Investigation On The Correlated States Of CrI₂ Ultra-thin Films

Comparing with three dimensional materials,two dimensional Van der Waals materials possess atomic thickness with quantum size effect,thus exhibiting fruitful physical phenomena.These exotic quantum states not only enable exploring the rich variety of unprecedented ordered states at monolayer limit,including topology,superconductivity,magnetism,ferroelectricity,etc.,but also show huge potential for high quality electronic device applications due to their excellent performance in mechanical and optical properties.Among these ordered states,Mott phase is the mother phase of high temperature superconductors and the basis of strong correlation physics,but is rarely studied in low dimensions.In this thesis,by using molecular beam epitaxy(MBE)and scanning tunneling microscopy(STM),we have successfully fabricated high quality CrI₂ films and investigated their Mott characteristics and magnetism in monolayer limit,as well as discussing the intrinsic defects in CrI₂ films.The main results are as follows:(1)We synthetize high quality CrI₂ films on 6H-Si C(0001)and HOPG substrate,which are achieved in a layer-by-layer growth mode with atomic flatness and controllable thicknesses.By using scanning tunneling microscopy,we determine the atomic structure of CrI₂ lattice with an energy band gap of?3.2e V,which remains unchanged at different layers.In addition,the spectroscopic mappings show no signature of standing waves near the defects or edge boundaries,implying strong correlation effect in CrI₂ films.(2)Our cooperation with density functional theory and dynamic mean field theory calculations shows that:given the 5 magnetic structures,with considering the electron correlation effect(U=4 e V),the ferromagnetic ground state has the lowest total energy with an energy gap of 3.0 e V,consistent with the experimental observations.This indicates the CrI₂ films a strong Mott insulator with the ferromagnetic ground state.Two satellite peaks in the d I/d V curve correspond to the d_Z² and d_x²-y² orbitals of lower Hubbard band in the framework of extended Hubbard model,in which both the on-site term(U)and the nearest neighbor Coulomb repulsion(V)are considered.The Mott characteristics of CrI₂ films are also corroborated by the spectral weight transfer and gap reduction at charge dopants near the Na and Zn adatoms.(3)We prepare a W tip covered by Cr layers and obtain an out-of-plane spin-polarized tip which is calibrated on the Co/Cu(111)system and remains stable within±2 T.With this magnetic tip,we focus on the d_x²-y² orbital of the lower Hubbard band at?1.3 e V.The differential conductance response under various applied magnetic field indicates a ferromagnetic ground state of CrI₂ films with a switching magnetic field residing between 0.1 T and 0.2 T.(4)We determine the lattice distortion of the intrinsic defect on CrI₂ films by high resolution topographies and interpret such defects as small polarons.Besides,by utilizing the STM tip,we also realize the manipulation of the defects,including moving,generation and erasing.