arXiv subjects

S. Mahapatra

Publications and source records attributed to S. Mahapatra.

At least 19 recordsLinked to original sources

arXiv2026

Fluorescence and Relaxation Dynamics of Cesium in Argon Matrices: Multiple Trapping Sites and Host-Guest Interactions

We investigate the fluorescence and relaxation dynamics of Cs atoms embedded in a cryogenic argon matrix using spectroscopy measurements combined with diatomic-in-molecule (DIM) simulations. The data reveal complex emission spectra, large Stokes shifts, and slow relaxation effects, indicating strong host-guest interactions and substantial lattice reorganization. Although the spectra are superimposed on a broad background, possibly due to low-symmetry, defect-related, or grain-boundary trapping sites, the main spectral structure is consistent with two dominant trapping environments that give rise to two triplet absorption features with distinct fluorescence behavior of the doublet and singlet components. Polarization measurements further suggest that these sites may differ in symmetry, although a unique structural assignment remains difficult

physics.atom-ph↗

arXiv2024

Analysis of growth of silicon thin films on textured and non-textured surface

Hydrogenated amorphous silicon alloy films are generally deposited by radio frequency plasma enhanced chemical vapor deposition (RF PECVD) technique on various types of substrates. Generally it is assumed that film quality remains unchanged when deposited on textured or non-textured substrates. Here we analyzed the difference in growth of thin film silicon layers when deposited in a textured and a non-textured surface. In this investigation characteristics of two solar cells were compared, where one cell was prepared on a textured surface ( Cell-A) while the other prepared on a non-textured surface (Cell-B). Defect analysis of the devices were carried out by simulation and device modeling. It shows that the intrinsic film deposited on a textured surface was more defective ($2.4\times 10^{17}$ cm$^{-3}$) than that deposited on a flat surface ($3.2\times 10^{16}$ cm$^{-3}$). Although the primary differences in these two cells were thickness of the active layer and nature of surface texturing, the simulation results show that thin film deposited on a textured surface may acquire an increased defect density than that deposited on a flat surface. Lower effective flux density of $SiH_{3}$ precursors on the textured surface can be one of the reasons for higher defect density in the film deposited on textured surface. An Improved light coupling can be achieved by using a thinner doped window layer. By changing the thickness from 15 nm to 3 nm, the short circuit current density increased from 16.4 mA/cm$^{2}$ to 20.96 mA/cm$^{2}$ and efficiency increased from $9.4\%$ to $12.32\%$.

cond-mat.mtrl-sci↗

arXiv2023

Small-scale cosmic ray anisotropy observed by the GRAPES-3 experiment at TeV energies

GRAPES-3 is a mid-altitude (2200 m) and near equatorial ($11.4^{\circ}$ North) air shower array, overlapping in its field of view for cosmic ray observations with experiments that are located in Northern and Southern hemispheres. We analyze a sample of $3.7\times10^9$ cosmic ray events collected by the GRAPES-3 experiment between 1 January 2013 and 31 December 2016 with a median energy of $\sim16$ TeV for study of small-scale ($<60^{\circ}$) angular scale anisotropies. We observed two structures labeled as A and B, deviate from the expected isotropic distribution of cosmic rays in a statistically significant manner. Structure `A' spans $50^{\circ}$ to $80^{\circ}$ in the right ascension and $-15^{\circ}$ to $30^{\circ}$ in the declination coordinate. The relative excess observed in the structure A is at the level of $(6.5\pm1.3)\times10^{-4}$ with a statistical significance of 6.8 standard deviations. Structure `B' is observed in the right ascension range of $110^{\circ}$ to $140^{\circ}$. The relative excess observed in this region is at the level of $(4.9\pm1.4)\times10^{-4}$ with a statistical significance of 4.7 standard deviations. These structures are consistent with those reported by Milagro, ARGO-YBJ, and HAWC. These observations could provide a better understanding of the cosmic ray sources, propagation and the magnetic structures in our Galaxy.

astro-ph.HE↗

arXiv2022

Effect of magnetic phase coexistence on spin-phonon coupling and magnetoelectric effect in polycrystalline Sm0.5Y0.5Fe0.58Mn0.42O3

The polycrystalline co-doped samples of Sm0.5Y0.5Fe0.58Mn0.42O3 were prepared by solid-state reaction route and its various physical properties with their correlations have been investigated. The dc magnetization measurements on the sample revealed a weak ferromagnetic (WFM) transition at TN=361 K that is followed by an incomplete spin reorientation (SR) transition at TSR1= 348 K. A first order magnetic transition (FOMT) around 292 K completes the spin reorientation transition and the material enters into a nearly collinear antiferromagnetic (AFM) state for T < 260 K. The compound exhibited magnetization reversal below the compensation temperature (Tcomp) = 92 K at low measured field of 100 Oe. At further low temperature below 71 K, the compound also exhibited Zero-field cooled memory effects confirming a reentrant spinglass state formation. Robust magnetodielectric (MD) magnetoelectric coupling has been established in the present material through field dependent dielectric and resistivity measurements. True ferroelectric transition with a considerable value of saturation polarization (= 0.06 micro C/cm2 at 15 K) have been found in the specimen below TFE= 108 K. We observed an intense spin-phonon coupling (SPC) across TSR and TN from the temperature dependent Raman spectroscopy and is responsible for the intrinsic magnetoelectric effect. This SPC also stabilizes the ferroelectric state below TFE in the material. The delicate interplay of the lattice (Phonons), charge and spins governs the observed features in the investigated physical properties of the material that makes the specimen a promising multifunctional material.

cond-mat.mtrl-sci↗

arXiv2021

A quenched 2-flavour Einstein-Maxwell-dilaton gauge-gravity model

We extend earlier work by introducing an Einstein-Maxwell-dilaton (EMD) action with two quark flavours. We solve the corresponding equations of motion in the quenched approximation (probe quark flavours) via the potential reconstruction method in presence of a background magnetic field in search for a self-consistent dual magnetic AdS/QCD model. As an application we discuss the deconfinement transition temperature confirming inverse magnetic catalysis, whilst for moderate values of the magnetic field also the entropy density compares relatively well with corresponding lattice data in the vicinity of the transition.

hep-th↗

arXiv2019

The angular resolution of GRAPES-3 EAS array after correction for the shower front curvature

The angular resolution of an extensive air shower (EAS) array plays a critical role in determining its sensitivity for the detection of point $\gamma$-ray sources in the multi-TeV energy range. The GRAPES-3 an EAS array located at Ooty in India (11.4$^{\circ}$N, 76.7$^{\circ}$E, 2200 m altitude) is designed to study $\gamma$-rays in the TeV-PeV energy range. It comprises of a dense array of 400 plastic scintillators deployed over an area of 25000 m$^2$ and a large area (560 m$^2$) muon telescope. A new statistical method allowed real time determination of the propagation delay of each detector in the GRAPES-3 array. The shape of shower front is known to be curved and here the details of a new method developed for accurate measurement of the shower front curvature is presented. These two developments have led to a sizable improvement in the angular resolution of GRAPES-3 array. It is shown that the curvature depends on the size and age of an EAS. By employing two different techniques, namely, the odd-even and the left-right methods, independent estimates of the angular resolution are obtained. The odd-even method estimates the best achievable resolution of the array. For obtaining the angular resolution, the left-right method is used after implementing the size and age dependent curvature corrections. A comparison of the angular resolution as a function of EAS energy by these two methods shows them be virtually indistinguishable. The angular resolution of GRAPES-3 array is 47$^{\prime}$ for energies E$>$5 TeV and improves to 17$^{\prime}$ at E$>$100 TeV and finally approaching 10$^{\prime}$ at E$>$500 TeV.

astro-ph.IM↗

arXiv2019

Exact results for an STU-model

The duality symmetries of the STU-model of Sen and Vafa are very restrictive. This is utilized to determine the holomorphic function that encodes its two-derivative Wilsonian effective action and its couplings to the square of the Weyl tensor to fifth order in perturbation theory. At fifth order some ambiguities remain which are expected to resolve themselves when proceeding to the next order. Subsequently, a corresponding topological string partition function is studied in an expansion in terms of independent invariants of $S$, $T$ and $U$, with coefficient functions that depend on an effective duality invariant coupling constant $u$, which is defined on a Riemann surface $\mathbb{C}$. The coefficient function of the invariant that is independent of $S$, $T$ and $U$ is determined to all orders by resummation. The other functions can be solved as well, either algebraically or by solving differential equations whose solutions have ambiguities associated with integration constants. This determination of the topological string partition function, while interesting in its own right, reveals new qualitative features in the result for the Wilsonian action, which would be difficult to appreciate otherwise. It is demonstrated how eventually the various ambiguities are eliminated by comparing the results for the effective action and the topological string. While we only demonstrate this for the leading terms, we conjecture that this will hold in general for this model.

hep-th↗

arXiv2019

Block Prefix Mechanism for Flow Mobility in PMIPv6 Based Networks

The next generation Internet is deemed to be heterogeneous in nature and mobile devices connected to the Internet are expected to be equipped with different wireless network interfaces. As seamless mobility is important in such networks, handover between different network types, called vertical handover, is an important issue in such networks. While proposing standards like Mobile IPv6 (MIPv6) and Proxy Mobile IPv6 (PMIPv6) for mobility management protocols, one important challenge being addressed by IETF work groups and the research community is flow mobility in multi-homed heterogeneous wireless networks. In this paper we propose and analyze a block prefix mechanism for flow mobility in PMIPv6 and conducted extensive analytical and simulation studies to compare the proposed mechanism with existing prefix based mechanisms for flow mobility in PMIPv6 reported in terms of important performance metrics such as handover latency, average hop delay, packet density, signaling cost and packet loss. Both analytical and simulation results demonstrate that the proposed mechanism outperforms the existing flow mobility management procedures using either shared or unique prefixes.

cs.NI↗

arXiv2018

Wafer-scale all-epitaxial GeSn-on-insulator on Si(111) by molecular beam epitaxy

All epitaxial GeSn-on-insulator (GeSnOI) heterostructures have been realized by conventional molecular beam epitaxy of both GeSn and a thin Gd2O3 insulating layer, on Si(111) substrates. Analysis of the crystal and surface quality by high-resolution transmission electron microscopy (HRTEM), high-resolution X-ray diffraction (HRXRD), and atomic force microscopy (AFM) revealed the formation of a continuous and fully-relaxed single-crystalline GeSn epilayer, however with a surface roughness of 3.5 nm. Stacking faults and reflection microtwins were also observed in cross-sectional HRTEM images. From Hall measurement, the concentration and mobility of holes, introduced by un-intentional p-type doping of the GeSn epilayers, were estimated to 8x10^16 cm-3 and 176 cm-2V-1s-1, respectively. In metal-semiconductor-metal (MSM) Schottky diodes, fabricated with these GeSnOI heterostructures, the dark current was observed to be lower by a decade, when compared to similar diodes fabricated with GeSn/Ge/Si(001) heterostructures. The results presented here are encouraging for the development of these engineered substrates for (opto)electronic applications.

physics.app-ph↗

arXiv2015

A simple technique for gamma ray and cosmic ray spectroscopy using plastic scintillator

A new and simple technique has been developed using plastic scintillator detectors for gamma ray and cosmic ray spectroscopy without single channel analyzer (SCA) or multichannel analyzer (MCA). In these experiments only a leading edge discriminator (LED) and NIM scalers have been used. Energy calibration of gamma spectra in plastic scintillators has been done using Co$^{60}$ and Cs$^{137}$ sources. The details experimental technique, analysis procedure and experimental results has been presented in this article.

physics.ins-det↗

arXiv2014

Deformations of special geometry: in search of the topological string

The topological string captures certain superstring amplitudes which are also encoded in the underlying string effective action. However, unlike the topological string free energy, the effective action that comprises higher-order derivative couplings is not defined in terms of duality covariant variables. This puzzle is resolved in the context of real special geometry by introducing the so-called Hesse potential, which is defined in terms of duality covariant variables and is related by a Legendre transformation to the function that encodes the effective action. It is demonstrated that the Hesse potential contains a unique subsector that possesses all the characteristic properties of a topological string free energy. Genus $g\leq3$ contributions are constructed explicitly for a general class of effective actions associated with a special-Kähler target space and are shown to satisfy the holomorphic anomaly equation of perturbative type-II topological string theory. This identification of a topological string free energy from an effective action is primarily based on conceptual arguments and does not involve any of its more specific properties. It is fully consistent with known results. A general theorem is presented that captures some characteristic features of the equivalence, which demonstrates at the same time that non-holomorphic deformations of special geometry can be dealt with consistently.

hep-th↗

arXiv2014

Spontaneous breaking of time reversal symmetry in strongly interacting two dimensional electron layers in silicon and germanium

We report experimental evidence of a remarkable spontaneous time reversal symmetry breaking in two dimensional electron systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si:P and Ge:P $δ-$layers, suggesting an effect driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping, providing a microscopic mechanism for spontaneous lifting of the time reversal symmetry. Our experiments suggest the emergence of a new many-body quantum state when two dimensional electrons are confined to narrow half-filled impurity bands.

cond-mat.str-el↗

arXiv2010

BPS black holes, the Hesse potential, and the topological string

The Hesse potential is constructed for a class of four-dimensional N=2 supersymmetric effective actions with S- and T-duality by performing the relevant Legendre transform by iteration. It is a function of fields that transform under duality according to an arithmetic subgroup of the classical dualities reflecting the monodromies of the underlying string compactification. These transformations are not subject to corrections, unlike the transformations of the fields that appear in the effective action which are affected by the presence of higher-derivative couplings. The class of actions that are considered includes those of the FHSV and the STU model. We also consider heterotic N=4 supersymmetric compactifications. The Hesse potential, which is equal to the free energy function for BPS black holes, is manifestly duality invariant. Generically it can be expanded in terms of powers of the modulus that represents the inverse topological string coupling constant, $g_s$, and its complex conjugate. The terms depending holomorphically on $g_s$ are expected to correspond to the topological string partition function and this expectation is explicitly verified in two cases. Terms proportional to mixed powers of $g_s$ and $\bar g_s$ are in principle present.

hep-th↗

arXiv2009

Determining the Sign of Δ_{31} by Future Long Baseline and Reactor Experiments

We study the determination of neutrino mass hierarchy, through neutrino experiments within the next ten years. The T2K neutrino oscillation experiment will start in 2009. In the experiment the high intensity ν_μ beam from JHF is directed to Super-Kamiokande (SK) detector 295 km away. The NOνA (off axies neutrino oscillation) experiment is being planned, with the ν_μ beam from Fermi-Lab directed to a site 610km away, which is 0,7,14 milliradian off-axies. Both the above experiments will measure ν_μ\toν_{e} oscillation probability. The double-CHOOZ experiment under construction detects \bar{ν_{e}}s emitted by nuclear reactors both through a near detector (150m) and a far detector (1.05 Km) and measure \bar{ν_{e}}\to\bar{ν_{e}} survival probability. In this paper, we outline a procedure to determine the sign of Δ_{31} from the simulated data of the above experiments.

hep-ex↗

arXiv2009

Subleading and non-holomorphic corrections to N=2 BPS black hole entropy

BPS black hole degeneracies can be expressed in terms of an inverse Laplace transform of a partition function based on a mixed electric/magnetic ensemble, which involves a non-trivial integration measure. This measure has been evaluated for black holes with various degrees of supersymmetry and for N=4 supersymmetric black holes all results agree. It generally receives contributions from non-holomorphic corrections. An explicit evaluation of these corrections in the context of the effective action of the FHSV model reveals that these are related to, but quantitatively different from, the non-holomorphic corrections to the topological string, indicating that the relation between the twisted partition functions of the latter and the effective action is more subtle than has so far been envisaged. The effective action result leads to a duality invariant BPS free energy and arguments are presented for the existence of consistent non-holomorphic deformations of special geometry that can account for these effects. A prediction is given for the measure based on semiclassical arguments for a class of N=2 black holes. Furthermore an attempt is made to confront some of the results of this paper to a recent proposal for the microstate degeneracies of the STU model.

hep-th↗

arXiv2008

The mixed black hole partition function for the STU model

We evaluate the mixed partition function for dyonic BPS black holes using the recently proposed degeneracy formula for the STU model. The result factorizes into the OSV mixed partition function times a proportionality factor. The latter is in agreement with the measure factor that was recently conjectured for a class of N=2 black holes that contains the STU model.

hep-th↗

arXiv2007

Low Temperature Investigation of Electrical Conduction in Polysilicon: Simulation and Experiment

Investigation of electrical conduction in polysilicon nanowires (polySiNW) with nanograins (5 to 20nm), based on Monte Carlo (MC) simulations and electrical measurements from 4K to 300K are presented. Some irregular Coulomb Oscillations (CO) are observed at temperatures lower than 200K showing several period widths due to the random distribution in grain size (5-20nm). A remarkable result consists in more effective oscillations observed at intermediate range of temperatures (between 25K and 150K) and high drain voltages. The temperature dependence of COs is explained by the fact that in a multiple asymmetric dot system at low temperature, COs are observed not at the lowest but at an intermediate temperature range, whereas the drain voltage dependence is due to an enhanced non-resonant tunneling. MC simulations have confirmed experimental observations.

cond-mat.mtrl-sci↗

arXiv2007

Black hole entropy functions and attractor equations

The entropy and the attractor equations for static extremal black hole solutions follow from a variational principle based on an entropy function. In the general case such an entropy function can be derived from the reduced action evaluated in a near-horizon geometry. BPS black holes constitute special solutions of this variational principle, but they can also be derived directly from a different entropy function based on supersymmetry enhancement at the horizon. Both functions are consistent with electric/magnetic duality and for BPS black holes their corresponding OSV-type integrals give identical results at the semi-classical level. We clarify the relation between the two entropy functions and the corresponding attractor equations for N=2 supergravity theories with higher-derivative couplings in four space-time dimensions. We discuss how non-holomorphic corrections will modify these entropy functions.

hep-th↗