Ferroelectric quantum criticality

Abstract

Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new effects, like the breakdown of the conventional Fermi-liquid theory of metals has been focused in narrow band electronic systems. Ferroelectric crystals provide a very different type of quantum criticality that arises purely from the crystalline lattice. In many cases the ferroelectric phase can be tuned to absolute zero using hydrostatic pressure or chemical or isotopic substitution. Close to such a zero temperature phase transition, the dielectric constant and other quantities change into radically unconventional forms due to the quantum fluctuations of the electrical polarization. The simplest ferroelectrics may form a text-book paradigm of quantum criticality in the solid-state as the difficulties found in metals due to a high density of gapless excitations on the Fermi surface are avoided. We present low temperature high precision data demonstrating these effects in pure single crystals of SrTiO 3 and KTaO 3 . We outline a model for describing the physics of ferroelectrics close to quantum criticality and highlight the expected 1/T 2 dependence of the dielectric constant measured over a wide temperature range at low temperatures. In the neighbourhood of the quantum critical point we report the emergence of a small frequency independent peak in the dielectric constant at approximately 2 K in SrTiO 3 and 3 K in KTaO 3 believed to arise from coupling to acoustic phonons. Looking ahead, we suggest that ferroelectrics could be used as systems in which to controllably build in extra complexity around the quantum critical point. For example, in ferroelectric or antiferroelectric materials supporting mobile charge carriers, quantum paraelectric fluctuations may mediate new effective electron-electron interactions giving rise to a number of possible states such as superconductivity.The study of quantum matter at low temperatures has given rise to a fascinating and often surprising catalogue of phenomena important to our understanding of nature 1 and to technological development 2 . In particular, the study of materials close to a continuous low temperature phase transition or so called quantum critical point forms an important branch of research within condensed matter physics. A chief reason for this is that close to such a transition, materials become highly degenerate and new states of matter are frequently found to emerge. In fact, it turns out that many materials end up being close to or within the quantum critical regime. This is because quantum critical phenomena can affect materials over a wide range of temperatures, pressures and other variables. In electrically conducting materials, the standard model of the metallic state, Landau's Fermi liquid theory is seen to breakdown close to the low temperature boundary between a magnetic and paramagnetic phase and is replaced with other forms of novel quantum liquid. For example, in some weakly magnet...

Valence fluctuations in Sn(Pb)2P2S6 ferroelectrics

“…So, according to the experimental data [6,8], the paraelectric ground state can be stable at P > 1.5 GPa for Sn 2 P 2 S 6 , and at y > 0.7 and normal pressure for (Pb y Sn 1−y ) 2 P 2 S 6 mixed crystals. At 0 K, the quantum critical point (QCP) can be reached by variation of pressure or composition [9]. Early the QCP have been obviously found in the quantum paraelectric SrTiO 3 at increase of 18 O isotope concentration only [10].…”

A ferroelectric quantum phase transition inside the superconducting dome of Sr1−xCaxTiO3−δ

“…This provides a new input for the ongoing debate on the microscopic origin of superconductivity 4,[8][9][10] . Calcium substitution leads to a higher dielectric constant at low temperature (Fig.…”

“…It turns into a ferroelectric upon substitution of a tiny fraction of strontium atoms with calcium 3 . The two orders may be accidental neighbours or intimately connected, as in the picture of quantum critical ferroelectricity 4 . Here, we show that in Sr 1−x Ca x TiO 3−δ (0.002 < x < 0.009, δ < 0.001) the ferroelectric order coexists with dilute metallicity and its superconducting instability in a finite window of doping.…”

“…In many unconventional superconductors, another electronic order competes with superconductivity. Here, the ferroelectric order is an obvious candidate 4,9 . It emerges in SrTiO 3 by isotopic substitution of 16 O oxygen atoms with 18 O (ref.…”

“…They predicted that such a substitution would enhance the highest critical temperature and shift it to lower doping. This is a quantum critical ferroelectric 4 scenario, in which the maximum T c is pinned to the destruction of the ferroelectric order. Our observation is in qualitative agreement with this scenario.…”