Macroscopic Polarization Research Articles

Effect of Nd doping on the refractive index and thermo-optic coefficient of GdVO4 single crystals

Undoped and Nd (0.2, 0.6, 0.8, 1.0, 1.2 and 1.6 at%)-doped GdVO4 single crystals have been grown by optical floating zone technique. XRD analysis reveals small lattice expansion especially along c axis on Nd doping. It has been observed that the extra-ordinary refractive index (RI) of undoped GdVO4 crystals is higher than the doped GdVO4 single crystals up to 1.0 at% Nd doping, beyond which it surpasses the value of the undoped crystal. This is due to the competing effect of lattice elongation and decrease in electron density on Nd doping. The increase in RI with Nd doping is attributed to macroscopic polarization effects. Moreover, the extra-ordinary refractive index (ne) only increases on increasing Nd concentration, whereas ordinary refractive index (no) remains unaffected within the measurement limits. The temperature-dependent RI measurements depicted a positive thermo-optic coefficient (TOC) for RI and birefringence, which is ~ 10−5/°C and is independent of doping concentration. TOC for ne is higher than no, which has been explained on the basis of anisotropic thermal expansion. An important set of relations between TOC and wavelength (532–1551 nm) has been established which is useful for estimating TOC in the temperature range of 30–150 °C.

A Solution Processed Ultrathin Molecular Dielectric for Organic Field-Effect Transistors

Leakage-free, thin molecular dielectric layers with sizable macroscopic polarization are valuable for organic field-effect transistor applications. Molecules that are designed for large polarizabil...

Local structure of relaxor ferroelectric SrxBa1−xNb2O6 from a pair distribution function analysis

Pair distribution function analysis of neutron-scattering data and of ab initio molecular dynamics results have been employed to study short-range structural correlations and their temperature dependence in a heavily disordered dielectric material ${\mathrm{Sr}}_{x}{\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{2}{\mathrm{O}}_{6}\phantom{\rule{4pt}{0ex}}(x=0.35,\phantom{\rule{0.28em}{0ex}}0.5$, and 0.61). Intrinsic disorder caused by a partial occupation of the cationic sites by differently sized Sr and Ba atoms and their vacancies introduces important local strains to the structure and directly influences the Nb-${\mathrm{O}}_{6}$ octahedra tilting. The resulting complex system of tilts is found to be both temperature and Sr-doping sensitive with the biggest tilt magnitudes reached at low temperatures and high strontium contents, where ferroelectric relaxor behavior appears. We find evidence for two Nb-${\mathrm{O}}_{6}$ subsystems with different variations of niobium-oxygen bond lengths, distinct dynamics, and disparate levels of deviation from macroscopic polarization direction. These findings establish a detailed picture of the local structure of ${\mathrm{Sr}}_{x}{\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{2}{\mathrm{O}}_{6}$ and provide a deeper insight into the origins of the materials dielectric properties.

Triplet superconductivity in coupled odd-gon rings

Shedding light on the nature of spin-triplet superconductivity has been a long-standing quest in condensed matter physics since the discovery of superfluidity in liquid 3He. Nevertheless, the mechanism of spin-triplet pairing is much less understood than that of spin-singlet pairing explained by the Bardeen-Cooper-Schrieffer theory or even observed in high-temperature superconductors. Here we propose a versatile mechanism for spin-triplet superconductivity which emerges through a melting of macroscopic spin polarization stabilized in weakly coupled odd-gon (e.g., triangle, pentagon, etc) systems. We demonstrate the feasibility of sustaining spin-triplet superconductivity with this mechanism by considering a new class of quasi-one-dimensional superconductors A2Cr3As3 (A = K, Rb, and Cs). Furthermore, we suggest a simple effective model to easily illustrate the adaptability of the mechanism to general systems consisting of odd-gon units. This mechanism provides a rare example of superconductivity from on-site Coulomb repulsion.

Dielectric and ferroic properties of metal halide perovskites

Halide perovskite semiconductors and solar cells respond to electric fields in a way that varies across time and length scales. We discuss the microscopic processes that give rise to the macroscopic polarization of these materials, ranging from the optical and vibrational response to the transport of ions and electrons. The strong frequency dependence of the dielectric permittivity can be understood by separating the static dielectric constant into its constituents, including the orientational polarization due to rotating dipoles, which connects theory with experimental observations. The controversial issue of ferroelectricity is addressed, where we highlight recent progress in materials and domain characterization but emphasize the challenge associated with isolating spontaneous lattice polarization from other processes such as charged defect formation and transport. We conclude that CH3NH3PbI3 exhibits many features characteristic of a ferroelastic electret, where a spontaneous lattice strain is coupled to long-lived metastable polarization states.

Photochromism-Induced Macroscopic Polarization Switching in (K,Na)NbO 3-Based Piezoceramics

The intriguing coupling interactions between photochromism and ferroelectric polarization were firstly observed in Sm doped (K0.5Na0.5)NbO3 piezoceramics. After 407 nm irradiation, the maximum polarization (Pmax) and remanent polarization (Pr) intensities were dramatically decreased, whereas the coercive field, leakage current and photocurrent showed the remarkable increase. The variations of the polarization and photocurrent can be reversibly converted by alternating light irradiation and thermal stimulus, suggesting good polarization switching performance. This work provides a new route to effectively modulate ferro-/piezoelectric and photocurrent properties of ferroelectric materials by the non-contact remote control of light.

Soft-mode driven dynamics in ferroelectrics -new insight from ultrafast terahertz and x-ray experiments

The coupled lattice and charge dynamics connected with soft-mode excitations are mapped by nonlinear terahertz spectroscopy and ultrafast x-ray diffraction. Sub-picometer lattice displacements induce electron relocations over 100 pm in polycrystaline aspirin and ferroelectric ammonium sulfate (AS). In AS, the electron motions induce a macroscopic polarization switching.

Spin tensor and its role in non-equilibrium thermodynamics

It is shown that the description of a relativistic fluid at local thermodynamic equilibrium depends on the particular quantum stress-energy tensor operator chosen, e.g., the canonical or symmetrized Belinfante stress-energy tensor. We argue that the Belinfante tensor is not appropriate to describe a relativistic fluid whose macroscopic polarization relaxes slowly to thermodynamic equilibrium and that a spin tensor, like the canonical spin tensor, is required. As a consequence, the description of a polarized relativistic fluid involves an extension of relativistic hydrodynamics including a new antisymmetric rank-two tensor as a dynamical field. We show that the canonical and Belinfante tensors lead to different predictions for measurable quantities such as spectrum and polarization of particles produced in relativistic heavy-ion collisions.

Thin-film stabilization of LiNbO3-type ZnSnO3 and MgSnO3 by molecular-beam epitaxy

In polar crystals, cooperative ionic displacement produces a macroscopic spontaneous polarization. Among such polar materials, LiNbO3-type wide bandgap oxides are particularly appealing because they offer useful ferroelectric properties and also potentially lead to multiferroic materials. Using molecular-beam epitaxy, we investigated the thin-film growth of high-pressure phase LiNbO3-type ZnSnO3 and discovered a polar oxide candidate, MgSnO3. We found that LiNbO3-type substrates play an essential role in the crystallization of these compounds, though corundum-type Al2O3 substrates also have the identical crystallographic arrangement of oxygen sublattice. Optical transmittance and electrical transport measurements revealed their potential as a transparent conducting oxide. Establishment of a thin-film synthetic route would be the basis for exploration of functional polar oxides and research on conduction at ferroelectric interfaces and domain walls.

Accurate Formula for the Macroscopic Polarization of Strongly Correlated Materials.

The many-body Berry phase formula for macroscopic polarization is approximated by a sum of natural orbital geometric phases with fractional occupation numbers accounting for the dominant correlation effects. This formula accurately reproduces the exact polarization in the Rice-Mele-Hubbard model across the band insulator-Mott insulator transition. A similar formula based on a reduced Berry curvature accurately predicts the interaction-induced quenching of Thouless topological charge pumping.

Electrochemical polarization around metallic particles — Part 2: The role of diffuse surface charge

Numerical simulations are carried out to study the induced-polarization response of a charged metallic sphere, which immersed in electrolyte solution is covered by a static diffuse layer. The metallic sphere itself is assumed to be perfectly conductive; electromigration and diffusion processes in bulk electrolyte and diffuse layer are modeled by the Poisson-Nernst-Planck system of partial differential equations. To include the effect of a fixed diffuse charge, we consider a constant electric [Formula: see text]-potential at the surface of the particle, which leads to the buildup of a static diffuse layer. Furthermore, a minor fraction of electroactive cations engages in oxidation-reduction reactions at the particle surface, which allows charges to be transferred across the solid-liquid interface. Upon excitation by a low-frequency electric field, we observe the coupling of three polarization processes in the composite material consisting of metallic particle and surrounding electrolyte. The first is related to the dynamic charging of field-induced diffuse layers immediately outside the two hemispheres of the sphere. The other two are volume-diffusion processes: (1) one driven by the reaction currents through the particle surface and (2) the other driven by the unequal electromigration transport of anions and cations through the static diffuse layer. Diffuse-layer relaxation and volume diffusion due to reaction currents can also be observed around uncharged metallic particles and clearly dominate the macroscopic polarization response. The [Formula: see text]-surface potential at the particle surface, and thus the static diffuse layer, only moderately change the relaxation of the field-induced diffuse layer: With the increasing magnitude of the [Formula: see text]-potential, we observe an increase of the low-frequency electrical conductivity of the particle in suspension, a reduction of its polarization magnitude, and a shift of its characteristic frequency toward lower frequencies. The volume-diffusion process due to the reaction currents remains practically unaffected by the static diffuse layer.

Macroscopic electric polarization and microscopic electron dynamics: Quantitative insight from femtosecond x-ray diffraction

Phenomena in the macroscopic world are described by classical physics while processes at the microscopic atomic scale are governed by the laws of quantum mechanics. The connection between microscopic and macroscopic physical quantities is far from being trivial and remains partly unexplained. This study presents a novel theoretical approach for calculating macroscopic polarizations from transient microscopic charge densities that are derived from ultrafast time-resolved x-ray diffraction experiments. This is important for understanding the properties of polar and ionic materials and, in particular, those of ferroelectrics with widespread technical applications.

Superposition principle for the simultaneous optimization of collective responses.

We study the dynamics of sets of independent systems, all of which are coupled to the same time-dependent external force. Using optimal control theory, we compute the most efficient temporal pulse shape for this force that can maximize simultaneously the collective response of these systems. This response can be a weighted sum of all amplitudes at the final interaction time. Remarkably, it turns out that for certain systems this optimal force for the collective response can be related to the individual forces that would optimize each system separately. We illustrate this superposition principle for the simultaneous optimization of collective responses with numerical and also analytical solutions for sets of damped linear and nonlinear oscillators. We also apply this principle to predict the optimal temporal profile of a laser pulse that can maximize the final macroscopic polarization (total dipole moment) of a set of quantum mechanical two-level atoms.

Polarization switching in the PbMg1/3Nb2/3O3 relaxor ferroelectric: An atomistic effective Hamiltonian study

An atomistic effective Hamiltonian technique is employed to investigate the polarization switching mechanism of the (metastable but long-living) ferroelectric state of ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ relaxor ferroelectric, resulting from the application of a dc electric field along the $[\overline{1}\overline{1}\overline{1}]$ direction---that is, opposite to the initial polarization. It is predicted that such switching is of inhomogeneous type. It involves the creation of intermediate short-range ordered, relaxor-like phases in between long-range ordered states inside which an infinite cluster exists and has dipoles near either the initial polarization direction (for shorter times) or the field's direction (for longer times). In contrast, dipoles belonging to finite clusters or being isolated can deviate away from [111] and $[\overline{1}\overline{1}\overline{1}]$, and, in fact, rotate in average from [111] to $[\overline{1}\overline{1}\overline{1}]$ when time increases. Such rotations govern the reversal of the polarization from [111] to $[\overline{1}\overline{1}\overline{1}]$ occurring within the intermediate relaxor-like states' region, while always resulting in the overall cancellation of any Cartesian component of the polarization that is perpendicular to [111]. These rotations occurring at the atomic scale also naturally imply that some fundamental assumptions of the original nucleation-limited-switching (NLS) model are not valid, despite the fact that we numerically further find that the whole temporal behavior of the macroscopic polarization can be well fitted by the general formula associated with NLS. In other words, we have stumbled into a novel switching or at least a switching that should be denoted as a generalized NLS model. Finally, three different electric-field regimes are predicted, with each of them having its own dependency of the switching time on the magnitude of the applied electric field and only one of them obeying the Merz's law. The existence of these three regimes is explained in terms of specific microscopic features.

Integration of ferroelectric Pb(Zr0.52Ti0.48)O3 thin films on conducting nanocrystalline diamond for high performance device applications

We report on the direct integration of a Pb(Zr0.52Ti0.48)O3 (PZT) thin film on a hot filament chemical vapor deposition grown conducting boron doped nanocrystalline diamond (B-NCD) film. A conducting B-NCD thin film with a grain size less than 100 nm and surface roughness close to 8 nm is used as a metallic bottom electrode. X-ray diffraction and Raman spectroscopy revealed the single phase perovskite ferroelectric nature of the pulsed laser deposited grown PZT thin film directly on the conducting B-NCD substrate with an electrical resistivity of 10 mΩ-cm. The PZT thin film grown on the B-NCD/Si substrate showed the high remanent polarization (2Pr) of 68 μC/cm2 and high dielectric constant of ∼1300 with a low leakage current density of ∼10−5 A/cm2. Macroscopic and nanoscale polarization switching experiments are performed to confirm the ferroelectric nature of the PZT/B-NCD capacitor. A very small degradation of remanent polarization of 10% even after 1010 switching cycles of the sample demonstrates the excellent ferroelectric performance of the multilayer.

Insight into crystal-structure dependent charge separation and photo-redox catalysis: A combined experimental and theoretical study on Bi(IO3)3 and BiOIO3

Solar-driven conversion for CO2 reduction and oxygen activation reactions show huge potentials for energetic and environmental applications. However, the influence of crystal structure of a photocatalyst on its photocatalytic performance has been seldom investigated so far. Herein, to probe the relationship between crystal structure and photocatalytic properties, two bismuth iodates, the centrosymmetric (CS) monoclinic Bi(IO3)3 and noncentrosymmetric (NCS) orthorhombic BiOIO3 are employed as the models. The photocatalytic reduction and oxidation capabilities of Bi(IO3)3 and BiOIO3 were surveyed by monitoring the CO2 reduction and oxygen activation reactions. The results revealed that BiOIO3 shows far superior photocatalytic activity than Bi(IO3)3, which can more efficiently convert CO2 into CO and produce larger amounts of O2− and OH. The experimental characterizations and DFT calculations co-uncovered that much more efficient charge separation and migration occur in BiOIO3 in compared to Bi(IO3)3, which are responsible for the obviously higher photoactivity of BiOIO3. This is mainly due to that the NCS crystal structure of BiOIO3 that gives rise to a large macroscopic polarization, facilitating the separation of photogenerated electron-hole pairs. The microscopic first hyperpolarizability for BiOIO3 was calculated to be 2.56 × 10−30 esu for the dominant component at the static limit and 15.73 × 10−30 esu at the wavelength 409.1 nm, which well verifies the strong polarization of BiOIO3. This study may furnish the perspective into designing high-performance photocatalytic materials on the basis of crystal structure engineering.

Polarization in the social sciences: Assortative mixing in social science collaboration networks is resilient to interventions

Academic collaboration in the social sciences is characterized by a polarization between hermeneutic and nomological researchers. This polarization is expressed in different publication strategies. The present article analyzes the complete co-authorship networks in a social science discipline in two separate countries over five years using an exponential random graph model. It examines whether and how assortative mixing in publication strategies is present and leads to a polarization in scientific collaboration. In the empirical analysis, assortative mixing is found to play a role in shaping the topology of the network and significantly explains collaboration. Co-authorship edges are more prevalent within each of the groups, but this mixing pattern does not fully account for the extent of polarization. Instead, a thought experiment reveals that other components of the complex system dampen or amplify polarization in the data-generating process and that microscopic interventions targeting behavior change with regard to assortativity would be hindered by the resilience of the system. The resilience to interventions is quantified in a series of simulations on the effect of microscopic behavior on macroscopic polarization. The empirical study controls for geographic proximity, supervision, and topical similarity (using a vector space model), and the interplay of these factors is likely responsible for this resilience. The paper also predicts the co-authorship network in one country based on the model of collaborations in the other country.

The relationship between covalent bonds and the optical response in a nonpolar family ATeMoO6 (A=Mg, Zn, Cd): A Berry-phase investigation

The macroscopic polarization, the refractive indices, birefringence, and the second-order nonlinear optical susceptibilities in a family of nonpolar materials ATeMoO6 (A = Mg, Zn, and Cd) have been investigated using the Berry-phase calculation. The microscopic origin of the optical response is further investigated by the variation of atomic charge, the Born effective charges, and the chemical bond analyses based on the Bader charge analysis and the Mulliken bond population analysis. The results show that the robust covalent bonds play an important role in determining the refractive indices and SHG response, and the anisotropic electron distributions on covalent Te‒O and Mo‒O bond give main contribution to the optical anisotropic birefringence.

Exploring ferroelectric and magnetic properties of Tb-substituted m = 5 layered Aurivillius phase thin films

Here, we report the effect of A-site substitution of Tb at the expense of Bi on the ferroelectric and magnetic properties in m = 5 layered 2-D Aurivillius Bi6Ti3Fe2O18 thin films. The nominal stoichiometry of the prepared compound is Tb0.40Bi5.6Fe2Ti3O18, Tb0.90Bi5.1Fe2Ti3O18, and Bi6Ti3Fe2O18. Phase examination reveals that only 0.40 mol. % is successfully substituted forming Tb0.40Bi5.6Fe2Ti3O18 thin films. Lateral and vertical piezoresponse switching loops up to 200 °C reveal responses for Bi6Ti3Fe2O18, Tb substituted Tb0.40Bi5.6Fe2Ti3O18, and Tb0.90Bi5.1Fe2Ti3O18 thin films along the in-plane (±42.31 pm/V, 88 pm/V and ±134 pm/V, respectively) compared with the out-of-plane (±6.15 pm/V, 19.83 pm/V and ±37.52 pm/V, respectively). The macroscopic in-plane polarization loops reveal in-plane saturation (Ps) and remanence polarization (Pr) for Bi6Ti3Fe2O18 of ±26.16 μC/cm2 and ±22 μC/cm2, whereas, ±32.75 μC/cm2 and ±22.11 μC/cm2, ±40.30 μC/cm2 and ±28.5 μC/cm2 for Tb0.40Bi5.6Fe2Ti3O18 and Tb0.90Bi5.1Fe2Ti3O18 thin films, respectively. No ferromagnetic signatures were observed for Bi6Ti3Fe2O18 and Tb0.40Bi5.6Fe2Ti3O18. However, a weak response was observed for the Tb0.90Bi5.1Fe2Ti3O18 at 2 K. Microstructural analysis of Tb0.90Bi5.1Fe2Ti3O18 revealed that it contains 4 vol. % Fe:Tb rich areas forming FexTbyOz, which accounts for the observed magnetic moment. This study demonstrates the importance of thorough microstructural analysis when determining whether magnetic signatures can be reliably assigned to the single-phase system. We conclude that Tb0.40Bi5.6Fe2Ti3O18 and Tb0.90Bi5.1Fe2Ti3O18 samples are not multiferroic but demonstrate the potential for Fe-RAM applications.

Water, the special liquid

Some prominent special properties of liquid water are summarized and related to the characteristics of the water molecule. It is shown that the substance appears to be anomalous only when compared to hard-sphere models with molecular interactions restricted to excluded-volume effects. Besides the electric charge distribution and the balanced number of hydrogen bond donating and accepting sites, the strength of the hydrogen bonds and also of the covalent bonds are identified significant factors in making water so special. As another major feature the small size of the water molecule is emphasized. It is often neglected but is relevant in every respect. It contributes, for example, to the large permittivity of water which plays an important role in the solvent behavior of water. The paper also demonstrates that the smallness of the water molecule is a key property in the molecular mobility of the liquid. The single-dipole (molecular) reorientation time is deduced from the experimentally available relaxation time of the macroscopic dielectric polarization and is discussed in terms of the so-called wait-and-switch model of dielectric relaxation. By comparison of the waiting periods of the model with structure relaxation times from ultraviolet and X-ray Brillouin spectra, close coupling between the reorientational motions and translational motions, i.e. the hydrogen network fluctuations in water, is disclosed. This finding confirms the idea of the wait-and-switch model. In addition, the waiting period is seen to nicely follow Eyring behavior, indicating an activated switch (or jump) mechanism.