Implicit and multigrid procedures for steady-state computations with upwind algorithms

A computational study for the convergence acceleration of Euler and Navier-Stokes computations with upwind schemes has been conducted in a unified framework. It involves the flux-vector splitting algorithms due to Steger-Warming and Van Leer, the flux-difference splitting algorithms due to Roe and Osher and the hybrid algorithms, AUSM (Advection Upstream Splitting Method) and HUS (Hybrid Upwind Splitting). Implicit time integration with line Gauss-Seidel relaxation and multigrid are among the procedures which have been systematically investigated on an individual as well as cumulative basis. The upwind schemes have been tested in various implicit-explicit operator combinations such that the optimal among them can be determined based on extensive computations for two-dimensional flows in subsonic, transonic, supersonic and hypersonic flow regimes. In this study, the performance of these implicit time-integration procedures has been systematically compared with those corresponding to a multigrid accelerated explicit Runge-Kutta method. It has been demonstrated that a multigrid method employed in conjunction with an implicit time-integration scheme yields distinctly superior convergence as compared to those associated with either of the acceleration procedures provided that effective smoothers, which have been identified in this investigation, are prescribed in the implicit operator.

Phonon renormalization in doped bilayer graphene

We report phonon renormalization in bilayer graphene as a function of doping. The Raman G peak stiffens and sharpens for both electron and hole doping as a result of the nonadiabatic Kohn anomaly at the Gamma point. The bilayer has two conduction and valence subbands, with splitting dependent on the interlayer coupling. This gives a change of slope in the variation of G peak position with doping which allows a direct measurement of the interlayer coupling strength.

Asymptotic behaviour of point processes with Markov-dependent intervals

This splitting techniques for MARKOV chains developed by NUMMELIN (1978a) and ATHREYA and NEY (1978b) are used to derive an imbedded renewal process in WOLD's point process with MARKOV-correlated intervals. This leads to a simple proof of renewal theorems for such processes. In particular, a key renewal theorem is proved, from which analogues to both BLACKWELL's and BREIMAN's forms of the renewal theorem can be deduced.

Phase transition in ammonium aluminium and ammonium chromium alums-A high pressure EPR study

An investigation of the phase transitions at high pressures in the alums mentioned in the title has been carried out using EPR of the Cr3+ ion (at the trivalent metal ion site). It is observed that at ambient as well as at high pressures there is a change of slope in the linear variations of the zero field splitting with temperature and that the low temperature phase is characterised by a large number of lines in the EPR spectra. The transition temperature shows a large positive shift with pressure, for both the alums. All these facts are explained in terms of our model of the origin of the trigonal field at the trivalent metal ion site as well as the details of the motion of NH4+ ion.

Theory For The Anomalous Hall Constant Of Mixed-Valence Systems

We show that the large anomalous Hall constants of mixed-valence and Kondo-lattice systems can be understood in terms of a simple resonant-level Fermi-liquid model. Splitting of a narrow, orbitally unquenched, spin-orbit split, f resonance in a magnetic field leads to strong skew scattering of band electrons. We interpret both the anomalous signs and the strong temperature dependence of Hall mobilities in CeCu2Si2, SmB6, and CePd3 in terms of this theory.

An infrared spectroscopic study of the incommensurate transitions and related phase changes in K2Pb[Cu(NO2)6] and [N(CH3)4]2MX4 (M = Mn, Co, Cu or Zn and X = Cl or Br)

K2Pb[Cu(NO2)6] and [N(CH3)4]2MX4 (M = Mn, Co, Cu or Zn and X = Cl or Br) undergo phase transitions which involve incommensurate phases. The transitions have been investigated by examining the changes in the NO2 and CH3 vibration bands in the i.r. spectra. Splitting and broadening of some of the bands across the incommensurate transitions are discussed in the context of geometrical restrictions in the incommensurate phases. The phase transitions have also been characterized using differential scanning calorimetry.

Non-bonded interactions in 2,2,4,4-tetramethyl-1,3-cyclobutanedithione and 2,2,4,4-tetramethyl-3-thio-1,3-cyclobutanedione

Electronic absorption and emission spectra as well as He(I) photoelectron spectra of 2,2,4,4-tetramethyl-,3-cyclobutanedithione and 2,2,4,4-tetramethyl-1-3-thio-1,3-cyclobutanedione have been interpreted on the basis of molecular orbital calculations. The results show that the non-bonded orbital of the dithione is split owing to through-bond interaction, the magnitude of splitting being 0.4 eV. The π* orbital of the dithione appears to be split by about 0.2 eV. Electronic absorption spectra show evidence for the existence of four n—π* transitions, arising out of the splitting of the orbitals referred to above, just as in the case of 2,2,4,4-tetramethyl-1,3-cyclobutanedione. Electronic and photoelectron spectra of the thio-dione show evidence for weak interaction between the C=S and C&.zdbnd;O groups, probably via π* orbitals. Infrared spectra of both the dithione and the thio-dione are consistent with the planar cyclobutane ring; the ring-puckering frequency responsible for non-bonded interactions is around 67 cm−1 in both the dithione and the thio-dione, the value not being very different from that in the dione. The 1,3-transannular distance is also similar in the three molecules.

Cr3+ electron paramagnetic resonance study of Sn(1-x)CrxO(2) (0.00 <= x <= 0.10)

This paper reports on the liquid-helium-temperature (5 K) electron paramagnetic resonance (EPR) spectra of Cr3+ ions in the nanoparticles of SnO2 synthesized at 600 degrees C with concentrations of 0%, 0.1%, 0.5%, 1%, 1.5%, 2.0%, 2.5%, 3.0%, 5.0%, and 10%. Each spectrum may be simulated as overlap of spectra due to four magnetically inequivalent Cr3+ centers characterized by different values of the spin-Hamiltonian parameters. Three of these centers belong to Cr3+ ions in orthorhombic sites, situated near oxygen vacancies, characterized by very large zero-field splitting parameters D and E, presumably due to the presence of nanoparticles in the samples. The fourth EPR spectrum belongs to the Cr3+ ions situated at sites with tetragonal symmetry, substituting for the Sn4+ ion, characterized by a very small value of D. In addition, there appears a ferromagnetic resonance line due to oxygen defects for samples with Cr3+ concentrations of <= 2.5%. Further, in samples with Cr3+ concentrations of >2.5%, there appears an intense and wide EPR line due to the interactions among the Cr3+ ions in the clusters formed due to rather excessive doping; the intensity and width of this line increase with increasing concentration. The Cr3+ EPR spectra observed in these nanopowders very different from those in bulk SnO2 crystals.

Sulphate ion dynamics in alpha-alums studied by esr at high pressures

The variation of zero-field splitting and linewidth of Cr3+ ion in KCr and KAI alums with hydrostatic pressure and with temperature is investigated. A model for the apparent phase transition is proposed on the basis of the reorientational motion of the SO2�4 groups.

X-ray and IR characterisation of Nasicon solid solution, Na1+xZr2SixP3−xO12

X-ray and IR studies on Nasicon solid solutions, Na1+xZr2SixP3−xO12, are carried out as a function of composition x. X-ray diffraction studies show that the unit cell volume increases as x increases and exhibits a maximum at x ≈ 2. On further increase in x the unit cell volume decreases. The infrared absorption peak positions and the splitting of these absorption peaks suggest a distortion of the PO4 and SiO4 tetrahedra. But the distortion is not large enough to change the local symmetry around the phosphorus or silicon ion from Td to C3v.

ESR studies of phase transitions in double propionates

Detailed ESR investigations of Mn2+ substituting for Ca2+ in Ca2Sr(C2H5COO)6 (DSP), Ca2Pb(C2H5COO)6 (DLP) and Ca2Ba(C2H5COO)6 (DBP), in single crystals and powders, over the temperature range from 200°C to -180°C have been carried out to study the successive phase transitions in these compounds. (DSP: [Tetragonal] ← 8.5°C → [tetragonal, ferroelectric] [tetragonal] ← -169°C → [monoclinic, ferroelectric]; DLP : [tetragonal] ← 60°C → [tetragonal, ferroelectric] ← -71.5°C → [monoclinic, ferroelectric]; [Cubic] ← -6°C → [orthorhombic] ← -75°C → [?]). Spectra have been analysed in terms of axial spin Hamiltonians and the temperature dependences of the parameters studied. In DSP and DLP across the I ↔ II transition, new physically and chemically inequivalent sites appear indicating the disappearance of the diad axes on which the propionate groups are located, bringing out the connection between the motional states of the propionate groups and the occurence of ferroelectricity. The II ↔ III transition also causes chemically inequivalent sites to develop, indicating that the transitions may not be isomorphous as believed previously. In DBP, the -6°C transition leads to (i) a doubling of both physically and chemically inequivalent sites (ii) a small (150 G at -6°C to 170 G at -8°C), but abrupt change in the magnitude of the zero-field splitting tensor D, and (iii) displacements of the orientations of the D tensors. Results are interpreted in terms of alternate rotations of the oxygen octahedra, showing participation of the carboxyl oxygens in the transition. No drastic changes in the parameters occur across the -75°C transition consistent with its second order nature. Similarities and dissimilarities of the ESR spectra of the three compounds in relation to the phase transitions, are discussed.

Analysis-By-Synthesis Based Switched Transform Domain Split Vq Using Gaussian Mixture Model

Using analysis-by-synthesis (AbS) approach, we develop a soft decision based switched vector quantization (VQ) method for high quality and low complexity coding of wideband speech line spectral frequency (LSF) parameters. For each switching region, a low complexity transform domain split VQ (TrSVQ) is designed. The overall rate-distortion (R/D) performance optimality of new switched quantizer is addressed in the Gaussian mixture model (GMM) based parametric framework. In the AbS approach, the reduction of quantization complexity is achieved through the use of nearest neighbor (NN) TrSVQs and splitting the transform domain vector into higher number of subvectors. Compared to the current LSF quantization methods, the new method is shown to provide competitive or better trade-off between R/D performance and complexity.

Sulphate ion dynamics in a-alums studied by esr at high pressures

The variation of zero-field splitting and linewidth of Cr3+ ion in KCr and KAI alums with hydrostatic pressure and with temperature is investigated. A model for the apparent phase transition is proposed on the basis of the reorientational motion of the SO2-4 groups.

Mossbauer lineshape in the presence of NMR transitions

The general expression for the Mössbauer lineshape in the presence of radio frequency perturbation derived earlier has been further extended. This involves the calculation of the off-diagonal matrix elements of the correlation function. The results show that there are additional transition lines owing to the nuclear magnetic resonance induced transition in the resonance region. These lines do not show any broadening or splitting. As an example the effect of the rf field on 57Fe nuclei is discussed.

The electronic factor in Bergman cyclization

Through-bond interactions in 1,4-dehydrobenzene preferentially stabilize the out-of-phase combination of the radical hydrids, The resultant splitting between the frontier orbitals is crucial in making Bergman cyclization a symmetry-allowed process. Orbital symmetry also inhibits the radical centers from forming a C-C bond, enabling the biradical to survive as a local minimum capable of intermolecular hydrogen abstraction, Both these factors, which are important in the design of DNA cleaving molecules, are confirmed through calculations on biradicals formed from diynes in which through-bond interactions stabilize the in-phase combination of hybrids at the radical centers.