Seminars Archive

One-dimensional metallic line defects in two-dimensional semiconductors

Abstract
Grain boundaries in two dimensional materials, such as graphene or layered transition metal dichalcogenides (TMDs), are one dimensional (1D) material ‘defects’. For most twist-angles between grains, the electronic structure of the grain boundary consists of localized states. For MoSe2 and MoTe2 grown by molecular beam epitaxy (MBE) on a MoS2 or HOPG/graphene a high density of grain boundaries with a 60° twist angle is observed. A 60° twist angle in a material with 120° rotation symmetry results in the formation of twins with mirror symmetry. Importantly, these twin-boundaries are metallic with a dispersive band. Here we investigate their properties by scanning tunneling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES) and discuss their 1D properties. 1D metals are known to behave fundamentally different from metals in higher dimensions. In particular, while excitations in 2D and 3D metals are described by a Fermi-liquid, electrons in 1D metals can only be excited collectively which gives rise to the Tomonaga-Luttinger liquid (TTL) formalism. Predictions arising from the TTL can be tested by ARPES, for example a suppression of the density of states close to the Fermi-level is observed. It is also known, that 1D metals are intrinsically unstable and should undergo a metal-to-insulator (Peierls) transition. Such a transition results in a charge density wave whose periodicity is measured in STM and is directly related to the Fermi-wave vector measured in ARPES.