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 $\mathrm{MnBi_2Te_4}$ an axion insulator?

  Contested

  Thin $\mathrm{MnBi_2Te_4}$ 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 $\mathrm{MnBi_2Te_4}$ surface Dirac cone appears gapless

  Contested

  Magnetism should open an energy gap in the surface electrons of $\mathrm{MnBi_2Te_4}$, 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.