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Shiv Nadar University

(SoNS) School of Natural Sciences

Studies on the effect of 10-50 atomic % Ca doping on structural distortions are reported in the present work. Bismuth layered structured ferroelectrics oxides of the composition Sr1-xCaxBi2Nb2O9; with x=0.0 - 0.5 were synthesized by the solid-state reaction method. X-ray diffraction analysis revealed the formation of perovskite ferroelectric ceramic sample and a few additional peaks were observed as concentration of calcium varied from 10 to 50%, gradual shift in peaks with doping is discussed with respect to observed anomalous change in lattice parameters. Also, transformation of structure from pseudo-orthorhombic to pseudo-tetragonal phase is observed at room temperature as concentration of calcium is increased. This suggests that structural distortions are pronounced in such a manner so as to stabilize the structure of unit cell, which may lead to high value of Curie temperature and remanent polarization useful in non-volatile random access memory (NvRAM) devices.

Physics

World University root node

Proceedings of SPIE - The International Society for Optical Engineering

Effect of calcium doping on lattice parameters resulting in structural distortions of SrBi2Nb2O9 ferroelectric ceramics

The formation of nanoquasicrystallites in stable icosahedral Al-Cu-Fe quasicrystalline thin films by implementing an indirect heating method from a single source is discussed in this paper. The final composition of the quasicrystalline thin films is found to be Al62.9Cu24.6Fe12.5 using x-ray fluorescence spectroscopy. The icosahedral nanoquasicrystallites formation is studied by glancing-angle x-ray diffraction and transmission electron microscopy. The size of the nanoquasicrystallites is estimated to be 4-20 nm. The density of nanoquasicrystallites is calculated as (1 ± 2) × 1012 cm-2. The improvement of the icosahedral phase formation with increasing annealing temperature is also verified by measuring the change in resistivity with temperature in the range 10-300 K. The resistivity of the best film, which is obtained by annealing the as-deposited films at 700 °C for 1h, is calculated to be ∼2000 μΩ cm at room temperature and ∼4000μΩ cm at 10 K.

Nanotechnology

Nanoquasicrystallites formation in stable icosahedral Al-Cu-Fe quasicrystalline thin films

This article focuses on the influence of chemical driving forces on the mixing and phase formation taking place at the interface of highly reactive metal/semiconductor systems under ion-beam irradiation. Ta/Si bilayers were irradiated with Ar, Kr, and Xe ions to fluences of (formula presented) and at temperatures between liquid nitrogen and 400 °C. The interface mixing and silicide formation were monitored as function of the ion mass and fluence by means of Rutherford backscattering spectrometry and x-ray diffraction. The interface broadening variance was found to depend linearly on the ion fluence and was explained with the help of a compound formation model involving local or global thermal spikes. The results are compared with those found in other silicide and germanide systems. The transition from local to global spikes was found to occur at the critical deposited damage energy of about 2.5 keV/nm. © 2001 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Atomic mixing and interface reactions in Ta/Si bilayers during noble-gas ion irradiation

We study electrical signature of defect clusters in KeV Ar ion-implanted n-silicon using Deep Level Transient Spectroscopy (DLTS) and isothermal capacitance spectroscopies such as time analyzed transient spectroscopy (TATS) and high resolution Laplace-DLTS. The samples are annealed at relatively low temperatures of 350 °C - 600 °C at which defect clusters are known to form and evolve. Contrary to the view that few dominant point-defect like traps are associated with defect clusters, our results show that the band gap may be replete with bands of multiple trap states; however their occupation and hence observation depends on experimental conditions dictated by dynamics of career capture and emission at these traps. Charge redistribution among multiple states and deepening of effective emission energy with capture are shown to be commonly occurring at these defects. Isothermal transient spectroscopy is shown to be appropriate tool for recognition of some of these features.

Materials Research Society Symposium - Proceedings

Electrical signature of ion-implantation induced defects in n-silicon in the defect cluster regime studied using DLTS and isothermal transient spectroscopies

Extending the supersymmetric method proposed by Tkachuk to the complex domain, we obtain general expressions for superpotentials allowing generation of quasi-exactly solvable PT-symmetric potentials with two known real eigenvalues (the ground state and first-excited state energies). We construct examples, namely those of complexified nonpolynomial oscillators and of a complexified hyperbolic potential, to demonstrate how our scheme works in practice. For the former we provide a connection with the sl(2) method, illustrating the comparative advantages of the supersymmetric one.

Fulltext

Modern Physics Letters A

PT-symmetric nonpolynomial oscillators and hyperbolic potential with two known real eigenvalues in a susy framework

We show that complex Lie algebras (in particular s1(2, ℂ)) provide us with an elegant method for studying the transition from real to complex eigenvalues of a class of non-Hermitian Hamiltonians: Complexified Scarf II, generalized Pöschl-Teller, and Morse. The characterizations of these Hamiltonians under the so-called pseudo-Hermiticity are also discussed. © 2002 Elsevier Science B.V. All rights reserved.

Physics Letters, Section A: General, Atomic and Solid State Physics

Non-Hermitian Hamiltonians with real and complex eigenvalues in a Lie-algebraic framework

The charge-density distribution in 2H-chromene-2-thione (2-thiocoumarin), C9H6OS, has been determined from X-ray diffraction data measured at 90 K using a CCD detector, to a resolution of sinθ/λ &lt; 1.08 Å-1. A multipolar-atom density model was fitted against 6908 reflections with I &gt; 2σ(I) [R(F) = 0.021, wR(F) = 0.022, goodness of fit = 1.81] in order to generate the difference Fourier maps. The topological properties of the molecular electron density in terms of the bond critical points and the evaluation of the dipole moment show that the molecular dipole moment in the crystal is higher than the corresponding value derived from theoretical calculations.

Chemistry

Acta Crystallographica Section B: Structural Science

Electron density study of 2H-chromene-2-thione

At present, there is an increasing interest in the iron di-silicide phase β-FeSi2, which is supposed to be a direct band gap semiconductor and one of the most promising materials for silicon-based optoelectronics, e.g., light-emitting devices, solar cells, and photo detectors. But this phase is very difficult to be produced. Here, the successful direct synthesis of this phase by ion beam mixing of Fe/Si bilayers at temperatures in the range of 400 to 600°C is reported. The aim of the experiments was to achieve a complete reaction of the deposited Fe layer with the Si substrate that results in the formation of a pure, single-phased β-FeSi2 surface layer. The obtained silicide layers, their structure and composition are investigated by conversion electron Mössbauer spectroscopy (CEMS), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD). The fraction of the β-FeSi2 formed is determined by CEMS as function of ion species, energy, fluence and temperature. Complete growth and formation of a single-phased β-FeSi2 layer was achieved by 205 keV Xe ion irradiation at a fluence of 2 × 1016 ions/cm2 at 600°C.

Hyperfine Interactions

Mössbauer optimization of the direct synthesis of β-FeSi2 by ion beam mixing of Fe/Si bilayers

This study deals with structural and optical properties of β-FeSi2 layers produced by direct ion beam mixing of Fe/Si bilayers with Xe ions. By irradiation of 35 nm Fe on Si, at 600 °C with 205 keV Xe to 2 × 1016 ions/cm2, the formation of ~ 105 nm single-phase β-FeSi2 layers was achieved. Their structures were analyzed by Rutherford backscattering spectroscopy. X-ray diffraction, conversion electron Mössbauer spectroscopy, high resolution transmission electron microscopy, and photo-absorption. The structural analyses revealed that the β-FeSi2 layers grow in the form of irregularly shaped crystal grains, with a pronounced surface morphology, but with a rather sharp silicide/silicon interface. The grains that originate from the interface are epitaxially oriented relative to the Si(100) substrate. Optical absorption, as compared with that in β-FeSi2 layers produced by ion beam synthesis or co-sputter deposition, indicates a direct band gap of 0.92 eV. A pronounced surface roughness of the ion beam mixed layers yielded photo-absorption approximately three times higher as compared with the other two sets of samples. The band gap stays nearly constant over the temperature range from 80 to 295 K. This is tentatively assigned to a high degree of structural disorder and stress induced in the ion beam mixed β-FeSi2 layers. © 2002 Elsevier Science B.V. All rights reserved.

Surface and Coatings Technology

Structural and optical properties of β-FeSi2 layers grown by ion beam mixing

We discuss certain features of pseudo-Hermiticity and weak pseudo-Hermiticity conditions and point out that, contrary to a recent claim, there is no inconsistency if the correct orthogonality condition is used for the class of pseudo-Hermitian, PT symmetric Hamiltonians of the type Hβ = [p + iβv(x)]2/2m + V (x). © 2002 Elsevier Science B.V. All rights reserved.