Magnetic topological insulators and axion electrodynamics

5 problems

• Axion quasiparticle and axion polariton in antiferromagnetic topological insulators

  Partially resolved

  In antiferromagnetic topological insulators, magnetic fluctuations should make the axion angle oscillate, producing a collective excitation that mixes with light, and its coupling to light has not been observed.

• Is even-layer MnBi2Te4 an axion insulator

  Contested

  Thin MnBi2Te4 with an even number of layers shows zero Hall resistance and high longitudinal resistance at zero field, which fits an axion insulator but also an ordinary insulator.

• Why the MnBi2Te4 surface Dirac cone appears gapless

  Contested

  Magnetism should open an energy gap in the surface electrons of MnBi2Te4, which is what allows zero-field quantized Hall effects, but several photoemission experiments see no gap.

• Why zero-field quantized Hall effects survive only near one kelvin

  Open

  Magnetic topological insulators show a perfectly quantized Hall resistance only at temperatures of order one kelvin, far below the temperature at which they become magnetic, and the reason is debated.

• Direct measurement of the quantized topological magnetoelectric effect

  Partially resolved

  An axion insulator should become electrically polarized in a magnetic field by an amount fixed only by the fine-structure constant, and this static effect has never been measured directly.