Ultra-Low Temperature Scanning Tunneling Spectroscopy Study of THE Local Electronic Structure of Two-Dimensional Dirac Fermion Systems

 

Niv Levy1, Tong Zhang1,2, Jungseok Chae1,2, and Joseph A. Stroscio1

 

1CNST, NIST, Gaithersburg, MD

2Maryland NanoCenter, UMD, College Park, MD

 

 

We  have developed a scanning tunneling microscope (STM) based on an ultra-high vacuum (UHV) compatible dilution refrigerator which operates in extreme conditions of ultra-high vacuum, ultra-low temperatures (10 mK),  and high magnetic fields (15 T).  The ultra-low temperatures allow us to achieve unparalleled energy resolution in tunneling spectroscopy measurements, where the energy resolution is given by the thermal broadening of the tip/sample Fermi levels.  We are currently using the system to study Dirac fermions in graphene and topological insulators (TI).  Studying both systems allows us to contrast the chiral pseudo-spin polarized states in graphene with the strongly electron-spin polarized helical surface states of a topological insulator.   Both material systems offer applications in future electronics, such as high mobilities in a single atomic carbon sheet of graphene, and unique electron-spin structures for spintronic applications with TIs.

Our graphene studies involve epitaxial graphene grown on SiC, which is a possible route towards large scale fabrication of graphene devices. Owing to the instrument's performance and its high energy resolution, we observed valley and spin splitting of the N=1 Landau level (LL) into a quartet of states. Unexpectedly, we observed half-filled LL states as well as the integer LL states.  For the TI studies, we are growing atomically flat Bi2Se3, Bi2Te3 and Sb2Te3 films on epitaxial graphene using molecular beam epitaxy (MBE).  We are characterizing and optimizing the growth dynamics by real time reflection high-energy electron diffraction (RHEED).  Initial STM studies of these TI films are currently being performed, which we will use to contrast Landau level quantization in TIs to the quantization we observed in graphene.