arXiv subjects

Sandeep Kumar

Publications and source records attributed to Sandeep Kumar.

At least 109 records · Page 6Linked to original sources

arXiv2022

Static and Dynamical, Fractional Uncertainty Principles

We study the process of dispersion of low-regularity solutions to the Schrödinger equation using fractional weights (observables). We give another proof of the uncertainty principle for fractional weights and use it to get a lower bound for the concentration of mass. We consider also the evolution when the initial datum is the Dirac comb in $\mathbb{R}$. In this case we find fluctuations that concentrate at rational times and that resemble a realization of a Lévy process. Furthermore, the evolution exhibits multifractality.

math.AP↗

arXiv2021

Topological phonons in an inhomogeneously strained silicon-6: Possible evidence of the high temperature spin superfluidity and the second sound of topological phonons

The superposition of topological phonons and flexoelectronic charge separation in an inhomogeneously strain Si give rise to topological electronic magnetism of phonons. The topological electronic magnetism of phonons is also expected to give rise to stationary spin current or spin superfluidity. In this experimental study, we present possible evidence of spin superfluidity in an inhomogeneously strained p-Si thin films samples. The spin superfluidity is uncovered using non-local resistance measurement. A resonance behavior is observed in a non-local resistance measurement at 10 kHz and between 270 K and 281.55 K, which is attributed to the second sound. The observation of second sound and spatially varying non-local resistance phase are the evidences for spin superfluidity. The spatially varying non-local resistance with opposite phase are also observed in Pt/MgO/p-Si sample. The overall non-local responses can be treated as a standing waveform from temporal magnetic moments of the topological phonons.

cond-mat.mtrl-sci↗

arXiv2021

Topological phonons in an inhomogeneously strained silicon-1: Evidence of long-distance spin transport and unidirectional magnetoresistance of phonons

Transverse acoustic waves in an inhomogeneous medium are analogues to electromagnetic waves and will exhibit topological behavior due to the Berry gauge potential in the momentum space due to inhomogeneity. The inhomogeneous (or gradient) medium can be created using an applied strain gradient in a semiconductor thin film (silicon) since the phonon frequency and dispersion will be a function of the local strain along the strain gradient direction. As a consequence, topological phonon mediated spin and heat transport can be engineered in the semiconductor thin films. Here, we present evidence of a long-distance (100 um) spin transport in the freestanding Si thin film sample under an applied strain gradient using transverse spin-Nernst effect measurement. The long-distance spin transport was attributed to the topological spin-Hall effect of phonons in an inhomogeneous medium. The inhomogeneous medium was validated using unidirectional magnetoresistance of phonons where the magnitude of the coefficient of the non-reciprocal response at room temperature was as large as reported in the BiTeBr at low temperatures. The topological phonons also manifested the topological Nernst effect. This work not only enhances the current understanding of inhomogeneous systems but also lays the foundation of the topological and spin phononics.

cond-mat.mtrl-sci↗

arXiv2021

Topological phonons in an inhomogeneously strained silicon-2: Evidence of spin-momentum locking

In this study, we report first experimental evidence of spin-momentum locking in the topological phonons in an inhomogeneously strained Si thin film. The spin-momentum locking in the topological phonons lead to a longitudinal spin texture or spatially inhomogeneous spin distribution in the freestanding sample structure. The spin texture was uncovered using location dependent Hall effect and planar Hall effect measurement. The charge carrier density and anomalous Hall resistance showed a linear behavior along the length of the sample. Similarly, the planar Hall resistance related with the spin dependent scattering was also found to be different at two different location along the length of the sample. The spin-momentum locking also gave rise to transverse thermal spin current and spin-Nernst magneto thermopower response, which was uncovered using angle dependent longitudinal second harmonic measurement. The magneto thermopower response was also a function of crystallography of the Si sample where the sign of the response was opposite for <110> and <100> aligned samples. The spin-momentum locking in topological phonons may give rise to large spin dependent response at and above room temperature, which can pave the way for energy efficient spintronics and spin-caloritronics devices.

cond-mat.mtrl-sci↗

arXiv2021

Topological phonons in an inhomogeneously strained silicon-4: Large spin dependent thermoelectric response and thermal spin transfer torque due to topological electronic magnetism of phonons

The superposition of flexoelectronic doping and topological phonons give rise to topological electronic magnetism of phonon in an inhomogeneously strained Si in the bilayer structure with metal. In case of ferromagnetic metal and Si bilayer structure, the flexoelectronic doping will also give rise to larger spin current, which will lead to large spin to charge conversion due to topological electronic magnetism of phonon. By applying a temperature difference to ferromagnetic metal/Si bilayer structure under an applied strain gradient, a large thermoelectric response can be generated. In this experimental study, we report a large spin dependent thermoelectric response at Ni80Fe20/Si bilayer structure. The spin dependent response is found to be an order of magnitude larger than that in Pt thin films and similar to topological insulators surface states in spite of negligible intrinsic spin-orbit coupling of Si. This large response is attributed to the flexoelectronic doping and topological electronic magnetism of phonons, which was uncovered using topological Nernst effect measurement. This alternative and novel approach of using inhomogeneous strain engineering to address both spin current density and spin to charge conversion can open a new window to the realization of spintronics and spin-caloritronics devices using metal and doped-semiconductor layered materials.

cond-mat.mtrl-sci↗

arXiv2021

Topological phonons in an inhomogeneously strained silicon-5: Inhomogeneous magnetoelectronic effect in a conductor

The spatially inhomogeneity in a magnetic crystal give rise to electric polarization, which is known as inhomogeneous magnetoelectric effect. Similarly, an inhomogeneous magnetoelectronic effect in a conducting multiferroic material give rise to spatially inhomogeneous magnetic moment and spin distribution due to spatially inhomogeneity in the charge carrier concentration. In this study, we present experimental evidence of inhomogeneous magnetoelectronic effect in Py/p-Si layered structure. The Py/p-Si layered structure exhibit electronic multiferroicity due to superposition of flexoelectronic charge carrier doping and topological phonons. It gives rise to spatially modulations in the spin density and magnetic moment, which are discovered using the Hall effect measurement. The charge carrier density as well as type of the charge carrier are found to be a function of spatial coordinate as well as direction of magnetic field. The observed modulations can also be interpreted as incommensurate SDW with wavelength of ~142 um. The inhomogeneous magnetoelectronic effect also give rise to magnetocaloric effect, which is uncovered using thermal hysteresis in the magnetoresistance measurement. This is a first experimental evidence of inhomogeneous magnetoelectronic effect, which is electronic counterpart of the magnetoelectric effect.

cond-mat.mtrl-sci↗

arXiv2021

Experimental evidence of hidden spin polarization in silicon by using strain gradient

The centrosymmetric materials with hidden spin polarization are considered to be the promising candidates for realization of energy efficient spintronics systems and devices. However, the control of hidden spin polarization and resulting transport behavior is not well understood. We hypothesized that inhomogeneous strain can be the external knob to study and control hidden spin polarization. In this work, we demonstrate a strain gradient mediated symmetry breaking to discover the hidden spin polarization in centrosymmetric Si lattice. The hidden spin polarization gives rise to magnetocrystalline anisotropy and local magnetic moment along <111> directions in the Si. The local magnetic moment gives rise to spin-acoustic phonon coupling, which is the underlying cause of observed spin-Hall effect in both n-Si and p-Si. Discovery of hidden magnetic moment in Si not only challenges the fundamental understanding of the origin of the magnetism but also presents a giant leap in realization of spintronics systems.

cond-mat.mes-hall↗

arXiv2021

SAU: Smooth activation function using convolution with approximate identities

Well-known activation functions like ReLU or Leaky ReLU are non-differentiable at the origin. Over the years, many smooth approximations of ReLU have been proposed using various smoothing techniques. We propose new smooth approximations of a non-differentiable activation function by convolving it with approximate identities. In particular, we present smooth approximations of Leaky ReLU and show that they outperform several well-known activation functions in various datasets and models. We call this function Smooth Activation Unit (SAU). Replacing ReLU by SAU, we get 5.12% improvement with ShuffleNet V2 (2.0x) model on CIFAR100 dataset.

cs.LG↗

arXiv2021

Closed-loop targeted optogenetic stimulation of C. elegans populations

We present a high-throughput optogenetic illumination system capable of simultaneous closed-loop light delivery to specified targets in populations of moving Caenorhabditis elegans. The instrument addresses three technical challenges: it delivers targeted illumination to specified regions of the animal's body such as its head or tail; it automatically delivers stimuli triggered upon the animal's behavior; and it achieves high throughput by targeting many animals simultaneously. The instrument was used to optogenetically probe the animal's behavioral response to competing mechanosensory stimuli in the the anterior and posterior soft touch receptor neurons. Responses to more than $10^4$ stimulus events from a range of anterior-posterior intensity combinations were measured. The animal's probability of sprinting forward in response to a mechanosensory stimulus depended on both the anterior and posterior stimulation intensity, while the probability of reversing depended primarily on the posterior stimulation intensity. We also probed the animal's response to mechanosensory stimulation during the onset of turning, a relatively rare behavioral event, by delivering stimuli automatically when the animal began to turn. Using this closed-loop approach, over $10^3$ stimulus events were delivered during turning onset at a rate of 9.2 events per worm-hour, a greater than 25-fold increase in throughput compared to previous investigations. These measurements validate with greater statistical power previous findings that turning acts to gate mechanosensory evoked reversals. Compared to previous approaches, the current system offers targeted optogenetic stimulation to specific body regions or behaviors with many-fold increases in throughput to better constrain quantitative models of sensorimotor processing.

q-bio.NC↗

arXiv2021

On the Schrödinger map for regular helical polygons in the hyperbolic space

The main purpose is to describe the evolution of $\Xt = \Xs \wedge_- \Xss,$ with $\X(s,0)$ a regular polygonal curve with a nonzero torsion in the 3-dimensional hyperbolic space. Unlike in the Euclidean space, a nonzero torsion implies two different helical curves. However, recent techniques developed by de la Hoz, Kumar, and Vega help us in describing the evolution at rational times both theoretically and numerically, and thus, the similarities and differences. Numerical experiments show that the trajectory of the point $\X(0,t)$ exhibits new variants of Riemann's non-differentiable function whose structure depends on the initial torsion in the problem. As a result, with these new solutions, it is shown that the smooth solutions (helices, straight line) in the hyperbolic space show the same instability as displayed by their Euclidean counterparts and curves with zero-torsion. These numerical observations are in agreement with some recent theoretical results obtained by Banica and Vega.

math.AP↗

arXiv2021

Vortex Filament Equation for a regular polygon in the hyperbolic plane

The aim of this article is twofold. First, we show the evolution of the vortex filament equation (VFE) for a regular planar polygon in the hyperbolic space. Unlike in the Euclidean space, the planar polygon is open and both of its ends grow exponentially, which makes the problem more challenging from a numerical point of view. However, with fixed boundary conditions, a finite difference scheme and a fourth-order Runge--Kutta method in time, we show that the numerical solution is in complete agreement with the one obtained from algebraic techniques. Second, as in the Euclidean case, we claim that, at infinitesimal times, the evolution of VFE for a planar polygon as the initial datum can be described as a superposition of several one-corner initial data. As a consequence, not only can we compute the speed of the center of mass of the planar polygon, but the relationship also allows us to compare the time evolution of any of its corners with that in the Euclidean case.

math.NA↗

arXiv2021

Intensity Prediction of Tropical Cyclones using Long Short-Term Memory Network

Tropical cyclones can be of varied intensity and cause a huge loss of lives and property if the intensity is high enough. Therefore, the prediction of the intensity of tropical cyclones advance in time is of utmost importance. We propose a novel stacked bidirectional long short-term memory network (BiLSTM) based model architecture to predict the intensity of a tropical cyclone in terms of Maximum surface sustained wind speed (MSWS). The proposed model can predict MSWS well advance in time (up to 72 h) with very high accuracy. We have applied the model on tropical cyclones in the North Indian Ocean from 1982 to 2018 and checked its performance on two recent tropical cyclones, namely, Fani and Vayu. The model predicts MSWS (in knots) for the next 3, 12, 24, 36, 48, 60, and 72 hours with a mean absolute error of 1.52, 3.66, 5.88, 7.42, 8.96, 10.15, and 11.92, respectively.

cs.LG↗

arXiv2021

Tropical cyclone intensity estimations over the Indian ocean using Machine Learning

Tropical cyclones are one of the most powerful and destructive natural phenomena on earth. Tropical storms and heavy rains can cause floods, which lead to human lives and economic loss. Devastating winds accompanying cyclones heavily affect not only the coastal regions, even distant areas. Our study focuses on the intensity estimation, particularly cyclone grade and maximum sustained surface wind speed (MSWS) of a tropical cyclone over the North Indian Ocean. We use various machine learning algorithms to estimate cyclone grade and MSWS. We have used the basin of origin, date, time, latitude, longitude, estimated central pressure, and pressure drop as attributes of our models. We use multi-class classification models for the categorical outcome variable, cyclone grade, and regression models for MSWS as it is a continuous variable. Using the best track data of 28 years over the North Indian Ocean, we estimate grade with an accuracy of 88% and MSWS with a root mean square error (RMSE) of 2.3. For higher grade categories (5-7), accuracy improves to an average of 98.84%. We tested our model with two recent tropical cyclones in the North Indian Ocean, Vayu and Fani. For grade, we obtained an accuracy of 93.22% and 95.23% respectively, while for MSWS, we obtained RMSE of 2.2 and 3.4 and $R^2$ of 0.99 and 0.99, respectively.

physics.ao-ph↗

arXiv2021

Ionization and transport in partially ionized multicomponent plasmas: Application to atmospheres of hot Jupiters

We study ionization and transport processes in partially ionized multicomponent plasmas. The plasma composition is calculated via a system of coupled mass action laws. The electronic transport properties are determined by the electron-ion and electron-neutral transport cross sections. The influence of electron-electron scattering is considered via a correction factor to the electron-ion contribution. Based on this data, the electrical and thermal conductivity as well as the Lorenz number are calculated. For the thermal conductivity, we consider also the contributions of the translational motion of neutral particles and of the dissociation, ionization, and recombination reactions. We apply our approach to a partially ionized plasma composed of hydrogen, helium, and a small fraction of metals (Li, Na, Ca, Fe, K, Rb, Cs) as typical for hot Jupiter atmospheres. We present results for the plasma composition and the transport properties as function of density and temperature and then along typical P-T profiles for the outer part of the hot Jupiter HD 209458b. The electrical conductivity profile allows revising the Ohmic heating power related to the fierce winds in the planet's atmosphere. We show that the higher temperatures suggested by recent interior models could boost the conductivity and thus the Ohmic heating power to values large enough to explain the observed inflation of HD 209458b.

physics.plasm-ph↗

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 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

Predicting Landfall's Location and Time of a Tropical Cyclone Using Reanalysis Data

Landfall of a tropical cyclone is the event when it moves over the land after crossing the coast of the ocean. It is important to know the characteristics of the landfall in terms of location and time, well advance in time to take preventive measures timely. In this article, we develop a deep learning model based on the combination of a Convolutional Neural network and a Long Short-Term memory network to predict the landfall's location and time of a tropical cyclone in six ocean basins of the world with high accuracy. We have used high-resolution spacial reanalysis data, ERA5, maintained by European Center for Medium-Range Weather Forecasting (ECMWF). The model takes any 9 hours, 15 hours, or 21 hours of data, during the progress of a tropical cyclone and predicts its landfall's location in terms of latitude and longitude and time in hours. For 21 hours of data, we achieve mean absolute error for landfall's location prediction in the range of 66.18 - 158.92 kilometers and for landfall's time prediction in the range of 4.71 - 8.20 hours across all six ocean basins. The model can be trained in just 30 to 45 minutes (based on ocean basin) and can predict the landfall's location and time in a few seconds, which makes it suitable for real time prediction.

cs.LG↗

arXiv2021

Prediction of Landfall Intensity, Location, and Time of a Tropical Cyclone

The prediction of the intensity, location and time of the landfall of a tropical cyclone well advance in time and with high accuracy can reduce human and material loss immensely. In this article, we develop a Long Short-Term memory based Recurrent Neural network model to predict intensity (in terms of maximum sustained surface wind speed), location (latitude and longitude), and time (in hours after the observation period) of the landfall of a tropical cyclone which originates in the North Indian ocean. The model takes as input the best track data of cyclone consisting of its location, pressure, sea surface temperature, and intensity for certain hours (from 12 to 36 hours) anytime during the course of the cyclone as a time series and then provide predictions with high accuracy. For example, using 24 hours data of a cyclone anytime during its course, the model provides state-of-the-art results by predicting landfall intensity, time, latitude, and longitude with a mean absolute error of 4.24 knots, 4.5 hours, 0.24 degree, and 0.37 degree respectively, which resulted in a distance error of 51.7 kilometers from the landfall location. We further check the efficacy of the model on three recent devastating cyclones Bulbul, Fani, and Gaja, and achieved better results than the test dataset.

cs.LG↗