Value Of Internal Field Research Articles

Structural, Physical, Theoretical and Spectroscopic Investigations of Mixed‐Valent Eu2Ni8Si3 and Its Structural Anti‐Type Sr2Pt3Al8

AbstractEu2Ni8Si3 and its anti‐type representative Sr2Pt3Al8 were synthesized from the elements. They crystallize in the tetragonal crystal system with space group P42/nmc and with lattice parameters of a=997.9(1) and c=747.6(1) pm (Eu2Ni8Si3) as well as a=1082.9(2) and c=823.3(2) pm (Sr2Pt3Al8). Both compounds were investigated via single crystal X‐ray diffraction, indicating slight Si/Ni mixing for the silicide. Sr2Pt3Al8 exhibits a temperature independent magnetic susceptibility, suggesting superimposed dia‐ and Pauli‐paramagnetic contributions. The independent Al and Pt sites of the platinide were further characterized by 27Al and 195Pt solid‐state NMR spectroscopy, which were assigned with the help of electronic structure calculations. ICOHP calculations and Bader charges were used to analyze the bonding situation. Eu2Ni8Si3 in contrast is paramagnetic with a ferromagnetic transition at TC=46.9(2) K and exhibits an effective magnetic moment of μeff=6.61(1) μB per Eu atom. The latter is in line with an intermediate valence that was further proven by 151Eu Mößbauer spectroscopic investigations. At 300 K, the refined Eu2+/Eu3+ ratios are 60 %/40 %, at 78 K 62 % and 38 % (Eu2+/Eu3+) are observed, being in line with the ratio deduced from the magnetic susceptibility. Finally, at 6 K a ratio of 68 % Eu2+ and 32 % Eu3+ was observed. Below the Curie temperature, the Eu2+ signal shows a full magnetic hyperfine splitting, with an internal magnetic field value of B0=28.4 T.

Induced electrostatic fields on optical gain in a polar quantum dot

Exciton binding energy, optical transition energy, emission wavelength and electrostatic built-in internal fields are studied with the various magnesium alloy compositions and the dot radii in ZnO/Zn1−xMgxO cylindrical quantum dot. The induced built-in internal electric fields are found to be around the order of MV/cm and the obtained values range from 0.41 to 1.5 MV/cm. The obtained values of internal fields are used to find the optical gain and it is found to be larger than GaN materials. The effects of adding Mg composition, the built-in internal fields and the spatial confinement have great influence on the exciton states.

Enhanced ferroelectric properties and crystal structure of TGS0.74P0.26 single crystals

The single crystals of triglycine sulpho phosphate (TGS0.74P0.26) were grown by slow evaporation method with 0.6 M of phosphoric acid in the growth solution. The crystal structure of the grown crystal is refined with 0.74 sulphur and 0.26 phosphorous occupancy with the formula of TGS0.74P0.26 using single crystal x-ray diffraction analysis. The dielectric, ferroelectric and pyroelectric properties of the TGS0.74P0.26 single crystals were investigated. The coercive field (Ec) and internal bias field (Ib) values are 8 kV/cm and 1.4 kV/cm, respectively and these values are found increased compared to the pure TGS and also low dielectric loss was observed. The pyroelectric coefficient of TGS0.74P0.26 crystal is 0.023 µC/cm2/°C and the pyroelectric figure of merit for current, voltage and detectivity are Fi = 105 pm/V, Fv = 0.055 m2/C and Fd = 22 µPa−1/2 respectively. Enhanced ferroelectric properties were observed when compared to the pure TGS crystals. The figure of merit for the detectivity (Fd) shows higher values when compared with that of the well-known materials such as PZT (11.5 µPa−1/2) and other lead free materials such as SBN (7.21 µPa−1/2) and CSBN (12 µPa−1/2). With these excellent properties, the TGS0.74P0.26 single crystal can be a promising material for the pyroelectric detector applications.

Infrared photoreflectance of InSb-based two-dimensional nanostructures

An IR photoreflectance (PR) study of two-dimensional nanoheterostructures based on InSb was performed by using Fourier-transform IR photomodulation spectroscopy. The studied structures, including InSb/AlxIn1−xSb quantum wells (QWs) and type-II nanostructures with monolayer-thick InSb insertions within bulk InAs layers, were grown via molecular beam epitaxy on GaAs (001) and InAs (001) substrates, respectively. The PR spectra of the InSb/AlxIn1−xSb heterostructures exhibited a series of signals from the QWs, including the spin-orbit split band, as well as the AlxIn1−xSb layers, which enabled control of the barrier height and composition. A comparison of the experimental results with calculations made by the effective mass approximation technique was used to identify several optical transitions observed within the QWs, including those related to the excited states not visible in the photoluminescence spectra. For the InSb/InAs monolayer nanostructures, an analysis of the PR spectra features allowed for determination of their energy spectrum, internal electric field value, and localization energy of holes up to room temperature.

3D long-range magnetic ordering in (C2H5NH3)2CuCl4 compound revealed by internal magnetic field from muon spin rotation and first principal calculation

We report the results of muon spin rotation measurement that reveal a 3D long-range magnetic ordering in the inorganic CuCl4 layers separated by (C2H5NH3) organic ligands. The measured temperature dependent internal magnetic field down to 300 mK shows the 3D long-range magnetic ordering with magnetic transition at the Néel temperature of TN = 10.06 K and critical exponent of 0.31. The ground state internal dipole field is calculated by considering muon zero-point motion effect at the muon stop position determined by first principal DFT calculation, in combination with magnetization density calculated on the basis of a specific magnetic structure model. The calculated result is shown in good agreement with the measured value of internal field at the ground state temperature, thereby justifying the magnetic structure model adopted for this system and explaining the 3D nature of magnetic ordering. This model may therefore be applicable to the study of other magnetic materials.

Electron-beam charging of dielectrics preirradiated with moderate-energy ions and electrons

The effects of charging of dielectric targets irradiated with moderate-energy electrons in a scanning electron microscope are examined. Considerable differences in the kinetics of charging of the reference samples and the samples preirradiated with ions and electrons are reported. These differences are attributed to the processes of radiation-induced defect formation in Al2O3 (sapphire) and SiO2 that are, however, dissimilar in nature. The contributions of surface structure modification and changes in the electrophysical parameters of the surface (specifically, the charge spreading effect) are revealed. Critical doses of irradiation with Ar+ ions and electrons inducing active defect formation in dielectric targets and critical values of internal charge fields producing a significant contribution to the temporal parameters of Al2O3 and SiO2 charging are determined.

Disordering effect on electronic mechanism of thermal destruction of GeE'-centers in glassy GeO2

Thermal destruction processes of paramagnetic GeE'-centers in two different types of GeO2 glass have been studied. By investigating the temperature dependence of the electron emission typical of these processes, we found that the electron energy is sensitive to the structural disorder. The obtained results obtained were interpreted within the framework of modified Perel-Karpus model for the ionization of local centers in electric field. It was found that the electron-phonon coupling for GeE'-centers is a parameter that depends on the structure. We have shown that relatively weak electron-phonon coupling and low values of internal electric field are characteristic features of glassy GeO2 with high degree of continual disorder. Thus, the structural disorder, that at least depends on the synthesis conditions, can be considered as an effective tool governing the fundamental and functional properties of the glass.

Internal field and self-polarization in lead zirconate titanate thin films

It has been shown that 300-nm-thick polycrystalline films of lead zirconate titanate (PZT), the compositions of which correspond to the region of morphotropic phase boundary, undergo anomalous changes in the composition and the microstructure as the annealing temperature increases. This causes substantial variations in the dielectric and piezoelectric responses, the spontaneous polarization and the internal field value. It has been demonstrated that the self-polarization varies nonmonotonically with increasing the internal field and is characterized by a threshold effect. The role of excess lead oxide and its influence on the formation of the self-polarization and the change in the effective permittivity of the ferroelectric layer has been discussed.

Distribution of Internal Random Fields in the Vicinity of Diffused Ferroelectric Phase Transition in K0.81(NH4)0.19H2PO4 Single Crystal

The analysis of P(E) hysteresis loops obtained near diffused ferroelectric phase transition in ferro-antiferroelectric K0.81(NH4)0.19H2PO4 mixed crystal gave the possibility to estimate the shape of random fields distribution function. It was found that this function is well approximated by a sum of two Gaussians with positive and negative average values of internal field. Supposition is made that double-peak distribution of random fields is due to charges (probably Takagi defects) on boundaries between polar regions and nonpolar matrix.

Study of magnetization reversal in LaCr1−xFexO3 compounds

Single phase samples of LaCr1−xFexO3 for x = 0–0.50 were prepared and their magnetic properties were studied to understand the magnetization reversal. Magnetization reversal was observed even for 5 at. % of Fe doping and it persisted up to x = 0.15. Ferromagnetic like behavior with a large coercive field of the order of 0.5 T was observed in the intermediate composition range of x = 0.20–0.40. However, for x = 0.45 and 0.50, magnetization reversal was again observed. Magnetization reversal was studied in detail by carrying out field cooled magnetization measurements for different applied magnetic fields. The mechanism of magnetization reversal for low Fe concentrations (x = 0.05–0.15) is basically due to the paramagnetic behavior of doped Fe ions under the influence of negative internal field arising from the antiferromagnetically ordered Cr3+ ions. The value of maximum negative internal field was estimated to be −3.5 kOe. On the other hand, the mechanism of magnetization reversal for x = 0.45 and 0.50 samples is found to be quite different. They could be quantitatively analyzed based on the model, where the competition between single ion anisotropy and Dzyaloshinsky–Moriya interaction is taken into account.

Iron oxide nanoparticles in NaA zeolite cages

Zeolite NaA samples with varying concentration of Fe3+ ions have been prepared by wet chemical method. Based on powder X-ray diffraction, 29Si and 27Al MAS NMR and Fe3+ EPR investigations, the formation of nano-sized ferric oxide particles inside the larger α-cages of zeolite NaA has been established. Both Mössbauer effect and magnetization measurements carried out down to 4.5 K established the superparamagnetic behaviour of these Fe2O3 particles with a blocking temperature of ≈20 K, where the magnetization values showed deviation for the zero field cooled and field cooled samples and the appearance of a very narrow magnetic hysteresis loop below this temperature. For all Fe3+ containing samples the room temperature Mössbauer spectrum is a broad quadrupole doublet with chemical shift, δ ≈ 0.33 mm/s and quadrupole splitting, ΔEq ≈ 0.68 mm/s. Variable temperature 57Fe Mössbauer effect measurements exhibited magnetic features below the blocking temperature and at 4.5 K, the observed spectrum is a broad magnetic sextet characterized by an internal hyperfine field value of ≈504 kOe along with a very weak central superparamagnetic quadrupole doublet.

Orientation dependence of electronic structure and optical gain of (11N)-oriented III-V-N quantum wells

A ten-band k·p Hamiltonian for III-V-N dilute nitride semiconductor quantum wells (QWs) grown on the (11N)-oriented substrates is presented. The energy dispersion curves, optical transition matrix elements, internal piezoelectric field, and optical gain of InGaAsN/GaAs on the (110), (111), (113), and (11∞)-oriented substrates are investigated including band-anti-crossing, strain, and piezoelectric field effects. The band structures and optical gain are sensitive to the substrate orientation. The fundamental transition energy is the largest for the (111)-oriented QW and the smallest for (11∞)-oriented QW. The absolute values of internal piezoelectric field in the well and barrier layers reach the maximum for the (111)-QW, and zero for the (110) and (11∞)-oriented QWs. There exists an injection current density turning point. When the injection current density is below the turning point, the (111)-oriented QW has the largest peak gain. At the larger injection current density, the (11∞)-oriented QW has the largest peak gain.

Mössbauer Spectroscopy of Fe-Ni-Nb-B Alloy in Weak Magnetic Field

Amorphous and nanocrystalline (Fe1–xNix)81Nb7B12 (x = 0, 0.25, 0.5, 0.75) alloys were measured by M?ssbauer spectrometry in the weak external magnetic field of 0.5 T. From structural analyses, ferromagnetic bcc-FeNi and fcc-FeNi and paramagnetic (Fe-Ni)23B6 phases were identified in the annealed samples. It was shown that in the external magnetic field the intensities of the 2nd and the 5th lines (A23 parameter) are the most sensitive M?ssbauer parameters. Rather small changes were observed in the values of internal magnetic field. Our results showed that the amorphous precursor is more sensitive to the influence of external magnetic field than the nanocrystalline alloy. All spectra of amorphous precursor showed the increase of A23 parameter and decrease of internal magnetic field values of about 1 T (±0.5 T) under influence of external magnetic field. In the case of nanocrystalline samples the tendency for the values of internal magnetic field is similar but the effect is not so pronounced. The measurements confirmed that even weak external magnetic field affected orientation of the net magnetic moments. Our results indicate that effect of the external magnetic field is stronger in the case of amorphous samples due to their disordered structure.

Porphyrins as Detectors of Internal Electric Field in Heme Proteins

Heme proteins display remarkable specificity in discriminating between diatomic ligands that are similar in structure, such as dioxygen, carbon monoxide, and nitric oxide. However, the factors affecting their ability to accomplish this are not known with certainty. We are exploring the influence of the internal electric field produced by the protein matrix on the binding affinities at its heme active site. Generally, the “structure determines function” paradigm is employed to formulate a steric mechanism. Here we present a unique “electrostatic structure” approach to this question, using myoglobin as a model protein. The porphyrin residing at the active site provides a spectroscopic report of the electrostatic environment. In a first step, the atoms comprising the protein matrix are treated as point charges, and their individual electric field contributions are calculated using Coulomb's Law and summed at the heme site. Point mutations surrounding the active site are introduced and the effect on the internal electric field value is noted. In addition, the fluctuation of the field due to natural breathing motions of the protein in solution is modeled, posing the possibility of alternating fields playing a role in ligand discrimination. Experimentally, hole-burning spectroscopy provides a way to extract internal electric field values. This technique uses the Stark effect which is an application of an external electric field perturb the energy of the porphyrin detector. Preliminary results are determined to be quite significant - on the order of megavolts per centimeter. Finally, an excited state analysis of our detector porphyrins using Gaussian09 is presented, including transition dipole moment and state energy values. These values play an important role in the interpretation of spectroscopic data.

Fatigue of lead titanate and lead zirconate titanate thin films with various values of coercive and internal bias fields

The dependence of the residue polarization on the number of switching cycles in thin ferroelectric films with various thicknesses and on substrates differing by values of the coercive and internal bias fields was experimentally studied. A model describing the features of the fatigue effect in these films is proposed.

Analysis of variation in geomagnetic field of Chinese mainland based on comprehensive model CM4

The fourth generation of comprehensive model of geomagnetic field CM4 is a numerical model in which earths magnetic field can completely be divided into internal and external parts. In this paper we have calculated and plotted the grid values of internal and external fields of Chinese mainland in every 10 a from 1960 to 2000 by combining CM4 with Taylor polynomial model based on all survey data during 1960—2000, and made more efforts to analyze the whole distribution of crustal anomaly field. Results show that internal field value of northern component X decreased with time. The amplitude was about 750 nT in the period from 1960 to 2000, the trend of external field variation increased in 1960, then gradually decreased about 32 nT until 2000. The internal field value of eastern component Y decreased first, then continuously increased, and its intensities decreased about 40 nT from 1960 to 2000. The trend of external field variation was increased, decreased and increased as time went on, and the external field increased up to about 3.8 nT since 1960. The internal field value of vertical component Z was similar to that of Y, its intensities totally increased about 600 nT. The trend of external field variation decreased and then increased with time, amplitude of which was about 4.6 nT. As for anomalous field, the distributions of components X and Z, total intensity F, and horizontal component H were all negative, the intensities of X and H decreased with longitude while those of Z and F increased. The distributions of Y and declination D were very similar and both of them had positive values in west-central region of China. Their intensities decreased with longitude. The distribution of inclination I was mainly negative, and its intensity increased with longitude.

Internal field effects on the lasing characteristics of InGaN/GaN quantum well lasers

A theoretical investigation of InxGa1−xN/GaN single quantum well lasers with x in the range 0.05⩽x⩽0.3 is carried out via self-consistent Schrödinger–Poisson calculations in the effective mass approximation in order to quantify the adverse effects of the internal electric field on the lasing characteristics of these heterostructures. We find a nonzero optimum internal field value that minimizes the threshold current density Jth, and whose amplitude depends on the quantum well width, In content, and cavity losses. We demonstrate that the complete elimination of the internal field in In0.2Ga0.8N/GaN blue laser diodes with typical cavity losses should result in a decrease in Jth by as much as a factor of 4. Furthermore, for a wide range of In contents and cavity losses, we find that the optimum well width that minimizes Jth ranges between 2.5 and 4 nm. Finally, we show that the longest lasing wavelength that can be achieved from an InGaN/GaN quantum well laser is in the range of 480–500 nm depending on cavity losses.

Electronic states and the Stark shift in narrow band InSb quantum spherical layer

Electron states and the Stark shift in narrow band quantum spherical InSb layers are considered in frameworks of Kane model with respect to non-parabolicity of the dispersion law. An exact analytical solution of the corresponding Klein–Gordon equation for the model of impenetrable walls is obtained, and in addition to that a transcendent equation for determination of electron energetic spectrum is derived as well. The influence of the electric field is considered in frames of the perturbation theory, it is also shown that the value of the Stark shift is proportional to the second power degree of the field. Dependences of the electron energy versus the internal field values, external radius, and electric field are presented. The obtained results are compared with results obtained for the case with standard dispersion.

Effective field investigation in arrays of polycrystalline ferromagnetic nanowires

Nanowire arrays have been used as prototypes to investigate basic issues such as size effect, shape anisotropy, and dipolar interaction on the magnetic properties. Under ideal conditions, the nanowires are approximated as perfect long cylinders. Here, coherent rotation as the magnetization reversal mode cannot completely interpret the experimental data. The internal magnetic field value, in nanowire arrays, decreases due to the wire inhomogeneities and the dipolar interaction between the wires. Realistic models must account for the imperfections due to the fabrication process. Instead of it, in this work, a modified ellipsoid-chain array model is proposed to describe magnetization reversal in nanowire arrays. From the angular dependence of the ferromagnetic resonance field presented previously in the literature and from our proper results here, we present experimental confirmations to the model.

Photoreflectance study of GaAsSb∕InP heterostructures

Photoreflectance (PR) spectroscopy experiments are reported on GaAsSb∕InP heterostructures. The GaAsSb PR spectrum is studied as a function of temperature and the transition nature is shown to change from Franz-Keldysh oscillations (FKO) at room temperature to a third derivative functional form (TDFF) line shape at low temperatures. Combining both analysis (FKO and TDFF) in the same sample, we derive internal electric field and phase values of the PR transition, together with accurate values for alloy band gap energy on the whole temperature range. Type II interface recombination is shown to reduce photovoltage effects as a function of temperature. FKO are found to appear for a very weak electric field (8kV∕cm) in the GaAsSb∕InP heterostructure, contrary to usual observations. This point is discussed in relation with the broadening parameter of the transition.