Department of Physics
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Item Third-order elastic constants and low-temperature lattice thermal expansion of the high-temperature structural material TiAl(IOP, 2001) Nair, Sindhu S.We have derived the expressions for the 12 third-order elastic constants for a tetragonal system by the method of homogeneous deformation which are used to determine the third-order elastic constants of TiAl. These third-order elastic constants are used to determine the low-temperature limit of the volume thermal expansion of TiAl. The value of C333 was calculated to be a high value. The high value of the third-order elastic constant C333 shows a marked anisotropy in the crystal. The low-temperature limit is found to be negative, indicating a negative volume thermal expansion down to zero kelvin.Item Micromagnetic simulations of the domain structure and the magnetization reversal of Co50Ni50/Pt multilayer dots(Elsevier, 2002-01) Nair, Sindhu S.The domain structure and the switching field of Co50Ni50/Pt multilayer dots, prepared by laser interference lithography, were micromagnetically simulated. The simulations were carried out with a three-dimensional simulation package, optimized for large-scale problems. The single-domain state is the lowest energy state for dots with a diameter below 75 nm. The switching field was computed by using suitable minimization techniques, and was used to analyze the effect of size, dot shape and edge defects.Item A Photoemission study of CoO films on Au (111) Surface Science(Elsevier, 2003-09) Nair, Sindhu S.Angle-resolved ultraviolet photoemission spectroscopy has been used to study thin layers of Co and CoO on an Au(1 1 1) substrate. A bulk-like CoO layer was obtained for a nominal coverage of 7 ML Co, with Co evaporated in an oxygen atmosphere of 10−4 Pa and the Au(1 1 1) substrate at 350 K. Low-energy electron diffraction of this layer exhibits a (1 × 1) structure, indicating an unreconstructed CoO(1 1 1) surface. Considering the instability of a bulk-like terminated CoO surface, we assume stabilization by OH termination. At low coverage, the CoO layer is oxygen deficient and exhibits metallic character. For the bulk-like CoO layers we found band-like oxygen states and no detectable dispersion for the states near the Fermi edge, which is in agreement with previous investigations of single-crystalline CoO.Item Growth mechanism and angle-resolved photoemission spectra of cobalt oxide (CoO) thin films on Ag(1 0 0)(Elsevier, 2004-09) Nair, Sindhu S.Using Auger electron spectroscopy we have studied the growth of cobalt oxide on Ag(1 0 0) substrate at different substrate temperatures (25, 170 and 250 °C). The films grown at 170 °C substrate temperature were found to grow closest to a layer-by-layer growth mode. In order to experimentally investigate the electronic structure of the CoO films angle-resolved UV photoelectron spectra were recorded for a 10 ML CoO film that are discussed and compared with available experimental results.Item Generalized Grüneisen parameters and low temperature limit of lattice thermal expansion of cadmium and zirconium(Springer, 2006-09) Nair, Sindhu S.The generalized Grüneisen parameters (γ′ j ) and (γ″ j ) for cadmium and zirconium were calculated from the second- and third-order elastic constants to determine the low temperature limit of the volume thermal expansion of these metals of hexagonal symmetry. The low temperature limit of cadmium and zirconium was calculated to be positive values indicating a positive volume expansion down to 0 K even though many Grüneisen gammas were found to be negative.Item Novel high contrast electrochromic polymer materials based on 3,4-propylenedioxythiophene(Elsevier, 2007-09) Nair, Sindhu S.Mono and di allyl and napthyl substituted 3,4-propylenedioxythiophenes were synthesized and polymerized electrochemically. All the monomers were characterized for their molecular structures, and the polymers were characterized for their electrochemical properties. The disubstituted derivatives showed higher contrast than the corresponding mono substituted derivatives. The allyl substituted polymers showed higher contrast and faster switching time than corresponding napthyl substituted derivatives. The presence of the allyl group as the pendant can be used for further functionalization of the polymer.Item Gravitational-wave detection using redshifted 21-cm observations(APS, 2009-06) Sarkar, Tapomoy GuhaA gravitational-wave traversing the line of sight to a distant source produces a frequency shift which contributes to redshift space distortion. As a consequence, gravitational waves are imprinted as density fluctuations in redshift space. The gravitational-wave contribution to the redshift space power spectrum has a different μ dependence as compared to the dominant contribution from peculiar velocities. This, in principle, allows the two signals to be separated. The prospect of a detection is most favorable at the highest observable redshift z. Observations of redshifted 21-cm radiation from neutral hydrogen hold the possibility of probing very high redshifts. We consider the possibility of detecting primordial gravitational waves using the redshift space neutral hydrogen power spectrum. However, we find that the gravitational-wave signal, though present, will not be detectable on superhorizon scales because of cosmic variance and on subhorizon scales where the signal is highly suppressed.Item The CMBR ISW and HI 21 cm cross-correlation angular power spectrum(IOP, 2009-08) Sarkar, Tapomoy GuhaThe late-time growth of large scale structures is imprinted in the CMBR anisotropy through the Integrated Sachs Wolfe (ISW) effect. This is perceived to be a very important observational probe of dark energy. Future observations of redshifted 21-cm radiation from the cosmological neutral hydrogen (HI) distribution hold the potential of probing the large scale structure over a large redshift range. We have investigated the possibility of detecting the ISW through cross-correlations between the CMBR anisotropies and redshifted 21-cm observations. Assuming that the HI traces the dark matter, we find that the ISW-HI cross-correlation angular power spectrum at an angular multipole ℓ is proportional to the dark matter power spectrum evaluated at the comoving wave number ℓ/r, where r is the comoving distance to the redshift from which the HI signal originated. The amplitude of the cross-correlation signal depends on parameters related to the HI distribution and the growth of cosmological perturbations. However, the cross-correlation is extremely weak as compared to the CMBR anisotropies and the predicted HI signal. Even in an ideal situation, the cross-correlation signal is smaller than the cosmic variance and a statistically significant detection is not very likely.Item Two-step growth of InGaN quantum dots and application to light emitters(Wiley, 2007-06) Gangopadhyay, SubhashisA two-step growth method for creating InGaN quantum dots (QDs) was developed by using a combination of an InxGa1–xN nucleation layer (NL) without island structures and an InyGa1–yN formation layer (FL) with an indium content lower than that of the InxGa1–xN NL. The realization of QDs was confirmed by micro-photoluminescence (μ-PL) measurements only for the sample with both the InxGa1–xN NL and the InyGa1–yN FL. The spectral position of the QD ensemble recombination was controlled mainly by the deposition time of the InxGa1–xN NL. Green (∼520 nm) and amber (∼600 nm) LEDs with the QD layers grown by the two-step growth method as the active region were also fabricated and compared with that having InGaN QW layers, reported previously. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Item Purpose-led Publishing logo. Evolution of Ge nanoislands on Si(110)-'16 × 2' surface under thermal annealing studied using STM(IOP, 2009-10) Gangopadhyay, SubhashisThe initial nucleation of Ge nanoclusters on Si(110) at room temperature (RT), annealing-induced surface roughening and the evolution of three-dimensional Ge nanoislands have been investigated using scanning tunneling microscopy (STM). A few monolayers (ML) of Ge deposited at room temperature lead to the formation of Ge clusters which are homogeneously distributed across the surface. The stripe-like patterns, characteristic of the Si(110)-'16 × 2' surface reconstruction are also retained. Increasing annealing temperatures, however, lead to significant surface diffusion and thus, disruption of the underlying '16 × 2' reconstruction. The annealing-induced removal of the stripe structures (originated from '16 × 2' reconstruction) starts at approximately 300 °C, whereas the terrace structures of Si(110) are thermally stable up to 500 °C. At approximately 650 °C, shallow Ge islands of pyramidal shape with (15,17,1) side facets start to form. Annealing at even higher temperatures enhances Ge island formation. Our findings are explained in terms of partial dewetting of the metastable Ge wetting layer (WL) (formed at room temperature) as well as partial relaxation of lattice strain through three-dimensional (3D) island growth.