Quantum Critical Ferroelectricity and Emergent Phenomena in KTaO3 and SrTiO3

Abstract

This thesis explores the quantum phase diagrams of KTaO3 and SrTiO3, both quantum paraelectrics near to a ferroelectric quantum critical point, including study of material properties which may be influenced by proximity to a ferroelectric quantum phase transition. A quantum critical phase transition is one where the system approaches criticality in the zero-temperature limit where the energy scale of quantum fluctuations is comparable to or greater than the energy of classical thermal fluctuations, leading to new system dynamics and a phase space of strong interactions. Data is presented on the dielectric susceptibility of KTaO3 under hydrostatic pressures up to 20 kbar. KTaO3 has no structural or ferroelectric phase transitions down to absolute zero, suggesting that low temperature features of interest are intrinsically related to the quantum phase transition rather than to elastic domain walls as has been suggested by some researchers in the case of SrTiO3 which undergoes an antiferrodistortive transition below a temperature of T = 105K [1–4]. We have extended the dielectric susceptibility measurements in KTaO3 and SrTiO3 down to 50mK and find evidence that the inverse susceptibility may vary as −T^4 in the zero-temperature limit, a result previously predicted [5] but never before observed. Resonant piezoelectric spectroscopy results are reported on KTaO3 which reveal the quantum polar-elastic nature of the material at low temperatures and is understood here as arising from the coupling of quantum polarization and strain fields. Measurements of thermal conductivity κ, specific heat C_p, and linear thermal expansion ΔL/L_0 are presented. We consider the Grüneisen ratio Γ(T), i.e. the ratio of the linear thermal expansion coefficient to the specific heat (α/C_p). The Grüneisen ratio is predicted to diverge as 1/T^2 in the low-temperature limit near a ferroelectric quantum phase transition, which would be in stark contrast to a nearly temperature independent Γ(T) found in materials far from quantum criticality such as BaTiO3. However, our measurements show that the predicted behaviour of Γ(T) is interrupted at low temperatures by the appearance of broad minima in the sample length, L(T), near 30-40K in both KTaO3 and SrTiO3. This leads to a negative linear thermal expansion coefficient, α(T), at temperatures below the minimum in L(T). The phonon mean free path l_ph in KTaO3 is derived from data on thermal conductivity and specific heat and literature values of the sound velocity vs, showing a peculiar plateau of l_ph ≈ 0.6 μm in a window of 10-22.5K before increasing rapidly to lengths of over 100 μm as T → 0. We discuss the potential emergence of polarization textures as being perhaps a natural outcome of proximity to a ferroelectric quantum phase transition, bridging the gap between the bulk-disordered paraelectric state and bulk-ordered ferroelectric state. Origin mechanisms for such polarization textures, and their relevance as a possible explanation for many of the behaviours observed in KTaO3 and SrTiO3, are considered and discussed. Work on cryogenic systems development is also presented, including the conversion of a 1K pumped He-4 system into a millikelvin adiabatic demagnetization refrigerator with an electrically-controlled gas-gap heat switch.