Phonon thermal Hall effect in insulators

5 problems

• Origin of the large phonon thermal Hall effect in cuprates

  Open

  In copper-oxide superconductors, phonons gain a large sideways heat flow below the doping where the pseudogap state appears and also in the undoped insulating parents. What gives phonons this handedness is unknown.

• Intrinsic versus defect-driven phonon thermal Hall effect in $\mathrm{SrTiO_3}$

  Open

  The effect could come from the phonons themselves or from the way they scatter off impurities. $\mathrm{SrTiO_3}$, where it is large, is a clean place to decide.

• How do phonons couple to a magnetic field in nonmagnetic insulators?

  Open

  Crystals such as quartz, which have no magnetic atoms, still show sideways phonon heat flow in a field, while glassy silica with the same chemistry shows none.

• Why do phonons in many insulators show a thermal Hall effect?

  Contested

  Sideways heat flow in a magnetic field appears in magnetic and non-magnetic insulators alike, with roughly similar size relative to the ordinary heat flow. No single coupling between phonons and the field explains all cases.

• Origin of the large thermal Hall effect in the pseudogap phase and Mott-insulating cuprates

  Open

  Hole-doped cuprates show a large negative thermal Hall conductivity when they enter the pseudogap regime, and an even larger signal can appear in undoped Mott-insulating compounds where mobile electrons are absent. Measurements suggest that phonons carry much of the heat Hall current, but phonons are neutral and need some coupling to acquire chirality. The open problem is to identify the microscopic source of this phonon thermal Hall response and explain its sign, size, temperature dependence, and doping dependence.