Our research bridges condensed matter physics and materials science. We perform first-principles calculations to design novel complex materials and predict their physical properties. We use materials design as a new approach to understand fundamental physics (such as magnetism and superconductivity) as well as to search for novel functional properties (for example, multiferroics). Our research has a strong connection to experiments. Clarifying phenomena in existing experiments and stimulating new experiments constitute the core part of our research.
Ab Initio Methods
We use ab initio or first-principles methods to solve many-body Schrödinger equation with Coulomb interaction for complex materials. The basic setup for the calculations is chemical composition and crystal structure, with no a priori assumptions. We focus on two important ab initio methods: density functional theory (DFT) and dynamical mean field theory (DMFT). Combining DFT and DMFT leads to one of the most sophisticated approaches to describing realistic strongly correlated materials, such as transition metal oxides.
Transition metal oxides exhibit rich exotic properties, such as metal-insulator transition, colossal magneto-resistance and high temperature superconductivity. The reason for the diverse phenomena is that different degrees of freedom in oxides, such as charge, spin and orbital, are all coupled to each other, leading to many competing phases. Small variations in crystal structure and chemical composition can induce substantial changes in electronic and magnetic properties of transition metal oxides.
With the advances in thin film growth techniques, we can now experimentally grow artificial heterostructure in which two dissimilar materials of nano meters thick can be deposited on top of each other in a "layer by layer" manner with atomic precision. The properties of the interface in heterostructures can be fundamentally different from either constituent, opening a new playground to search for novel physical properties and new materials.
Here 'strong correlation' refers to the collective phenomena that arise from the Coulomb interaction in many-electron systems (for example, solids). The canonical model to describe such 'strong correlation' is the Hubbard model and its extension. We use first-principles calculations to construct an extended (or multi-orbital) Hubbard model for realistic complex materials and approximately solve it using sophisticated many-body techniques. Our goal is to understand how to control collective phenomena and engineer physical properties of strongly correlated materials.