Frequency Dependence Of Real Part Research Articles

Effect of CdSe/ZnS quantum dots doping on the ion transport behavior in nematic liquid crystal

The tunability of ion transport behavior in the liquid crystal (LC) system doped with nanomaterials offers a stable and potential technology for designing the next generation application devices. Earlier research efforts, including theoretical and experimental approaches, explore ion density and the effect on mesomorphic properties of several LC nanocomposite system using different variables such as doping concentration, ionic contamination and type of material. In the present work, considering the simplest nematic 5CB LC and core–shell type CdSe/ZnS quantum dots (QDs), the temperature-dependent ion transport behavior and electrical properties have been experimentally measured with varying QD concentration by dielectric spectroscopy technique. The mobile ion concentration and diffusion constant are effectively analyzed using two different approaches: modified Sawada model and Uemura formalism. Utilizing the experimental dielectric data in the pure and doped nematic LC systems, we described the ion trapping and releasing nature of QDs with respect to concentration level and observed the significant contribution of the two types of mobile ions differing in size and mobility. The study also demonstrates that the doping of QDs effectively traps two kinds of ions while the ion releasing phenomenon occurs only for one type of ion with high doping concentration. The effective interaction between QD ligands and molecules creates a tactoid-like ellipsoidal shape of the nanoparticles which in turn enhances the mobile ion density in the higher doped samples. Additionally, the diffusive motion of ions varies linearly with temperature. We have investigated the two plateau regions in the frequency-dependent real part of conductivity with Jonscher power law and observed the significant contribution of the first type of ion to the conduction process in all the studied samples. These experimental findings in this studied system will facilitate the advanced understanding of ion interaction for nanomaterials in other complex LC matrix.

Negative capacitance of nanocomposites with CoFeZr nanoparticles embedded into silica matrix

The study presents frequency dependences of real part of admittance Z′(f) and phase shift angle θ(T,f) in nanogranular films containing CoFeZr nanoparticles with “core-shell” structure embedded into SiO2 matrix. The 3 μm thicknesses (Co41Fe49Zr10)x(SiO2)100−x films with 20 ≤ x ≤ 80at.% were deposited in vacuum chamber evacuated either with pure Ar (Set 1 samples) or Ar + O2 gas mixture (Set 2) using ion-beam sputtering technique. After characterization by X-Ray diffraction, Mössbauer spectroscopy, scanning electron microscopy and magnetization studies, the films both of Set1 and Set 2 samples were subjected by admittance measurements at 300 K in the frequency range of 0.1–1000 kHz. Mössbauer spectroscopy have shown that oxidized CoFeZr nanoparticles in Set 2 samples contain semiconducting iron-based oxides with Fe3+ charge states of iron ions. The observed Z′(f) and θ(f) dependencies for the Set2 films below the xc have shown dielectric regime of carrier transport. They also exhibited that at weak AC electric fields inductive-like contribution to reactive part of admittance (with positive θ values) prevails over the capacitive one f > 10 kHz. This effect of the so-called “negative capacitance” was explained by the delay of current, formed by electrons hopping between nanoparticles, relative to applied bias voltage. This delay is forced by formation of dipoles of charged FeCoZr nanoparticles with native Fe-based oxide “shells” around them that results in the increase of mean life time of hopping electrons on nanoparticles with “core-shell” structure.

Consequences of R3+ cationic radii on the dielectric and magnetic behavior of RCrO3 perovskites

In this study, the consequences of R3+ cationic radii on the dielectric, conductivity and magnetic response of selected RCrO3 perovskites are presented. The frequency-dependent real part of dielectric constant (e′) and loss tangent (tan δ) were studied at 300 K and observed that at low frequency the values are high. From the conductivity (σAC) studies, we found that values of frequency exponent (s) are in the range 0.21–0.51 for the measured frequency range, which clearly indicates that the conduction mechanism is due to small polaron hopping. The magnetic studies reveal that all the synthesized samples exhibit magnetic phase transition (paramagnetic to antiferromagnetic) below the critical Neel temperature (TN) which is associated to Cr3+–O2−–Cr3+ exchange interaction and the value of TN decreases uniformly with decreasing R3+ cationic radii. Some samples show interesting magnetic behavior, along with isothermal hysteresis curve proposing the spin reorientation phase transition of Cr3+ magnetic ordering.

Magnetic Spin Susceptibility of Graphene in Ferromagnetic State: A Tight-Binding Model Study

We report here a tight-binding model study of frequency-dependent ferromagnetic spin susceptibility of the graphene system. The tight-binding Hamiltonian consists of electron hoppings up to third-nearest-neighbors, substrate and impurity effects in the presence of Coulomb interaction of electrons separately at two in-equivalent A and B sub-lattices of graphene. To calculate magnetic susceptibility, we calculate the two-particle electron Green’s functions by using Zubarev’s double time Green’s function technique. The electron occupations at A and B sub-lattices for both up and down spins are computed numerically and self-consistently. The frequency-dependent real part of ferromagnetic susceptibility of the system is computed numerically by taking [Formula: see text] grid points of the electron momentum. The susceptibility displays a sharp peak at the neutron momentum transfer energy at low energies and another higher energy resonance peak appearing at substrate-induced gap. The [Formula: see text]-peak shifts to a higher energy with the increase of momentum [Formula: see text]. The susceptibility shows that the high energy peak shifts to higher energies due to the corresponding increase of substrate-induced gap observed experimentally. It is observed that the Coulomb interaction suppresses the substrate-induced gap, but the impurity doping at A site enhances the substrate-induced gap, while doping at B site suppresses it.

Microscopic theory of substrate-induced gap effect on real AFM susceptibility in graphene

We address here a tight-binding model study of frequency-dependent real part of antiferromagnetic susceptibility for the graphene systems. The Hamiltonian consists of electron hopping upto third nearest-neighbours, substrate and impurity effects in the presence of electron–electron interactions at A and B sublattices. To calculate susceptibility, we evaluate the two-particle electron Green’s function by using Zubarev’s Green’s function technique. The frequency-dependent real part of antiferromagnetic susceptibility of the system is computed numerically by taking $$1000 \times 1000$$ grid points of the electron momentum. The susceptibility displays a sharp peak at the neutron momentum transfer energy at low energies and another higher energy peak appearing at substrate-induced gap. The evolution of these two peaks is investigated by varying neutron wave vector, Coulomb correlation energy, substrate-induced gap, electron hopping integrals and A- and B-site electron doping concentrations.

Computational study of structural, optoelectronic and nonlinear optical properties of dynamic solid-state chalcone derivatives

In the present study, we use the state of art density functional theory (DFT) techniques to calculate the structural, optoelectronic and nonlinear optical (NLO) properties for two novel chalcone derivatives. The geometrical structures of chalcone derivatives compound 1 and 2 are optimized using periodic boundary conditions (PBC) in solid-state phase as well as isolated single molecular geometry in the gas phase. The reasonable agreement is found among experimental, solid-state and gas phase single molecular geometries, which provide us, further confidence to explore the potential of above-entitled derivatives as good functional materials for electro-optical applications. For instance, the frequency dependent real parts of dielectric functions are calculated for compound 1 and 2. The maximum value of real part of the dielectric function for compound 1 and 2 at 0eV are computed as 4.35 and 6.68 for the polarization vectors of (001) directions, respectively, which reveals the fact that the compound 1 and 2 might be good charge transport materials. The reflectivities of the compound 1 and 2 are 0.64 and 0.45 revealing that the compound 2 might be more efficient material for organic photovoltaic (OPV) applications. The results of the refractive index improved by doping the strong electron withdrawing groups (EWGs) shows that the compound 2 might be good refractor of the photon as compared to compound 1. The calculated values for static second-order polarizability are 3498 and 10464 a. u. and for frequency dependent second harmonic generations are 2557 and 6429 a. u. for compound 1 and 2, respectively, which indicates their significant potential for possible nonlinear optical applications.

Existence of negative permittivity in carbon coated iron nanoparticle - PDMS composites

We prepared polydimethylsiloxane (PDMS)-carbon coated iron nanoparticle (CCFeNP) composites (varying filler content) to investigate the dielectric behaviour, in the frequency range 100 Hz–100 MHz. The sample structures were studied by means of Transmission Electron Microscopy (TEM), Scanning electron microscopy (SEM) and X-ray diffraction (XRD). The ac conductivity, impedance, permittivity and dielectric loss of PDMS-CCFeNP composites were analysed. The real part of permittivity/epsilon (εʹ) for the cured composites varied inversely with frequency and switched from positive to negative. The 10 wt% composites exhibit negative permittivity at higher frequency, whereas for 50 wt% permittivity switches from a highly positive to negative at very low frequency and remained negative thereafter. The shifting of switching frequency towards lower frequencies with increasing filler loading is attributed to the increased inductive nature of the composites. The unique frequency dispersion of permittivity is explained by interband transition and Drude model (multiband nature of the electronic excitations). The well dispersed filler particles in PDMS matrix gave the high permittivity of 833 for 30 wt% composite at 1 kHz. The frequency dependence of real part of ac conductivity (σacʹ) and impedance studies were carried out to elucidate the capacitive-inductive transition and dielectric resonance in the composites.

Attenuation of free spheroidal oscillations of the Earth after the M = 9 Earthquake in Sumatra and the super-deep Earthquake in the Sea of Okhotsk: I. the Admissible Q-factor range for the fundamental mode and overtones of the free spheroidal oscillations

The problem of reconstructing the depth distribution of density and the depth and frequency dependences of the mechanical Q-factor in the Earth’s mantle from the entire set of the present-day seismic and astrometric data on the travel times and periods of seismic waves and the amplitudes and phases of forced nutations is considered. The solution of the problem is refined by including the new data about the attenuation of the free oscillations of the Earth excited by the Sumatra earthquake (M = 9) and super-deep earthquake in the Sea of Okhotsk. The actual accuracy of the Q-factor is studied in the first part of the paper. To this end, we analyze (1) the convergence of the Q-factor estimated from the time series of different length shifted along the time axis and (2) the convergence of the results based on the different data. Since the accuracy of identifying all the periods and attenuation factors for the free oscillations from the Sumatra earthquake is significantly higher than for the earthquake with M = 9 in Japan, the data are only compared for the Sumatra and Okhotsk events. The signal-to-noise ratio (SNR) is analyzed based on the records from different Global Seismographic Network (GSN) stations including the station in Obninsk, the Kurchatov station in Kazakhstan, and the main ERM and MAJO stations in Japan. It is found that the highest SNR was observed in Obninsk. The inverse problem of reconstructing the density and Q-factor is solved for the frequency dependent real parts of the shear moduli with allowance for the most accurate data about the attenuation factors for the fundamental spheroidal modes of the free oscillations of the Earth.

Dielectric Behaviour of (Ba1-xNax)(Ti1-xNbx)O3 Ceramics Obtained by a Conventional and Mechanochemical Syntheses

The (Ba1-xNax)(Ti1-xNbx)O3 (BNTNx) ferroelectric ceramics have been prepared by a solid state reaction with the use of a conventional method. A mechanochemical treatment based on the high-energy planetary ball milling has been used as an alternative method of synthesizing of the ceramics. The morphology of the investigated samples was characterized by a scanning electron microscopy (SEM). The characterization of electrical properties of the BNTNx samples within the temperature range from 130 K to 600 K were performed by means of a dielectric spectroscopy method at the frequency ranging from 0.1 Hz to 10 MHz. The temperature and frequency dependences of real part of dielectric permittivity showed the changes of the character of the paraelectric–ferroelectric (PE–FE) phase transition for the ceramic samples obtained by both methods. These functional materials turned out to be either classical or relaxor ferroelectrics.

Spectrophotometric and electrical properties of imperatorin: an organic molecule

Imperatorin (molecular formula = C16H14O4, molecular mass = 270) an organic molecule was isolated from ethyl acetate extract of the root parts of the plant Prangos pabularia. The optical study was carried out by ultraviolet–visible spectroscopy, and this compound showed an indirect allowed transition. The optical band gap (E g ) was found around 3.75 eV. Photoluminescence shows various good emission bands. The frequency-dependent real part of the complex ac conductivity was found to follow the universal dielectric response: σ ac (ω) α ω s [where σ ac (ω) is the frequency-dependent total conductivity, ω is the frequency, and s is the frequency exponent]. From ac conductivity data analysis, correlated barrier hopping charge-transport mechanism is the dominant electrical transport process shown by this compound. The good emission, less absorption, wide band gap and good electrical properties shown by this compound project them as a bright choice for organic electronic devices.

Temperature-induced delocalization of charge carriers and semiconductor to metal-like phase in SrFeO3−δ

Perovskite SrFeO3−δ, a Ruddlesden–Popper class of system exhibits interesting electronic and magnetic properties. Influence of oxygen vacancies on the electrical response of nanocrystalline SrFeO2.91 as a function of temperature is investigated using impedance spectroscopy technique. A change observed in the Nyquist plot at 383 K has been analyzed in terms of localized and delocalized eg electrons. An unusual and interesting temperature-induced semiconductor to metal-like transition is observed in the frequency-dependent real part of dielectric permittivity. Dependence of frequency on the real and imaginary parts of impedance with respect to temperature supports the presence of semiconductor to metal-like transition in SrFeO2.91.

Single crystal growth by gel technique and characterization of lithium hydrogen tartrate

Single crystal growth of lithium hydrogen tartrate by gel encapsulation technique is reported. Dependence of crystal count on gel density, gel pH, reactant concentration and temperature are studied and the optimum conditions for these crystals are worked out. The stoichiometric composition of the grown crystals is determined using EDAX/AES and CH analysis. The grown crystals are characterized by X-ray diffraction, FTIR and Uv-Visible spectroscopy. It is established that crystal falls under orthorhombic system and space group P222 with the cell parameters as: a=10.971Å, b=13.125Å and c=5.101Å; α=90.5o, β=γ=90°. The morphology of the crystals as revealed by SEM is illustrated. Crystallite size, micro strain, dislocation density and distortion parameters are calculated from the powder XRD results of the crystal. UV–vis spectroscopy shows indirect allowed transition with an optical band gap of~4.83eV. The crystals are also shown to have high transmittance in the entire visible region. Dependence of dielectric constant, dielectric loss and conductivity on frequency of the applied ac field is analyzed. The frequency-dependent real part of the complex ac conductivity is found to follow the universal dielectric response: σac (ω)~ωs. The trend in the variation of frequency exponent with frequency corroborates the fact that correlated barrier hopping is the dominant charge-transport mechanism in the present system.

Crystal structure, morphological, optical and electrical investigations of Oxypeucedanin micro crystals: an isolated compound from a plant

The structural, optical and dielectric properties of micro-crystals of Oxypeucedanin (C16H14O5), isolated from ethyl acetate extract of the roots of the plant Prangos pabularia is presented. These isolated crystals were characterized by using X-ray diffraction (XRD), scanning electron microscopy, optical microscope, ultraviolet (UV)–visible spectroscopy and dielectric spectroscopy techniques. XRD of these crystals confirm that they have monoclinic structure (space group = P21/c). The frequency-dependent real part of the complex ac conductivity was found to follow the universal dielectric response: σac (ω)–ω s . The trend in the variation of the frequency exponent (s) with frequency (ω) supports the fact that correlated barrier hopping is the dominant charge transport mechanism for this compound. From the UV spectroscopy, the optical band gap (E g ) has been found to be around 3.76 eV and is following indirect allowed transition. The observed optical and electric properties predict these crystals as an open choice for modern flexible electronics.

Structural characterization, electrical and dielectric relaxations in Dy-doped zirconia

Optically active 2mol% dysprosium doped zirconia, Zr0.98Dy0.02O2, was synthesised by conventional solid state route. Rietveld analysis of XRD data of this composition revealed that monoclinic phase dominates at room temperature. SEM micrograph depicted the presence of spherical grains with average grain size of 200nm. The FTIR spectrum of this system indicated a homogeneous distribution of Dy particles among the zirconia structure. The Raman and UV spectra of the system showed various peaks corresponding to different phases present in the system. The electrical conductivity and dielectric properties of this system were measured in temperature range 573–1073K and in the frequency range 20Hz–1MHz. The real part of conductivity spectra was found to follow Jonscher power law. The temperature dependent total conductivity was found to show three different regions, with different activation energy in the entire temperature range. The scaling behaviour of the conductivity spectra also confirms the observed temperature dependence. The temperature and frequency dependence of real part of permittivity and tangent loss indicated the presence of orientational and space charge polarization processes in the system.

Temperature and frequency dependences of real part of impedance in the FeCoZr-doped PZT nanogranular composites

The study of carrier transport in granular nanocomposite films consisting of ferromagnetic Fe45Co45Zr10 alloy nanoparticles embedded in a low-conductive Pb81Sr4(Na5Bi5)15(Zr57.5Ti42.5)O3 matrix has been performed. Real part of the impedance of the nanocomposites was studied as a function of composition, temperature and frequency, for the samples with 25 ≤ x ≤ 80 at.%, deposited by ion-beam sputtering in Ar + O2 gas mixture at the oxygen pressures of $P_{O_2 } $ = 2.4·10−3 and 3.7·10−3 Pa. It has been proved that approaching the threshold concentration, x C in the FeCoZr alloy resulted in the transition from high-ohmic to low-ohmic state with the x increase, without the change of hopping mechanism of carrier transport.

The transition from ferromagnet to cluster-glass in La1−xTbxMn1/2Sc1/2O3

We report the structural and magnetic properties of the La1−xTbxMn1/2Sc1/2O3 series. LaMn1/2Sc1/2O3 shows a long range ferromagnetic ordering in agreement with a fully polarized Mn-sublattice. The substitution of La with Tb produces structural changes which affect the magnetic properties. This substitution leads to a contraction in the unit cell volume that mainly reduces the M–O–M bond angle (M = Mn, Sc). The bending of this angle decreases the Mn–O–Mn superexchange interaction and enhances the competition between nearest-neighbour and next-nearest-neighbour interactions. Accordingly, the magnetic ground state changes from ferromagnetic to a glassy magnetic state. Large thermal irreversibility between zero-field-cooled and field-cooled conditions is observed for all samples. The study of the dynamic magnetic properties has been performed using the frequency dependent real part of the ac magnetic susceptibility. The use of both the Vogel–Fulcher law and the conventional critical slowing down law yields similarly good accuracies in the fits. The relaxation times obtained from both laws concur with the existence of a cluster-glass for x ≥ 0.5 samples.

Synthesis, magnetic, optical, and electrical transport properties of the nanocomposites of polyaniline with some rare earth chlorides

Nanocomposites of polyaniline with some rare earth chlorides like neodymium chloride, europium chloride, and dysprosium chloride were synthesized by chemical oxidative polymerization of aniline. Morphological, structural, thermal, optical, magnetic, and electrical properties of the samples were characterized by scanning electron microscope, x-ray diffractometer, Fourier transform infrared spectrometer, thermogravimetric analyzer, differentiating scanning calorimeter, optical absorption spectroscopy, room temperature magnetic susceptibility measurement, and low temperature electrical transport measurement. Nanocomposites were thermally more stable than pure polyaniline and they were more crystalline than pure polyaniline. Magnetic susceptibilities of the samples were field dependent. Three-dimensional variable range hopping charge transport mechanism was followed by the samples. The dc magnetoconductivity of the composites can be explained in terms of forward interference effect and wave function shrinkage effect. dc conductivity of Europium chloride-polyaniline composite shows a transition from forward interference model to wave function shrinkage model. Ac conductivity of the investigated samples can be explained in terms of correlated barrier hopping model. Permittivity of the sample is dominated by the grain boundary behavior in low frequency and by the grain phase in the high frequency and frequency dependence of real part of impedance is explained by Maxwell–Wagner capacitor model.

Spin-glass behavior in ferromagnetic Fe[Fe(CN)6]⋅xH2O nanoparticles

The magnetic properties of the nanoparticles of hexacyanometallate based molecular magnetic compound Fe[Fe(CN)6]⋅xH2O have been investigated using dc magnetization and the frequency dependent real part of ac susceptibility χac′ techniques. The magnetic properties of these nanoparticles with an average particle diameter ∼50 nm have been compared with that of the bulk polycrystalline sample. A reduction in the Curie temperature as well as low coercivity and remanent magnetization have been found for the nanoparticles compared to those for the bulk polycrystalline sample. The maximum magnetization for the nanoparticles is also found to be smaller as compared to that for the bulk polycrystalline sample. For the nanoparticles, the peak temperature Tp in the χac′ vs T curve has been found to shift to a higher temperature with increasing frequency and this frequency dependence is described well by a power law. The fitting yields the relaxation time constant τ0=1.3×10−13 s, the freezing temperature Tg=10.63 K, and the critical exponent zν=5.5±0.3. For the bulk polycrystalline sample, no such shift in the ac susceptibility peak temperature has been observed. The time dependence of the thermoremanent magnetization has been observed only for the nanoparticles, which could be represented well with a stretched exponential law. The observed magnetic behavior of the nanocrystalline sample indicates the presence of disordered spins (leading to a spin-glass behavior) in these ferromagnetically ordered particles.

Dielectrophoresis of DNA: Time- and Frequency-Dependent Collections on Microelectrodes<tex>$^dagger$</tex>

This paper reports measurements that characterize the collection of DNA onto interdigitated microelectrodes by high-frequency dielectrophoresis. Measurements of time-dependent collection of 12 kilobase pair plasmid DNA onto microelectrodes by dielectrophoresis show significant reduction in the response as the frequency increases from 100 kHz to 20 MHz. Collection time profiles are quantitatively measured using fluorescence microscopy over the range 100 kHz to 5 MHz and are represented in terms of two parameters: the initial dielectrophoretic collection rate, and the initial to steady-state collection transition. Measured values for both parameters are consistent with trends in the frequency-dependent real part of the effective polarizability measured for the same plasmid DNA using dielectric spectroscopy. The experimentally measured parameters are qualitatively compared with trends predicted by theory that takes into account dielectrophoretic particle movement and diffusion. The differences between experiment and theory are discussed with suggested improvements to theoretical models, for example, including the effects of electrohydrodynamically driven fluid motion.

Dynamics of spin clusters in amorphousFe2O3

The dynamic magnetic properties of powdered x-ray amorphous Fe 2 O 3 sample with mean particle size ~12.0 nm (σ=± 0.36) have been investigated using frequency dependent real part of ac susceptibility Xac' and time dependent dc magnetization studies. A frequency-dependent peak was observed in Xac' vs T measurements. The frequency dependence of the T p , the temperature at which the maximum in the ac susceptibility curve is observed, is described well by the Vogel-Fulcher law as well as the power law. The fitting with the Vogel-Fulcher law yields the relaxation time constant T 0 =5× 10 -8 s and the interaction parameter T 0 =34.0± 0.2 K. Whereas, the fitting with the conventional critical slowing-down law, the power law, yields T 0 =3.5× 10 -11 s, the freezing temperature τ g =35.1± 0.1 K and critical exponent zv=5.3± 0.4. The value of τ 0 =5× 10 -8 s obtained from the Vogel-Fulcher law fit gives an evidence of the presence of interacting spin clusters in our system and that obtained from the power law fit suggests the freezing of these spin clusters at ~35.1 K. Time dependence of the thermoremanent magnetization (TRM), represented well by the stretched exponential function, supports the presence of relaxing spin clusters with no unique relaxation time. The TRM is found to scale according to T log 10 (t/τ 0 ), with T 0 =3.5× 10 -11 s.