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| Figure 1: A Coulomb drag experiment measures the interactions between charges in two closely spaced layers. The experiment entails running a current through the “drive” layer (here, the top layer) and measuring the resulting flow of charge in the “drag” layer (the bottom layer). The panels indicate three (of many) possible drag scenarios associated with two sheets of bilayer graphene (grey). At left, exciton pairs form between holes (red) in the drive layer and electrons (green) in the drag layer, giving rise to a large drag effect. At center, holes drag electrons in the same direction (positive drag) because of momentum transfer between the charges in different sheets. At right, holes drag electrons in the opposite direction (negative drag), an observation in bilayer graphene that is yet to be explained. |
Topics: Atomic Physics, Bose-Einstein Condensate, Condensed Matter Physics, Quantum Mechanics
Superfluids (fluids with zero viscosity) and superconductors (materials with zero resistance) have a common ingredient: bosons. These particles obey Bose-Einstein statistics, allowing a collection of them at low temperatures to collapse into a single quantum-mechanical state, or Bose-Einstein condensate. Bosons in superconductors consist of two paired electrons, but the pairing is weak and only occurs at low temperatures. In a quest to build devices that carry electricity with low dissipation at higher temperatures, researchers have therefore explored the possibility of engineering electrical condensates [1] out of strongly bound pairs of electrons and holes, or excitons. Now, two research groups have, independently, fabricated and characterized a graphene-based device that is thought to be a promising platform for realizing an exciton condensate [2, 3]. Neither group has yet found evidence for such a condensate—the ultimate goal of such experiments. But their measurements lay the groundwork for future searches.
Excitons form in semiconductors and insulators. The binding energy between the exciton’s electron and hole can be quite strong, greatly exceeding their thermal energy at room temperature. Unfortunately, excitons recombine quickly, too fast to allow a condensate to form. Although excitons coupled to light confined within a cavity can form hybrid particles (exciton-polaritons) that do live long enough to condense [4], such condensates require a continuous input of light.
APS Viewpoint: Chasing the Exciton Condensate
Michael S. Fuhrer, Alex R. Hamilton
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