Quantum oscillations in insulators

5 problems

• Origin of conductance oscillations in the InAs/GaSb hybridization gap

  Open

  Quantum wells of InAs and GaSb form an insulator whose small energy gap comes from mixing of electron and hole bands, yet their conductance oscillates in a magnetic field when the electron density is tuned into the gap. Whether this is the band-mixing mechanism proposed for $\mathrm{SmB_6}$ or something else is unknown.

• How can an insulator without a Fermi surface show quantum oscillations?

  Open

  Quantum oscillations normally require a sharp boundary between filled and empty electron states, which an insulator lacks. Proposed explanations range from field effects on the insulating gap to new particles that carry no charge.

• Are the bulk quantum oscillations in $\mathrm{SmB_6}$ intrinsic?

  Contested

  Some $\mathrm{SmB_6}$ crystals show magnetization oscillations typical of metal electrons although their resistance says the bulk is insulating. Other groups attribute the oscillations to surface states or to aluminum inclusions left from crystal growth.

• What carries Landau quantization in insulating monolayer $\mathrm{WTe_2}$?

  Open

  Monolayer $\mathrm{WTe_2}$, an insulator in its interior, shows resistance oscillations starting at a weak magnetic field, as if it contained very mobile particles. What these particles are is unknown.

• Does insulating $\mathrm{YbB_{12}}$ host a Fermi surface of neutral fermions?

  Contested

  $\mathrm{YbB_{12}}$ shows oscillations in resistance and magnetization, plus heat capacity and heat conduction that grow linearly with temperature as in a metal, while remaining electrically insulating.