Weyl and Dirac semimetals: anomaly-driven transport

5 problems

• Is the charge-density wave in (TaSe4)2I an axion insulator?

  Contested

  When a charge-density wave (a periodic ripple in electron density) gaps out the Weyl points of (TaSe4)2I, its sliding motion should act like the hypothetical axion particle; one experiment reported this and later work questioned it.

• Is negative longitudinal magnetoresistance a chiral-anomaly signature?

  Contested

  Many semimetals conduct better when a magnetic field points along the current, as the chiral anomaly predicts, but uneven current flow inside the sample (current jetting) can produce the same effect.

• Thermoelectric signature of the mixed axial-gravitational anomaly

  Contested

  A temperature gradient along a magnetic field in a Weyl semimetal should drive an extra current tied to a gravitational quantum anomaly, and the single reported observation needs confirmation.

• Quantized circular photogalvanic effect in chiral multifold semimetals

  Contested

  In chiral crystals such as RhSi and CoSi, circularly polarized light should drive a current whose growth rate equals a whole number (a topological charge) times fundamental constants, and this quantization has not been confirmed.

• Origin of Hall plateaus in bulk $\mathrm{ZrTe_5}$

  Contested

  Thick crystals of $\mathrm{ZrTe_5}$ show Hall-resistance plateaus resembling the quantum Hall effect, which normally needs a two-dimensional sample, and whether a field-induced charge-density wave causes them is unsettled.