arXiv subjects

A. Bhattacharya

Publications and source records attributed to A. Bhattacharya.

At least 91 records · Page 5Linked to original sources

arXiv2015

Spectral weight redistribution in (LaNiO3)n/(LaMnO3)2 superlattices from optical spectroscopy

We have studied the optical properties of four (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ superlattices (SL) ($n$=2, 3, 4, 5) on SrTiO$_3$ substrates. We have measured the reflectivity at temperatures from 20 K to 400 K, and extracted the optical conductivity through a fitting procedure based on a Kramers-Kronig consistent Lorentz-Drude model. With increasing LaNiO$_3$ thickness, the SLs undergo an insulator-to-metal transition (IMT) that is accompanied by the transfer of spectral weight from high to low frequency. The presence of a broad mid-infrared band, however, shows that the optical conductivity of the (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ SLs is not a linear combination of the LaMnO$_3$ and LaNiO$_3$ conductivities. Our observations suggest that interfacial charge transfer leads to an IMT due to a change in valence at the Mn and Ni sites.

cond-mat.str-el↗

arXiv2015

Pathway to the Galactic Distribution of Planets: Combined Spitzer and Ground-Based Microlens Parallax Measurements of 21 Single-Lens Events

We present microlens parallax measurements for 21 (apparently) isolated lenses observed toward the Galactic bulge that were imaged simultaneously from Earth and Spitzer, which was ~1 AU West of Earth in projection. We combine these measurements with a kinematic model of the Galaxy to derive distance estimates for each lens, with error bars that are small compared to the Sun's Galactocentric distance. The ensemble therefore yields a well-defined cumulative distribution of lens distances. In principle it is possible to compare this distribution against a set of planets detected in the same experiment in order to measure the Galactic distribution of planets. Since these Spitzer observations yielded only one planet, this is not yet possible in practice. However, it will become possible as larger samples are accumulated.

astro-ph.EP↗

arXiv2015

OGLE-2011-BLG-0265Lb: a Jovian Microlensing Planet Orbiting an M Dwarf

We report the discovery of a Jupiter-mass planet orbiting an M-dwarf star that gave rise to the microlensing event OGLE-2011-BLG-0265. Such a system is very rare among known planetary systems and thus the discovery is important for theoretical studies of planetary formation and evolution. High-cadence temporal coverage of the planetary signal combined with extended observations throughout the event allows us to accurately model the observed light curve. The final microlensing solution remains, however, degenerate yielding two possible configurations of the planet and the host star. In the case of the preferred solution, the mass of the planet is $M_{\rm p} = 0.9\pm 0.3\ M_{\rm J}$, and the planet is orbiting a star with a mass $M = 0.22\pm 0.06\ M_\odot$. The second possible configuration (2$σ$ away) consists of a planet with $M_{\rm p}=0.6\pm 0.3\ M_{\rm J}$ and host star with $M=0.14\pm 0.06\ M_\odot$. The system is located in the Galactic disk 3 -- 4 kpc towards the Galactic bulge. In both cases, with an orbit size of 1.5 -- 2.0 AU, the planet is a "cold Jupiter" -- located well beyond the "snow line" of the host star. Currently available data make the secure selection of the correct solution difficult, but there are prospects for lifting the degeneracy with additional follow-up observations in the future, when the lens and source star separate.

astro-ph.EP↗

arXiv2014

Oscillatory Non-collinear Magnetism Induced by Interfacial Charge Transfer in Metallic Oxide Superlattices

Interfaces between correlated complex oxides are promising avenues to realize new forms of magnetism that arise as a result of charge transfer, proximity effects and locally broken symmetries. We report upon the discovery of a non-collinear magnetic structure in superlattices of the ferromagnetic metallic oxide La2/3Sr1/3MnO3 (LSMO) and the correlated metal LaNiO3 (LNO). The exchange interaction between LSMO layers is mediated by the intervening LNO, such that the angle between the magnetization of neighboring LSMO layers varies in an oscillatory manner with the thickness of the LNO layer. The magnetic field, temperature, and spacer thickness dependence of the non-collinear structure are inconsistent with the bilinear and biquadratic interactions that are used to model the magnetic structure in conventional metallic multilayers. A model that couples the LSMO layers to a helical spin state within the LNO fits the observed behavior. We propose that the spin-helix results from the interaction between a spatially varying spin susceptibility within the LNO and interfacial charge transfer that creates localized Ni2+ states. This provides a new approach to engineering non-collinear spin textures in metallic oxide heterostructures that can be exploited in devices based on both spin and charge transport.

cond-mat.mtrl-sci↗

arXiv2014

Origin and Evolution of Structure and Nucleosynthesis for Galaxies in the Local Group

The Milky Way is the product of a complex evolution of generations of mergers, collapse, star formation, supernova and collisional heating, radiative and collisional cooling, and ejected nucleosynthesis. Moreover, all of this occurs in the context of the cosmic expansion, the formation of cosmic filaments, dark-matter halos, spiral density waves, and emerging dark energy. In this review we summarize observational evidence and discuss recent calculations concerning the formation, evolution nucleosynthesis in the galaxies of the Local-Group. In particular, we will briefly summarize observations and simulations for the dwarf galaxies and the two large spirals of the Local Group. We discuss how galactic halos form within the dark matter filaments that define a super-galactic plane. Gravitational interaction along this structure leads to streaming flows toward the two dominant galaxies in the cluster. These simulations and observations also suggest that a significant fraction of the Galactic halo formed as at large distances and then arrived later along these streaming flows. We also consider the insight provided by observations and simulations of nucleosynthesis both within the galactic halo and dwarf galaxies in the Local Group.

astro-ph.GA↗

arXiv2014

On The Prediction of The Statistical Model

The number of quark flavours $n_{f}$, has been derived to be equal to 6 in agreement with the experimental observations. The colour factor ratio $\frac{C_{A}}{C_{F}}$ has also been computed and is found to be in exact agreement with the corresponding QCD prediction. Further, the $e^{+}e^{-}$ pair annihilation cross section into hadrons at very high energy is found to be a function only of the fractal dimension of the hadron.

hep-ph↗

arXiv2013

A Super-Jupiter orbiting a late-type star: A refined analysis of microlensing event OGLE-2012-BLG-0406

We present a detailed analysis of survey and follow-up observations of microlensing event OGLE-2012-BLG-0406 based on data obtained from 10 different observatories. Intensive coverage of the lightcurve, especially the perturbation part, allowed us to accurately measure the parallax effect and lens orbital motion. Combining our measurement of the lens parallax with the angular Einstein radius determined from finite-source effects, we estimate the physical parameters of the lens system. We find that the event was caused by a $2.73\pm 0.43\ M_{\rm J}$ planet orbiting a $0.44\pm 0.07\ M_{\odot}$ early M-type star. The distance to the lens is $4.97\pm 0.29$\ kpc and the projected separation between the host star and its planet at the time of the event is $3.45\pm 0.26$ AU. We find that the additional coverage provided by follow-up observations, especially during the planetary perturbation, leads to a more accurate determination of the physical parameters of the lens.

astro-ph.EP↗

arXiv2013

Composite Fermion Approach to Diquark and Heavy-Light Baryon Masses

A composite Fermion (CF) model of quasi particle has been used to describe a diquark. Considering baryons in quark-diquark configuration,the masses of the heavy light baryons like $Λ_{c}^{+}$, $Σ_{c}^{+}$, $Ξ_{c}^{0}$, $Ω_{c}^{0}$ and $Λ_{b}^{+}$, $Σ_{b}^{+}$, $Ξ_{b}^{0}$, $Ω_{b}^{-}$ have been computed using this CF model of the diquark. The results are found to be in good agreement with the corresponding experimental findings. It has been suggested that the diquark can be well described in the framework of CF model in a gauge invariant way.

hep-ph↗

arXiv2013

Correlation between defect and magnetism of Ar9+ implanted and un-implanted Zn0.95Mn0.05O thin films suitable for electronic application

Sol-gel derived thin films of Zn0.95Mn0.05O have been implanted with Ar9+ ions with doses viz. 5x10e14 ions/cm2 (low), 1x10e15 ions/cm2 (intermediate) and 1x10e16 ions/cm2 (high). Structural, morphological, optical and magnetic properties of the films have been investigated. Structural study confirmed single phase, wurtzite structure of the films. The absence of impurity phase has been confirmed from several measurements. Ion implantation induces a large concentration of point defects into the films as identified from optical study. All films exhibit well above room temperature (RT) intrinsic ferromagnetism (FM) as evidenced from field and temperature dependent magnetization measurements. The magnetization attains the maximum value for high dose of Ar9+ ion implanted film. It shows RT saturation magnetization (MS) value of 0.69emu/gm. The observed FM has been correlated with proportion of intrinsic defects, such as, zinc and oxygen vacancies and the values of MS. Defect induced formation of bound magnetic polaron actually controls the FM. The utility of these films in transparent spin electronic device has also been exhibited.

cond-mat.mtrl-sci↗

arXiv2013

Anamolous reduction of magnetic coercivity of graphene oxide and reduced graphene oxide upon cooling

In this report we present the temperature evolution of magnetic coercivity of graphene oxide (GO) and reduced graphene oxide (RGO). We report an anamolous decrease in coercivity of GO and RGO with decreasing temperature. We could explain this anamolous behavior invoking the inherent presence of ripple in graphene. We observe antiferromagnetic and ferromagnetic behavior at room temperature for GO and RGO respectively, but at low temperatures both shows paramagnetic behavior.

cond-mat.mtrl-sci↗

arXiv2012

Interface Ferromagnetism in a SrMnO3/LaMnO3 Superlattice

Resonant soft x-ray absorption measurements at the O K edge on a SrMnO3/LaMnO3 superlattice show a shoulder at the energy of doped holes, which corresponds to the main peak of resonant scattering from the modulation in the doped hole density. Scattering line shape at the Mn L3,2 edges has a strong variation below the ferromagnetic transition temperature. This variation has a period equal to half the superlattice superperiod and follows the development of the ferromagnetic moment, pointing to a ferromagnetic phase developing at the interfaces. It occurs at the resonant energies for Mn3+ and Mn4+ valences. A model for these observations is presented, which includes a double-exchange two-site orbital and the variation with temperature of the hopping frequency tij between the two sites.

cond-mat.str-el↗

arXiv2011

Rapidity and Pseudorapidity distributions of the Various Hadron-Species Produced in High Energy Nuclear Collisions : A Systematic Approach

With the help of a phenomenological approach outlined in the text in some detail, we have dealt here with the description of the plots on rapidity and pseudorapidity spectra of some hadron-secondaries produced in various nucleus-nucleus interactions at high energies. The agreement between the measured data and the attempted fits are, on the whole, modestly satisfactory excepting a very narrow central region in the vicinity of y=$η$=0. At last, hints to how the steps suggested in the main body of the text to proceed with the description of the measured data given in the plots could lead finally to a somewhat systematic methodology have also been made.

hep-ph↗

arXiv2011

Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices

We demonstrate that delta-doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AF) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Delta-doped carriers are inserted into a manganite superlattice (SL) by a digital-synthesis technique. Theoretical consideration of these additional carriers show that they cause a local enhancement of ferromagnetic (F) double-exchange with respect to AF superexchange, resulting in local canting of the AF spins. This leads to a highly modulated magnetization, as measured by polarized neutron reflectometry. The spatial modulation of the canting is related to the spreading of charge from the doped layer, and establishes a fundamental length scale for charge transfer, transformation of orbital occupancy and magnetic order in these manganites. Furthermore, we confirm the existence of the canted, AF state as was predicted by de Gennes [P.-G. de Gennes, Phys. Rev. 118, 141 (1960)], but had remained elusive.

cond-mat.str-el↗

arXiv2011

Fractal Space Time and Variation of Fine Structure Constant

The effect of fractal space time of the quantum particles on the variation of the fine structure constant $α$ has been studied. The variation of fine structure constant has been investigated around De Broglie length $λ$ and compton length $λ_{c}$ and it has been suggested that the variation may be attributed to the dimensional transition of the particle trajectories between these two quantum domains. Considering the Fractal universe with a small inhomogeneity in the mass distribution in the early universe, the variation of the fine structure constant have been investigated between matter and radiation dominated era. The fine structure constant shows a critical behaviour with critical exponent which is fractional and shows a discontinuity. It has been suggested that the variation of the fine structure constant may be attributed to the intrinsic scale dependance of the fundamental constants of nature.

astro-ph.CO↗

arXiv2011

Resonant soft x-ray scattering from La(1-x)Sr(x)MnO(3) quantum wire arrays

We describe a strategy for using resonant soft x-ray scattering (RSXS) to study the electronic structure of transition metal oxide quantum wires. Using electron beam lithography and ion milling, we have produced periodic, patterned arrays of colossal magnetoresistance (CMR) phase La(1-x)Sr(x)MnO(3) consisting of ~ 5000 wires, each of which is 80 nm in width. The scattered intensity exhibits a series of peaks that can be interpreted as Bragg reflections from the periodic structure or, equivalently, diffraction orders from the grating-like structure. RSXS measurements at the Mn L(2,3) edge, which has a large magnetic cross section, show clear evidence for a magnetic superstructure with a commensurate period of five wires, which we interpret as commensurately modulated antiferromagnetism. This superstructure, which is accompanied by non-trivial reorganization of the magnetization within each wire, likely results from classical dipole interactions among the wires. We introduce a simple, exactly soluble, analytic model of the scattering that captures, semi-quantitatively, the primary features in the RSXS data; this model will act as a foundation for forthcoming, detailed studies of the magnetic structure in these systems.

cond-mat.str-el↗

arXiv2011

Band gap engineering by functionalization of BN sheet

From first principles calculations, we investigate the stability and physical properties of single layer h-BN sheet chemically functionalized by various groups viz. H, F, OH, CH3, CHO, CN, NH2 etc. We find that full functionalization of h-BN sheet with these groups lead to decrease in its electronic band gap, albeit to different magnitudes varying from 0.3 eV to 3.1 eV, depending upon the dopant group. Functionalization by CHO group, in particular, leads to a sharp decrease in the electronic band gap of the pristine BN sheet to ~ 0.3 eV, which is congenial for its usage in transistor based devices. The phonon calculations on these sheets show that frequencies corresponding to all their vibrational modes are real (positive), thereby suggesting their inherent stability. The chemisorption energies of these groups to the B and N atoms of the sheet are found to lie in the range of 1.5 -6 eV.

cond-mat.mtrl-sci↗

arXiv2011

A Comparative Study Between a Micromechanical Cantilever Resonator and MEMS-based Passives for Band-pass Filtering Application

Over the past few years, significant growth has been observed in using MEMS based passive components in the RF microelectronics domain, especially in transceiver components. This is due to some excellent properties of the MEMS devices like low loss, excellent isolation etc. in the microwave frequency domain where the on-chip passives normally tend to become leakier and degrades the transceiver performance. This paper presents a comparative analysis between MEMS-resonator based and MEMS-passives based band-pass filter configurations for RF applications, along with their design, simulation, fabrication and characterization. The filters were designed to have a center frequency of 455 kHz, meant for use as the intermediate frequency (IF) filter in superheterodyne receivers. The filter structures have been fabricated in PolyMUMPs process, a three-polysilicon layer surface micromachining process.

cs.OH↗

arXiv2011

Control of octahedral rotations in (LaNiO$_3$)$_{n}$/(SrMnO$_3$)$_m$ superlattices

Oxygen octahedral rotations have been measured in short-period (LaNiO$_3$)$_n$/(SrMnO$_3$)$_m$ superlattices using synchrotron diffraction. The in-plane and out-of-plane bond angles and lengths are found to systematically vary with superlattice composition. Rotations are suppressed in structures with $m>n$, producing a nearly cubic form of LaNiO$_3$. Large rotations are present in structures with $m<n$, leading to reduced bond angles in SrMnO$_3$. The metal-oxygen-metal bond lengths decrease as rotations are reduced, in contrast to behavior previously observed in strained, single layer films. This result demonstrates that superlattice structures can be used to stabilize non-equilibrium octahedral behavior in a manner distinct from epitaxial strain, providing a novel means to engineer the electronic and ferroic properties of oxide heterostructures.

cond-mat.mtrl-sci↗