Supercluster of Electrons in Ultrathin TaSe₂ Nanocrystals

Abstract

We discover a supercluster of electrons in the ultrathin TaSe₂ crystals synthesized by our developed de-chalcogenide method. The supercluster has an unexpected structure containing 61 electrons denoted by \\sqrt61×\\sqrt61, in addition to a \\sqrt49×\\sqrt49 supercluster based on \\sqrt7×\\sqrt7 clusters expected in triangle lattices. This new hierarchal structure and successive clustering cannot be explained by a conventional electron clustering mechanism assisted by background lattice distortions, i.e., charge-density-wave formation mechanism based on Fermi surface nesting. We propose an alternative possible formation mechanism based on saddle-point nesting, taking into account a trigonal symmetry as well as lattice distortion energy.