Phonon thermal Hall effect in insulators

4 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 SrTiO3

  Open

  The effect could come from the phonons themselves or from the way they scatter off impurities. SrTiO3, 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.