Topological Phases of Matter
Selected Publications
Global topology of Dirac fermions in magic-angle graphene
Our theory reveals the topological origin of the mismatch between the number of left- and right-handed Dirac quasiparticles in twisted bilayer graphene, which is a step toward identifying new topological phases of matter.
Junyeong Ahn, S. Park, and B.-J. Yang, "Failure of Nielsen-Ninomiya Theorem and Fragile Topology in Two-Dimensional Systems with Space-Time Inversion Symmetry: Application to Twisted Bilayer Graphene at Magic Angle", Physical Review X 9, 021013 (2019).
Theory of monopole nodal lines
This work establishes a theoretical framework for studying the monopole charge of nodal lines by introducing the concept of the Stiefel-Whitney and Euler classes, which allowed us to discover the unique band structure serving as evidence of monopole charges in both theoretical proposals and experimental observations.
Junyeong Ahn, D. Kim, Y. Kim, and B.-J. Yang, “Band topology and linking structure of nodal line semimetals with Z2 monopole charges”, Physical Review Letters 121, 106403 (2018).
New phase diagram of quantum spin Hall insulators
We discover a unique topological phase transition in two-dimensional non-centrosymmetric systems with time-reversal and twofold rotation symmetries. Instead of a direct transition between insulating phases, a 2D Weyl semimetal phase emerges during the transition. This unconventional phase transition can be observed in materials like HgTe/CdTe quantum wells and few-layer black phosphorus.
Junyeong Ahn and B.-J. Yang, “Unconventional Topological Phase Transition in Two-Dimensional Systems with Space-Time Inversion Symmetry”, Physical Review Letters 118, 156401 (2017).
Topological Correspondence between Hermitian and Non-Hermitian Systems
This work shows that certain topological properties of quantum systems with and without dissipation, known as non-Hermitian and Hermitian systems, respectively, are related. Our discovery provides a fresh insight into non-equilibrium dynamics, with potential applications in understanding materials with gain and loss.
J. Y. Lee, Junyeong Ahn, H. Zhou, and A. Vishwanath, "Topological Correspondence between Hermitian and Non-Hermitian Systems: Anomalous Dynamics", Physical Review Letters 123, 206404 (2019).
Unconventional Majorana fermions on the surface
This work introduces new types of Majorana fermions that appear on the surface of topological superconductors. They include higher-spin Majorana fermions, Majorana Fermi lines, and a group of four interconnected fermions, whose band structures are unique to superconductors and distinct from those of the surface states of topological insulators.
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Junyeong Ahn and B.-J. Yang, "Unconventional Majorana fermions on the surface of topological superconductors protected by rotational symmetry", Physical Review B 103, 184502 (2021).