Topological Evolution Of Correlated Band Structures And Heavy-fermion-like Behavior In A Molybdenum-based Metal Organic Framework Material

Microscopic porous materials have received extensive attention since their discov-ery.Among them,metal organic framework(MOF)materials are being studied more and more,not only in gas storage and adsorption,catalysts and other chemical fields,but also in the field of physics in recent years,due to their structural topological diversity,adjustable performance and many other excellent properties.This thesis mainly studies the topological evolution of the correlated energy band of the three-dimensional MOF material C48S36Mo6 at room temperature and the heavy-fermion-like behavior at low temperature.The first chapter takes gas adsorption and storage as examples to introduce the application of MOF materials in the chemical industry,and the attention and research it has received in the field of physics in recent years,including low-dimensional mag-netism and spin liquids.In addition,this chapter also introduces the topology-related physics,including KT phase transition,quantum Hall effect,quantum spin Hall effect,and the classification of topological insulators and topological semi-metallic materials.The second chapter introduces the band structure calculation methods for the electronic structures of condensed materials.There are band structure theories under single elec-tron approximation,including free electron approximation,near-free electron approxi-mation,tight-binding model,and multi-electron density functional theory.The density functional introduction focuses on the Born-Oppenheimer approximation,the Hartree-Fock approximation,the Hohenberg-Kohn theorem,the exchange correlation functional and the correlation U model.Due to its special micro-porous structure characteristics,MOF materials have at-tracted wide interest in the field of physics,especially in the field of low-dimensional magnetism and spin liquid research in recent years.In the third chapter of this thesis,based on the stability of the ZIF-8 as the subclass of MOFs,and the potential application of two-dimensional transition metal molybdenum-based materials,a new molybdenum-based MOFs material C48S36Mo6 with ZIF-8 configuration is proposed.It is composed of organic ligands[C16S12]8-which are connected to each other through Mo4+ metal ions.Using the first-principles calculation method,we simulated the changes of var-ious magnetic state energies and corresponding band structures of C48S36Mo6 with Coulomb correlation U.The results reveal heavy-fermion-like electronic behaviour that results from highly localized impurity-like Mo-d electrons and tiny energy dif-ference(<0.4 meV/atom)between antiferromagnetic and ferromagnetic state.Consid-ering thermal fluctuation and weak magnetic exchange energy,C48S36Mo6 is a non-magnetic metal at room temperature,and magnetic insulator with a correlation-driven metal-insulator transition at low temperature.Our Wannier functions analysis indicates that topological properties of energy bands around the Fermi level can be perturbed by correlation U,leading to moving of close Dirac nodal lines in three dimensional mo-mentum space.Further introducing of spin-orbit coupling opens a small inverted energy gap of 3 meV at original nodal line due to bulk inversion symmetry.If a gate voltage of 48 meV is applied,the Fermi level will fall into the small energy gap of band inversion,and the system will realize a phase transition from Kondo metal to topological Kondo insulator.