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Search Results: 1 - 10 of 1887 matches for " Sugata Mukherjee "
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A Comparative Study of Electronic Properties of Bulk MoS2 and Its Monolayer Using DFT Technique: Application of Mechanical Strain on MoS2 Monolayer  [PDF]
Sohail Ahmad, Sugata Mukherjee
Graphene (Graphene) , 2014, DOI: 10.4236/graphene.2014.34008
Abstract: Electronic structure calculation of bulk and monolayer MoS2 has been performed using plane wave pseudopotential method based on density functional theory. The indirect band gap in the bulk MoS2 was found to be 0.9 eV, whereas in the monolayer-MoS2 the band gap of 1.57 eV was found to be direct one. The calculated physical parameters of monolayer MoS2 are found to be very close to the bulk MoS2 and compare well with available experimental and other theoretical results. The calculated density of states (DOS) may help explain this change in the nature of band gap in bulk and in monolayer MoS2. A further variation in band gap has been observed in MoS2 monolayer on applying biaxial strain.
Electronic Properties of Boron and Nitrogen doped graphene: A first principles study
Sugata Mukherjee,T. P. Kaloni
Physics , 2012, DOI: 10.1007/s11051-012-1059-2
Abstract: Effect of doping of graphene either by Boron (B), Nitrogen (N) or co-doped by B and N is studied using density functional theory. Our extensive band structure and density of states calculations indicate that upon doping by N (electron doping), the Dirac point in the graphene band structure shifts below the Fermi level and an energy gap appears at the high symmetric K-point. On the other hand, by B (hole doping), the Dirac point shifts above the Fermi level and a gap appears. Upon co-doping of graphene by B and N, the energy gap between valence and conduction bands appears at Fermi level and the system behaves as narrow gap semiconductor. Obtained results are found to be in well agreement with available experimental findings.
A Heuristic Reputation Based System to Detect Spam Activities in a Social Networking Platform, HRSSSNP  [PDF]
Manoj Rameshchandra Thakur, Sugata Sanyal
Social Networking (SN) , 2013, DOI: 10.4236/sn.2013.21005
Abstract: The introduction of the social networking platform has drastically affected the way individuals interact. Even though most of the effects have been positive, there exist some serious threats associated with the interactions on a social networking website. A considerable proportion of the crimes that occur are initiated through a social networking platform [1]. Almost 33% of the crimes on the internet are initiated through a social networking website [1]. Moreover activities like spam messages create unnecessary traffic and might affect the user base of a social networking platform. As a result preventing interactions with malicious intent and spam activities becomes crucial. This work attempts to detect the same in a social networking platform by considering a social network as a weighted graph wherein each node, which represents an individual in the social network, stores activities of other nodes with respect to itself in an optimized format which is referred to as localized data set. The weights associated with the edges in the graph represent the trust relationship between profiles. The weights of the edges along with the localized data set are used to infer whether nodes in the social network are compromised and are performing spam or malicious activities.
Remote Presence: Technologies for ‘Beaming’ Teachers Where They Cannot Go
Sugata Mitra
Journal of Emerging Technologies in Web Intelligence , 2009, DOI: 10.4304/jetwi.1.1.55-59
Abstract: There are schools where good teachers do not want to go. The reasons for this are discussed in the context of both developing and developed economies. It is proposed that learning in such areas, particularly for children, needs the support of appropriate technology. The concept of “presence” of a mediator is discussed in the context of distance education. Some results from experiences in teaching at a distance over Skype are discussed along with some of the key technology related issues for such remote teaching.A design is presented for an experimental ROV (remotely operated vehicle) device for distance education and its possible implications discussed. The term “Instructional Robotics” is suggested.
Peculiar early-type galaxies in the SDSS Stripe82
Sugata Kaviraj
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.16714.x
Abstract: We explore the properties of `peculiar' early-type galaxies (ETGs) in the local Universe, that show (faint) morphological signatures of recent interactions such as tidal tails, shells and dust lanes. Standard-depth (51s exposure) multi-colour galaxy images from the Sloan Digital Sky Survey (SDSS) are combined with the significantly (2 mags) deeper monochromatic images from the public SDSS Stripe82 to extract, through careful visual inspection, a robust sample of nearby, luminous ETGs, including a subset of ~70 peculiar systems. 18% of ETGs exhibit signs of disturbed morphologies (e.g. shells), while 7% show evidence of dust lanes and patches. The peculiar ETG population is found to preferentially inhabit low-density environments (outskirts of clusters, groups or the field). An analysis of optical emission-line ratios indicates that the fraction of peculiar ETGs that are Seyferts or LINERs (19.4%) is twice the corresponding values in their relaxed counterparts (10.1%). LINER-like emission is the dominant type of nebular activity in all ETG classes, plausibly driven by stellar photoionisation associated with recent star formation. An analysis of UV-optical colours indicates that, regardless of the luminosity range being considered, the fraction of peculiar ETGs that have experienced star formation in the last Gyr is a factor of ~1.5 higher than that in their relaxed counterparts. The spectro-photometric results strongly suggest that the interactions that produce the morphological peculiarities also induce low-level recent star formation which, based on the recent literature, are likely to contribute a few percent of the stellar mass over the last 1 Gyr. The catalogue of galaxies that forms the basis of this paper can be obtained at: http://www.mssl.ucl.ac.uk/~ska/stripe82/skaviraj_stripe82.dat or on request from the author.
The significant contribution of minor mergers to the cosmic star formation budget
Sugata Kaviraj
Physics , 2013, DOI: 10.1093/mnrasl/slt136
Abstract: We estimate an empirical lower limit for the fraction of cosmic star formation that is triggered by minor mergers in the local Universe. Splitting the star-formation budget by galaxy morphology, we find that early-type galaxies (ETGs) host ~14% of the budget, while Sb/Sc galaxies host the bulk (~53%) of the local star formation activity. Recent work indicates that star formation in nearby ETGs is driven by minor mergers, implying that at least ~14% of local star formation is triggered by this process. A more accurate estimate can be derived by noting that an infalling satellite likely induces a larger starburst in a galaxy of 'later' morphological type, both due to higher availability of gas in the accreting galaxy and because a bigger bulge better stabilizes the disk against star formation. This enables us to use the star formation in ETGs to estimate a lower limit for the fraction of star formation in late-type galaxies (LTGs) that is minor-merger-driven. Using a subsample of ETGs that is mass and environment-matched to the LTGs (implying a similar infalling satellite population), we estimate this limit to be ~24%. Thus, a lower limit for the fraction of cosmic star formation that is induced by minor mergers is ~35% (14% [ETGs] + 0.24 x 86% [LTGs]). The observed positive correlation between black hole and galaxy mass further implies that a similar fraction of black hole accretion may also be triggered by minor mergers. Detailed studies of minor-merger remnants are therefore essential, to quantify the role of this important process in driving stellar mass and black hole growth in the local Universe.
The importance of minor-merger-driven star formation and black-hole growth in disk galaxies
Sugata Kaviraj
Physics , 2014, DOI: 10.1093/mnras/stu338
Abstract: We use the SDSS Stripe 82 to empirically quantify the stellar-mass and black-hole growth triggered by minor mergers in local spiral (disk) galaxies. Since major mergers destroy disks and create spheroids, morphologically disturbed spirals are likely remnants of minor mergers. Disturbed spirals exhibit enhanced specific star formation rates (SSFRs), the enhancement increasing in galaxies of 'later' morphological type (which have more gas and smaller bulges). By combining the SSFR enhancements with the fraction of time spirals spend in this 'enhanced' mode, we estimate that ~40% of the star formation in local spirals is directly triggered by minor mergers. The disturbed spirals also exhibit higher nuclear-accretion rates, implying that minor mergers enhance the growth rate of the central black hole. However, the specific accretion rate shows a lower enhancement than that in the SSFR, suggesting that the coupling between stellar-mass and black-hole growth is weak in minor-merger-driven episodes. Given the significant fraction of star formation that is triggered by minor mergers, this weaker coupling may contribute to the large intrinsic scatter observed in the stellar vs. black-hole mass relation in spirals. Combining our results with the star formation in early-type galaxies -- which is minor-merger-driven and accounts for ~14% of the star formation budget -- suggests that around half of the star formation activity in the local Universe is triggered by the minor-merger process.
Recent star formation in local, morphologically disturbed spheroidal galaxies on the optical red sequence
Sugata Kaviraj
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.17139.x
Abstract: We combine GALEX (ultra-violet; UV) and SDSS (optical) photometry to study the recent star formation histories of ~100 field galaxies on the optical red sequence, a large fraction of which exhibit widespread signs of disturbed morphologies in deep optical imaging that are consistent with recent merging events. More than 70% of bulge-dominated galaxies in this sample show tidal features at a surface brightness limit of 28 mag arcsec^-2. We find that, while they inhabit the optical red sequence, they show a wide spread in their UV colours (~4 mags), akin to what has been discovered recently in the general early-type population. A strong correlation is found between UV colour and the strength of the tidal distortions, such that the bluest galaxies are more distorted. This strongly suggests that the blue UV colours seen in many nearby early-types are driven by (low-level) merger-induced star formation within the last 3 Gyrs, contributing less than 10% of the stellar mass. If the ongoing mergers in this sample, which have a median mass ratio of 1:4, are representative of the nearby red merger population, then less than 25% of the new stellar mass in the remnants is typically added through merger-induced star formation. While the dust extinction in the inter-stellar medium (ISM) in these galaxies is small [E(B-V)<0.1], the local dust content of the star-forming regions is, on average, a factor of 3 higher. Finally, we use our theoretical machinery to provide a recipe for calculating the age of the most recent star formation event (t2) in nearby (z<0.1) red early-type galaxies: Log (t2) [Gyrs] ~ 0.6.[(NUV-u)-(g-z)-1.73], where NUV, u, g and z are the observed photometric magnitudes of the galaxies in the GALEX/SDSS filtersets.
Systole and $λ_{2g-2}$ of a hyperbolic surface
Sugata Mondal
Mathematics , 2013,
Abstract: We apply topological methods to study eigenvalues of the Laplacian on closed hyperbolic surfaces. For any closed hyperbolic surface $S$ of genus $g$, we get a geometric lower bound on ${\lambda_{2g-2}}(S)$: ${\lambda_{2g-2}}(S) > 1/4 + {\epsilon_0}(S)$, where ${\epsilon_0}(S) > 0$ is an explicit constant which depends only on the systole of $S$
On topological upper-bounds on the number of small cuspidal eigenvalues
Sugata Mondal
Mathematics , 2014,
Abstract: Let $S$ be a noncompact, finite area hyperbolic surface of type $(g, n)$. Let $\Delta_S$ denote the Laplace operator on $S$. As $S$ varies over the {\it moduli space} ${\mathcal{M}_{g, n}}$ of finite area hyperbolic surfaces of type $(g, n)$, we study, adapting methods of Lizhen Ji \cite{Ji} and Scott Wolpert \cite{Wo}, the behavior of {\it small cuspidal eigenpairs} of $\Delta_S$. In Theorem 2 we describe limiting behavior of these eigenpairs on surfaces ${S_m} \in {\mathcal{M}_{g, n}}$ when $({S_m})$ converges to a point in $\overline{\mathcal{M}_{g, n}}$. Then we consider the $i$-th {\it cuspidal eigenvalue}, ${\lambda^c_i}(S)$, of $S \in {\mathcal{M}_{g, n}}$. Since {\it non-cuspidal} eigenfunctions ({\it residual eigenfunctions} or {\it generalized eigenfunctions}) may converge to cuspidal eigenfunctions, it is not known if ${\lambda^c_i}(S)$ is a continuous function. However, applying Theorem 2 we prove that, for all $k \geq 2g-2$, the sets $${{\mathcal{C}_{g, n}^{\frac{1}{4}}}}(k)= \{ S \in {\mathcal{M}_{g, n}}: {\lambda_k^c}(S) > \frac{1}{4} \}$$ are open and contain a neighborhood of ${\cup_{i=1}^n}{\mathcal{M}_{0, 3}} \cup {\mathcal{M}_{g-1, 2}}$ in $\overline{\mathcal{M}_{g, n}}$. Moreover, using topological properties of nodal sets of {\it small eigenfunctions} from \cite{O}, we show that ${{\mathcal{C}_{g, n}^{\frac{1}{4}}}}(2g-1)$ contains a neighborhood of ${\mathcal{M}_{0, n+1}} \cup {\mathcal{M}_{g, 1}}$ in $\overline{\mathcal{M}_{g, n}}$. These results provide evidence in support of a conjecture of Otal-Rosas \cite{O-R}.
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