Paraelectric System Research Articles

Quantum fluctuations lead to glassy electron dynamics in the good metal regime of electron doped KTaO3

One of the central challenges in condensed matter physics is to comprehend systems that have strong disorder and strong interactions. In the strongly localized regime, their subtle competition leads to glassy electron dynamics which ceases to exist well before the insulator-to-metal transition is approached as a function of doping. Here, we report on the discovery of glassy electron dynamics deep inside the good metal regime of an electron-doped quantum paraelectric system: KTaO3. We reveal that upon excitation of electrons from defect states to the conduction band, the excess injected carriers in the conduction band relax in a stretched exponential manner with a large relaxation time, and the system evinces simple aging phenomena—a telltale sign of glassy dynamics. Most significantly, we observe a critical slowing down of carrier dynamics below 35 K, concomitant with the onset of quantum paraelectricity in the undoped KTaO3. Our combined investigation using second harmonic generation technique, density functional theory and phenomenological modeling demonstrates quantum fluctuation-stabilized soft polar modes as the impetus for the glassy behavior. This study addresses one of the most fundamental questions regarding the potential promotion of glassiness by quantum fluctuations and opens a route for exploring glassy dynamics of electrons in a well-delocalized regime.

Direct Visualization of Distorted Twin Boundaries in Ce-Doped GdFeO3.

Utilizing advanced transmission electron microscopy (TEM), the structure at the (110)-type twin boundary (TB) of Ce-doped GdFeO3 (C-GFO) has been investigated with picometer precision. Such a TB is promising to generate local ferroelectricity within a paraelectric system, while precise knowledge about its structure is still largely missing. In this work, a direct measurement of the cation off-centering with respect to the neighboring oxygen is enabled by integrated differential phase contrast (iDPC) imaging, and up to 30 pm Gd off-centering is highly localized at the TB. Further electron energy loss spectroscopy (EELS) analysis demonstrates a slight accumulation of oxygen vacancies at the TB, a self-balanced behavior of Ce at the Gd sites, and a mixed occupation of Fe2+ and Fe3+ at the Fe sites. Our results provide an informative picture with atomic details at the TB of C-GFO, which is indispensable to further push the potential of grain boundary engineering.

Synthesis of a New Ferroelectric Relaxor Based on a Combination of Antiferroelectric and Paraelectric Systems.

Relaxor ferroelectric-based energy storage systems are promising candidates for advanced applications as a result of their fast speed and high energy storage density. In the research field of ferroelectrics and relaxor ferroelectrics, the concept of solid solution is widely adopted to modify the overall properties and acquire superior performance. However, the combination between antiferroelectric and paraelectric materials was less studied and discussed. In this study, paraelectric barium hafnate (BaHfO3) and antiferroelectric lead hafnate (PbHfO3) are selected to demonstrate such a combination. A paraelectric to relaxor ferroelectric, to ferroelectric, and to antiferroelectric transition is observed by varying the composition x in the (Ba1-xPbx)HfO3 solid solution from 0 to 100%. It is noteworthy that ferroelectric phases can be realized without primal ferroelectric material. This study creates an original solid solution system with a rich spectrum of competing phases and demonstrates an approach to design relaxor ferroelectrics for energy storage applications and beyond.

Fingerprints of Critical Phenomena in a Quantum Paraelectric Ensemble of Nanoconfined Water Molecules.

We have studied the radio frequency dielectric response of a system consisting of separate polar water molecules periodically arranged in nanocages formed by the crystal lattice of the gemstone beryl. Below T = 20-30 K, quantum effects start to dominate the properties of the electric dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature-dependent real dielectric permittivity ε'(T). When analyzing in detail the temperature evolution of the reciprocal permittivity (ε')-1 down to T ≈ 0.3 K and comparing it with the data obtained for conventional quantum paraelectrics, like SrTiO3, KTaO3, we discovered clear signatures of a quantum-critical behavior of the interacting water molecular dipoles: Between T = 6 and 14 K, the reciprocal permittivity follows a quadratic temperature dependence and displays a shallow minimum below 3 K. This is the first observation of "dielectric fingerprints" of quantum-critical phenomena in a paraelectric system of coupled point electric dipoles.

Optical phonon contribution to the thermal conductivity of a quantum paraelectric

Motivated by recent experimental findings, we study the contribution of a quantum critical optical phonon branch to the thermal conductivity of a paraelectric system. We consider the proximity of the optical phonon branch to transverse acoustic phonon branch and calculate its contribution to the thermal conductivity within the Kubo formalism. We find a low temperature power law dependence of the thermal conductivity as T α , with 1 < α < 2, (lower than T 3 behavior) due to optical phonons near the quantum critical point. This result is in accord with the experimental findings and indicates the importance of quantum fluctuations in the thermal conduction in these materials.

Spin ordering and physical properties of NaPrFeWO6 and NaSmFeWO6 with polar double perovskite structure

NaPrFeWO6 and NaSmFeWO6 have been synthesized as polycrystalline powders by means of a sol-gel route. Both samples adopt a noncentrosymmetric structure derived from the simple perovskite ABO3 and characterized by a layered ordering of A-site atoms (Na and rare earth) and a rock-salt ordering of B-site atoms (Fe and W) as deduced from x-ray and neutron powder diffraction techniques. Magnetization measurements revealed antiferromagnetic ordering below TN ≈23 K in both compounds but weak spontaneous magnetization is observed in the hysteresis loops of the samples above and below TN revealing a parasitic ferromagnetic component that comes from minor ferromagnetic impurities. The magnetic structure of NaPrFeWO6 is formed by two antiferromagnetic sublattices (rare earth and Fe) that follow the same propagation vector k = (½, 0, ½). The magnetic structure is purely collinear with the moments oriented along the b-axis. It presents competitive magnetic interactions and reduced ordered moments at low temperatures. The rare earth moments in the zig-zag chains running along a (parallel to the ab plane) are ordered following a sequence typical of an E-type arrangement (up-up-down-down). Each Fe2+ atom has four nearest Pr3+ neighbors and it is coupled antiferromagnetically to three of them and ferromagnetically to the fourth one. Strong neutron absorption from Sm atoms hinders obtaining reliable neutron patterns of NaSmFeWO6 for Rietveld analysis but the comparison of neutron patterns above and below TN detected the same magnetic peaks indicating a similar magnetic structure. Dielectric measurements are compatible with a paraelectric system with a strong Maxwell-Wagner contribution from depletion layers and electrical properties are dominated by leakage currents.

Oxygen vacancy formation energies in PbTiO3/SrTiO3 superlattice

The defect stability in a prototypical perovskite oxide superlattice consisting of SrTiO$_3$ and PbTiO$_3$ (STO/PTO) is determined using first principles density functional theory calculations. Specifically, the oxygen vacancy formation energies E$_v$ in the paraelectric and ferroelectric phases of a superlattice with four atomic layers of STO and four layers of PTO (4STO/4PTO) are determined and compared. The effects of charge state, octahedral rotation, polarization, and interfaces on the E$_v$ are examined. The formation energies vary layer-by-layer in the superlattices, with E$_v$ being higher in the ferroelectric phase than that in the paraelectric phase. The two interfaces constructed in these oxide superlattices, which are symmetrically equivalent in the paraelectric systems, exhibit very different formation energies in the ferroelectric superlattices and this can be seen to be driven by the coupling of ferroelectric and rotational modes. At equivalent lattice sites, E$_v$ of charged vacancies is generally lower than that of neutral vacancies. Octahedral rotations (a$^0$a$^0$c$^-$) in the FE superlattice have a significant effect on the E$_v$, increasing the formation energy of vacancies located near the interface but decreasing the formation energy of the oxygen vacancies located in the bulk-like regions of the STO and PTO constituent parts. The formation energy variations among different layers are found to be primarily caused by the difference in the local relaxation at each layer. These fundamental insights into the defect stability in perovskite superlattices can be used to tune defect properties via controlling the constituent materials of superlattices and interface engineering.

Investigation of magnetic, optical and electrical properties of La3Ba3Cu6O14 synthesized by molten flux method

The great deal of novel rare earth based semiconducting lanthanum barium copper oxide La3Ba3Cu6O14 (LBC-336) were synthesized by low temperature molten salt synthesis (MSS) due to “dissolution precipitation mechanism”. Here, we reported one pot synthesis of product by direct precipitation from a molten KOH-NaOH mixture at 450 °C and single phase La3Ba3Cu6O14 with tetragonal crystal system. The particle size of 140–200 nm were observed in both PXRD pattern and HRSEM micrographs and it showed a cubic morphology. The semiconducting nature was extracted from various parameters like optical band gap (1.8 eV), AC conductivity (0.70 eV), DC conductivity (0.70 eV), and also Hall effect parameters like the charge carrier concentration values n=6.0×1026 m−3 and it proved as a p-type semiconductor. The electrical phase trasition temperature from ferroelectric to antiferroelectric system (Tm=420 K) and anti-ferroelectric – paraelectric system (depolarization temperature Td=673 K) which attributed to the space charge polarization contributed to the conduction mechanism. The magnetic phase transitions were from ferromagnetic to ferrimagnetic system (Curie temperature (Tc=70 K)) and it led to soft magnetic material and also held good for superconductor application upto 70 K.

High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe12O19

High quality single crystals of BaFe$_{12}$O$_{19}$ were grown using the floating zone technique in flowing oxygen pressurized to 100 atm. Single crystal neutron diffraction was used to determine the nuclear and magnetic structure of BaFe$_{12}$O$_{19}$ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe$^{3+}$ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90-300 K. The inverse dielectric permittivity, $1/\varepsilon$, along the c-axis shows a $T^2$ temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in $1/\varepsilon$. These features resemble those of classic quantum paraelectrics such as SrTiO$_3$. The presence of the upturn in $1/\varepsilon$ indicates that BaFe$_{12}$O$_{19}$ is a critical quantum paraelectric system with Fe$^{3+}$ ions involved in both magnetic and electric dipole formation.

High Carrier Mobility Coupled with Quantum Paraelectric Fluctuation

Photoconductivity and photo-magneto-transport properties have been measured in quantum paraelectric and ferroelectric SrTiO 3 crystals. We have found a large isotope effect on the conductivity of photo-carrier in this system. This isotope effect strongly suggests a close correlation between the quantum fluctuations and the giant photoconductivity in the quantum paraelectric system. Here we propose the possibility of the control of the carrier motion by using the external electric field. These results may become a key to understand the behavior of the carrier in the dense electric dipoles with so called as quantum fluctuation.

Isotope Effect on Photoconductivity in Quantum Paraelectric SrTiO3

Photoconductivity and various photo-magnetotransport properties have been studied in quantum paraelectric SrTiO 3 crystals. SrTi 16 O 3 shows giant photoconductivity, as reported before. The photodoped carrier in the 16 O system is of electron type and has high mobility beyond 10 4 cm 2 V -1 s -1 at 5 K. Its number can reach the order of 10 15 cm -3 . In contrast, isotope-substituted SrTi 18 O 3 is not a good photoconductor. In addition, the photocarrier's mobility in the 18 O sample is negligibly small. These results suggest a strong correlation between the quantum fluctuation and the giant photoconductivity in this quantum paraelectric system. In other words, the study of photocarrier dynamics is a novel approach to the investigation of quantum fluctuation in dielectric materials.

Giant Photoconductivity in Quantum Paraelectric Oxides

Photoconductivity and photo-magneto-transport properties have been measured in quantum paraelectric SrTiO3 crystals. We have found a large isotope effect in the photocurrent value in this system. This isotope effect suggests an important contribution of the quantum fluctuations for the giant photoconductivity in the quantum paraelectric system.

Photoinduced Phenomena in Quantum Paraelectric Oxides by Ultraviolet Laser Irradiation

A photo-induced effect in the quantum dielectric system, such as SrTiO3, KTaO3, and CaTiO3, has been investigated under rather small external DC electric field. The dielectric measurement was performed under ultraviolet (UV) laser excitation combined with DC electric field. The gigantic photo-induced effects of dielectric properties of the quantum paraelectric systems have been firstly observed in the perovskite-type materials. The photoinduced dielectric effect was obtained as a function of DC electric field under constant UV irradiation.

Systematic approach for comparing paraelectric tunneling models to resonance data

In paraelectric systems, reorientable impurity dipoles can give rise to resonance absorption of microwaves at liquid-helium temperatures. Several current models exist to explain paraelectricity in terms of tunneling between neighboring dipole orientations. In this paper we divide tunneling parameter space into several regions in which the effects of an additional electric field dependence facilitates data analysis of paraelectric resonance spectra. We give results for the two standard dipole models and for two field orientations, producing 32 multiple-energy-level diagrams and sum rules. These diagrams and sum rules greatly facilitate data analysis and allow a strong test of the theory.

Unusual paraelectric system, KBr:Li+

We have measured the paraelectric resonance absorption of lithium-doped KBr over the frequency range 42-217 GHz with electric fields applied along the $〈100〉$, $〈111〉$, and $〈110〉$ orientations. Many lines are observed, with zero-field splittings (ZFS) ranging from 52 to 161 GHz for the $^{7}\mathrm{Li}^{+}$ isotope. When $^{6}\mathrm{Li}^{+}$ replaces $^{7}\mathrm{Li}^{+}$, a large isotope shift of the ZFS (typically about 40%) is observed, indicating that ${\mathrm{Li}}^{+}$ is tunneling in some multiwell potential. This paraelectric system is unusual in several respects: no significant changes in the relative line intensities are observed between 4.2 and 1.4 K, impurities such as O${\mathrm{H}}^{\ensuremath{-}}$ can suppress the observed signal, and quenching low-concentration samples increases the signal intensity by factors of 10-1000. We attempted to fit the data to the standard tunneling models and had limited success with the $〈110〉$ model; several (but not all) strong lines in each field orientation could be fit very well. However, when both the weak and strong lines are considered, too many ZFS are observed, which suggests that this ${\mathrm{Li}}^{+}$ system is far too complex to be explained by any of the standard tunneling models. A comparison with other experiments and some suggestions for future work are included.

Phase transition to spontaneous coherent state in paraelectric systems

Phase transition to spontaneous coherent state in paraelectric systems

Infrared absorption of OH - doped LiF

The low temperature IR spectra of OH - doped LiF shows no additional absorption in the vicinity of the main stretching band. This suggests that it is an “on-center” paraelectric system and that this feature suppresses most manifestations of this type of impurities.

A paraelectric system with two electric dipole moments: KI: OH−

Paraelectric resonance in KI: O${\mathrm{H}}^{\ensuremath{-}}$ at 1.5 and 4.2\ifmmode^\circ\else\textdegree\fi{}K has been measured from 12 to 150 GHz, using a combination of fixed- and swept-frequency techniques. The data confirm the existence of a low-lying excited multiplet of energy levels 140 GHz above the ground-state multiplet and a simple two-multiplet tunneling model is shown to be in good agreement with the main features of the experimental data. The dipole moment of O${\mathrm{H}}^{\ensuremath{-}}$ in the ground-state multiplet is 1.25 $e$ \AA{}, and 2.15 $e$ \AA{} in the excited multiplet, while the dipole orientation in each is $〈110〉$. The nearest-neighbor tunneling parameters are dominant, having magnitudes of 8.25 and 13.5 GHz in the lower and upper multiplets, respectively, with a small admixture of second-nearest-neighbor tunneling. The observation of a second multiplet has been interpreted as evidence for a coupled dipole system in which the intrinsic dipole moment of O${\mathrm{H}}^{\ensuremath{-}}$ and the off-center dipole moment can combine in several ways. The swept-frequency experiments, in addition to providing an accurate measurement of some zero-field splittings, show that the absorption line shape is symmetric in frequency at low electric fields. This result implies that internal-electric-field broadening is not a dominant contribution to the linewidths observed in this system.

Large electrostrictive effect in doped alkali halide crystals

The electrostriction of alkali halide crystals containing substitutional impurities (the paraelectric systems KCl:Li+, NaBr:F−, RbCl:Ag+, RbBr:Ag+, KCl:OH−, and the system KBr:Li+) has been measured for concentrations of the order 1017–1018 cm−3. Some of these systems exhibit a giant almost temperature-independent electrostrictive effect that does not saturate.

The Influence of the Dipole-Dipole Interactions on the Phonon Induction

Abstract It is shown that the none quilibrium state of the dipole-dipole reservoir in paramagnetic and paraelectric systems can stimulate the generation of the coherent spontaneous radiation essentially. At the same time the uncoherent spontaneous radiation and coherent induced (in the Einstein sense) radiation canN′t be amplified in this conditions essentially. Moreover the electromagnetic and acoustic maser-effects and uncoherent apontaneous radiation completely disappear in the limit case of enough small absolute values of the dipole-dipole system (DDS) temperature. The nuclear polarization is also absent at enough low absolute electron-nuclear and nuclear-nuclear DDS temperatures. All of these consequences follow from the obvious trace property of the product of n-particle density matrix operators and operators of observable quantities constituted from m-particle operators: this trace is identically equal to zero if m > n (all n or m particles are different).