Emergent nonsymmorphic Dirac semimetal and its carrier dynamics in doped spin-orbit-coupled Mott insulator

Abstract

After the discovery of graphene, which is the first Dirac semimetal, intensive theoretical and experimental studies have been done to discover and characterize Dirac semimetals. For example, investigations of gapless surface states arising from the nontrivial bulk topology of Dirac semimetals and unique responses to an applied electric and/or magnetic field of Dirac semimetals have been performed. While there are many studies about Dirac semimetals, not much is known about correlated Dirac semimetals or Dirac semimetals in strongly-correlated materials. It is thus interesting to investigate the possible emergence of a Dirac semimetal in strongly-correlated systems such as the lightly-doped Mott insulator, which is particularly promising for exploring the interplay between the electron correlation and other emergent degrees of freedom.

In this thesis, using a symmetry analysis, a realistic five-orbital tight-binding model derived from density-functional theory calculations, a mean-field Hubbard model, and a Boltzmann transport theory, we study the lightly-doped spin-orbit-coupled Mott insulator Sr2(Ir,Tb)O4 and (Sr,La)2IrO4. We demonstrate that a Dirac semimetal protected by the nonsymmorphic crystalline symmetry emerges even in the presence of the strong spin-orbit coupling and electron correlation. Remarkably, we reveal that a correlation-induced symmetry-breaking order leads to a phase transition from a Dirac line-node (DLN) semimetal to a Dirac point-node (DPN) semimetal. We then show that the correlation effect is also manifest in the temperature-dependent Dirac carrier dynamics, enabling the distinction between the DLN and DPN semimetals. Finally, we find that our results are well consistent with the available angle-resolved photoemission spectroscopy and terahertz spectroscopy data. The emergent nonsymmorphic Dirac semimetal presented in this thesis allows the relativistic electrodynamics governed by the extremely small scattering rate even at room temperature, thus providing an intriguing opportunity to explore new emergent and collective phenomena of correlated Dirac materials.