"The effect of the external electric field in 2D materials: on the left the electrons are shown as figurines that stay away from each other; on the right, when the electric field is turned on, represented by the jazz band, the electrons pair with each other and move in a perfect synchronized way""Electric field control of electronic states in strongly correlated electron materials has been a holy grail of modern condensed matter research."Antonio Castro Neto
Researchers from the National University of Singapore have demonstrated how to manipulate electrons in thin semiconductors by confining them in a device made from atomically thin materials and then changing both the external electric and magnetic fields. As they are so thin, such 2D superconducting materials could improve upon conventional superconductors for applications including portable magnetic resonance imaging (MRI) machines.
With it being difficult to control the motion of electrons directly, many semi-conductor materials depend on chemical doping, where some foreign material is embedded to release or absorb electrons, thus creating a change in the electron concentration that can be used to drive currents. However, this can result in irreversible chemical change in the material, with the foreign atoms disrupting its natural ordering.
In this study, published in Nature [Li et al. Nature (2015) DOI: 10.1038/nature16175], the effects of chemical doping were replicated using only external electric and magnetic fields applied to titanium diselenide (TiSe2) encapsulated with boron-nitride (hBN). The team was able to control the behavior of the electrons both accurately and reversibly, allowing for measurements that had only been theoretical to date. The thinness of the materials was crucial, as the electrons were confined within the material as a two-dimensional layer, with the electric and magnetic fields having a strong and uniform effect. The material also has to be thin enough to avoid shielding the electric field in the bulk material, and 2D materials do not experience electrical screening.
Electric field control of electronic states in strongly correlated electron materials has long been seen as pivotal for condensed matter research, but it is only with the isolation of graphene and developments in 2D materials that this has been achieved. This research showed how spatially modulated electronic states are fundamental to the appearance of 2D superconductivity. As team leader Antonio Castro Neto points out, “we could also drive the material into a state called superconductivity, in which electrons move throughout the material without any heat or energy loss”.
In terms of MRIs, these have to be large due to the 3D superconducting materials used to create the large magnetic fields needing to be cooled to extremely low temperatures and also isolated from the environment using large vacuum chambers. With 2D superconducting materials, it is possible to isolate the superconducting element without the need for the chambers. The team expects that similar results will also apply to other 2D materials as they have similar electronic phases, and that the technique will allow for new experiments to provide insight into high-temperature superconductivity and other solid-state phenomena.