Department of Physics

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We report experiments on a pump-probe configuration to elucidate the formation of a plasma channel by the hydrodynamic evolution of air breakdown in laser focus. A stable air breakdown was produced by focusing a picosecond laser pulse to create a shock driven plasma channel in the laser focus for propagating a nanosecond pulse. A four fold increase in the lifetime of the channel estimated by monitoring the temporal evolution of the fluorescence of a spectral line at 504.5 nm of N+ transition 3p 3S-3s 3P0 is reported. Assuming plasma in local thermal equilibrium plasma temperature of ∼8.2 eV and an electron density of ∼1.4 × 1018 cm-3 were determined using a Stark broadening of 649.2 nm line of NII transition 3d 3D 0-4p3D in the channel. An enhancement in the electron density of the plasma channel was observed at the 7 ns delay of the nanosecond laser pulse relative to the picosecond laser pulse. © 2007 American Institute of Physics.

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Journal of Applied Physics

Increasing lifetime of the plasma channel formed in air using picosecond and nanosecond laser pulses

The sintering behaviour of magnetic bulk glassy compact has been addressed. Rapidly quenched ribbons of Fe71Nb3.7Cu1Al3Mn0.8Si13.5B7 alloy were ball milled to powder and consolidated using binder to get bulk glassy compacts. Structural and crystallization behaviour of glassy and heat-treated powders was evaluated. Differential scanning calorimetry (DSC) of the powders showed transition from amorphous to nanocrystalline phase at 800 K. Magnetic evaluation of the bulk glassy cores revealed best soft magnetic property at a sintering temperature of 775 K which was due to the formation of α-Fe(Si,Al) and Fe3(Si,Al) nanoparticles evidenced by transmission electron microscopy (TEM) and X-ray analysis. However, at this temperature the glassy compact exhibited low value of remanence, which was attributed to the demagnetization field due to porosity in the material that developed from the escaped mineral binder. At subsequent higher sintering temperature of 785 K the glassy core exhibited inferior soft magnetic properties with a low coercivity due to the formation of strongly anisotropic boride phases. Remanence value increased due to reduced demagnetization effect arising from a more compact morphology. Such reduced porosity was due to the coalescence of powder particle as observed from scanning electron microscopy (SEM). © 2006 Elsevier B.V. All rights reserved.

Journal of Materials Processing Technology

Sintering behaviour of magnetic bulk glassy cores obtained from Fe71Nb3.7Cu1Al3Mn0.8Si13.5B7 alloy powder

We have investigated the magnetic properties of a Co36 Fe36 Si4 B20 Nb4 alloy developed for high temperature soft magnetic applications. Studies of the crystallization of the alloy, prepared in the form of amorphous ribbons by melt-spinning, were performed using differential scanning calorimetry and X-ray diffraction. These studies, and further validation by transmission electron microscopy, suggested the formation of (CoFe)2 Si nanoparticles dispersed in an amorphous matrix during the first stage of crystallization which occurred at around 845 K. The formation of this nanocrystalline state by annealing at temperatures between 825 and 875 K not only raised the Curie temperature of the material to above 1000 K but also gave superior soft magnetic properties with a coercivity less than 50 A/m (625 mOe). However, at higher annealing temperatures, formation of strongly anisotropic phases with out of plane domain magnetization, observed by magnetic force microscopy, resulted in deterioration of the soft magnetic properties. © 2007 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials

Soft magnetic properties of a high temperature CoFeSiBNb nanocrystalline alloy

Magnetic Barkhausen emissions (MBE) and magnetic hysteresis loop techniques have been used to correlate the magnetic properties with creep behaviour in modified 9Cr-1Mo steel. Formation of massive carbides like M23C6, Laves phase (Fe2Mo) and Z-phase are the cause of creep failure in the present material. The root mean square voltage of MBE signal increased from the secondary stage of creep. This was attributed to the growth of carbides. Microcracks generated by massive and brittle precipitates induced demagnetizing fields that restricted domain wall movement, resulting in continuous reduction of remanence during creep. © 2007 Acta Materialia Inc.

Scripta Materialia

Magnetic evaluation of creep in modified 9Cr-1Mo steel

Magnetic and mechanical properties were evaluated for various heat-treated Cu-strengthened HSLA-100 steels. After austenitizing at 1183 K for one hour and then water quenching, the material was aged at different temperatures ranging between 623 K and 973 K for one hour. Scanning and transmission electron microscopes with an energy dispersive X-ray attachment were used for microstructural analysis. Nanosize coherent copper precipitates caused an increase in hardness with increase in ageing temperature. The steel exhibited maximum hardness at 773 K followed by decrease in hardness with over-ageing. Copper precipitates coarsened and lost coherency during over-ageing. Ageing behaviour again exhibited an increase in hardness at 973 K due to the formation of new martensite islands. Magnetic hysteresis loop and magnetic Barkhausen emissions techniques were used to characterize the aged materials. No correlation was observed between the magnetic parameters and hardness. The results were explained by the fact that nanosize Cu precipitates, the size of which is much smaller than the domain wall width, did not influence the magnetic domain dynamics.

Philosophical Magazine

Magnetic and mechanical properties of Cu-strengthened aged HSLA-100 steel

We exploit a simple and accurate matrix method to analyze the effects of introducing a linear chirp either in thickness or in refractive index of the cladding layers on the propagation characteristics (loss and dispersion) of 1D photonic band gap planar Bragg reflection waveguides (BRWs). We show that an appropriate chirp in the otherwise periodic claddings of finite extent BRWs could be gainfully exploited to tailor its leakage loss and waveguide dispersion features. In particular, we theoretically demonstrate that for some reported sample BRWs, leakage loss and waveguide dispersion could be significantly reduced by a factor of 30-50 and by about two orders of magnitude, respectively as compared to un-chirped BRWs. Furthermore, we also show that in contrast to un-chirped BRWs, how chirped BRWs could be designed with attractive feature like much less number of cladding layers and nearly wavelength independent losses. Our analysis and proposal should serve as a useful design tool to tailor the propagation characteristics of BRWs. © 2007 Springer Science+Business Media, LLC.

Optical and Quantum Electronics

Loss and dispersion tailoring in 1D photonics band gap Bragg reflection waveguides: Finite chirped claddings as a design tool

We perform an ab initio study of the motion of the nano sized water dimer through a single-walled carbon nanotube (SWCNT), the stability of an encapsulated one-dimensional (1D) water chain inside SWCNT, and the H2 O -induced structural, energetic, electronic, and optical properties of the SWCNTs. The adsorption of the water molecules is caused by the dispersion forces, i.e., the van der Waals (vdW) interactions. Thus, the role of the vdW interactions in the estimation of the BE for the weakly bound adsorbates cannot be ignored as has been done in several earlier publications. We find that a single H2 O molecule or single water dimer or a 1D chain of water dimers is trapped inside the medium-sized (6,6) carbon nanotube placed in vacuum. However, the H2 O molecule or water dimer may be transmitted in case the tube is surrounded by water or water vapor at high vapor pressure at high temperatures. On the other hand, a chain of single H2 O molecules or more number of the encapsulated H2 O molecules is very weakly coupled to the wide (10,10) carbon nanotube and can, thus, easily transmit through the carbon nanotube in agreement with the recent experiments. Further, appreciable adsorption both inside and on the surface of the (10,10) carbon nanotube is predicted in concurrence with the experiments. The small (medium-sized) diameter tubes will adsorb strongly (accommodate) the water molecules outside (inside) the nanotubes. The H2 O adsorption converts the conducting small-diameter zigzag (5,0) tube into a semiconductor. Further, the adsorption reduces the band gap of the semiconducting achiral zigzag (10,0) nanotube but increases the band gap of a chiral semiconducting (4,2) tube. The adsorbed H2 O molecules increase the electrical conductivity in agreement with the experiment. The overall peak structure in the optical absorption for the pristine tube is not altered significantly by the adsorption except for small alterations in the energy locations and the relative intensities of the peaks in the achiral tubes. © 2007 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Ab initio study of H2 O and water-chain-induced properties of carbon nanotubes

The results of an ab initio study of the properties of the ice nanotubes in isolation or inside the single-walled carbon nanotubes (SWCNTs) are reported. We consider both the van der Waals (vdW) interactions and the atomic vibrational entropy effects and find that all the ice nanotubes having different m -polygonal (m=4-8) cross sections either in isolation or lying inside the (10,10) SWCNTs are stable. The octagonal ice nanotube is the most stable one followed by the hexagonal ice nanotube, a result in disagreement with the earlier workers. Thus, striking differences between the results of an ab initio and the empirical potential studies are observed. The ice nanotubes are coupled to the carbon nanotube only by the vdW interactions. Our analysis of the computed binding energy and the structural parameters shows that the ice tube inside the wide carbon nanotube experimentally observed by Maniwa [J. Phys. Soc. Jpn. 71, 2863 (2002)] has a hexagonal structure. The electron energy gap for the ice nanotubes lie in the ultraviolet in the energy range, 3.32-3.92 eV. The optical absorption for the isolated hexagonal and octagonal ice nanotubes appears in the ultraviolet region in the energy range, 3.7-4.0 eV. The optical absorption of the (10,10) carbon nanotube is not affected significantly by the encapsulated hexagonal and octagonal ice nanotubes with the exception that all the absorption peaks are displaced toward the low-energy side by about 0.1-0.2 eV. © 2007 The American Physical Society.

Ab initio study of ice nanotubes in isolation or inside single-walled carbon nanotubes

We study a generic open quantum system where the coupling between the system and its environment is of an energy-preserving quantum nondemolition (QND) type. We obtain the general master equation for the evolution of such a system under the influence of a squeezed thermal bath of harmonic oscillators. From the master equation it can be seen explicitly that the process involves decoherence or dephasing without any dissipation of energy. We work out the decoherence-causing term in the high- and zero-temperature limits and check that they match with known results for the case of a thermal bath. The decay of the coherence is quantified as well by the dynamics of the linear entropy of the system under various environmental conditions. We make a comparison of the quantum statistical properties between QND and dissipative types of evolution using a two-level atomic system and a harmonic oscillator. © 2007 IOP Publishing Ltd.

Journal of Physics A: Mathematical and Theoretical

Dynamics of decoherence without dissipation in a squeezed thermal bath

We have investigated the ac magnetic susceptibility and magnetic heating of aqueous suspensions of γ- Fe2 O3 nanoparticles embedded in alginate hydrogel matrix and isolated γ- Fe2 O3 and Fe3 O4 nanoparticles coated with tetramethyl ammonium hydroxide. All three ferrofluids were characterized by measuring the dc magnetization, ac susceptibility, and magnetic heating. We found that significant Ńel relaxation is present in all samples, but only the isolated nanoparticle ferrofluids show any significant feature associated with Brownian relaxation near the freezing temperature of the carrier liquid. The heating rate of the ferrofluids varies systematically with the magnitude of the Brownian relaxation peak, despite similar values of the absolute magnetization. These results highlight the importance of the Brownian relaxation for heating applications incorporating magnetic nanoparticles. © 2007 American Institute of Physics.