On a cold winter’s morning we go to the kitchen, put the kettle on and soon after we can enjoy a comforting hot drink. This miracle is due to one of the fundamental processes in electrical conductors: the electron-phonon interaction. Phonons are the thermal vibrations of the atoms. The electrons that carry the current disturb the atomic nuclei, they start to swing more energetically, and the result is what we know as Joule heating.

In Joule heating, however, the atomic vibrations attain a certain elevated temperature and settle there. In a recent paper in Physical Review Letters we show theoretically that there is a fundamental threshold, corresponding to a current density several orders of magnitude higher than that in a lightbulb filament, above which this equilibration is no longer possible. The result: the explosive emission of a blast of travelling lattice waves without anything to reabsorb them. The phonons generated in this process are monochromatic (i.e. they are closely clustered around a single frequency) and coherent. Since the process hinges on stimulated emission, it feeds on itself and hence the unstoppable nature of the event. The phonon beams generated in this way are the sound analogue of a laser.

Since the now-violent motion of the atoms scatters the electrons very strongly, the current drops to about the threshold value and stays there. The upshot is that there is a fundamental current density that cannot be exceeded without dramatic consequences.

This critical current is much too high for macroscopic wires. But it is eminently possible in atomic wires and nanowires. While the effect poses a fundamental limit to the functionality of these devices, it also opens up potential applications, such as the possible use of the phonon jets to manipulate and image lattice defects and interfaces by scattering off them.

This effect was predicted with pen and paper in 2017 but it is only now that its direct demonstration became possible in real-time quantum-mechanical simulations of current-driven dynamics in metallic atomic chains.

The team for this work brings together researchers from QUB and TCD with a long-standing collaboration on electron transport and current-induced magneto-mechanical effects, spanning a period of over 20 years. The team members are Dr Maria Stamenova (TCD), Dr Plamen Stamenov (TCD) and Dr Tchavdar Todorov (QUB). The work, however, took place in an extended context involving lively discussions with other QUB colleagues, notably Dr Ray McQuaid, Dr Amit Kumar and Professor Marty Gregg, as well as Professor Stefano Sanvito at TCD, one of the original TCD-QUB team leaders.

This wider team together with Dr Dan Dundas (QUB) – a long-standing collaborator on current-driven dynamics - is working towards a joint project between the two centres to take this research forward along combined theoretical and experimental lines.