Time and place: 2018.8.24, 16:00 pm, Wuhan National High Magnetic Field Center A319

Presenter: Ryuichi Shindou

Title: Theories of topological spin-nematic excitonic insulators in graphite under high magnetic field and transport scaling in disordered semimetals

Abstract：

In the first part of my talk, I will argue that three-dimensional topological excitonic insulator is realized in graphite under high magnetic field. Graphite under high magnetic field exhibits consecutive metal-insulator (MI) transitions as well as re-entrant insulator-metal (IM) transition at low temperature. A part of the experiment was discovered more than 30 years ago, while the identities of the low-temperature insulating phases are still unclear by now. We identify these enigmatic insulator phases with excitonic insulator phases, where electron and hole pocket(s) form spin-triplet excitonic pairings. We show that the re-entrant IM transition in the graphite experiment can be naturally explained by an enhanced quantum spin fluctuation in the presence of smaller electron and hole pocket(s). We further argue that the odd-parity spin-triplet excitonic pairing reconstructs chiral surface Fermi arc state of electron and that of hole into a 2+1 massless surface Dirac fermion (topological excitonic insulator).

In the second part of my talk, I will talk about transport scaling theories in disordered Weyl semimetal. In electronic band structure of solid state material, two band touching points with linear dispersion (called as `Weyl node') appear in pair in the momentum space. When they annihilate with each other, the system undergoes a quantum phase transition from Weyl semimetal (WSM) phase to a band insulator (BI) phase. The continuous phase transition is recently discovered in solid state materials. The phase transition is described by a critical theory with a `magnetic dipole' like object in the momentum space. I will argue that the critical theory hosts a new disorder-driven quantum multicritical phenomena. Based on the renormalization group argument, we clarify transport scaling properties around the Weyl node around the quantum multicritical point as well as the direct phase line between BI and WSM phases.