Letteratura scientifica selezionata sul tema "Quantum paraelectrics"

In this article, two concepts of temperature, i.e., Burns temperature for relaxor ferroelectrics and quantum temperature scale for quantum paraelectrics, are reviewed briefly. Since both temperatures describe the deviation of the dielectric constant from Curie–Weiss law, their relationship is discussed. Finally the concept of quantum temperature scale is extended to demonstrate the evolution process of quantum paraelectric behavior to relaxor ferroelectric behavior.

The dielectric and magnetic polarizations of quantum paraelectrics and paramagnetic materials have in many cases been found to initially increase with increasing thermal disorder and hence, exhibit peaks as a function of temperature. A quantitative description of these examples of “order-by-disorder” phenomena has remained elusive in nearly ferromagnetic metals and in dielectrics on the border of displacive ferroelectric transitions. Here, we present an experimental study of the evolution of the dielectric susceptibility peak as a function of pressure in the nearly ferroelectric material, strontium titanate, which reveals that the peak position collapses toward absolute zero as the ferroelectric quantum critical point is approached. We show that this behavior can be described in detail without the use of adjustable parameters in terms of the Larkin–Khmelnitskii–Shneerson–Rechester (LKSR) theory, first introduced nearly 50 y ago, of the hybridization of polar and acoustic modes in quantum paraelectrics, in contrast to alternative models that have been proposed. Our study allows us to construct a detailed temperature–pressure phase diagram of a material on the border of a ferroelectric quantum critical point comprising ferroelectric, quantum critical paraelectric, and hybridized polar-acoustic regimes. Furthermore, at the lowest temperatures, below the susceptibility maximum, we observe a regime characterized by a linear temperature dependence of the inverse susceptibility that differs sharply from the quartic temperature dependence predicted by the LKSR theory. We find that this non-LKSR low-temperature regime cannot be accounted for in terms of any detailed model reported in the literature, and its interpretation poses an empirical and conceptual challenge.

Un SrTiO3 paraélectrique quantique est un matériau situé à proximité d'un point critique quantique de la transition ferroélectrique dans lequel la température critique vers l'état ferroélectrique est supprimée jusqu'à 0 K. Cependant, la compréhension du comportement de la transition de phase à proximité de ce point reste problématique. Ici, nous étudions les solutions solides basées sur le SrTiO3 pour approcher les régions pré-critiques du diagramme de phase et étudions le résultat de la coexistence des fluctuations quantiques et du mouvement thermique. Cela permettra la découverte de nouvelles déclarations de phase et de nouvelles propriétés physiques, résultant de la concurrence des régimes quantiques et classiques. En utilisant la concentration x de Pb dans la solution solide PbxSr1-xTiO3 comme paramètre d'accord et par l'application de la combinaison des méthodes de spectroscopie Raman et diélectrique, on approche le point critique quantique et on étudie l'interaction des phénomènes classiques et quantiques dans la zone de criticité. On obtient la température critique de PbxSr1-xTiO3 et l'évolution des propriétés dynamiques du système en fonction de x pour révéler le mécanisme de la transition. On montre que la transition ferroélectrique se produit progressivement par l'émergence de nanorégions polaires. On étudie également la transition structurelle cubique à tétragonale, qui se produit à des températures plus élevées, et montre que ses propriétés sont presque indépendantes de la concentration et ne sont pas affectées par la criticité quantique. On étudie la structure cristalline et la dynamique du réseau de solutions solides paraélectriques quantiques de BaxSr1-xTiO3 à l'aide de la diffraction des rayons X, de la spectroscopie Raman et de la spectroscopie infrarouge-térahertz (THz-IR) dans une gamme de température de 4-300 K. La diffraction des rayons X et la spectroscopie Raman révèlent la transition de phase structurale non polaire cubique à tétragonal à environ 100K. En même temps, les spectres Raman manifestent la présence de modes polaires, TO2 et TO4, normalement interdits en phase paraélectrique. L'émergence de ces modes indique l'apparition des nanorégions polaires dans une large plage de températures. Les modes deviennent plus intensifs à basse température, la dépendance en température de leurs intensités lors du refroidissement révèle un changement de pente en forme de coude, de plat à raide, indiquant l'activation des nanorégions polaires. Les spectres de transmission THz-IR montrent que les fréquences au carré des modes mous polaires TO1, responsables de la transition ferroélectrique, suivent le comportement de Cochran à haute température. Cependant, à basse température, ceux-ci ne disparaissent pas mais saturent à la température de Curie extrapolée, démontrant la caractéristique de plateau en dessous de 20K. Ce comportement, cohérent avec la saturation connue de la constante diélectrique, indique que la transition vers la phase ferroélectrique dans BaxSr1-xTiO3 est supprimée par les fluctuations quantiques et que le système reste dans l'état paraélectrique quantique à très basse température. Nous étudions également en détail les propriétés diélectriques du PbxSr1-xTiO3 et montrons que dans la composition avec x = 0,005, un plateau lisse est observé dans la dépendance à la température de la permittivité diélectrique. La hauteur du plateau dépend de la concentration en Pb et diminue progressivement lorsque x augmente. Ce plateau est dû aux fluctuations quantiques aléatoires des ions qui dominent à basses températures et concentrations. À x plus élevé, les fluctuations thermiques deviennent plus prononcées; donc le plateau disparaît
A quantum paraelectric SrTiO3 is a material situated in close proximity to a quantum critical point of ferroelectric transition in which the critical temperature of ferroelectric state is suppressed down to 0 K. However, the understanding of the behaviour of the phase transition in the vicinity of this point remains challenging. Here we study the solid solutions based on the SrTiO3 to approach the pre-critical regions of the phase diagram and study the outcome of the coexistence of quantum fluctuations and thermal motion. It will allow the discovery of the novel phase statements and physical properties, occurring due to competition of quantum and classical regimes. We study the crystal structure and lattice dynamics of quantum paraelectric BaxSr1 xTiO3 solid solutions using X-Ray diffraction, Raman and terahertz-infrared (THz-IR)-spectroscopies in a temperature range 4-300K. The X-Ray diffraction and Raman spectroscopy reveal the cubic-to-tetragonal non-polar structural phase transition at about 100K. At the same time, Raman spectra manifest the presence of polar modes, TO2 and TO4, normally prohibited in paraelectric phase. Emergence of these modes indicates the appearance of the polar nanoregions in a broad temperature range. The modes become more intensive at low temperatures, the temperature dependence of their intensities on cooling reveals the kink-like change of the slope from flat to steep, indicating on activation of polar nanoregions. The transmission THz-IR-spectra show, that squared frequencies of the polar TO1 soft modes, responsible for the ferroelectric transition, follow the Cochran’s behavior at high temperatures. However, at low temperatures, it does not vanish at extrapolated Curie temperature but saturates, demonstrating the plateau feature below 20K. This behavior, coherent with the known saturation of the dielectric constant, indicates that transition to ferroelectric phase in BaxSr1-xTiO3 is suppressed by quantum fluctuations and system stays in the quantum paraelectric state at very low temperatures. Using the concentration of Pb in PbxSr1-xTiO3 solid solutions as a tuning parameter and applying the combination of Raman and dielectric spectroscopy methods we approach the quantum critical point in PbxSr1-xTiO3 and study the interplay of classical and quantum phenomena in the region of criticality. We obtain the critical temperature of PbxSr1-xTiO3 and the evolution of the temperature-dependent dynamical properties of the system as a function of x to reveal the mechanism of the transition. We show that the ferroelectric transition occurs gradually through the emergence of the polar nanoregions. We study also the cubic-to-tetragonal structural transition, occurring at higher temperatures, and show that its properties are almost concentration-independent and not affected by the quantum criticality. We also study the dielectric properties for the PbxSr1-xTiO3 in detail and show that in the composition with x = 0.005, a smooth plateau is observed in the temperature dependence of the dielectric permittivity. The height of the plateau depends on the Pb concentration and gradually decreases when x increases. This plateau arises due to random quantum fluctuations of the ions which dominate at low temperatures and concentrations. At higher x, the thermal fluctuations become more pronounced; therefore the plateau disappears

The thesis is organized as follows. In the rest of this introductory chapter we review in more detail the main relevant experiments, and point to some implications of these for our model building. Chapter 2 is devoted to a general discussion of models to be treated in subsequent chapters. First we recall the well-known displacive versus orderdisorder regime aspects of the standard classical model for structural phase transitions, and discuss the extent to which these aspects pertain to the case of QPE perovskites. Correspondingly, we divide the possible models into two classes, those based on continuous and those based on discrete degrees of freedom. As an example of the former class, we propose a Landau-Ginzburg-Wilson hamiltonian for SrTi03 , including elastic couplings, which leads to discuss the existence of an incommensurate phase. Considering the latter class of models, we try to identify their most important ingredients, taking as relevant degrees of freedom the discrete Ti - 0 bond variables. In Chapter 3, the classical incommensurate ferroelectricity arising from elastic couplings is investigated in detail, and the effect of quantum fluctuations is discussed on a heuristic level. In Chapter 4 we then investigate three idealized 2D quantum discrete lattice models. The first is a plain quantum four-state clock model, while the second includes an ice-type constraint. Both models are studied by means of a numerical Path Integral Monte Carlo simulation (PIMC), and the corresponding phase diagrams are determined with reasonable accuracy. On the technical side, in order to overcome the pathologically slow 1 / m convergence in number of Trotter slices in case of the constrained model, a special method has been invented, which is described in detail in Appendix A. In the last section of chapter 4 we describe our third model, which consists of endowing the constrained four-state clock model with an additional physical effect, namely the possibility of bond hopping and 'bond vacancies'. For this third model we have so far not been able to set up an accurate numerical simulation technique, allowing us to determine the complete phase diagram in the parameter space at finite temperatures. Some general considerations are presented, based on analogies with Andreev and Lifshitz's work [18] on quantum crystals of H e^4 , whose main idea is briefly summarized in Appendix B, for the sake of completeness. A particular zero temperature and zero coupling case of this more complete third model is studied numerically, by means of a Variational Monte Carlo technique, the details of which are described in Appendix C. Finally the last, fifth, chapter is devoted to discussion and conclusions.

Among the lead-free perovskite-structure materials, strontium titanate (SrTiO3—ST) and potassium tantalate (KTaO3—KT), pure or modified, are of particular importance. They are both quantum paraelectrics with high dielectric permittivity and low losses that can find application in tunable microwave devices due to a dependence of the permittivity on the electric field. Factors as Sr/Ti and K/Ta ratio in ST and KT ceramics, respectively, can alter the defect chemistry of these materials and affect the microstructure. Therefore, if properly understood, cation stoichiometry variation may be intentionally used to tailor the electrical response of electroceramics. The scientific and technological importance of the stoichiometry variation in ST and KT ceramics is reviewed and compared in this chapter. The differences in crystallographic phase assemblage, grain size, and dielectric properties are described in detail. Although sharing crystal chemical similarities, the effect of the stoichiometry is markedly different. Even if the variation of Sr/Ti and K/Ta ratios did not change the quantum-paraelectric nature of ST and KT, Sr excess impedes the grain growth and decreases the dielectric permittivity in ST ceramics, while K excess promotes the grain growth and increases the dielectric permittivity in KT ceramics.

We show that the quantum paraelectric material SrTiO3 displays an exceptionally strong electro-optic tunability at cryogenic temperatures. This has wide-reaching implications for a variety of applications in quantum science.

SDS-structures based on silicon nanolayers, paraelectris and ferroelectrics are the basis for nanoscale quantum, radio, optic and neuromorphic integrated electronics compatible with industrial CMOS silicon process for multifunctional chips.

Abstract: We report the observation of a long-lived birefringent state in quantum paraelectric KTaO3 induced by THz pump pulses in a two-THz-photon process. We relate this long-lived state to the proximity of the ferroelectric phase transition.