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Tuesday, February 27, 2018
3:30 PM - 4:30 PM
CNLS Conference Room (TA-3, Bldg 1690)

Q-Mat Seminar

Disorder-induced topological phase transitions & topological magnonic states

Ying Su
Hong Kong University of Science and Technology

Topological states of matter have been a subject of intensive studies in recent years because of their exotic properties such as the existence of topologically protected edge and surface states. First, I will discuss about the disorder-induced topological phase transition, which was firstly found in topological Anderson insulator (TAI) with time-reversal symmetry (TRS). Here we show that when the TRS is broken, the physics of TAI becomes even richer. The pattern of two-terminal conductance and nonequilibrium local current distribution display novel TAI phases characterized by nonzero Chern numbers, indicating the occurrence of multiple chiral edge modes. Tuning either disorder or Fermi energy (in both topologically trivial and nontrivial phases), drives transitions between these distinct TAI phases, characterized by jumps of the quantized conductance from 0 to e2/h and from e2/h to 2e2/h. An effective theory based on the Born approximation yields an accurate description of different TAI phases in parameter space. Then I will discuss the topological states of magnons, the quanta of low-energy excitations of magnetic materials. Weyl magnons are featured by nontrivial magnon band crossings at paired Weyl nodes of opposite chirality, and they are shown to exist in pyrochlore ferromagnets and stacked honeycomb ferromagnets. There exist the magnon arcs with an equal energy contour around the Weyl nodes on sample surfaces due to the topological protection. Using the Aharonov-Casher effect associated with the interaction between magnetic moments and electric fields, the magnon motion can be quantized into magnonic Landau levels. The zeroth magnonic Landau level is chiral. Under a magnetic field gradient, Weyl magnons propagate unidirectionally from one Weyl node to the other with opposite chirality, resulting in the magnonic chiral anomaly. The magnonic chiral anomaly can be detected through the linear dependence of spin and heat conductance on the electric field gradient.

Host: Shizeng Lin