature of electrons and responsible for processes such as Joule heating [1]) and the average force. The average force contains, amongst other contributions, the electron wind force [2]. The electron wind force has the remarkable feature of being non-conservative [3 - 5] and can therefore do net work on atoms around closed paths. Understanding this new energy exchange mechanism between the current and the atomic motion will be necessary in the continued miniturization of electronic devices and may also be exploited for the manipulation of matter at the atomistic level, and/or in the development of nanoscale motors.

Non-conservative dynamics are investigated in a system containing many degrees of freedom: defect-free metallic nanowires. Electron-ion interactions are modelled at the level of the Ehrenfest approximation, which neglects Joule heating. This leaves work by the non-conservative current-induced forces as the only energy injection mechanism into the atomic motion, enabling us to isolate and study its effect. As a result of the competition between the non-conservative forces and electronic friction the ionic kinetic energies saturate at a bias-dependent steady-state. The dependence of the ionic saturation kinetic energy on length, bias and mass, observed in the non-equilibrium non-adiabatic molecular dynamics simulations can be understood with the help of a simple analytical model. Two key results are that the saturation kinetic energy per atom under non-conservative current-induced dynamics dies out with increasing atomic wire length and with decreasing atomic mass. The results therefore define the limit in which low dimensional metallic conductors should be expected to be most stable against this novel mechanism for energy transfer from current into atomic motion. This material also highlights the benefit of simple preliminary steady-state calculations in anticipating aspects of brute force dynamical simulations, and provides rule of thumb criteria for the design of stable quantum wires.


[1] M. Galperin, M. A. Ratner, and A. Nitzan, J. Phys.: Cond. Matt. 19, 103201 (2007).
[2] R. Landauer and J. W. F. Woo, Phys. Rev. B 10, 1266 (1974).
[3] S. R. Sorbello, Sol. Stat. Phys. 51, 159 (1997).
[4] D. Dundas, E. J. McEniry, and T. N. Todorov, Nature Nanotech. 4, 99 (2009).
[5] J.-T. Lü, M. Brandbyge, and P. Hedegård, Nano. Lett. 10, 1657 (2010).

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