New Technique for Measuring the Layer-Resolved Charge Density

Recently Benjamin M. Hunt and his colleagues developed a new technique for measuring the layer-resolved charge density, from which they can map layer polarization of the valley or spin quantum numbers in bilayer graphine and other two dimensional materials.

Large magnetic field causes the single-particle energy spectrum of a two-dimensional electron system (2DES) to collapse into Landau levels (LLs) containing degenerate states. The presence of such states makes simulating interacting electron problems difficult. Therefore an experimental input to constrain the possible ground states are required when there is degeneracy. The Bernal bilayer graphene (B-BLG) zero energy Landau level (ZLL) provides an extreme example of such degeneracy. Previous experiments to show the valley quantum number has only been probed indirectly in semiconductor quantum wells, graphene monolayer and bilayers and transition metal dichalcogenides. Direct probes of spin in 2dimensional electron systems(2DESs) has been already demonstrated. One of the key open questions in bilayer graphine is resolving the order in which the eight components fill as electrons are added. In this study, Benjamin M Hunt and his colleagues demonstrated direct measurement of valley and orbital levels in bilayer graphite. They have detected that the four valley and orbital components have different weights on the two layers of the bilayer. 

By using Hunt’s technique one can probe layer, valley, and spin polarization quantitatively in other atomic layered materials, including twisted bilayer graphene and both homobilayer and heterobilayer of transition metal dichalcogenide

In their experiment they have used devices consist of hexagonal boron nitride encapsulated B-BLG flakes fitted with metal top and bottom gates. They have applied bias to both top and bottom gates to tune the layer polarization and total charge density. They have measured deviations between total density and layer density at high magnetic field as corrections to the measured gate capacitances. They use this difference to infer the fillings as a function of both the total electron density and applied perpendicular electric field. By using these methods they were able determine the layer polarization of van der Waals bilayers and used it to constrain a detailed model of symmetry breaking in the bilayer graphene ZLL.

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