Research
Strongly Correlated systems out of equilibrium
As a strongly correlated system is "pushed out" of equilibrium, one may raise the question of how the system responses. This problem is hard to be solved because an additional degree of freedom, time, is involved. Our study of a Kondo impurity system shows that the spectral function in response to a quench of changing Coulomb repulsion evolves in time with two time-scales inversely proportional to two energy-scales respectively. The first one is shown in the shifting of high energy peaks. The second one is the reconstruction of the low-energy resonance. The observation is expected to be validated in experiments, e.g. time-resolved PES. H. T. M. Nghiem and T. A. Costi, PRL2017.
Strongly correlated materials with spin-orbit coupling
Every electron has its own intrinsic angular momentum as known as spin, resulting their own magnetic moment. However, an electron moving around the nucleus also experiences a magnetic field due to its motion in the reference frame of the electron. These two give an amount of energy known as the spin-orbit coupling, which is a relativistic effect. On the other hand, for certain materials, valence electrons are likely to interact with each other, the large term is the onsite interaction when two electrons stay at the same atom, characterized by a strength U, thus the name "strongly correlated systems" is coined. There are systems where both the spin-orbit coupling and the interaction are nonnegligible, which may exhibit new phenomena. It is our motivation to study such systems, starting from simple model such as the three-orbital (t2g orbital) model, aiming to study realistic materials such as the irididum oxides.