Ferroelectric quantum phase transitions and polar elasticity

Ferroelectrics tuned to the neighbourhood of zero temperature phase transitions present an unexpected and novel form of criticality due to the quantum fluctuations of electrical dipole fields. For displacive materials crystalizing in three spatial dimensions with multiaxial order-parameters, these fluctuations appear to exist in an effective four-dimensional space leading to non-classical temperature dependences of the electrical susceptibility and thermal expansion. The coupling of the polarization and strain fields leads to a quantum polar-elastic regime characterized by a low-temperature peak in the susceptibility in paraelectrics near to ferroelectric quantum critical points. We present experimental and theoretical results of the electrical susceptibility and Grüneissen ratio and explore the phase diagram of KTaO3 and SrTiO3 using pressure and uniaxial stress tuning. At temperatures well below the peak, we discuss evidence for the emergence of a liquid of polarization textures exhibiting slow dynamics and a departure from the standard model of ferroelectric quantum criticality. An investigation of the insulating 'vacuum' state is likely to aid our understanding of forms of unconventional superconductivity as recently detected in electron-doped bulk and interface quantum ferroelectrics.