Impurity Clusters Research Articles

Electronic structures and impurity cluster features in Mg-Zn-Y alloys with a synchronized long-period stacking ordered phase

Photoemission and inverse-photoemission spectroscopy (PES and IPES) measurements were carried out on polycrystalline Mg97Zn1Y2, Mg85Zn6Y9, and Mg75Zn10Y15 alloys with a synchronized long-period stacking ordered phase together with non-doped Mg to investigate the valence- and conduction-band electronic structures, respectively. It was found from the valence-band PES spectra that the non-doped Mg includes at most 25% of an intrinsic oxidation portion. With increasing impurity concentration, the valence-band density of states (DOS) very slightly changes, while the conduction-band DOS drastically varies. The contributions of the impurity elements are clarified using a density functional theory calculation for the Mg85Zn6Y9 alloy. The core-level PES measurements were also performed at the Mg 2s, 2p, Zn 3p, 3d, and Y 3d levels to study the chemical natures of the corresponding elements. The Y 3d core spectrum exhibits mostly three doublets in Mg97Zn1Y2, while mostly only one in the heavily doped alloys, reflecting a coexistence of three different sites of the Y atoms; ones in the Zn6Y8 L12 clusters, ones in their fragments, and isolated ones in Mg97Zn1Y2.

Defect characterization of UMG mc-Si solar cells using LBIC and luminescence imaging techniques

ABSTRACTUpgraded metallurgical-grade silicon (UMG Si) solar cells with different ranges of efficiencies were characterized through electroluminescence imaging (ELi) and light-beam induced current (LBIC) measurements. The results showed a good correlation between the EL intensity and the efficiency of the solar cells. ELi images gave a bright contrast at the defects, grain boundaries and intragrain defects, and dark contrast inside the grain bodies. Metallic impurities are much more present in some cells due to the directional solidification of the Si ingot. Local short-circuit current mapping with LBIC measurements revealed a bright zone in the neighborhoods of the defects due to the depletion of impurities. Internal quantum efficiencies (IQE) and effective diffusion lengths (Leff) were calculated using different excitation wavelengths. High resolution LBIC measurements revealed micrometric clusters of impurities around intragrain defects.

Effect of the Lattice and Spin-Phonon Contributions on the Temperature Behavior of the Ground State Splitting of Gd3+ in SrMoO4

The temperature behavior of the EPR spectra of the Gd3+ impurity center in single crystals of SrMoO4 in the temperature range T = 99–375 K is studied. The analysis of the temperature dependences of the spin Hamiltonian b 2 0 (T) = b2(F) + b2(L) and P 2 0 (T) = P2(F) + P2(L) (for Gd157) describing the EPR spectrum and contributing to the Gd3+ ground state splitting ΔE is carried out. In terms of the Newman model, the values of b2(L) and P2(L) depending on the thermal expansion of the static lattice are estimated; the b2(F) and P2(F) spin-phonon contributions determined by the lattice ion oscillations are separated. The analysis of b 2 0 (T) and P 2 0 (T) is evidence of the positive contribution of the spin-phonon interaction; the model of the local oscillations of the impurity cluster with close frequencies ω describes well the temperature behavior of b2(F) and P2(F).

Phonon excitations in a single crystal Mg85Zn6Y9 with a synchronized long-period stacking ordered phase

Inelastic X-ray scattering (IXS) experiments were carried out at room temperature at BL35XU of the SPring-8 on a single crystal Mg85Zn6Y9 alloy with a synchronized long-period stacking ordered (LPSO) phase of purely 18R type. The IXS measurements were carried out in both the longitudinal and transverse directions along the Γ-K and Γ-M (momentum vector q⊥ lattice axis c), and Γ-A (q∥c) directions for the energy transfers ω below 40 meV. Peaks arising from longitudinal and transverse acoustic (LA and TA) modes were clearly observed in the IXS spectra. The dispersion relations of the LA and TA modes in the Mg85Zn6Y9 LPSO alloy are mostly similar to those of previous INS data on single crystal pure Mg. New impurity-derived dispersion-less excitation modes are observed along all of the crystal directions at about 5, 10, and 17 meV. Partial vibrational density of states (vDOS) was obtained using an abinitio molecular dynamics simulation, and it was found that the ∼10 meV peak mainly originates from the phonon excitations inside the L12-type Zn6Y8 clusters and the ∼17 meV peak from those between the cluster and the host Mg atoms, and a small shoulder at 5 meV consists of motions of all the elements. These findings clarify that the dispersion-less modes mainly correspond to localized stretching and twisting modes around the impurity clusters in the LPSO alloys.

Влияние решеточного и спин-фононного вкладов на температурное поведение расщепления основного состояния Gd-=SUP=-3+-=/SUP=- в SrMoO-=SUB=-4-=/SUB=-

AbstractThe temperature behavior of the EPR spectra of the Gd^3+ impurity center in single crystals of SrMoO_4 in the temperature range T = 99–375 K is studied. The analysis of the temperature dependences of the spin Hamiltonian b _2 ^0 ( T ) = b _2( F ) + b _2( L ) and P _2 ^0 ( T ) = P _2( F ) + P _2( L ) (for Gd^157) describing the EPR spectrum and contributing to the Gd^3+ ground state splitting Δ E is carried out. In terms of the Newman model, the values of b _2( L ) and P _2( L ) depending on the thermal expansion of the static lattice are estimated; the b _2( F ) and P _2( F ) spin-phonon contributions determined by the lattice ion oscillations are separated. The analysis of b _2 ^0 ( T ) and P _2 ^0 ( T ) is evidence of the positive contribution of the spin-phonon interaction; the model of the local oscillations of the impurity cluster with close frequencies ω describes well the temperature behavior of b _2( F ) and P _2( F ).

An Intra and Inter Service Dependent Security Aware Packet Scheduling in Mobile Ad-Hoc Networks

An improved security-aware packet scheduling algorithm (ISAPS) was proposed for efficient packet scheduling and obtaining high level security irrespective of the system workload. In ISAPS, packets from multiple wireless modes are assumed as independent. Many service oriented packets in Mobile Ad-hoc Networks (MANET) are dependent one another, scheduling packets based on service dependency improves overall performance of networks In this paper, Intra service dependent security aware packet scheduling algorithm (ISDSPS) is proposed to enhance scheduling performance by clustering the packets. However, some impurity clusters occurred during clustering. To overcome this issue an intra and inter service dependent security aware packet scheduling algorithm (IISDSPS) is proposed for identifying similar type of packets among clusters. IISDSPS maximizes the security levels as well as schedulability performance by providing priority to packets based on intra and inter service dependency. Simulation results proved that IISDSPS algorithm outperforms than existing algorithms in terms of guarantee ratio, average security level, packet delivery ratio and End to End delay.

Electron paramagnetic resonance study of Ce doped partially stabilized ZrO2 crystals

ZrO2 (PSZ) solid solutions crystals stabilized with yttrium and cerium oxides have been studied using electron paramagnetic resonance (EPR) in the X and Q ranges. Zr3+ have been observed centers in the as-annealed ZrO2 crystals stabilized only by yttrium oxide (2.8mol% Y2O3). Another type of paramagnetic-O-centers appear as a result of CeO2 addition to ZrO2 crystals along with yttrium oxide. To estimate the concentration of Ce3+ ions in PZS crystals, we recorded the EPR spectra in the presence of a reference at 7K. Paramagnetic Ce3+ ions have been identified and their relative amount in the PSZ crystals before and after high-temperature heat treatment has been assessed. Annealing in air leads decreases the concentration of Ce3+ ions for all the test compositions and changes the color of the crystals from red to white. After annealing of the sample 2.0Y0.8Ce3Zr, the amount of paramagnetic Ce3+ ions decreased approximately twofold. Paramagnetic centers from Ce3+ have not been detected in the specimen with a low cerium content of 0.1mol% after annealing which indicates the complete transition of Ce3+ to the Ce4+ state. We show that the forming cerium paramagnetic centers are bound by strong exchange interactions. No angular dependence of the EPR lines of the paramagnetic Ce3+ cations on the applied external magnetic field has been observed. Probable origin of the absence of angular dependence is that the impurity rare-earth ions are located close to one another, forming impurity clusters with an effective spin of Seff=1/2.

The effects of acid pretreatment and surface stresses on the evolution of impurity clusters and graphene formation on Cu foil

In this study, the effects of acid pretreatment together with the surface stresses are studied systematically to control the densities of Si and Ca based impurities on the surface and to understand their effects on the final graphene morphology formed on Cu foil via CVD. In order to investigate the surface coverage and morphology of graphene, in addition to scanning electron microscope (SEM) studies, graphene grown on Cu foils are oxidized in air to reveal the graphene domains under the optical microscope (OM). Graphene formation is also confirmed using Raman spectroscopy directly on the Cu foil. Our studies show that the coverage and the morphology of the final graphene are strongly affected by the acid pretreatment because it removes the surface oxide layer and surface Ca impurities inherently present on the copper foil. Furthermore, it is found that the surface elastic stresses have also direct influence on the sizes and distribution of the surface impurities which affect the final morphology of graphene.

DFT-based Monte Carlo Simulations of Impurity Clustering at CeO2(111)

The interplay between energetics and entropy in determining defect distributions at ceria(111) is studied using a combination of DFT+U and lattice Monte Carlo simulations. Our main example is fluorine impurities, although we also present preliminary results for surface hydroxyl groups. A simple classical force-field model was constructed from a training set of DFT+U data for all symmetrically inequivalent (F−)n(Ce3+)n nearest-neighbor clusters with n = 2 or 3. Our fitted model reproduces the DFT energies well. We find that for an impurity concentration of 15% at 600 K, straight and hooked linear fluorine clusters are surprisingly abundant, with similarities to experimental STM images from the literature. We also find that with increasing temperature the fluorine cluster sizes show a transition from being governed by an attractive potential to being governed by a repulsive potential as a consequence of the increasing importance of the entropy of the Ce3+ ions. The distributions of surface hydroxyl groups a...

Cluster size convergence of the density matrix embedding theory and its dynamical cluster formulation: A study with an auxiliary-field quantum Monte Carlo solver

We investigate the cluster size convergence of the energy and observables using two forms of density matrix embedding theory (DMET): the original cluster form (CDMET) and a new formulation motivated by the dynamical cluster approximation (DCA-DMET). Both methods are applied to the half-filled one- and two-dimensional Hubbard models using a sign-problem free auxiliary-field quantum Monte Carlo (AFQMC) impurity solver, which allows for the treatment of large impurity clusters of up to 100 sites. While CDMET is more accurate at smaller impurity cluster sizes, DCA- DMET exhibits faster asymptotic convergence towards the thermodynamic limit (TDL). We use our two formulations to produce new accurate estimates for the energy and local moment of the two-dimensional Hubbard model for U/t = 2, 4, 6. These results compare favourably with the best data available in literature, and help resolve earlier uncertainties in the moment for U/t = 2.

Investigation of the structural, optical and magnetic properties of Fe doped ZnO thin films coated on glass by sol-gel spin coating method

Fe doped ZnO thin films with varying Fe concentrations say 0, 3, 5, 7, 10, 15, and 20 at% were prepared on the glass substrate by sol-gel spin coating technique and their structural, morphological, optical and magnetic properties were investigated. XRD results indicated that the prepared samples were single crystalline films with hexagonal wurtzite structure. Secondary phases or impurity clusters were not observed for the samples. SEM images of the prepared samples showed clear fibrous structures. AFM analysis revealed the increase of film roughness for the Fe doped films. Tauc's plot was used to find the band gap of the prepared samples which was found to increase with the increase of Fe concentration and thus showed a redshift. PL spectra showed NBE, violet, blue and green emissions for the undoped and Fe doped films. Magnetic measurements confirmed a clear ferromagnetic behavior for all the prepared samples.

Equilibrium Grain Boundary Segregation and Clustering of Impurities in Colloidal Polycrystalline Monolayers.

We investigate the segregation of impurities to grain boundaries in colloidal polycrystalline monolayers using video microscopy. A model colloidal alloy is prepared by embedding large spherical impurities in a polycrystalline monolayer of small host colloidal hard spheres, which stops grain growth at a finite grain size. The size ratio between the impurities and the host particles determines whether they behave as interstitial or substitutional impurities in the bulk crystal, akin to those in real alloys. We find that the partitioning of impurities between the grains and the grain boundaries is in very good agreement with the Langmuir-McLean adsorption model for equilibrium grain boundary segregation. This enables the direct measurement of the free energy of adsorption for the two types of impurities. Near saturation, we characterize the spatial distribution of the adsorbed impurities and find that it strongly depends on their interstitial or substitutional nature. This is because the relative importance of clustering and mixing due to nonadditivity is determined by geometrical constraints imposed by the crystalline host lattice.

Electrophobic interaction induced impurity clustering in metals

We introduce the concept of electrophobic interaction, analogous to hydrophobic interaction, for describing the behavior of impurity atoms in a metal, a “solvent of electrons”. We demonstrate that there exists a form of electrophobic interaction between impurities with closed electron shell structure, which governs their dissolution behavior in a metal. Using He, Be and Ar as examples, we predict by first-principles calculations that the electrophobic interaction drives He, Be or Ar to form a close-packed cluster with a clustering energy that follows a universal power-law scaling with the number of atoms (N) dissolved in a free electron gas, as well as W or Al lattice, as Ec ∝ (N2/3−N). This new concept unifies the explanation for a series of experimental observations of close-packed inert-gas bubble formation in metals, and significantly advances our fundamental understanding and capacity to predict the solute behavior of impurities in metals, a useful contribution to be considered in future material design of metals for nuclear, metallurgical, and energy applications.

Magnetization reversal of the transverse domain wall confined between two clusters of magnetic impurities in a ferromagnetic planar nanowire

Numerical simulations have been used to investigate the polarity reversal of the transverse domain wall in rectangular magnetic nanowires and the stabilization of the domain wall position after occurring the polarity reversal. In order to control the wall position we have considered two clusters of magnetic impurities, identical and equidistant from the nanowire width axis. Traps of pinning and blocking for the transverse domain wall can be originated from magnetic impurities, consisting of a local variation of the exchange constant. Under suitable excitation amplitudes it is possible to switch the polarity of the transverse domain wall by applying a nanosecond axial magnetic field pulse in a fast and controllable way.

The effect of double impurity cluster of Ni and Co in TiO2 bulk; a DFT study

The geometrical structure, electronic properties and stability energy of different arrangements of double cluster of Ni-Ni, Co-Co and Ni-Co in rutile and anatase bulk were investigated. Our calculated results showed that Co-Co preferred to state in the nearest distance (nn arrangement) in both anatase and rutile unit cells. While, the Ni-Ni and Ni-Co clusters showed the most stability with the nn form in rutile structure, they preferred the n arrangement (next to the nearest neighbor) in anatas one. These results can be explained by the crystalline structure of rutile that provides the stronger metal-metal interaction by lower distance and also the activity of Co rather than Ni. The band structure results showed a high, moderate and slight decline in the band gap of anatase and rutile in the presence of Co-Co, Ni-Co and Ni-Ni clusters, respectively. An interesting result was shown by altering the direct band gap of pure rutile to the indirect band gap in effect of Co-Co cluster that introduces the Co/rutile structure with an obvious photo-activity. Ni-Ni and Ni-Co impurities show an inverse behavior in anatase phase by changing the indirect band gap of pure anatase to the direct band gap. This may be the main reason to explain the lower photo-efficiency of Ni/TiO2 than pure TiO2 under UV illumination. In the last section we show that the Co impurity energetically prefer to state in TiO2 bulk versus the Ni one by energy of 5 eV.

Osoblyvosti protikannya strumu pry ul'trazvukovomu navantazhenni v syl'no kompensovanykh nyz'koomnykh krystalakh CdTe:Cl

To elucidate the mechanism of influence of ultrasound on the temperature, T, dependence of conductivity (q(T)) in low-resistance CdTe:Cl (NCl ≈ 1024 m−3) single crystals of the n-type, the Hall effect and the relaxation kinetics of q(t) at the ultrasound (fUS ∼ 10 MHz, WUS ∼ 104 W/m2) switching-on and -off have been studied in a temperature interval from 77 to 300 K. A completely reversible dynamical influence of ultrasound is revealed for the first time. It has different characters for the low (LT, T < 180 K) and high (HT, T > 200 K) temperature intervals. Acoustically stimulated changes in the HT region are found to be in-significant: the mobility of charge carriers decreases a little, and long-term processes of q(t) relaxation are not observed. In the LT region, the relative acoustically stimulated changes grow; in particular, the duration of q(t) relaxation processes increases, and they reveal a two-stage character. To explain this phenomenon, the model of a heterogeneous semiconductor containing clusters of impurity defects in vicinities of dislocations is applied. A mechanism is proposed that relates the “instant” increase of q(t) with the acoustically stimulated reduction of the amplitude of fluctuations of the large-scale potential owing to the enlargement of the effective electronic radius of dislocation impurity clusters. Long-term (50–500 s) temperature-dependent relaxation processes are governed by the diffusive reconstruction of the point-defect structure in the cluster bulk, including the transformation of acceptor (V2−CdCl+Te)− complexes into neutral (V2−Cd2Cl+Te)0 ones.

Local compressibilities of Cr3+ octahedral clusters in the natural and synthetic emerald crystals

The local linear compressibilities of Cr3+ octahedral clusters in the natural and synthetic emerald (Be3Al2Si6O18: Cr3+) crystals are investigated by analyzing their pressure-induced R-line shifts. The results show that the local compressibility of Cr3+ cluster in the synthetic emerald is slightly smaller than that in the natural emerald, suggesting that the atom arrangement of Cr3+ impurity clusters in the synthetic emerald is slightly tighter than that in the natural one. The point is supported by the observed cubic field parameters Dq in the two emerald crystals. The local compressibilities of Cr3+ clusters in both the natural and synthetic emeralds are also slightly different from that of the whole host beryl (Be3Al2Si6O18) crystal. The causes are discussed.

Disorder-induced vortex lattice melting in a Bose-Einstein condensate

We study the vortex lattice dynamics in presence of single impurity as well as random impurities or disorder. We show that in presence of a single impurity the vortex lattice gets distorted and the distortion depends on the position of the single impurity with respect to the positions of the vortices in the impurity free Abrikosov vortex lattice and also the strength of the impurity potential. We then show that a new type of giant hole with hidden vortices inside it can be created in the vortex lattice by a cluster of impurities. In presence of random impurity potential or disorder the vortex lattice melts. We show that the vortex lattice also melts in presence of pseudorandom potential generated by the superposition of two optical lattices. The absence of long-range order in the melted vortex lattice is demonstrated from the structure factor profile and the histogram of the distances between each pair of the vortices.

Study of Clustering of Impurities Doped in Si Crystals Depending on Annealing Conditions and Concentration by Using Soft X-ray Photoelectron Spectroscopy

Study of Clustering of Impurities Doped in Si Crystals Depending on Annealing Conditions and Concentration by Using Soft X-ray Photoelectron Spectroscopy

Defect evolution and photoluminescence in anion-defective alumina single crystals exposed to high doses of gamma-rays

A method of luminescent UV and VUV spectroscopy was used to study the evolution of color centers in anion-defective alumina single crystals exposed to high doses of gamma-radiation. A sharp drop in the intensity of the emission bands and, therefore, the concentration of F+ and F-centers associated with the formation of aggregate F2-type centers was found. The aggregate centers create an additional emission band in the range of (1.8–2.8) eV. When the crystals are exposed to middle and high doses, the photoluminescence (PL) intensity is the highest in the emission band of F22+-centers, which indicates a high concentration of the aggregates from singly charged oxygen vacancies (of F+-centers). When PL of the crystals exposed to high doses is excited with synchrotron radiation of the VUV range, a wide emission band in the red and near infrared (NIR) regions is registered. The centers related presumably to impurity defects, their aggregates and clusters consisting of several oxygen vacancies are responsible for this emission band.