Temperature-dependent valence-band photoemission spectra of La1-xSrxMnO3

We have studied the temperature dependence of the photoemission spectra of La1-xSrxMnO3 (x=0.0, 0.2, and 0.4) and found that the spectral line shape dramatically changes in the entire valence-band region, particularly for x=0.2 and 0.4. By contrast, the spectra of La0.6Sr0.4CoO3 show no significant temperature dependence. From comparison between the temperature-and composition-(x) dependent spectral changes and the temperature-composition phase diagram of La1-xSrxMnO3, we suggest that the changes are related to the degree of hole localization on oxygen p orbitals, which is influenced by electron-lattice coupling and magnetic correlations.

Muon-spin rotation spectra in the mixed phase of high-T-c superconductors: Thermal fluctuations and disorder effects

We study muon-spin rotation (mu SR) spectra in the mixed phase of highly anisotropic layered superconductors, specifically Bi2+xSr2-xCaCu2O8+delta (BSCCO), by modeling the fluid and solid phases of pancake Vortices using liquid-state and density functional methods. The role of thermal fluctuations in causing motional narrowing of mu SR line shapes is quantified in terms of a first-principles theory of the flux-lattice melting transition. The effects of random point pinning are investigated using a replica treatment of liquid-state correlations and a replicated density functional theory. Our results indicate that motional narrowing in the pure system, although substantial, cannot account for the remarkably small linewidths obtained experimentally at relatively high fields and low temperatures. We find that satisfactory agreement with the mu SR data for BSCCO in this regime can be obtained through the ansatz that this ''phase'' is characterized by frozen short-range positional correlations reflecting the structure of the liquid just above the melting transition. This proposal is consistent with recent suggestions of a ''pinned liquid'' or ''glassy'' state of pancake Vortices in the presence of pinning disorder. Our results for the high-temperature liquid phase indicate that measurable linewidths may be obtained in this phase as a consequence of density inhomogeneities induced by the pinning disorder. The results presented here comprise a unified, first-principles theoretical treatment of mu SR spectra in highly anisotropic layered superconductors in terms of a controlled set of approximations. [S0163-1829(99)08033-9].

Ab initio studies of solvent effect on conformational equilibria and vibrational spectra of dipropionamide

Systematic ab initio molecular orbital studies of the conformational equilibria and vibrational spectra of dipropionamide using the basis sets 6-31g(d) and 6-31++G(d,p) have been carried out. The vibrational spectra of dipropionamide have been satisfactorily interpreted taking into account the agreement between the calculated frequencies, infrared and Raman band intensities and the shifts in the spectra of deuterated molecules with those observed. The previous assignments of most of the vibrational bands are well confirmed, a few bands need reassignment, however. The solvent effects were investigated by self-consistent reaction field theory using dipole and self-consistent isodensity polarized continuum model methods. The introduction of a dielectric medium has only a marginal effect on the conformational equilibria and vibrational spectra. However, the calculated changes in geometry and vibrational spectra on going from the gas phase to the solution phase are in accord with the increasing weight of the dipolar resonance structure in polar solvents. (C) 2002 Elsevier Science B.V. All rights reserved.

Excited state structure and dynamics of p-benzoquinone and bromanil from time-resolved resonance Raman spectra and simulation

p-Benzoquinone and its halogen substituted derivatives are known to have differing reactivities in the triplet excited state. While bromanil catalyzes the reduction of octaethylporphyrin most efficiently among the halogenated p-benzoquinones, the reaction does not take place in presence of the unsubstituted p-benzoquinone (T. Nakano and Y. Mori, Bull. Chem. Soc. Jpn., 67, 2627 (1994)). Understanding of such differences requires a detailed knowledge of the triplet state structures, normal mode compositions and excited state dynamics. In this paper, we apply a recently presented scheme (M. Puranik, S. Umapathy, J. G. Snijders, and J. Chandrasekhar, J. Chem, Phys., 115, 6106 (2001)) that combines parameters from experiment and computation in a wave packet dynamics simulation to the triplet states of p-benzoquinone and bromanil. The absorption and resonance Raman spectra of both the molecules have been simulated. The normal mode compositions and mode specific excited state displacements have been presented and compared. Time-dependent evolution of the absorption and Raman overlaps for all the observed modes has been discussed in detail. In p-benzoquinone, the initial dynamics is along the C=C stretching and C-H bending modes whereas in bromanil nearly equal displacements are observed along all the stretching coordinates.

Simulation of the infrared spectra of thioacetamide by the extended molecular mechanics method

The infrared spectrum of the matrix-isolated species of thioacetamide has been simulated using the extended molecular mechanics method. The equilibrium structure, vibrational frequencies, dipole moment and infrared absorption intensities of thioacetamide have been calculated in good agreement with the experiment. The vibrational frequencies and infrared absorption intensities for the isotopic molecules (CH2CSNH2)-C-13, (CH3CSNH2)-N-15 and (CH2CSND2)-C-13 have also been calculated consistent with the experiment. The infrared spectra of the matrix isolated species of N- and C- deuterated isotopomers of thioacetamide, CH3CSND2 and CD3CSNH2 have also been simulated in satisfactory agreement with the experimental spectra.

Simulation of the infrared spectra of N-methylformamide and N-methylacetamide by the extended molecular mechanics method

The infrared spectra of the matrix isolated species of N-methylformamide (NMF) and N-methylacetamide (NMA) and their N-deuterated molecules have been simulated by the extended molecular mechanics method using an empirical force field which includes charges and charge fluxes as coulombic potential parameters. The structural parameters and dipole. moments of NMF and NMA have. also been computed in satisfactory agreement with the experiment. Good agreement between experimental and calculated vibrational frequencies and infrared absorption band intensities for NMF and NMA and their deuterated molecules has been obtained. The vibrational assignments of NMF and NMA are-discussed taking also into account the infrared absorption intensities.

Microwave spectrum and structure of C(6)H(5)CCH center dot center dot center dot H(2)S complex

Rotational spectra of C(6)H(5)CCH center dot center dot center dot H(2)S, C(6)H(5)CCH center dot center dot center dot H(2)(34)S, C(6)H(5)CCH center dot center dot center dot HDS, C(6)H(5)CCH center dot center dot center dot D(2)S and C(6) H(5)CCD center dot center dot center dot H(2)S complexes have been observed using a pulsed nozzle Fourier transform microwave spectrometer. The observed spectrum is consistent with a structure in which hydrogen sulfide is located over the phenyl ring pi cloud and the distance between the centers of masses of the two monomers is 3.74 +/- 0.01 angstrom. In the complex, the H(2)S unit is shifted from the phenyl ring center towards the acetylene group. The vibrationally averaged structure has an effective Cs symmetry. Ab initio calculations were performed at MP2/aug-cc-pVDZ level of theory to locate the possible geometries of the complex. The calculations reveal the experimentally observed structure to be more stable than a coplanar arrangement of the monomers, which was observed for the C(6)H(5)CCH center dot center dot center dot H(2)O complex. Atoms in molecule theoretical analysis shows the presence of S-H center dot center dot center dot pi hydrogen bond. For the parent isotopologue, each transition frequency was found to split into two resulting from an interchange of the equivalent hydrogens of H(2)S unit in the complex. (C) 2011 Elsevier Inc. All rights reserved.

Spin State Selective Excitation of Single Quantum Transitions Using Multiple Quantum Spectroscopy- an Aid to Analyse Complex Proton NMR spectra of Oriented Molecules

NMR spectra of molecules oriented in liquid-crystalline matrix provide information on the structure and orientation of the molecules. Thermotropic liquid crystals used as an orienting media result in the spectra of spins that are generally strongly coupled. The number of allowed transitions increases rapidly with the increase in the number of interacting spins. Furthermore, the number of single quantum transitions required for analysis is highly redundant. In the present study, we have demonstrated that it is possible to separate the subspectra of a homonuclear dipolar coupled spin system on the basis of the spin states of the coupled heteronuclei by multiple quantum (MQ)−single quantum (SQ) correlation experiments. This significantly reduces the number of redundant transitions, thereby simplifying the analysis of the complex spectrum. The methodology has been demonstrated on the doubly 13C labeled acetonitrile aligned in the liquid-crystal matrix and has been applied to analyze the complex spectrum of an oriented six spin system.

Effect of Te addition on the optical properties of As(2)S(3) thin film

Bilayer thin films of Te/As(2)S(3) were prepared from Te and As(2)S(3) by thermal technique under high vacuum. Optical constants were calculated by analysing the transmission spectrum in the spectral range 400-1100 nm. The optical band gap decreases with the addition of Te to As(2)S(3). The decrease of optical band gap has been explained on the basis of density of states and the increase in disorder in the system. We have irradiated the as-deposited films using a diode pumped solid state laser of 532 nm wavelength to study photo-diffusion of Te into As(2)S(3). The changes were characterised by Fourier Transform Infrared and X-ray Photoelectron Spectroscopy (XPS). The optical band gap is found to be decreased with the light irradiation which is proposed due to homopolar bond formation. The core level peaks in XPS spectra give information about different bond formation. (C) 2011 Elsevier B.V. All rights reserved.

Spin-polarized STM spectra of Dirac electrons on the surface of a topological insulator

We provide a theory for the tunneling conductance G(V) of Dirac electrons on the surface of a topological insulator as measured by a spin-polarized scanning tunneling microscope tip for low-bias voltages V. We show that if the in-plane rotational symmetry on the surface of the topological insulator is broken by an external field that does not couple to spin directly (such as an in-plane electric field), G(V) exhibits an unconventional dependence on the direction of the magnetization of the tip, i.e., it acquires a dependence on the azimuthal angle of the magnetization of the tip. We also show that G(V) can be used to measure the magnitude of the local out-of-plane spin orientation of the Dirac electrons on the surface. We explain the role of the Dirac electrons in this unconventional behavior and suggest experiments to test our theory.

Effect of pairing fluctuations on low-energy electronic spectra in cuprate superconductors

We describe here a minimal theory of tight-binding electrons moving on the square planar Cu lattice of the hole-doped cuprates and mixed quantum mechanically with their own Cooper pairs. The superconductivity occurring at the transition temperature T(c) is the long-range, d-wave symmetry phase coherence of these Cooper pairs. Fluctuations, necessarily associated with incipient long-range superconducting order, have a generic large-distance behavior near T(c). We calculate the spectral density of electrons coupled to such Cooper-pair fluctuations and show that features observed in angle resolved photoemission spectroscopy (ARPES) experiments on different cuprates above T(c) as a function of doping and temperature emerge naturally in this description. These include ``Fermi arcs'' with temperature-dependent length and an antinodal pseudogap, which fills up linearly as the temperature increases toward the pseudogap temperature. Our results agree quantitatively with experiment. Below T(c), the effects of nonzero superfluid density and thermal fluctuations are calculated and compared successfully with some recent ARPES experiments, especially the observed bending or deviation of the superconducting gap from the canonical d-wave form.

Fourier descriptors for morphological analysis of vectorcardiograms

The use of Fourier shape descriptors for morphological studies of vectorcardio-grams (VCGs) is p resented . The FDs can effectively be used as features for classf-fication of VCGs of different clinical categories . In addition , they provide cli-nically significant qualitative shape information for use by the Cardiologist. The initial result sofanalysisof nwrmal and abnormal VCGs areencouraging.