3,250 research outputs found
Large negative magnetoresistance in a ferromagnetic shape memory alloy : Ni_{2+x}Mn_{1-x}Ga
5% negative magnetoresistance (MR) at room temperature has been observed in
bulk Ni_{2+x}Mn_{1-x}Ga. This indicates the possibility of using
Ni_{2+x}Mn_{1-x}Ga as magnetic sensors. We have measured MR in the
ferromagnetic state for different compositions (x=0-0.2) in the austenitic,
pre-martensitic and martensitic phases. MR is found to increase with x. While
MR for x=0 varies almost linearly in the austenitic and pre-martensitic phases,
in the martensitic phase it shows a cusp-like shape. This has been explained by
the changes in twin and domain structures in the martensitic phase. In the
austenitic phase, which does not have twin structure, MR agrees with theory
based on s-d scattering model.Comment: 3 pages, 3 figures, Appl. Phys. Lett 86, 202508 (2005
Spectral functions in doped transition metal oxides
We present experimental photoemission and inverse photoemission spectra of
SrTiO representing electron doped systems. Photoemission
spectra in presence of electron doping exhibit prominent features arising from
electron correlation effects, while the inverse photoemssion spectra are
dominated by spectral features explainable within single-particle approaches.
We show that such a spectral evolution in chemically doped correlated systems
is not compatible with expectations based on Hubbard or any other similar
model. We present a new theoretical approach taking into account the
inhomogeneity of the `real' system which gives qualitatively different results
compared to standard `homogeneous' models and is in quantitative agreement with
experiments.Comment: 10 pages; 1 tex file+4 postscript files (to appear in Europhysics
Letters
Electronic excitations on metal surfaces and nano-structures
Collective electron excitations on a free-electron-like metal surface like Al have been studied by X-ray photoemission spectroscopy. In this review, we discuss photoemission studies on quantization of electron states due to confinement in nano-structures like epitaxial Na films on Al(111). The electronic structure of Ar nanobubbles embedded in the sub-surface region of Al has been discussed. For Al, an asymmetric line-shape is observed for both monopole surface and bulk plasmon in good agreement with theory. The relative contributions of the intrinsic, extrinsic and the interference processes to the surface plasmon intensity are determined from theoretical plasmon line-shape calcu lations and angle-dependent photoemission. The characteristics of the multipole plasmon mode are also discussed. Using angle resolved photoemission, Na thin films on Al(111) have been studied for different thicknesses. We find features in the valence band spectra that behave like quantum well resonances in a narrow photon energy range where the overlayer collective excitations are observed. These resonances are observed because of quantum confinement due to a potential step at the Na/Al interface and the dynamical enhancement of the electric field in the overlayer. In case of argon nano-bubbles in Al, we find that the Ar 2p binding energy and the Doniach-Sunjic asymmetry of the core level line shape vary systematically as functions of Ar+ implantation energy and number of ions bombarded (fluence). These observations are explained by relating the strength of Al conduction electron screening of the core -hole created in the photoemission final state to the size of the Ar nano-bubbles
Premartensite to martensite transition and its implications on the origin of modulation in Ni2MnGa ferromagnetic shape memory alloy
We present here results of temperature dependent high resolution synchrotron
x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our
results show that the incommensurate martensite phase results from the
incommensurate premartensite phase, and not from the austenite phase assumed in
the adaptive phase model. The premartensite phase transforms to the martensite
phase through a first order phase transition with coexistence of the two phases
in a broad temperature interval (~40K), discontinuous change in the unit cell
volume as also in the modulation wave vector across the transition temperature
and considerable thermal hysteresis in the characteristic transition
temperatures. The temperature variation of the modulation wave vector q shows
smooth analytic behaviour with no evidence for any devilish plateau
corresponding to an intermediate or ground state commensurate lock-in phases.
The existence of the incommensurate 7M like modulated structure down to 5K
suggests that the incommensurate 7M like modulation is the ground state of
Ni2MnGa and not the Bain distorted tetragonal L10 phase or any other lock-in
phase with a commensurate modulation. These findings can be explained within
the framework of the soft phonon model
Optimal operating conditions and characteristics of acetone/CaF_2 detector for inverse photoemission spectroscopy
Performance and characteristics of a band-pass photon detector using acetone
gas and CaF_2 window (acetone/CaF_2) have been studied and compared with an
ethanol/MgF_2 detector. The optimal operating conditions are found to be 4 mbar
acetone pressure and 745+/-20 V anode voltage. The count rate obtained by us is
about a factor of 3 higher than what has been reported earlier for the acetone
detector. Unlike other gas filled detectors, this detector works in the
proportional region with very small dead time (4 micro sec). A detector
band-pass of 0.48+/-0.01 eV FWHM is obtained.Comment: Review of Scientific Instruments 76, 066102 (2005
Influence of Ni doping on the electronic structure of Ni_2MnGa
The modifications in the electronic structure of Ni_{2+x}Mn_{1-x}Ga by Ni
doping have been studied using full potential linearized augmented plane wave
method and ultra-violet photoemission spectroscopy. Ni 3d related electron
states appear due to formation of Ni clusters. We show the possibility of
changing the minority-spin DOS with Ni doping, while the majority-spin DOS
remains almost unchanged. The total magnetic moment decreases with excess Ni.
The total energy calculations corroborate the experimentally reported changes
in the Curie temperature and the martensitic transition temperature with x.Comment: 4 pages, 4 figures, accepted in Phys. Rev.
Ground-based detections of thermal emission from CoRoT-1b and WASP-12b
We report a new detection of the H-band thermal emission of CoRoT-1b and two
confirmation detections of the Ks-band thermal emission of WASP-12b at
secondary eclipses. The H-band measurement of CoRoT-1b shows an eclipse depth
of 0.145%\pm0.049% with a 3-{\sigma} percentile between 0.033% - 0.235%. This
depth is consistent with the previous conclusions that the planet has an
isother- mal region with inefficient heat transport from dayside to nightside,
and has a dayside thermal inversion layer at high altitude. The two Ks band
detections of WASP-12b show a joint eclipse depth of 0.299%\pm0.065%. This
result agrees with the measurement of Croll & collaborators, providing
independent confirmation of their measurement. The repeatability of the
WASP-12b measurements also validates our data analysis method. Our
measurements, in addition to a number of previous results made with other
telescopes, demonstrate that ground-based observations are becoming widely
available for characterization of atmospheres of hot Jupiters.Comment: 20 pages, including 8 figures and 1 table. Accepted for publication
in Ap
Phase-Dependent Properties of Extrasolar Planet Atmospheres
Recently the Spitzer Space Telescope observed the transiting extrasolar
planets, TrES-1 and HD209458b. These observations have provided the first
estimates of the day side thermal flux from two extrasolar planets orbiting
Sun-like stars. In this paper, synthetic spectra from atmospheric models are
compared to these observations. The day-night temperature difference is
explored and phase-dependent flux densities are predicted for both planets. For
HD209458b and TrES-1, models with significant day-to-night energy
redistribution are required to reproduce the observations. However, the
observational error bars are large and a range of models remains viable.Comment: 8 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Modulated structure in the martensite phase of Ni1.8Pt0.2MnGa: a neutron diffraction study
7M orthorhombic modulated structure in the martensite phase of Ni1.8Pt0.2MnGa
is reported by powder neutron diffraction study, which indicates that it is
likely to exhibit magnetic field induced strain. The change in the unit cell
volume is less than 0.5% between the austenite and martensite phases, as
expected for a volume conserving martensite transformation. The magnetic
structure analysis shows that the magnetic moment in the martensite phase is
higher compared to Ni2MnGa, which is in good agreement with magnetization
measurement
Coke Oven Emission Standards - A Comparative Assessment
In USA, Clean Air Act(CAA) came into existence from 1963 onwards. Consent decrees, state implementation plans and Occupational Safety and Health Administration(OSHA) regulations were put in vogue for coke oven emissions in 1970s. These were different for different batteries. Progressively charging practices were changed and many developments were also carried in door, lid, and off-take arrangements. Extensive research (both technical and financial) was carried out for the development of regul-atory alternatives in 1980s. Finally CAAA-1990 came into extistence after lot of negotiations.
Whereas in India, there were almost no standards (except CO and particulate matter emissions) existed before 1997. The philosophy behind the new standards is new to coke oven designers, suppliers, builders and operators. The environmental aspects so far remained focused on the ear-lier standards only. Adapting new concept would take time for all the players in the industry. The paper elaborates the methodology adopted for evaluation of coke oven emi-ssion standards in USA with respect to the same followed in India and at the end comes out with a proposal for
acceptance by the Regulatory Authorities
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