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S. K. Bose

Publications and source records attributed to S. K. Bose.

At least 19 recordsLinked to original sources
arXiv2021

Development and accuracy evaluation of Coded Phase-shift 3D scanner

In this paper, we provide an overview of development of a structured light 3D-scanner based on combination of binary-coded patterns and sinusoidal phase-shifted fringe patterns called Coded Phase-shift technique. Further, we describe the experiments performed to evaluate measurement accuracy and precision of the developed system. A study of this kind is expected to be helpful in understanding the basic working of current structured-light 3D scanners and the approaches followed for their performance assessment.

eess.IV↗
arXiv2018

Synaptic dynamics in complex self-assembled nanoparticle networks

We report a detailed study of neuromorphic switching behaviour in inherently complex percolating networks of self-assembled metal nanoparticles. We show that variation of the strength and duration of the electric field applied to this network of synapse-like atomic switches allows us to control the switching dynamics. Switching is observed for voltages above a well-defined threshold, with higher voltages leading to increased switching rates. We demonstrate two behavioral archetypes and show how the switching dynamics change as a function of duration and amplitude of the voltage stimulus. We show that the state of each synapse can influence the activity of the other synapses, leading to complex switching dynamics. We further demonstrate the influence of the morphology of the network on the measured device properties, and the constraints imposed by the overall network conductance. The correlated switching dynamics, device stability over long periods, and the simplicity of the device fabrication provide an attractive pathway to practical implementation of on-chip neuromorphic computing.

cond-mat.dis-nn↗
arXiv2017

On recent advances in 2D Constrained Delaunay triangulation algorithms

In this article, recent works on 2D Constrained Delaunay triangulation(CDT) algorithms have been reported. Since the review of CDT algorithms presented by de Floriani(Issues on Machine Vision, Springer Vienna, pg. 95--104, 1989), different algorithms for construction and applications of CDT have appeared in literature each concerned with different aspects of computation and different suitabilities. Therefore, objective of this work is to report an update over that review article considering contemporary prominent algorithms and generalizations for the problem of two dimensional Constrained Delaunay triangulation.

cs.CG↗
arXiv2015

The Atomic Hypothesis

A review is presented of the origin and development of the atomic hypothesis from antiquity till about the first millennium of the common era.

physics.hist-ph↗
arXiv2012

Half-metallicity and magnetism of GeTe doped with transition metals V, Cr and Mn: a theoretical study from the viewpoint of application in spintronics

This work presents results for the magnetic properties of the compound GeTe doped with 3d transition metals V, Cr and Mn from the viewpoint of potential application in spintronics. We report a systematic density-functional study of the electronic structure, magnetic and cohesive properties of these ternary compounds in both rock salt and zinc blende structures. In both cases, it is the Ge sublattice that is doped with the three transition metals. Some of these compounds are found to be half-metallic at their optimized cell volumes. For these particular cases, we calculate both exchange interactions and the Curie temperatures in order to provide some theoretical guidance to experimentalists trying to fabricate materials suitable for spintronic devices. Discussions relating our results to the existing experimental studies are provided whenever applicable and appropriate. Apparent discrepancy between experimental observations and our theoretical result for the case of Mn-doping is discussed in detail, pointing out various physical reasons and possible resolutions of the apparent discrepancy.

cond-mat.mtrl-sci↗
arXiv2011

Pressure dependence of Curie temperature and resistivity in complex Heusler alloys

Using first-principles electronic structure calculations, we have studied the dependence of the Curie temperature on external hydrostatic pressure for random Ni2MnSn Heusler alloys doped with Cu and Pd atoms, over the entire range of dopant concentrations. The Curie temperatures are calculated by applying random-phase approximation to the Heisenberg Hamiltonian whose parameters are determined using the linear response and multiple scattering methods, based on density-functional theory. In (Ni1-x,Pdx)2MnSn alloys, the Curie temperature is found to increase with applied pressure over the whole concentration range. The crossover from the increase to the decrease of the Curie temperature with pressure takes place for Cu concentrations larger than about 70% in (Ni1-x,Cux)2MnSn Heusler alloys. The results for the reference Ni2MnSn Heusler alloy agree well with a previous theoretical study of E. Sasioglu, L. M. Sandratskii and P. Bruno Phys. Rev. B 71 214412 (2005) and also reasonably well with available experimental data. Results for the spin-disorder-induced part of the resistivity in (Ni1-x,Pdx)2MnSn Heusler alloys, calculated by using the disordered local moment model, are also presented. Finally, a qualitative understanding of the results, based on Anderson's superexchange interaction and Stearn's model of the indirect exchange interaction between localized and itinerant d electrons, is provided.

cond-mat.mtrl-sci↗
arXiv2010

Magnetism of mixed quaternary Heusler alloys: (Ni,T)$_{2}$MnSn (T=Cu,Pd) as a case study

The electronic properties, exchange interactions, finite-temperature magnetism, and transport properties of random quaternary Heusler Ni$_{2}$MnSn alloys doped with Cu- and Pd-atoms are studied theoretically by means of {\it ab initio} calculations over the entire range of dopant concentrations. While the magnetic moments are only weakly dependent on the alloy composition, the Curie temperatures exhibit strongly non-linear behavior with respect to Cu-doping in contrast with an almost linear concentration dependence in the case of Pd-doping. The present parameter-free theory agrees qualitatively and also reasonably well quantitatively with the available experimental results. An analysis of exchange interactions is provided for a deeper understanding of the problem. The dopant atoms perturb electronic structure close to the Fermi energy only weakly and the residual resistivity thus obeys a simple Nordheim rule. The dominating contribution to the temperature-dependent resistivity is due to thermodynamical fluctuations originating from the spin-disorder, which, according to our calculations, can be described successfully via the disordered local moments model. Results based on this model agree fairly well with the measured values of spin-disorder induced resistivity.

cond-mat.mtrl-sci↗
arXiv2010

First principles theoretical studies of half-metallic ferromagnetism in CrTe

Using full-potential linear augmented plane wave method (FP-LAPW) and the density functional theory, we have carried out a systematic investigation of the electronic, magnetic, and cohesive properties of the chalcogenide CrTe in three competing structures: rock-salt (RS), zinc blende (ZB) and the NiAs-type (NA) hexagonal. Although the ground state is of NA structure, RS and ZB are interesting in that these fcc-based structures, which can possibly be grown on many semiconductor substrates, exhibit half-metallic phases above some critical values of the lattice parameter. We find that the NA structure is not half-metallic at its equilibrium volume, while both ZB and RS structures are. The RS structure is more stable than the ZB, with an energy that is lower by 0.25 eV/atom. While confirming previous results on the half-metallic phase in ZB structure, we provide hitherto unreported results on the half-metallic RS phase, with a gap in the minority channel and a magnetic moment of 4.0 $μ_{B}$ per formula unit. A comparison of total energies for the ferromagnetic (FM), non-magnetic (NM), and antiferromagnetic (AFM) configurations shows the lowest energy configuration to be FM for CrTe in all the three structures. The FP-LAPW calculations are supplemented by linear muffin-tin orbital (LMTO) calculations using both local density approximation (LDA) and LDA+U method. The exchange interactions and the Curie temperatures calculated via the linear response method in ZB and RS CrTe are compared over a wide range of the lattice parameter. The calculated Curie temperatures for the RS phase are consistently higher than those for the ZB phase.

cond-mat.mtrl-sci↗
arXiv2010

Exchange interactions and Curie temperatures in Cr-based alloys in Zinc Blende structure: volume- and composition-dependence from first-principles calculations

We present calculations of the exchange interactions and Curie temperatures in Cr-based pnictides and chalcogenides of the form CrX with X=As, Sb, S, Se and Te, and the mixed alloys CrAs$_{50}$X$_{50}$ with X=Sb, S, Se, and Te. The calculations are performed for Zinc Blende (ZB) structure for 12 values of the lattice parameter between 5.44 and 6.62 Å, appropriate for some typical II-VI and III-V semiconducting substrates. Electronic structure is calculated via the linear muffin-tin-orbitals (LMTO) method in the atomic sphere approximation (ASA), using empty spheres to optimize ASA-related errors. Whenever necessary, the results have been verified using the full-potential version of the method, FP-LMTO. The disorder effect in the As-sublattice for CrAs$_{50}$X$_{50}$ (X=Sb, S, Se, Te) alloys is taken into account via the coherent potential approximation (CPA). Exchange interactions are calculated using the linear response method for the ferromagnetic (FM) reference states of the alloys, as well as the disordered local moments (DLM) states. These results are then used to estimate the Curie temperature from the low and high temperature side of the ferromagnetic/paramagnetic transition. Estimates of the Curie temperature are provided, based on the mean field and the more accurate random phase approximations. Dominant antiferromagnetic exchange interactions for some low values of the lattice parameter for the FM reference states in CrS, CrSe and CrTe prompted us to look for antiferromagnetic (AFM) configurations for these systems with energies lower than the corresponding FM and DLM values. Results for a limited number of such AFM calculations are discussed, identifying the AFM[111] state as a likely candidate for the ground state for these cases.

cond-mat.mtrl-sci↗
arXiv2009

Robust coupling of superconducting order parameter in a mesoscale NbN-Fe-NbN epitaxial structure

We report an unconventional and promising route to self-assemble distributed superconductor-ferromagnet-superconductor (S-F-S) Josephson Junctions on single crystal [100] MgO. These structures consist of [110] epitaxial nano-plaquettes of Fe covered with superconducting NbN films of varying thickness. The S-F-S structures are characterized by strong magnetoresistance (MR) anisotropy for the in-plane and out-of-plane magnetic fields. The stronger in-plane MR suggests decoherence of S-F-S junctions whose critical current follows a (1-T/Tc) and (1-T/Tc)1/2 dependence for T Tc and T<<Tc respectively, in accordance with the theory of supercurrent transport in such junctions.

cond-mat.supr-con↗
arXiv2008

Lattice-mismatch-induced granularity in CoPt-NbN and NbN-CoPt superconductor-ferromagnet heterostructures: Effect of strain

The effect of strain due to lattice mismatch and of ferromagnetic (FM) exchange field on superconductivity (SC) in NbN-CoPt bilayers is investigated. Two different bilayer systems with reversed deposition sequence are grown on MgO (001) single crystals. While robust superconductivity with high critical temperature (T_c ~ 15.3 K) and narrow transition width DelT_c ~ 0.4 K) is seen in two types of CoPt-NbN/MgO heterostructures where the magnetic anisotropy of CoPt is in-plane in one case and out-of-plane in the other, the NbN-CoPt/MgO system shows markedly suppressed SC response. The reduced SC order parameter of this system, which manifests itself in Tc, temperature dependence of critical current density J_c (T), and angular (Phi) variation of flux-flow resistivity Rho_f is shown to be a signature of the structure of NbN film and not a result of the exchange field of CoPt. The Rho_f (H,T,Phi) data further suggest that the domain walls in the CoPt film are of the Neel type and hence do not cause any flux in the superconducting layer. A small, but distinct increase in the low-field critical current of the CoPt-NbN couple is seen when the magnetic layer has perpendicular anisotropy.

cond-mat.supr-con↗
arXiv2008

Correlations between morphology, crystal structure and magnetization of epitaxial cobalt-platinum films grown with pulsed laser ablation

The effects of growth rate (G_r), deposition temperature (T_d), film thickness (t_F), and substrate induced strain (epsilon) on morphological, crystallographic and magnetic characteristics of equiatomic CoPt epitaxial films synthesized with PLD are investigated. The (001) substrates of MgO, STO and LAO provide different degree of epitaxial strain for growth of the disordered face centered cubic (fcc) and ordered face centered tetragonal (L1_0) phases of CoPt. The films deposited at T_d~600 ^0C on all three substrates are fcc with in-plane magnetization and a narrow hysteresis loop of width~200 Oe. The L1_0 phase, stabilized only at T_d~700 ^0C becomes predominantly c-axis oriented as T_d is increased to 800 ^0C. While the crystallographic structure of the films depends solely on the T_d, their microstructure and magnetization characteristics are decided by the growth rate. At the higher G_r (~1A/sec) the L1_0 films have a maze-like structure which converts to a continuous film as the t_F is increased from 20 to 50 nm. The H_c of these films increases as the L1_0 phase fraction grows with T_d and its orientation becomes out of the film plane. The evolution of microstructure with T_d is remarkably different at lower growth rate (~0.4A /sec). Here the structure changes from a self-similar fractal pattern to an assembly of nano-dots as the T_d is raised from 700 to 800 ^0C, and is understood in terms of the imbalance between strain and interfacial energies. MFM of such films reveals no distinct domain walls within the nano-islands while a clear contrast is seen between the islands of reversed magnetization. The simple picture of coherent rotation of moment appears incompatible with the time dependence of the remanent magnetization in these films.

cond-mat.mtrl-sci↗
arXiv2007

Linear response results for phonons and electron-phonon coupling in hcp Sc - spin fluctuations and implications for superconductivity

We present a study of the electronic structure, phonon frequencies and electron-phonon coupling in hcp Sc under pressure. The electron-phonon coupling constant is found to increase steadily with pressure in the hcp phase, until the pressure reaches a value where the hcp phase becomes unstable. Calculations for the normal pressure $c/a$ ratio predict a phase change somewhere between calculated pressures of 22 and 30 GPa. The calculated frequencies for the equilibrium hcp lattice parameters are in good agreement with the inelastic neutron scattering results. From the measured value of the electronic specific heat constant and the calculated values of the Fermi level density of states and electron-phonon coupling constant, we conclude that the electron-paramagnon coupling constant in hcp Sc should be comparable to the electron-phonon coupling constant. This indicates that the spin fluctuation effects are strong enough to suppress superconductivity completely in hcp Sc. Based on estimates of the electron-paramagnon coupling constants and the calculated or estimated electron-phonon coupling constants, we argue that the hcp phase may become superconducting with a very low transition temperature immediately prior to the transition to the Sc-II phase and that the Sc-II phase should indeed be superconducting.

cond-mat.supr-con↗
arXiv2006

Phonon Spectra and Thermal Properties of Some fcc Metals Using the Embedded-Atom Method

By employing the analytic embedded-atom potentials of Mei {\it et al.} [Phys. Rev. B 43, 4653 (1991)] we have calculated the phonon dispersion spectra for six fcc metals: Cu, Ag, Au, Ni, Pd and Pt. We have also investigated thermal properties of these metals within the quasiharmonic approximation. Results for the lattice constants, coefficients of linear thermal expansion, isothermal and adiabatic bulk moduli, heat capacities at constant volume and constant pressure, Debye temperatures and Grüneisen parameters as a function of temperature are presented. The computed results are compared with the available experimental data. The comparison shows a generally good agreement between the calculated and experimental values for all thermodynamic properties studied. Isothermal and adiabatic bulk moduli and the specific heats are reproduced reasonably well, while the Grüneisen parameter and Debye temperature are underestimated by about 10%. The calculated phonon frequencies for Ag and Cu agree well with the results from inelastic neutron scattering experiments. However, there is considerable room for improvement in the phonon frequencies for Ni, Pd, Pt and Au, particularly at high phonon wave vectors close to the Brillouin zone boundary. The coefficient of linear thermal expansion is underestimated in most cases except for Pt and Au. The results are good for Pt up to 1000K and for Au up to 500 K.

cond-mat.mtrl-sci↗
arXiv2006

Momentum-resolved electron-phonon interaction in lead determined by neutron resonance spin-echo spectroscopy

Neutron resonance spin-echo spectroscopy was used to monitor the temperature evolution of the linewidths of transverse acoustic phonons in lead across the superconducting transition temperature, $T_c$, over an extended range of the Brillouin zone. For phonons with energies below the superconducting energy gap, a linewidth reduction of maximum amplitude $\sim 6 μ$eV was observed below $T_c$. The electron-phonon contribution to the phonon lifetime extracted from these data is in satisfactory overall agreement with {\it ab-initio} lattice-dynamical calculations, but significant deviations are found.

cond-mat.supr-con↗
arXiv2005

Electronic structure, electron-phonon coupling and superconductivity of isotypic noncentrosymmetric crystals Li$_2$Pd$_3$B and Li$_2$Pt$_3$B

Electronic structure of recently discovered isotypic ternary borides Li$_2$Pd$_3$B and Li$_2$Pt$_3$B, with noncentrosymmetric crystal structures, is studied with a view to understanding their superconducting properties. Estimates of the Fermi-surface averaged electron-phonon matrix element and Hopfield parameter are obtained in the rigid ion approximation of Gaspari and Gyorffy [Phys. Rev. Lett. {\bf 28} (1972) 801]. The contribution of the lithium atoms to the electron-phonon coupling is found to be negligible, while both boron and palladium atoms contribute equally strongly to the Hopfield parameter. There is a significant transfer of charge from lithium, almost the entire valence charge, to the B-Pd(Pt) complex. The electronic structure and superconducting properties of Li$_2$Pd$_3$B, thus, can be understood from the viewpoint of the compound being composed of a connected array of B-Pd tetrahedra decoupled from the backbone of Li atoms, which are connected by relatively short bonds. Our results suggest that conventional s-wave electron-phonon interaction without explicit consideration of SO coupling can explain qualitatively the observed $T_c$ in Li$_2$Pd$_3$B. However, such an approach is likely to fail to describe superconductivity in Li$_2$Pt$_3$B.

cond-mat.supr-con↗
arXiv2005

Superconductivity in Boron under pressure - why are the measured T$_c$'s so low?

Using the full potential linear muffin-tin orbitals (FP-LMTO) method we examine the pressure-dependence of superconductivity in the two metallic phases of Boron: bct and fcc. Linear response calculations are carried out to examine the phonon frequencies and electron-phonon coupling for various lattice parameters, and superconducting transition temperatures are obtained from the Eliashberg equation. In both bct and fcc phases the superconducting transition temperature T$_c$ is found to decrease with increasing pressure, due to stiffening of phonons with an accompanying decrease in electron-phonon coupling. This is in contrast to a recent report, where T$_c$ is found to increase with pressure. Even more drastic is the difference between the measured T$_c$, in the range 4-11 K, and the calculated values for both bct and fcc phases, in the range 60-100 K. The calculation reveals that the transition from the fcc to bct phase, as a result of increasing volume or decreasing pressure, is caused by the softening of the X-point transverse phonons. This phonon softening also causes large electron-phonon coupling for high volumes in the fcc phase, resulting in coupling constants in excess of 2.5 and T$_c$ nearing 100 K. We discuss possible causes as to why the experiment might have revealed T$_c$'s much lower than what is suggested by the present study. The main assertion of this paper is that the possibility of high T$_c$, in excess of 50 K, in high pressure pure metallic phases of boron cannot be ruled out, thus substantiating the need for further experimental investigations of the superconducting properties of high pressure pure phases of boron.

cond-mat.supr-con↗