Non-topographic current contrast in scanning field emission microscopy.

In scanning field emission microscopy (SFEM), a tip (the source) is approached to few (or a few tens of) nanometres distance from a surface (the collector) and biased to field-emit electrons. In a previous study (Zanin et al. 2016 Proc. R. Soc. A 472, 20160475. (doi:10.1098/rspa.2016.0475)), the field-emitted current was found to change by approximately 1% at a monatomic surface step (approx. 200 pm thick). Here we prepare surface domains of adjacent different materials that, in some instances, have a topographic contrast smaller than 15 pm. Nevertheless, we observe a contrast in the field-emitted current as high as 10%. This non-topographic collector material dependence is a yet unexplored degree of freedom calling for a new understanding of the quantum mechanical tunnelling barrier at the source site that takes into account the properties of the material at the collector site.

Mechanical dissipation via image potential states on a topological insulator surface.

Joule energy loss due to resistive heating is omnipresent in today's electronic devices whereas quantum-mechanical dissipation is largely unexplored. Here, we experimentally observe a suppression of the Joule dissipation in Bi2Te3 due to topologically protected surface states. Instead, a different type of dissipation mechanism is observed by pendulum atomic force microscopy, which is related to single-electron tunnelling resonances into image potential states that are slightly above the Bi2Te3 surface. The application of a magnetic field leads to the breakdown of the topological protection of the surface states and restores the expected Joule dissipation process. Nanomechanical energy dissipation experienced by the cantilever of the pendulum atomic force microscope provides a rich source of information on the dissipative nature of the quantum-tunnelling phenomena on the topological insulator surface, with implications for coupling a mechanical oscillator to the generic quantum material.

Spin-polarised electrons in a one-magnet-only Mott spin junction.

The current flowing through a Mott spin junction depends on the relative spin orientation of the two ferromagnetic layers comprising the "source" and "drain" sides of the junction. The resulting current asymmetry is detected as giant or tunnelling magnetoresistance depending on whether the two ferromagnets are separated by a metal or an insulator. Based on the fundamental principles of reciprocity for spin-dependent electron scattering, one can envisage a one-magnet-only spin junction in which the source is non-magnetic, and the spin information is encoded by the spin polarisation of the electrons that have crossed or are backscattered from the drain magnetic layer. The practical significance of using an unpolarised source is that the state of the magnetic layer can be modified without affecting the process of probing it. Whether this reciprocity is realised in the actual junctions is not yet known. Here, we demonstrate a nano-sized, one-magnet-only Mott spin junction by measuring the finite spin polarisation of the backscattered electrons. Based on this finding, we conclude that since the junction acts as a spin filter, the magnetic layer must experience a spin transfer that could become detectable in view of the high current densities achievable in this technology.