Imaging Double-Layer Quantum Hall Exciton Condensates – Part I

When two quantum Hall layers are brought into close proximity, electrons in one layer and holes in the other layer can bind to form interlayer excitons. These excitons obey Bose-Einstein exchange statistics and can form a Bose-Einstein condensate at sufficiently low temperatures. It has been theoretically predicted that upon the addition of charge carriers to such an interlayer exciton condensate, the charge carriers fractionalize into vortex-antivortex pairs, and the ground state is described by a lattice of fractionally charged vortices and antivortices [1]. Past experimental work on exciton condensates has been focused on macroscopically averaged transport and optical measurements, which are unable to provide nanoscale information on key aspects of these states, such as their excitations and response to the addition of charge carriers. In this first of a series of two talks, we describe how we use scanning tunneling microscopy/spectroscopy (STM/STS) to understand exciton formation in double layer graphene. When the total filling factor of the double layer system is an integer, STS shows a clear spectroscopic gap, indicative of exciton formation. Spectroscopy as a function of the total carrier density and the carrier imbalance between the two layers reveals a phase diagram of compressible and incompressible phases in the two layers, forming a basis for imaging of exciton condensate states.