arXiv subjects

Sunil Kumar

Publications and source records attributed to Sunil Kumar.

At least 55 records · Page 3Linked to original sources

arXiv2021

A Vector Space Approach to Generate Dynamic Keys for Hill Cipher

In this paper, a variant of the Hill cipher is proposed. In the classical Hill cipher, an invertible matrix is used for encryption but the scheme is vulnerable to the known-plaintext attack which can reveal the matrix. In our proposed cryptosystem, each plaintext block is encrypted by a new invertible key matrix that thwarts the known-plaintext attack. To generate the invertible matrices which serve as the dynamic keys we make use of the vector spaces, randomly generated basis and non-singular linear transformation. Resulting cipher is secure against the known-plaintext attack.

cs.CR↗

arXiv2021

Optical and Terahertz Response of Carbon Nanostructures

In the last three decades or so, we have witnessed an extraordinary progress in the research and technology of carbon-based nanomaterials. Among the peculiar highlights are the discoveries of fullerene, the carbon nanotubes and the magnificent simple scotch tape exfoliated graphene. The unique photophysical properties of these different allotropic forms of the nanocarbon have opened up vast application possibilities in many fields of science and technology, with particular emphasis on optoelectronics and photonics. A prerequisite for many of these applications is a thorough understanding of the nature of the elementary and coupled excitations and also various dynamical processes involving them. Here, we present an overview of the recent excitement with the carbon nanostructures, in particular, the quantum dots, nanotubes and graphene. We discuss some of their very interesting properties investigated through optical and THz spectroscopic tools. At optical frequencies, the light emitting properties, the nonlinearities and ultrafast response have been presented, while, the low-energy response has been considered in terms of studies obtained by using THz time-domain spectroscopy. Finally, we conclude with some of the future prospects on the photophysics of carbon nanosystems in realistic applications.

physics.app-ph↗

arXiv2021

Optical Damage Threshold and THz Generation Efficiency of (Fe,CoFeB)/(Ta,Pt) Spintronic Emitters

THz pulses are generated from femtosecond pulse-excited ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall effect. The contribution from ultrafast demagnetization/remagnetization is extremely weak, in the comparison. The highest possible THz signal strength from spintronic THz emitters is limited by the optical damage threshold of the corresponding heterostructures. The THz generation efficiency does not saturate with the excitation fluence even up till the damage threshold. Bilayer (Fe, CoFeB)/(Pt, Ta) based FM/NM spintronic heterostructures have been studied for an optimized performance for THz generation when pumped by sub-50 fs amplified laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an efficient THz source. The optimized FM/NM spintronic heterostructure on a quartz substrate, having alpha-phase Ta as the nonmagnetic layer, show the highest damage threshold as compared to those with Pt, irrespective of their generation efficiency. The damage threshold of the Fe/Ta heterostructure on quartz substrate is ~85 GW/cm2.

physics.app-ph↗

arXiv2021

Sub-bandgap activated charges transfer in a graphene-MoS2-graphene heterostructure

Monolayers of transition metal dichalcogenides are semiconducting materials which offer many prospects in optoelectronics. A monolayer of molybdenum disulfide (MoS2) has a direct bandgap of 1.88 eV. Hence, when excited with optical photon energies below its bandgap, no photocarriers are generated and a monolayer of MoS2 is not of much use in either photovoltaics or photodetection. Here, we demonstrate that large size MoS2 monolayer sandwiched between two graphene layers makes this heterostructure optically active well below the band gap of MoS2. An ultrafast optical pump-THz probe experiment reveals in real-time, transfer of carriers between graphene and MoS2 monolayer upon photoexcitation with photon energies down to 0.5 eV. It also helps to unravel an unprecedented enhancement in the broadband transient THz response of this tri-layer material system. We propose possible mechanism which can account for this phenomenon. Such specially designed heterostructures, which can be easily built around different transition metal dichalcogenide monolayers, will considerably broaden the scope for modern optoelectronic applications at THz bandwidth.

cond-mat.mes-hall↗

arXiv2021

Network-centric indicators for fragility in global financial indices

Over the last two decades, financial systems have been studied and analysed from the perspective of complex networks, where the nodes and edges in the network represent the various financial components and the strengths of correlations between them. Here, we adopt a similar network-based approach to analyse the daily closing prices of 69 global financial market indices across 65 countries over a period of 2000-2014. We study the correlations among the indices by constructing threshold networks superimposed over minimum spanning trees at different time frames. We investigate the effect of critical events in financial markets (crashes and bubbles) on the interactions among the indices by performing both static and dynamic analyses of the correlations. We compare and contrast the structures of these networks during periods of crashes and bubbles, with respect to the normal periods in the market. In addition, we study the temporal evolution of traditional market indicators, various global network measures and the recently developed edge-based curvature measures. We show that network-centric measures can be extremely useful in monitoring the fragility in the global financial market indices.

q-fin.ST↗

arXiv2021

THz pulses from optically excited Fe, Pt and Ta based Spintronic Heterostructures

Spintronic heterostructures are considered to be the new generation THz sources for their capability in producing high power and broadband THz radiation. Here, we provide a brief review on the state-of-the-art in this field. The optically excited bi- and tri-layer combinations of ferromagnetic and nonmagnetic thin films have become increasingly popular. Towards optimizing the THz conversion efficiency and broadband gapless spectrum from these THz emitters, various control parameters need to be taken into consideration. The inverse spin Hall effect in the heavy metal layer of the heterostructure is primarily responsible for the generation of THz pulses. A few new results on iron, platinum and tantalum based heterostructures have also been reported here. It is observed that the Ta(2nm)/Fe(2nm)/Pt(2nm) tri-layer heterostructure generates ~40(250)% stronger THz signal as compared to the counterpart Fe(2nm)/Pt(2nm) (Fe(3nm)/Ta(2nm)) bi-layer heterostructure.

physics.app-ph↗

arXiv2020

Spectrum of Landau Levels in GaAs Quantum Wells

We have studied the optical and electrical spectra from an n i p LED as a function of magnetic field. This sample incorporated three GaAs quantum wells in the intrinsic region. This device had excess n type doping and as a result. The quantum wells were populated by a two dimension Landau electron gas. The broad B0 field emission band evolved into a series of discrete features in the presence of a magnetic field. These were identified as interband transitions between the different values of l. Landau levels associated with the sub-bands, with the selection rule. An energy splitting between the two polarised components was observed for each Landau level transition. This was found to be equal to the sum of the conduction and valence band spin splittings. We used the know value of electron g factor to determine the valence band spin splittings. Our experimental values were compared to the numerically calculated values and were found to be in reasonable agreement.

cond-mat.mes-hall↗

arXiv2020

Enhancement in optically induced ultrafast THz response of MoSe2MoS2 heterobilayer

THz conductivity of large area MoS2 and MoSe2 monolayers as well as their vertical heterostructure, MoSe2MoS2 is measured in the 0.3-5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSe2MoS2 heterobilayer is enhanced many folds when optically excited above the direct band gap energies of the constituting monolayers. The free carriers generated in the heterobilayer evolve with the characteristic times found in each of the two monolayers. Surprisingly, the same enhancement is recorded in the ultrafst THz reflectivity of the heterobilayer when excited below the MoS2 bandgap energy. A mechanism accounting for these observations is proposed.

physics.app-ph↗

arXiv2020

Group ring based public key cryptosystems

In this paper, we propose two cryptosystems based on group rings and existing cryptosystem. First one is Elliptic ElGamal type group ring public key cryptosystem whose security is greater than security of cryptosystems based on elliptic curves discrete logarithmic problem (ECDLP). Second is ElGamal type group ring public key cryptosystem, which is analogous to ElGamal public key cryptosystem but has comparatively greater security. Examples are also given for both the proposed cryptosystems.

math.GR↗

arXiv2020

Magnetic field-dependent resistance crossover and logarithmic to non-saturating magnetoresistance in topological insulator Bi$_2$Te$_3$

We report a metal-insulator like transition in single crystalline 3D topological insulator Bi2Te3 at a temperature of 230K in presence of an external magnetic field applied normal to the surface. This transition becomes more prominent at larger magnetic field strength with the residual resistance value increasing linearly with the magnetic field. At low temperature, the magnetic field dependence of the magnetoresistance shows a transition from logarithmic to linear behavior and the onset magnetic field value for this transition decreases with increasing temperature. The logarithmic magnetoresistance indicates the weak anti-localization of the surface Dirac electrons while the high temperature behavior originates from the bulk carriers due to intrinsic impurities. At even higher temperatures beyond~230 K, a completely classical Lorentz model type quadratic behavior of the magnetoresistance is observed. We also show that the experimentally observed anomalies at ~230K in the magneto-transport properties do not originate from any stacking fault in Bi2Te3.

cond-mat.mes-hall↗

arXiv2020

A perspective on correlation-based financial networks and entropy measures

In this brief review, we critically examine the recent work done on correlation-based networks in financial systems. The structure of empirical correlation matrices constructed from the financial market data changes as the individual stock prices fluctuate with time, showing interesting evolutionary patterns, especially during critical events such as market crashes, bubbles, etc. We show that the study of correlation-based networks and their evolution with time is useful for extracting important information of the underlying market dynamics. We, also, present our perspective on the use of recently developed entropy measures such as structural entropy and eigen-entropy for continuous monitoring of correlation-based networks.

q-fin.CP↗

arXiv2020

Photo-Seebeck effect in single-crystalline bismuth telluride topological insulator

Bismuth telluride is a low energy bulk band-gap topological system with conducting surface states. Besides its very good thermoelectric properties, it also makes a very good candidate for broadband photodetectors. Here, we report temperature-dependent photo-Seebeck effect in a bulk single crystalline bismuth telluride. On light illumination, an electrically biased sample shows distinguishable contributions in the measured current due to both the Seebeck effect and the normal photo-generated carriers within a narrow layer of the sample. Detailed experiments are performed to elucidate the distinction between the Seebeck contribution and the photogenerated current. The temperature-dependence of the photocurrent without Seebeck contribution shows a sign reversal from negative to positive at a specific temperature depending on the wavelength of photoexcitation light.

cond-mat.mes-hall↗

arXiv2019

Flow Field Characteristics of a Supersonic Jet Influenced by Downstream Microjet Fluidic Injection

Mixing characteristics of a supersonic jet influenced by a Downstream Microjet Fluidic Injection (DMFI) system are numerically investigated. The DMFI system is built on the observation of a previous experimental study that utilized transverse fluidic injection from four equally spaced injection ports placed on an injection tube at a distance downstream of a 1.5 Mach number nozzle. The measurements from these previous experiments demonstrated thickening and mixing enhancement of the jet shear layer as a result of fluidic injection. The current numerical study examines the underlying physics of the flow field, as well as the effectiveness of the DMFI system at smaller mass flow rate ratios compared to those utilized in the previous experiments. Results indicate good agreement with the trend observed by the experimental study, and considerable improvement in enhancement of the jet mixing is observed. The observed mixing enhancement is attributed to the presence of the microjet tube and fluidic injection and the consequent generation of streamwise vortices, as well as the natural separating bow shock due to the transverse flow injection. DMFI system is shown to enhance the early mixing, resulting in attenuation of the downstream turbulence production. Furthermore, the DMFI system is demonstrated to be an effective method of mixing enhancement for supersonic jets with a potential for reducing the jet noise radiation.

physics.flu-dyn↗

arXiv2019

Singlet Pathway to the Ground State of Ultracold Polar Molecules

Starting from weakly bound Feshbach molecules, we demonstrate a two-photon pathway to the dipolar ground state of bi-alkali molecules that involves only singlet-to-singlet optical transitions. This pathway eliminates the search for a suitable intermediate state with sufficient singlet-triplet mixing and the exploration of its hyperfine structure, as is typical for pathways starting from triplet dominated Feshbach molecules. By selecting a Feshbach state with a stretched singlet hyperfine component and controlling the polarization of the excitation laser, we assure coupling to only a single hyperfine component of the $\textrm{A}^{1}Σ^{+}$ excited potential, even if the hyperfine structure is not resolved. Similarly, we address a stretched hyperfine component of the $\textrm{X}^{1}Σ^{+}$ rovibrational ground state, and therefore an ideal three level system is established. We demonstrate this pathway with ${}^{6}\textrm{Li}{}^{40}\textrm{K}$ molecules. By exploring deeply bound states of the $\textrm{A}^{1}Σ^{+}$ potential, we are able to obtain large and balanced Rabi frequencies for both transitions. This method can be applied to other molecular species.

cond-mat.quant-gas↗

arXiv2019

Broken translational and rotational symmetries in LiMn1.5Ni0.5O4 spinel

In condensed matter physics broken symmetries and emergence of quasi-particles are intimately linked to each other. Whenever a symmetry is broken, it leaves its fingerprints, and that may be observed indirectly via its influence on the other quasi-particles. Here, we report the strong signature of broken rotational symmetry induced due to long range-ordering of spins in Mn - sublattice of LiMn1.5Ni0.5O4 below Tc ~ 113 K reflected with the marked changes in the lattice vibrations using Raman scattering. In particular, the majority of the observed first-order phonon modes show a sharp shift in frequency in the vicinity of long range magnetic-ordering temperature. Phonons exist in a crystalline system because of broken translational symmetry, therefore any renormalization in the phonon-spectrum could be a good gauge for broken translational symmetry. Anomalous evolution of the few modes associated with stretching of Mn/NiO6 octahedra in the intermediate temperature range (~ 60-260 K) marked the broken translational symmetry attributed to the charge ordering. Interestingly same modes also show strong coupling with magnetic degrees of freedom, suggesting that charge-ordering and magnetic transition may be linked to each other.

cond-mat.mtrl-sci↗

arXiv2018

Q-PET: PET with 3rd Eye Quantum Entanglement based Positron Emission Tomography

In the present ongoing study, we are proposing a prototype model for positron emission tomography detection technology by introduction of a new discriminatory window parameter. It can be a new generation PET detection technique. We introduced Polarization Measurement of the annihilation photons(generated from the annihilation of positron and electron) as an additional parameter in proposed prototype, to correlate annihilation photons of a particular annihilation event. The motivation behind this introduction is Quantum Entanglement relation between the two annihilation photons. These two oppositely emitted photons are linearly polarized at right angle to each other. Simulations studies for this research work are undergoing and some preliminary results are presented here.

physics.ins-det↗

arXiv2018

Design, Fabrication and Characterization of nanoplasmonic lattice for trapping of ultracold atoms

Ultracold atom-traps on a chip enhances the practical application of atom traps in quantum information processing, sensing, and metrology. Plasmon mediated near-field optical potentials are promising for trapping atoms. The combination of plasmonic nanostructures and ultracold atoms has the potential to create a two dimensional array of neutral atoms with lattice spacing smaller than that of lattices created from interfering light fields -- the optical lattices. We report the design, fabrication and characterization of a nano-scale array of near-field optical traps for neutral atoms using plasmonic nanostructures. The building block of the array is a metallic nano-disc fabricated on the surface of an ITO-coated glass substrate. We numerically simulate the electromagnetic field-distribution using Finite Difference Time Domain method around the nanodisc, and calculate the intensity, optical potential and the dipole force for $^{87}$Rb atoms. The optical near-field generated from the fabricated nanostructures is experimentally characterized by using Near-field Scanning Optical Microscopy. We find that the optical potential and dipole force has all the desired characteristics to trap cold atoms when a blue-detuned light-field is used to excite the nanostructures. This trap can be used for effective trapping and manipulation of isolated atoms and also for creating a lattice of neutral atoms having sub-optical wavelength lattice spacing. Near-field measurements are affected by the influence of tip on the sub-wavelength structure. We present a deconvolution method to extract the actual near-field profile from the measured data.

physics.optics↗

arXiv2018

Rare earth size dependence on structural, electronic and magnetic properties of R2NiMnO6 double perovskites

Rare earth Manganite Nickelite double perovskites are prepared by solgel assisted combustion route, where, R varies from La, Pr, Nd, Sm, Gd, Tb, Dy, Y, and Ho. The samples have been systematically investigated using powder xray diffraction, Raman spectroscopy, ultraviolet visible spectroscopy, magnetization, and synchrotron based xray absorption spectroscopy measurements. All compounds in the family crystallize in the monoclinic structure and the monoclinic distortion enhances with decreasing trivalent rare earth radii. The magnetic ordering temperature, Tc, decreases from 270 K for La to 80 K for Ho samples as the rare earth radii decrease from 0.116nm for La to 0.102nm in case of Ho. An additional anomaly is observed in samples containing Nd, Sm, Tb, and Dy at lower temperatures, which originates from the 3d to 4f coupling between Mn and Ni and Nd, Sm, Tb and Dy magnetic moments. Further, high saturation magnetization is achieved for all samples indicating that they are atomically ordered and have less anti site disorders. Upon decreasing the size of rare earth ions, the local structure shows an expansion of NiO6 octahedra and almost unchanged MnO6 octahedra. Xray absorption near edge spectroscopy reveals that majority of Ni and Mn ions are in positive 2 and positive 4 valence states in all the samples. Raman spectra of RNMO show a softening of phonon modes resulting in the elongation of Ni to O and Mn to O bond lengths. Finally, a correlation between lattice parameters, structural distortion, octahedral tilting, superexchange angle, and electronic band gap, Curie temperature, and the rare earth ionic radius is established.

cond-mat.mtrl-sci↗