Relaxors, ferroelectric hafnia and polar textures

5 problems

• What stabilizes ferroelectricity and wake-up in thin-film HfO2

  Partially resolved

  Hafnium oxide, already used in computer chips, turned out in 2011 to be ferroelectric when made as films a few dozen atoms thick, although its bulk crystal is not. Why the polar crystal phase survives and why the response grows with repeated switching, called wake-up, are open.

• Structure and origin of polar nanoregions in relaxor ferroelectrics

  Contested

  In relaxors such as lead magnesium niobate, the dipoles never line up into large domains; instead small polar patches, called polar nanoregions, appear and slow down on cooling. What these patches are and why they form is still argued.

• Stability and deterministic switching of polar vortices and skyrmions

  Partially resolved

  In thin layered oxide films, electric dipoles can curl into vortices and skyrmions (stable swirling patterns) a few dozen atoms wide. Why they are stable, and how much field, energy and time it takes to write or erase a single one, is open.

• Origin of ultrahigh piezoelectricity in relaxor-PbTiO3 single crystals

  Contested

  Crystals of lead magnesium niobate mixed with lead titanate deform several times more per applied voltage than conventional piezoelectric ceramics. Whether this comes from easy rotation of the dipole direction or from the disordered polar patches is debated.

• How sliding ferroelectric bilayers switch and why fatigue is weak

  Open

  Stacking two layers of boron nitride or of a semiconductor like MoS2 in a particular offset creates an electric dipole between them that flips when one layer slides by one atomic spacing. How the sliding proceeds through moving boundaries between domains, and what limits its speed and endurance, is unclear.