Dielectric Susceptibility Research Articles

Flexoinduced ferroelectricity in low-dimensional transition metal dichalcogenides

We developed a Landau-type theory for the description of polar phenomena in low-dimensional transition metal dichalcogenides (TMDs), specifically exploring the flexoelectric origin of the polarization induced by a spontaneous bending and by inversion symmetry breaking due to the interactions with substrate. We consider the appearance of the spontaneous out-of-plane polarization due to the flexoelectric coupling with the strain gradient of the spontaneous surface rippling and surface-induced piezoelectricity. Performed calculations proved that the out-of-plane spontaneous polarization, originating from the flexoelectric effect in a rippled TMD, is bistable and reversible by a nonuniform electric field. In contrast, the spontaneous polarization induced by a misfit strain and symmetry-sensitive surface-induced piezoelectric coupling, cannot be reversed by an external electric field. Special attention is paid to the spectral analysis of the linear dielectric susceptibility and gain factor, which enhancement is critically important for the observation of the polar phenomena in low-dimensional TMDs by the surface-enhanced vibrational spectroscopy.

Polarization and dielectric susceptibility of a monolayer coronene like nano-structure: Monte Carlo study

In this paper, we study the dielectric properties of a monolayer coronene like nano-structure with mixed spins (S=±2, σ=±5/2), using Monte Carlo simulations. The ground state phase diagrams in different planes are studied. In addition, we examine the blocking temperature for different exchange coupling interactions. Moreover, we explore the effects of the crystal field on the total polarizations. Finally, the electric hysteresis cycles are given for different values of temperatures and exchange coupling interactions.

Ultrafast hydrogen bond dynamics of liquid water revealed by terahertz-induced transient birefringence

The fundamental properties of water molecules, such as their molecular polarizability, have not yet been clarified. The hydrogen bond network is generally considered to play an important role in the thermodynamic properties of water. The terahertz (THz) Kerr effect technique, as a novel tool, is expected to be useful in exploring the low-frequency molecular dynamics of liquid water. Here, we use an intense and ultrabroadband THz pulse (peak electric field strength of 14.9 MV/cm, centre frequency of 3.9 THz, and bandwidth of 1–10 THz) to resonantly excite intermolecular modes of liquid water. Bipolar THz field-induced transient birefringence signals are observed in a free-flowing water film. We propose a hydrogen bond harmonic oscillator model associated with the dielectric susceptibility and combine it with the Lorentz dynamic equation to investigate the intermolecular structure and dynamics of liquid water. We mainly decompose the bipolar signals into a positive signal caused by hydrogen bond stretching vibration and a negative signal caused by hydrogen bond bending vibration, indicating that the polarizability perturbation of water presents competing contributions under bending and stretching conditions. A Kerr coefficient equation related to the intermolecular modes of water is established. The ultrafast intermolecular hydrogen bond dynamics of water revealed by an ultrabroadband THz pump pulse can provide further insights into the transient structure of liquid water corresponding to the pertinent modes.

ELECTRICALLY CONTROLLED BIREFRINGENCE AND DIELECTRIC SUSCEPTIBILITY OF HELICAL NANOSTRUCTURES OF FERROELECTRIC LIQUID CRYSTALS

The relation between birefringence and dielectric susceptibility of helical nanostructures of ferroelectric liquid crystals in an electric field was studied experimentally and theoretically.

Quantum critical phenomena in a compressible displacive ferroelectric

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.

Multiresponsive Spin Crossover Driven by Rotation of Tetraphenylborate Anion in an Iron(III) Complex

Dynamic molecular materials, which involve mechanical motions in crystals, usually exhibit tunable properties related to conformational polymorphs. Herein, we describe a solvent-free molecular crys...

Nonlinear optical properties of n-type asymmetric double $$\delta $$-doped quantum wells: role of high-frequency laser radiation, doping concentration and well width

A numerical investigation on the nonlinear optical rectification, second and third harmonic generation coefficients in asymmetric double n-type $$\delta $$ -doped GaAs quantum well is performed in order to identify the influence of non-resonant intense laser radiation, doping concentration and the change in well widths. The energy eigenvalues and the corresponding eigenfunctions are determined by using effective-mass and parabolic band approximations. The working analytical expressions for the optical coefficients are derived from the iterative solving of compact-density matrix description of dielectric susceptibility. The obtained results reveal that the position and amplitude of the nonlinear optical rectification, second and third harmonic generation coefficients can be altered by modifying the external field as well as the compositional and geometrical setups.

Polar vibrational and dielectric properties of monolayer transition metal dichalcogenides from macroscopic equations

Long wavelength polar vibrations in monolayer (ML) transition metal dichalcogenides (TMDs) are systematically studied for in-plane and out-of-plane motions, using two pairs of macroscopic equations deduced from a microscopic dipole lattice model accounting for local field effects (LFEs) and electronic polarization (EP). Longitudinal and transverse optical modes and out-of-plane modes are derived, and the analytical expressions describe previous first-principles calculations very well. Owing to the LFEs, the in-plane dielectric susceptibilities of ML TMDs are one order of magnitude greater than the out-of-plane susceptibilities. Furthermore, the effects of the dielectric environment on the polar vibrations are studied. Both EP and LFEs should be accounted for obtaining an accurate evaluation of dielectric susceptibility and key lattice-dynamical properties such as Born charge and phonon dispersion. A two-dimensional (2D) Lyddane–Sachs–Teller relation and a frequency–susceptibility relation are derived for in-plane and out-of-plane motions, relating the 2D dielectric functions or susceptibilities to the polar phonon frequencies. The results are also compared in detail with those of ML hexagonal boron nitride.

Strain engineering of oxide thin films for photocatalytic applications

Photocatalytic materials are pivotal for the implementation of disruptive clean energy applications such as conversion of H$_{2}$O and CO$_{2}$ into fuels and chemicals driven by solar energy. However, efficient and cost-effective materials able to catalyze the chemical reactions of interest when exposed to visible light are scarce due to the stringent electronic conditions that they must satisfy. Chemical and nanostructuring approaches are capable of improving the catalytic performance of known photoactive compounds however the complexity of the synthesized nanomaterials and sophistication of the employed methods make systematic design of photocatalysts difficult. Here, we show by means of first-principles simulation methods that application of biaxial stress, $\eta$, on semiconductor oxide thin films can modify their optoelectronic and catalytic properties in a significant and predictable manner. In particular, we show that upon moderate tensile strains CeO$_{2}$ and TiO$_{2}$ thin films become suitable materials for photocatalytic conversion of H$_{2}$O into H$_{2}$ and CO$_{2}$ into CH$_{4}$ under sunlight. The band gap shifts induced by $\eta$ are reproduced qualitatively by a simple analytical model that depends only on structural and dielectric susceptibility changes. Thus, epitaxial strain represents a promising route for methodical screening and rational design of photocatalytic materials.

The composition induced crossover in nonlinear dielectric response in (1\u2009\u2212\u2009x) Pb(Zr0.70Ti0.30)O3\u2013x BiMn2O5 (x\u2009=\u20090, 0.02, 0.055, 0.11, 0.15, 0.22, and 1) ceramics

Ceramics based on lead zirconate-titanate Pb(Zr0.70Ti0.30)O3 (PZT), bismuth manganite (BM) and the composite (1 − x) PZT–x BM were obtained. A systematic study of linear and nonlinear dielectric susceptibilities temperature and frequency dependences was conducted. A composition-induced crossover in nonlinear dielectric susceptibility was detected. The so-called scaled nonlinear susceptibility, a3, was obtained from experimental values of the real part of the linear and third-order nonlinear dielectric susceptibilities. The value for a3 was negative for low BM content in the whole temperature range specific to discontinuous ferroelectric phase transitions and relaxor behavior. In contradiction, a3 was positive for BM and for the composite with sufficiently high BM content. The positive a3 was attributed to a subsystem of dipoles created by trapped charges. Hopping charge transfer, which is a dominant mechanism for conduction in manganites, is responsible for the positive a3 and for the colossal magnitude of the imaginary part of the linear dielectric susceptibility. The concentration of the ions was determined using scanning electron microscopy. The chemical disorder and the precipitation of ions determined using time of flight–secondary ion mass spectrometry correspond to a crossover in the electrical features.

FREQUENCY AND TEMPERATURE ANALYSIS OF THE CLAUSIUSMOSSOTTI FACTOR OF A KEROSENE-BASED FERROFLUID IN LOW FREQUENCY FIELD

Based on the complex dielectric susceptibility measurements, over the frequency range 10 kHz to 2 MHz and at different temperatures ranging from 25 0C to 80 0C, for a kerosenebased ferrofluid sample, the Claussius-Mossotti complex factor, K, was analyzed. Using the K obtained values and the Schwarz theoretical model, we have proposed an original method, very useful for determining the activation energy Ea of the Schwarz relaxation process from ferrofluid. The results show that the Ea decreases with the increase of temperature for all investigated frequencies. This behavior can be correlated with the deformation of the atmosphere of counterions from the surface of nanoparticles, which determines the polarization of ferrofluid in the low frequency field.

FREQUENCY AND TEMPERATURE ANALYSIS OF THE CLAUSIUSMOSSOTTI FACTOR OF A KEROSENE-BASED FERROFLUID IN LOW FREQUENCY FIELD

Based on the complex dielectric susceptibility measurements, over the frequency range 10 kHz to 2 MHz and at different temperatures ranging from 25 0C to 80 0C, for a kerosenebased ferrofluid sample, the Claussius-Mossotti complex factor, K, was analyzed. Using the K obtained values and the Schwarz theoretical model, we have proposed an original method, very useful for determining the activation energy Ea of the Schwarz relaxation process from ferrofluid. The results show that the Ea decreases with the increase of temperature for all investigated frequencies. This behavior can be correlated with the deformation of the atmosphere of counterions from the surface of nanoparticles, which determines the polarization of ferrofluid in the low frequency field.

Spin-Orbital Glass Transition in a Model of a Frustrated Pyrochlore Magnet without Quenched Disorder.

We show theoretically that spin and orbital degrees of freedom in the pyrochlore oxide Y_{2}Mo_{2}O_{7}, which is free of quenched disorder, can exhibit a simultaneous glass transition, working as dynamical disorder for each other. The interplay of spins and orbitals is mediated by the Jahn-Teller lattice distortion that selects the choice of orbitals, which then generates variant spin exchange interactions ranging from ferromagnetic to antiferromagnetic ones. Our MonteCarlo simulations detect the power-law divergence of the relaxation times and the negative divergence of both the magnetic and dielectric nonlinear susceptibilities, resolving the long-standing puzzle on the origin of the disorder-free spin glass.

Analytical solution of spontaneous polarization based on a Landau-Devonshire eight-order potential

The analytical solutions of spontaneous polarization, phase transition temperature TC, and ferroelectric limit temperature Th have been solved based on the Landau-Devonshire eight-order potential. According to the analytical solutions, the eighth-order potential gives a more reasonable and thinner superheating zone (Th-TC). The analytical solutions can be applied to deduce other analytical solutions of the functional properties directly, e. g., dielectric susceptibility and pyroelectric coefficient. It can also be used to evaluate different Landau-Devonshire potentials as demonstrated for BaTiO3.

Generation and manipulation of isotropic droplets in nematic medium using switchable dielectrophoresis.

Dielectrophoresis (DEP) in a medium with anisotropic dielectric susceptibility is very different from typical DEP in an isotropic medium: The direction of particle actuation can be switched depending on the direction of the susceptibility tensor of the medium. However, the understanding of switchable DEP (SDEP) in an anisotropic medium is still in its infant stage. Here, we investigate SDEP using heat-generated isotropic droplets in a nematic liquid crystal (LC) medium. We demonstrate that the location of the generation of isotropic droplets can be partially controlled by controlling the temperature gradient within the LC cell using dielectric loss. The SDEP actuation of isotropic droplets is also highly dependent on the location of the isotropic droplets. Using this method, we fabricated different array patterns of isotropic and nematic phase separations under different applied signals.

Large Polarization and Susceptibilities in Artificial Morphotropic Phase Boundary PbZr1−xTixO3 Superlattices

AbstractThe ability to produce atomically precise, artificial oxide heterostructures allows for the possibility of producing exotic phases and enhanced susceptibilities not found in parent materials. Typical ferroelectric materials either exhibit large saturation polarization away from a phase boundary or large dielectric susceptibility near a phase boundary. Both large ferroelectric polarization and dielectric permittivity are attained wherein fully epitaxial (PbZr0.8Ti0.2O3)n/(PbZr0.4Ti0.6O3)2n (n = 2, 4, 6, 8, 16 unit cells) superlattices are produced such that the overall film chemistry is at the morphotropic phase boundary, but constitutive layers are not. Long‐ (n ≥ 6) and short‐period (n = 2) superlattices reveal large ferroelectric saturation polarization (Ps = 64 µC cm−2) and small dielectric permittivity (εr ≈ 400 at 10 kHz). Intermediate‐period (n = 4) superlattices, however, exhibit both large ferroelectric saturation polarization (Ps = 64 µC cm−2) and dielectric permittivity (εr = 776 at 10 kHz). First‐order reversal curve analysis reveals the presence of switching distributions for each parent layer and a third, interfacial layer wherein superlattice periodicity modulates the volume fraction of each switching distribution and thus the overall material response. This reveals that deterministic creation of artificial superlattices is an effective pathway for designing materials with enhanced responses to applied bias.

Photocarrier-Induced Active Control of Second-Order Optical Nonlinearity in Monolayer MoS2.

Atomically thin transition metal dichalcogenides (TMDs) in their excited states can serve as exceptionally small building blocks for active optical platforms. In this scheme, optical excitation provides a practical approach to control light-TMD interactions via the photocarrier generation, in an ultrafast manner. Here, it is demonstrated that via a controlled generation of photocarriers the second-harmonic generation (SHG) from a monolayer MoS2 crystal can be substantially modulated up to ≈55% within a timeframe of ≈250 fs, a set of performance characteristics that showcases the promise of low-dimensional materials for all-optical nonlinear data processing. The combined experimental and theoretical study suggests that the large SHG modulation stems from the correlation between the second-order dielectric susceptibility χ(2) and the density of photoexcited carriers in MoS2 . Indeed, the depopulation of the conduction band electrons, at the vicinity of the high-symmetry K/K' points of MoS2 , suppresses the contribution of interband electronic transitions in the effective χ(2) of the monolayer crystal, enabling the all-optical modulation of the SHG signal. The strong dependence of the second-order optical response on the density of photocarriers reveals the promise of time-resolved nonlinear characterization as an alternative route to monitoring carrier dynamics in excited states of TMDs.

Enhanced magnetoelectric effect near a field-driven zero-temperature quantum phase transition of the spin-1/2 Heisenberg-Ising ladder.

The magnetoelectric effect of a spin-1/2 Heisenberg-Ising ladder in the presence of external electric and magnetic fields is rigorously examined by taking into account the Katsura-Nagaosa-Balatsky mechanism. It is shown that an applied electric field may control the quantum phase transition between a Néel (stripy) ordered phase and a disordered paramagnetic phase. The staggered magnetization vanishes according to a power law with an Ising-type critical exponent 1/8, the electric polarization exhibits a weak singularity, and the dielectric susceptibility shows a logarithmic divergence at this particular quantum phase transition. The external electric field may alternatively invoke a discontinuous phase transition accompanied with abrupt jumps of the dielectric polarization and susceptibility on the assumption that the external magnetic field becomes nonzero.

On dielectric relaxation equation for anisotropic polarizable reacting fluid mixtures

In this paper a linear theory for dielectric relaxation phenomena in polarizable reacting fluid mixtures is developed, in the frame of thermodynamics of irreversible processes with internal variables. The microscopic irreversible phenomena giving rise to dielectric relaxation are described splitting the total specific polarization in two irreversible parts and introducing one of these partial specific polarizations as internal variable in the thermodynamic state vector. The phenomenological equations for these fluid mixtures are derived and, in the linear case, a generalized Debye equation for dielectric relaxation phenomena is derived. Special cases are also treated. Linear theories for polarizable continuous media with dielectric relaxation phenomena were derived in the same frame of non-equilibrium thermodynamics with internal variables in previous papers by one of the authors (LR). A phenomenological theory for these phenomena was developed by Maugin for complex materials, using microscopic considerations and introducing articular partial polarizations per unit mass. The obtained results in this paper have applications in several fields of applied sciences, as, for instance, in medicine and biology, where complex fluids presenting dielectric relaxation, are constitued by different types of molecules, with own dielectric susceptibility and relaxation time.

A computational study of the electronic structure and optical properties of the complex TeO2/TeO3 oxides as advanced materials for nonlinear optics

Electronic structure of series of tellurium oxide crystals within the TeO2—TeO3 binary system is studied with generalized gradient approximation to DFT, hybrid DFT-HF method with the PBE0 and B3LYP exchange-correlation functionals and with quasiparticle G0W0 approach. Comparison with available experimental data revealed significant underestimation of the band gap values within DFT. The hybrid DFT-HF method leads to slightly overestimated values of the bandgap, and the best agreement with experimental data provides the ‘one-shot’ G0W0 calculations starting from Kohn–Sham solutions. The electronic structure of tellurium oxides is discussed in details. It is found that the bandgap value decreases proportionally to fraction of tellurium atoms in octahedral coordination. This change is due to formation of gap states by 5s(Te) electrons which do not participate in Te(VI)–O bonding. Dielectric susceptibilities are calculated within Random Phase approximation for the series of tellurium oxides and high nonlinear properties of the compounds are predicted by empirical Miller’s rule.