Topological semimetals

Remarkable discovery of graphene and topological insulators added another principle of quantum matter based on topological distinctions. Unlike ordinary quantum phases of matter, topological phases are not determined by a local order associated with broken symmetry but rather by a topological ingredient reflecting “twist” of electronic wave functions. The topological semimetals host the electronic states in which electron’s momentum is locked to its (pseudo) spins. Due to the unusual (pseudo) spin texture, the resulting Berry curvature introduces novel electrical properties including dissipation-less spin currents and anomalous Berry’s phase. This opens new avenue for designing materials by introducing the concept of quantum entanglement of wave functions that has not been explored before.

Quantum transport evidence of isolated nodal-line fermions

Anomalous transport responses, dictated by the nontrivial band topology, are the key for application of topological materials to advanced electronics and spintronics. We found that, in slightly hole-doped SrAs3, the single-loop nodal-line states are well-isolated from the trivial states and entirely determine the transport responses. The characteristic torus-shaped Fermi surface and the associated encircling Berry flux of nodal-line fermions are clearly manifested by quantum oscillations of the magnetotransport properties and the quantum interference effect resulting in the two-dimensional behaviors of weak antilocalization.

"Quantum transport evidence of isolated topological nodal-line fermions", Hoil Kim, Jong Mok Ok, Seyeong Cha, Bo Gyu Jang, Chang Il Kwon, Yoshimitsu Kohama, Koichi Kindo, Won Joon Cho, Eun Sang Choi, Youn Jung Jo, Woun Kang, Ji Hoon Shim, Keun Su Kim, and Jun Sung Kim,

Nat. Commun. 13, 7188 (2022).

Broken valley-symmetry states of a Dirac semimetal

The strong anisotropy in the Fermi surface is found to be essential for realizing the valley-selective interlayer conduction under the rotating magnetic field. We found a signature of the broken valley symmetry at high magnetic fields, which is the first experimental observation in quasi-2D Dirac systems. Our findings demonstrate that a Bi square net is a new building block in material design for new-type Dirac fermions as well as the valley-based electronics.

"Valley-polarized interlayer conduction of anisotropic Dirac fermions in SrMnBi2", Y. J. Jo, Joonbum Park, G. Lee, Man Jin Eom, E. S. Choi, Ji Hoon Shim, W. Kang, and Jun Sung Kim,

Phys. Rev. Lett. 113, 156602 (2014).

First anisotropic Dirac semimetal in a square lattice

We reported for the first time that a bulk material SrMnBi₂ hosts highly anisotropic Dirac fermions. Unlike most of Dirac materials having the isotropic Dirac dispersion, SrMnBi₂ has the strong momentum-dependent Fermi velocity and the sizable spin-orbit-coupling gap.

“Anisotropic Dirac Fermions in a Bi square net of SrMnBi2”, Joonbum Park, G. Lee, F. Wolff-Fabris, Y. Y. Koh, M. J. Eom, Y. K. Kim, M. A. Farhan, Y. J. Jo, C. Kim, J. H. Shim, J. S. Kim,

Phys. Rev. Lett. 107 126402 (2011).