A theoretical understanding of the behavior of materials is a great intellectual challenge, as well as the key to creating new technologies. I explore the frontier of quantum materials for which the Landau Fermi-liquid theory and the Landau-Ginzburg theory of phase transition often fail. In these materials, standing on two pillars of electron correlation and topological properties of electron wavefunctions, complex and unexpected phenomena, such as superconductivity and topological phases, emerge, opening exciting potential applications for quantum information science. My main research interests lie in exploring the physics of quantum materials, their applications for quantum information science, and methodology (theory, algorithm, codes) development.
Methodology Development for Quantum Materials
One of the most intriguing questions in condensed matter physics is how to understand, from first principles, the electronic structure of correlated electron systems with partially-filled d- and f- subshells. Microscopically, electrons in open d- and f- subshells tend to be localized on the short timescale, but itinerant on the long timescale. This coexistence of localized and itinerant characteristics is not captured sufficiently by conventional band-picture-based ab initio approaches, such as density functional theory and quasi-particle GW, and makes a theoretical understanding of these materials challenging. To answer this important open question, we proposed a new diagrammatically motivated ab initio approach based on ab initio linearized quasiparticle self-consistent GW and dynamical mean field theory (ab initio LQSGW+DMFT) in this article. Ab initio LQSGW+DMFT is a parameter-free electronic structure method for correlated electron systems that enables an electronic structure calculation of archetypical correlated electron systems, including paramagnetic Mott insulators La2CuO4 , Hund metal FeSe , and Hund insulator FeSb2 . Based on this proposal, an ab initio software package named “ComDMFT” has been distributed under GPLv3.0. This is the first open-source package supporting any simplified form of GW+EDMFT as well as enabling multiple methods in one-platform, which is beneficial in supporting the theoretical advancement of the electronic structure of correlated electron systems.
Emergent Phenomena in Quantum Materials
Microscopically, electrons in open d- and f - subshells tend to be localized on a short timescale but itinerant on a long timescale. This coexistence of localized and itinerant characters results in competition among different forms of long-range order, making correlated electron systems extremely sensitive to small changes in their control parameters, resulting in large responses. This makes theoretical understanding of these materials challenging, though it opens up exciting potential applications such as oxide electronics, high-temperature superconductors, and spintronic devices
I explore emergent phenomena in quantum materials by applying cutting-edge ab initio and model Hamiltonian methodologies. To illustrate, In this article, by using the GW plus extended dynamical mean-field theory, the valence-skipping charge order transition is shown to be driven by V . Most interestingly, the instability to this transition is significantly enhanced in the spin-freezing crossover regime, thereby lowering the critical V to the formation of charge order. Our finding unveils another feature of the Hund’s metal, and has potential implications to the broad range of multiorbital systems as well as the recently discovered charge order in iron-pnictides.
Exotic Physics of Dirac Fermions
The conical electronic structure along with the chiral nature of Dirac fermions gives rise to various unusual phenomena under external potentials. In two-dimensional Dirac fermion systems, we found another surprising, counter-intuitive electron-transport phenomenon: electron supercollimation that can be induced by one-dimensional disorder potentials. In the fluctuating direction of the external one-dimensional disorder potential, an electron wave packet is guided to propagate virtually undistorted, independently of its initial motion. The more disorder there is, the better the supercollimation is. This phenomenon is distinct from known systems wherein an electron wavepacket is further spread by disorder and hindered in the potential fluctuating direction. To our knowledge, this was not seen in any other systems. This study inspired another discovery, the supercollimation of pseudospin-1 electromagnetic waves. More information can be found here.
Qubits in Solid State Environments
Quantum computation has great potential to exceed the computational efficiency of classical computers for certain tasks, such as factoring large numbers. In order to implement quantum computation, all of the DiVincenzo criteria, including the ability to initialize qubits and long decoherence times of qubits, must be fulfilled simultaneously. In this research direction, We confronted puzzles hidden in two different qubit candidates that hold keys to their improved performance. The first issue pertained to the decoherence time in superconducting qubits: the origin of flux noise with a power spectrum scaling of 1/ f (f is frequency). It was known that the local moment in the Josephson junction is the source of magnetic flux noise in superconducting qubits; however, the origin of the local moments at the non-magnetic metal-insulator interface was puzzling. In this study, we showed that interface disorder localizes a substantial fraction of the metal-induced gap states and that electron interaction causes them to bear local moments at a generic metal-insulator interface. We suggested the necessity of controlling interface disorder for the performance of superconducting qubits. The second issue was the optical initialization mechanism of spin qubits in nitrogen-vacancy (N-V) center, which was puzzling. Theoretically, it is a nontrivial problem due to its degenerate ground-state nature; in the degenerate ground-state system, conventional band-picture-based ab initio approaches do not adequately capture electron correlation. We proposed a general scheme for the study of the initialization mechanism of spin qubits in materials by using exact diagonalization of a many-electron Hamiltonian with parameters derived from ab initio GW in this paper. This yielded two different initialization routes.