Department of Physics

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Low energy optical modes of molecular beam epitaxy-grown In1-x Gax N thin films with 0≤x≤0.6 are investigated using infrared reflectance measurements. We found that the reflectance of the films for wave vectors in the range from 600 to 800 cm-1 is determined by the high energy E1 (LO) -plasmon coupled modes. In the higher energy regime of the UV-visible reflectance spectrum of InN, critical points with energies 4.75, 5.36, and 6.12 eV belonging to A and B structures are observed. The energies of these critical points increase with increasing values of x, similar to the band gap energy of these films. © 2010 American Institute of Physics.

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Applied Physics Letters

Investigation of E1 (LO) phonon-plasmon coupled modes and critical points in In1-x Gax N thin films by optical reflectance measurements

Nonlinear optical (NLO) activity of a molecule in general is connected to fundamental issues like charge transfer, conjugation, polarization, and recently suggested and less investigated diradical character. Extensively studied molecules for negative second order hyperpolarizabilities are the centrosymmetric squaraine derivatives (SQ) which have absorption in the region greater than 600 nm. These have no formal electronic structure but can be represented as a mixture of zwitterionic and diradicaloid valence bond resonance structures. In this work we have designed and synthesized arylamino SQ, 1?10, which bias the valence bond resonance picture completely to the zwitterionic type and totally eliminates the diradicaloid contribution (estimated using computational studies to be zero). Due to this, the lowest energy transitions are blue-shifted to the region less than 500 nm. We have carried out degenerate four wave mixing (DFWM) studies for estimation of ? values of these molecules using a 800 nm wavelength laser. The ? values are measured under nonresonant conditions and also at intensities where two photon absorption does not play any role. Nonlinear absorption properties of these molecules are studied through Z scan technique. The ? values, though smaller than the SQ having absorption in the red region, are reasonably large and range from ?1.2 to ?6.9 - 10 ?33 esu. High level computational techniques and model molecules have been utilized to understand the transition and the NLO activity in these molecules. © 2010 American Chemical Society.

Journal of Physical Chemistry C

Third order nonlinear optical properties of squaraine dyes having absorption below 500 nm: A combined experimental and theoretical investigation of closed shell oxyallyl derivatives

The giant magneto-impedance of melt spun CoxFe72-xNb4Si4B20(x=10, 20, 36, 50) amorphous and nanostructured ribbons have been investigated. Alloys have been optimized at the driving current amplitude, frequency and found that amorphous ribbon of nominal composition of Co36Fe36Nb4Si4B20 shown maximum GMI ratio of 13%. The behaviour of the driving current amplitude on the GMI behaviour was studied and the sample was optimized for driving current amplitude, Iac=10 mA. The frequency dependence of the GMI behaviour was studied for the ribbon sample Co36Fe36Nb4Si4B20 at frequency in the range of 100 kHz-1.2 MHz of the optimized driving current amplitude and it was found that the sample showed the maximum GMI behaviour at f=700 kHz. The optimized samples were Joule heated at the current density J=0-35 A/m2 for a period of 1 min. The GMI ratio initially increased then progressively deteriorated with J, but after a certain range it shows up to 16% of improvement in the magneto-impedance value due the increase of nanocrystalline volume fraction. The asymmetry in the GMI profile was observed for the sample Joule heated at J=1-5 A/m2 for 1 min. © 2009.

Journal of Magnetism and Magnetic Materials

Magneto-Impedance behavior of Co-Fe-Nb-Si-B-based ribbons

The 6.5wt% Silicon steel has always a technological importance owing to its soft magnetic properties and low core losses. In this research, this steel is prepared in the form of continuous ribbons of 25 mm width and 35 mm thickness by rapid solidification technique using melt spinner. The as-cast ribbons show single phase of α-FeSi as observed from x-ray diffraction. By transmission electron microscopy (TEM), the alloy shows disordered A2 structure in as-cast ribbon while B2 ordered phase forms from the A2 matrix at annealed condition. Two magnetization temperatures are observed at 670 and 760°C due to the phase transformation of B2 and A2 structures, respectively. The alloy showed enhancement in the soft magnetic properties with reduction in coercivity from 1.90Oe in the as-cast state to 0.58Oe when annealed at 850°C. The lowering of coercivity is due to the formation of ordered B2 phase during annealing. © 2010 TIIM, India.

Transactions of the Indian Institute of Metals

Development of rapidly solidified 6.5 wt% silicon steel for magnetic applications

The martensitic transition and the ferro- to paramagnetic transition have been studied in a series of Ga excess Ni-Mn-Ga specimens [Ni 2-xMnGa1+x (0.4≤ x≤ 0.9)] by differential scanning calorimetry and magnetization measurements. The martensitic transition exhibits a hysteresis whose width is similar to Ni2MnGa, indicating that the transition is thermoelastic. The latent heat of transformation is comparable with other Ni-Mn-Ga alloys. A substantial increase in the martensitic transition temperature is observed due to Ga doping. Interestingly, the x-ray diffraction pattern of all the compositions studied show a modulated martensitic structure in the martensitic phase. © (2010) Trans Tech Publications.

Materials Science Forum

Structural, thermal and magnetic properties of Ga excess Ni-Mn-Ga

Superparamagnetic iron oxide nanoparticles (SPIONs) were synthesized from Fe3+/Fe2+ salts in the presence of different coating bio-materials such as glycine, gelatin, gum arabic and mixture of gelatin and gum Arabic using NaOH as precipitating agent. The average diameter of crystallites for different samples is in the range of 7.8-13.5 nm deducted from X-ray diffraction (XRD). The size and size distribution of superparamagnetic iron oxide nanoparticles are also determined from the measured magnetization curve fitted by magnetic core and non-magnetic shell model based on the Langevin function with a Schulz size distribution. Particle size extracted from magnetization curve is consistent with crystallite size obtained from XRD. Further, non-magnetic shell is of the order of ~0.7 nm thick. The thickness of biocompatible protective layer is found to be ~4.5 nm. Fourier transform infrared (FTIR) spectra reveal the binding of coating materials on the surface of SPIONs. Atomic force microscope (AFM) images indicate the formation of colloidal cluster of size ~120 nm. Coated outer shell of organic materials used in the present study not only provided the stability to the nanoparticles in solution but also can be highly useful for biomedical applications due to their known bio-compatibility. © 2010 American Scientific Publishers.

Advanced Science Letters

Preparation and characterization of biocompatible and Water-Dispersible Superparamagnetic Iron Oxide Nanoparticles (SPIONs)

Conventionally, it is assumed that a single molecule has only one absolute electronic property. Molecular electronics, molecular machines, intelligent drugs are some of the fields that require atomic scale control of molecular properties. In spite of remarkable achievements in the last two decades, absolute control of molecular properties has not been achieved. Here, experimental evidences argue against assigning a fixed property to a molecule. A molecule might structurally accommodate itself to the new environment and its electronic properties might change accordingly. By isolating electronic properties of all the conformers of a Rose Bengal molecule one by one, a map of its complete electronic properties is drawn here. The existing concept of one absolute electronic property of a molecule is true only for those molecules, which have only one stable conformer. For others, depending on the conformer, multiple distinct electronic properties may co-exist, leading to a variable output in an electronic characterization. Thus, we present a generalized method for characterizing/resolving collective electronic properties that emerge statistically. The method could be used for designing molecular switches for collective and evolutionary information processing. © 2010 the Owner Societies.

Physical Chemistry Chemical Physics

A new approach to extract multiple distinct conformers and co-existing distinct electronic properties of a single molecule by point-contact method

We extend our recent work and study implications of the standard model with four generations (SM4) for rare B and K decays. We again take seriously the several 2-3σ anomalies seen in B, Bs decays and interpret them in the context of this simple extension of the SM. SM4 is also of course of considerable interest for its potential relevance to dynamical electroweak symmetry breaking and to baryogenesis. Using experimental information from processes such as B→Xsγ, Bd and Bs mixings, indirect CP-violation from KL→ππ etc. along with oblique corrections, we constrain the relevant parameter space of the SM4, and find mt′ of about 400-600 GeV with a mixing angle |Vt′b*Vt ′s| in the range of about 0.05-1.4×10-2 and with an appreciable CP-odd associated phase, are favored by the current data. Given the unique role of the CP asymmetry in Bs→ψ due to its gold-plated nature, correlation of that with many other interesting observables, including the semileptonic asymmetry (ASL) are studied in SM4. We also identify several processes, such as B→Xsνν̄, KL→π0νν̄ etc., that are significantly different in SM4 from the SM. Experimentally the very distinctive process Bs→μ+μ- is also discussed; the branching ratio can be larger or smaller than in SM, (3.2→4.2)×10-9, by a factor of O(3). © 2010 The American Physical Society.

Physical Review D - Particles, Fields, Gravitation and Cosmology

Standard model with four generations: Selected implications for rare B and K decays

We study universality in the kinetics of spinodal phase separation in unstable thin liquid films, via simulations of the thin film equation. It is shown that, in addition to morphology and free energy, the number density of local maxima in the film profile can also be used to identify the early, late, and intermediate stages of spinodal phase separation. A universal curve between the number density of local maxima and rescaled time describes the kinetics of the early stage in d=2 and 3. The Lifshitz-Slyozov exponent of -1/3 describes the kinetics of the late stage in d=2 even in the absence of coexisting equilibrium phases. © 2010 The American Physical Society.

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

Kinetics of spinodal phase separation in unstable thin liquid films

Ultrahigh-energy neutrons and pions are likely to be produced in particle interactions inside cosmic ray sources and subsequently decay to neutrinos and other secondary particles [π±→μ ±νμ(ν̄μ),μ±→e ±ν̄μ(νμ)νe(ν̄e)]. In high magnetic fields of the cosmic acceleration sites, the ultrahigh-energy-charged particles may lose energy significantly due to synchrotron radiation before decay. We show that for gamma-ray bursts in the internal shock model, the flux of very high-energy antineutrinos (ν̄e) produced from decaying ultrahigh-energy neutrons can be more than the total neutrino flux produced in pion decay depending on the values of their Lorentz factors, luminosities, and variability times. © 2010 The American Physical Society.