5640 S Ellis Ave
Speaker: Amir Yacoby (Professor, Department of Physics, Harvard University)
Abstract: Quantum materials support elementary excitations that differ considerably from their underlying electronic constituents. Such novel excitations are often a result of the competition between repulsive Coulomb interactions and the kinetic energy of electrons. In two dimensions, the application of a strong magnetic field can suppress the kinetic energy of electrons leading to the integer and fractional quantum Hall effect where a myriad of novel phases with unique elementary excitations exist. Examples include excitations that carry spin without charge, charged topological spin textures known as Skyrmion, excitations that do not obey Bose or Fermi statistics and could even have non-Abelian character, electrons that carry quantized magnetic flux known as Composite Fermions and excitations that carry fractionalized charge.
Fractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue toward fractionalized and non-abelian excitations even at zero magnetic field. Early theoretical studies have predicted their existence in systems with energetically flat Chern bands and highlighted the critical role of a particular quantum band geometry. Magic angle twisted bilayer graphene (MATBG) supports flat Chern bands at zero magnetic field, and therefore offers a promising route toward stabilizing zero-field FCIs. Here I will describe our recent observation of FCI states at low magnetic fields in MATBG enabled by high-resolution local compressibility measurements. Our findings highlight the interplay between symmetry, topology and interactions that leads to a competition between ordered electronic solid phases and fractionalized electronic liquid phases. Our findings strongly suggest that FCIs may be realized at zero magnetic field and pave the way for the exploration and manipulation of anyonic excitations in moiré systems.
Location: William Eckhardt Research Center (ERC) 161 & Zoom