Browsing by Author "Kothari, Rahul"

Now showing 1 - 6 of 6
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    F+ center exchange mechanism and magnetocrystalline anisotropy in Ni-doped 3C-SiC
    (Elsivier, 2022) Narayanan, Harish Kumar; Babu, P D; Singh, S K; Kothari, Rahul; Moharana, Gyanti P
    Towards the development of a magnetic semiconductor suitable for spintronic device applications in extreme environments, we explored the possibility of inducing magnetic interaction in SiC by doping Nickel. The X-ray diffraction and Raman Spectroscopy studies confirm the incorporation of Ni into the host lattice. The magnetic measurements and electron spin resonance studies indicate the presence of room temperature ferromagnetic interaction in the system. The Curie temperature of 1, 3, and 5% Ni-doped samples have been found to be 420 K, 520 K, and 540 K respectively. Electron spin resonance study reveals that the valence state of Ni is 2+, which implies the creation of vacancies at both Silicon (VSi) and Carbon (VC) sites as they are tetravalent. The change in magnetization of the system with an increase in dopant concentration is consistent with the variation in the number of vacancies and free holes. The analysis of magnetization data using the Law of approach to saturation shows that the anisotropic constant decreases with an increase in temperature. The long-range magnetic interaction in the system is explained using the F+ center exchange mechanism.
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    Full-sky bispectrum in redshift space for 21cm intensity maps
    (IOP Publishing, 2020) Kothari, Rahul; Durrer, Ruth; Jalilvand, Mona
    We compute the tree-level bispectrum of 21cm intensity mapping after reionisation. We work in directly observable angular and redshift space, focusing on equal-redshift correlations and thin redshift bins, for which the lensing contribution is negligible. We demonstrate the importance of the contributions from redshift-space distortions which typically dominate the result. Taking into account the effects of telescope beams and foreground cleaning, we estimate the signal to noise, and show that the bispectrum is detectable by both SKA in single-dish mode and HIRAX in interferometer mode, especially at the lower redshifts in their respective ranges.
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    Probing Cosmology beyond ΛCDM using the SKA
    (Indian Academy of Sciences, 2023) Ghosh, Shamik; Jain, Pankaj; Kothari, Rahul
    The cosmological principle states that the Universe is statistically homogeneous and isotropic at large distance scales. Currently, there exist many observations which indicate a departure from this principle. It has been shown that many of these observations can be explained by invoking superhorizon cosmological perturbations and may be consistent with the Big Bang paradigm. Remarkably, these modes simultaneously explain the observed Hubble tension, i.e., the discrepancy between the direct and indirect measurements of the Hubble parameter. We propose several tests of the cosmological principle using SKA. In particular, we can reliably extract the signal of dipole anisotropy in the distribution of radio galaxies. The superhorizon perturbations also predict a significant redshift dependence of the dipole signal, which can be well tested by the study of signals of reionization and the dark ages using SKA. We also propose to study the alignment of radio galaxy axes as well as their integrated polarization vectors over distance scales ranging from a few Mpc to Gpc. We discuss data analysis techniques that can reliably extract these signals from data.
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    Superhorizon perturbations: A possible explanation of the hubble–lemaître tension and the large-scale anisotropy of the universe
    (IOP Publishing, 2022) Tiwari, Prabhakar; Kothari, Rahul; Jain, Pankaj
    Current cosmological observations point to a serious discrepancy between the observed Hubble parameter obtained using direct versus cosmic microwave background radiation measurements. Besides this so-called Hubble– Lemaî tre tension, we also find considerable evidence in diverse cosmological observables that indicate violation of the cosmological principle. In this paper, we suggest that both these discrepancies are related and can be explained by invoking superhorizon perturbations in the universe. We implement this by considering a single superhorizon mode and showing that it leads to both a dipole in large-scale structures and a shift in the Hubble–Lemaî tre parameter. Furthermore, the shift is found to be independent of redshift up to a certain distance. This is nicely consistent with the data.
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    Superhorizon Perturbations: A Possible Explanation of the Hubble–Lemaître Tension and the Large-scale Anisotropy of the Universe.
    (The American Astronomical Society., 2022) Tiwari, Prabhakar; Kothari, Rahul; Jain, Pankaj
    Current cosmological observations point to a serious discrepancy between the observed Hubble parameter obtained using direct versus cosmic microwave background radiation measurements. Besides this so-called Hubble– Lemaître tension, we also find considerable evidence in diverse cosmological observables that indicate violation of the cosmological principle. In this paper, we suggest that both these discrepancies are related and can be explained by invoking superhorizon perturbations in the universe. We implement this by considering a single superhorizon mode and showing that it leads to both a dipole in large-scale structures and a shift in the Hubble–Lemaître parameter. Furthermore, the shift is found to be independent of redshift up to a certain distance. This is nicely consistent with the data.
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    Torsion driven inflationary magnetogenesis
    (American Physical Society, 2020) Kothari, Rahul; Saketh, M. V. S.; Jain, Pankaj
    We show that the breaking of the conformal invariance of the electromagnetic Lagrangian, which is required for inflationary magnetogenesis, arises naturally in the Poincaré gauge theory. We use the minimal coupling prescription to introduce the electromagnetic gauge fields as well as non-Abelian gauge fields in this theory. Due to the addition of non-Abelian gauge fields, we show that the solar constraints on this model can be naturally evaded. We find that in the minimal version of this model the generated magnetic field is too small to explain the observations. We discuss some generalizations of the gravitational action, including the Starobinsky model and a model with conformal invariance. We show that such generalizations naturally generate the kinetic energy terms required for magnetogenesis. We propose a generalization of the minimal model by adding a potential term, which is allowed within the framework of this model, and show that it leads to sufficiently large magnetic fields.