arXiv subjects

Dipankar Saha

Publications and source records attributed to Dipankar Saha.

At least 19 recordsLinked to original sources

arXiv2026

Propagation Mechanism-Aware Near-Field Spatially Non-Stationary Channel Estimation and Environment Mapping

Extremely large aperture arrays (ELAAs) benefit the dual functions of integrated sensing and communication (ISAC) systems by enabling high-throughput data streams and high angular resolution with near-field spatial diversity. However, near-field spherical wavefront effects and spatial non-stationarity (SNS) bring challenges to both communication and sensing. This paper studies near-field spatially non-stationary channel estimation and environment mapping by jointly accounting for multi-bounce, blockage-induced partial visibility, and hybrid reflection-scattering propagation. We propose a unified parametric sensing channel model that represents the SNS phenomenon (due to partial array blockage, diffraction, and specular reflection) through spatially varying visibility and amplitude of each multipath across the array. To regularize the spatially varying delays caused by propagation mechanisms, we incorporate geometric constraints (GCs) based on environmental interaction points, embedding them into the model as absolute propagation delays. We then develop a GC-space-alternating generalized expectation-maximization (GC-SAGE) algorithm to estimate near-field channel parameters and locate environment scatterers/reflectors. Moreover, the GC-SAGE calculates per antenna path amplitudes based on the delays determined by the coordinates of scatterers/reflectors and transceivers, thereby effectively detecting channel SNS. Both ray-based simulation and field measurement are used to validate the proposed approach.

eess.SP↗

arXiv2024

SPICE-PIDE: A Methodology for Design and Optimization of Integrated Circuits

In application-specific designs, owing to the trade-off between power consumption and speed, optimization of various circuit parameters has become a challenging task. Several of the performance metrics, viz. energy efficiency, gain, performance, and noise immunity, are interrelated and difficult to tune. Such efforts may result in a great deal of manual iterations which in turn increase the computational overhead. Thus, it is important to develop a methodology that not only explores large design space but also reduces the computational time. In this work, we investigate the viability of using a SPICE and Python IDE (PIDE) interface to optimize integrated circuits. The SPICE simulations are carried out using 22 nm technology node with a nominal supply voltage of 0.8 V. The SPICE-PIDE optimizer, as delineated in this work, is able to provide the best solution sets considering various performance metrics and design complexities for 5 transistor level converters.

cs.OH↗

arXiv2024

Spectroscopic Signature of Local Alloy Fluctuations in InGaN/GaN Multi-Quantum-Disk Light Emitting Diode Heterostructures and Its Impact on the Optical Performance

Inhomogeneity-governed carrier localization has been investigated in three sets of InGaN/GaN multi-quantum-disk light-emitting diode (LED) structures grown by plasma-assisted molecular beam epitaxy (PAMBE) under different process conditions. A temperature-dependent study of the luminescence peak positions reveals that samples prepared under certain process conditions exhibit a thermal distribution of carriers from the localized states that show the typical S-shaped dependence in luminescence characteristics. The absence of an S-shaped nature in the other sample prepared with relatively higher In-flux infers a superior homogeneity in alloy composition. Further investigation manifested superior optical properties for the samples where the S-shape nature is found to be absent.

physics.app-ph↗

arXiv2024

Adsorption of Mo and O at S-vacancy on ReS2 surface of ReS2/MoTe2 vdW heterointerface

Applications like high density information storage, neuromorphic computing, nanophotonics, etc. require ultra-thin electronic devices which can be controlled with applied electric field. Of late, atomically thin two-dimensional (2D) materials and van der Waals (vdW) heterointerface of those have emerged as suitable candidates for such ultra-low power nanoelectric devices. In this work, employing density functional theory (DFT), the monolayer ReS2 / monolayer MoTe2 vdW heterostructure with Sulphur vacancy is studied to examine various ground state electronic properties. Changes in effective band gap owing to defect-induced states and modulation of the energy gap value with Molybdenum (Mo) and Oxygen (O) adsorption at the defect site are examined. Since two-dimensional (2D) material based nanoscaled devices exhibit promising switching between non-conducting and conducting states, determining the role of defect-induced states and the adsorption of atoms/molecules on surfaces is crucial. Here, a detailed theoretical study to determine surface properties and relative energetic stability of the vdW heterostructures is carried out. The charge re-distribution between the constituent layers is also analyzed by obtaining Electron Difference Density (EDD) for different heterointerfaces. Nonetheless, the efficacy of switching between non-conducting and conducting states is assessed based on adsorption energy of adatoms binding at the defect site.

cond-mat.mtrl-sci↗

arXiv2023

A New 22 nm ULPLS Architecture to Detect 70 mV Minimum Input, Suitable for IOT Applications

Modern applications such as energy harvesting, signal monitoring in bio-medical sensing, portable point of care devices, etc. which involve state of the art mixed signal subsystems require robust ultra low power operation. Here in this work, a novel ultra low power level shifter (ULPLS) is proposed for sensing voltage signals in sub-threshold region. The proposed architecture is implemented in 22 nm technology using a dual power supply. The high and low supply voltages (VddH & VddL) are set as 0.8 V and 0.4 V respectively. The key design features of ULPLS include a current limiting PMOS diode, a voltage divider, and an enhanced pull up network. The ULPLS exhibits a low power dissipation of ~ 22.84 nW with a minimum ~ 70 mV detection of input signal. The robustness of the design has been examined via worst case and Monte Carlo analyses.

eess.SP↗

arXiv2021

First-principles based simulations of electronic transmission in ReS$_{2}$/WSe$_{2}$ and ReS$_{2}$/MoSe$_{2}$ type-II vdW heterointerfaces

Electronic transmission in monolayer ReS$_{2}$ and ReS$_{2}$ based van der Waals (vdW) heterointerfaces are studied here. Since ReS$_{2}$/WSe$_{2}$ and ReS$_{2}$/MoSe$_{2}$ type-II vdW heterostructures are suitable for near infrared (NIR)/short-wave infrared (SWIR) photodetection, the role of interlayer coupling at the heterointerfaces is examined in this work. Besides, a detailed theoretical study is presented employing density functional theory (DFT) and nonequilibrium Green's function (NEGF) combination to analyse the transmission spectra of the two-port devices with ReS$_{2}$/WSe$_{2}$ and ReS$_{2}$/MoSe$_{2}$ channels and compare the near-equilibrium conductance values.Single layer distorted1T ReS$_{2}$ exhibits formation of parallel chains of 'Re' - 'Re' bonds, leading to in-plane anisotropy. Owing to this structural anisotropy, the charge carrier transport is very much orientation dependent in ReS$_{2}$. Therefore, this work is further extended to investigate the role of clusterized 'Re' atoms in electronic transmission.

cond-mat.mtrl-sci↗

arXiv2020

Exotic compositional ordering in Mn-Ni-As intermetallics

Recent advances in tools for crystal structure analysis enabled us to describe a new phenomenon in structural chemistry, which, to this day, has remained hidden. Here we describe a crystal structure with an incommensurate compositional modulation, Mn0.6Ni0.4As. The sample adopts the NiAs type structure, but in contrast to a normal solid solution, we observe that manganese and nickel separate into layers of MnAs and NiAs with thickness of 2-4 face-shared octahedra. Experimentally, results are obtained by combination of 3D electron diffraction, scanning transmission electron microscopy and neutron diffraction. The distribution of octahedral units between the manganese and nickel layers is perfectly described by a modulation vector q = 0.360(3) c*. An additional periodicity is thus present in the compound. Positional modulation is observed of all elements as a consequence of the occupational modulation.

cond-mat.mtrl-sci↗

arXiv2019

Near-direct bandgap $WSe_2$/$ReS_2$ type-II pn heterojunction for enhanced ultrafast photodetection and high-performance photovoltaics

PN heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current rectifiers, photodetectors, and photovoltaic devices. However, a direct or near-direct bandgap at the heterointerface that can significantly enhance optical generation, for high light absorbing few/multi-layer vdW materials, has not yet been shown. In this work, for the first time, few-layer group-6 transition metal dichalcogenide (TMD) $WSe_2$ is shown to form a sizeable (0.7 eV) near-direct bandgap with type-II band alignment at its interface with the group-7 TMD $ReS_2$ through density functional theory calculations. Further, the type-II alignment and photogeneration across the interlayer bandgap have been experimentally confirmed through micro-photoluminescence and IR photodetection measurements, respectively. High optical absorption in few-layer flakes, large conduction and valence band offsets for efficient electron-hole separation and stacking of light facing, direct bandgap $ReS_2$ on top of gate tunable $WSe_2$ are shown to result in excellent and tunable photodetection as well as photovoltaic performance through flake thickness dependent optoelectronic measurements. Few-layer flakes demonstrate ultrafast response time (5 $μ$s) at high responsivity (3 A/W) and large photocurrent generation and responsivity enhancement at the heterostructure overlap region (10-100X) for 532 nm laser illumination. Large open-circuit voltage of 0.64 V and short-circuit current of 2.6 $μ$A enables high output electrical power. Finally, long term air-stability and a facile single contact metal fabrication process makes the multi-functional few-layer $WSe_2$/$ReS_2$ heterostructure diode technologically promising for next-generation optoelectronic applications.

physics.app-ph↗

arXiv2018

The Avian Compass can be Sensitive even without Sustained Quantum Coherence

Theoretical studies indicating the presence of long-lived coherence in the radical pair system have engendered questions about the utilitarian role of sustained coherence in the avian compass. In this manuscript, we investigate this for a realistic multi-nuclear radical pair system, along with the related question of its sensitivity to the geomagnetic field. Firstly, we find that sustenance of long-lived coherence is unlikely in a realistic hyperfine environment. Secondly, probing the role of the hyperfine interactions on the compass sensitivity, we establish the hyperfine anisotropy as an essential parameter for the sensitivity. Thereby, we are able to identify a parameter regime where the compass would exhibit sensitivity even without sustained coherence.

quant-ph↗

arXiv2017

Effect of Tensile Strain in GaN Layer on the Band Offsets and 2DEG Density in AlGaN/GaN Heterostructures

We have addressed the existing ambiguity regarding the effect of process-induced strain in the underlying GaN layer on AlGaN/GaN heterostructure properties. The bandgaps and offsets for AlGaN on strained GaN are first computed using a cubic interpolation scheme within an empirical tight-binding framework. These are then used to calculate the polarization charge and two-dimensional electron gas density. Our bandstructure calculations show that it is not possible to induce any significant change in band offsets through strain in the GaN layer. The charge-density calculations indicate that such strain can, however, modulate the polarization charge and thereby enhance the 2DEG density at the AlGaN/GaN hetero-interface substantially, by as much as 25% for low Al mole fraction.

physics.app-ph↗

arXiv2017

Quantum Biomimetic Modeling of Diamond NV$^{-}$ Center Spin Dynamics

The spin dynamics of the diamond NV$^{-}$ center turns out to be similar to that of the chemical compass responsible for avian magnetoreception. We present a simulation framework based on quantum master equation for the former that is based upon the Radical Pair model of the latter. We show that this framework captures all the experimentally studied behavior of the NV$^{-}$ center spin system and can therefore be a predictive modeling tool.

physics.atom-ph↗

arXiv2017

On the functional window of the avian compass

The functional window is an experimentally observed property of the avian compass that refers to its selectivity around the geomagnetic field strength. We show that the radical-pair model, using biologically feasible hyperfine parameters, can qualitatively explain the salient features of the avian compass as observed from behavioral experiments: its functional window, as well as disruption of the compass action by an RF field of specific frequencies. Further, we show that adjustment of the hyperfine parameters can tune the functional window, suggesting a possible mechanism for its observed adaptability to field variation. While these lend strong support to the radical-pair model, we find it impossible to explain quantitatively the observed width of the functional window within this model, or even with simple augmentations thereto. This suggests that a deeper generalization of this model may be called for; we conjecture that environmental coupling may be playing a subtle role here that has not been captured accurately. Lastly, we examine a possible biological purpose to the functional window; assuming evolutionary benefit from radical-pair magnetoreception, we conjecture that the functional window is simply a corollary thereof and brings no additional advantage.

physics.bio-ph↗

arXiv2016

Fringe field control of one-dimensional room temperature quantum transport in site controlled AlGaN/GaN lateral nanowires

We have demonstrated effective fringe field control of one-dimensional electron gas (1-DEG) in AlGaN/GaN lateral nanowires. The nanowires are site controlled and formed by a combination of dry and anisotropic wet etching. The nanowire dimensions are well controlled and can have a very high length/width aspect ratio of 10 um/5 nm or larger. The transport is controlled by a fringe gate and shows room temperature quantum transport where gradual filling of 1-D subbands gets manifested as oscillations in the transconductance. The fringe gate threshold voltage for depletion of one-dimensional electron gas is found to increase with increasing drain voltage indicating efficient control of 1-DEG. The transport characteristics and fringe field operation are explained by taking into account quantum capacitance in addition to the conventional geometric capacitance. The effect of nanowire width and fringe gate position is also discussed.

cond-mat.mes-hall↗

arXiv2015

Large excitonic binding energy in GaN based superluminescent light emitting diode on naturally survived sub-10 nm lateral nanowires

We demonstrate a novel method for nanowire formation by natural selection during wet chemical etching in boiling Phosphoric acid. It is observed that wire lateral dimensions of sub-10 nm and lengths of 700 nm or more have been naturally formed during the wet etching. The dimension variation is controlled through etching times wherein the underlying cause is the merging of the nearby crystallographic hexagonal etch pits. The emission processes involving excitons are found to be efficient and lead to enhanced emission characteristics. The exciton binding energy is augmented by using quantum confinement whereby enforcing greater overlap of the electron-hole wave-function. The surviving nanowires are nearly defect-free, have large exciton binding energies of around 45 meV and a small temperature variation of the output electroluminescent light. We have observed superluminescent behaviour of the LEDs formed on these nanowires. There is no observable efficiency roll off till current densities of 400 A/cm2. The present work thus provides an innovative and cost effective manner of device fabrication on the formed nanowires and proves the immediate performance enhancement achievable.

cond-mat.mes-hall↗

arXiv2015

State Transitions and Decoherence in the Avian Compass

The radical pair model has been successful in explaining behavioral characteristics of the geomagnetic compass believed to underlie the navigation capability of certain avian species. In this study, the spin dynamics of the radical pair model and decoherence therein are interpreted from a microscopic state transition point of view. This helps to elucidate the interplay between the hyperfine and Zeeman interactions that enables the avian compass, and the distinctive effects of nuclear and environmental decoherence on it. Using a quantum information theoretic quantifier of coherence, we find that nuclear decoherence induces new structure in the spin dynamics without materially affecting the compass action; environmental decoherence, on the other hand, completely disrupts it.

physics.bio-ph↗

arXiv2014

Row-Based Dual Vdd Assignment, for a Level Converter Free CSA Design and Its Near-Threshold Operation

Subthreshold circuit designs are very much popular for some of the ultra low power applications, where the minimum energy consumption is the primary concern. But, due to the weak driving current, these circuits generally suffer from huge performance degradation. Therefore, in this paper, we primarily targeted to analyze the performance of a Near-Threshold Circuit (NTC), which retains the excellent energy efficiency of the subthreshold design, while improving the performance to a certain extent. A modified row-based dual Vdd 4-operand CSA (Carry Save Adder) design has been reported in the present work using 45 nm technology. Moreover, to find out the effectiveness of the near-threshold operation of the 4-operand CSA design; it has been compared with the other design styles. From the simulation results, obtained for the frequency of 20 MHz, we found that the proposed scheme of CSA design consumes 3.009*10-7 Watt of Average Power (Pavg), which is almost 90.9 % lesser than that of the conventional CSA design. Whereas, looking at the perspective of maximum delay at output, the proposed scheme of CSA design provides a fair 44.37 % improvement, compared to that of the subthreshold CSA design.

cs.AR↗

arXiv2014

Effect of line defects on the electrical transport properties of monolayer MoS$_{2}$ sheet

We present a computational study on the impact of line defects on the electronic properties of monolayer MoS2. Four different kinds of line defects with Mo and S as the bridging atoms, consistent with recent theoretical and experimental observations are considered herein. We employ the density functional tight-binding (DFTB) method with a Slater-Koster type DFTB-CP2K basis set for evaluating the material properties of perfect and the various defective MoS2 sheets. The transmission spectra is computed with a DFTB-Non-Equilibrium Greens Function (NEGF) formalism. We also perform a detailed analysis of the carrier transmission pathways under a small bias and investigate the phase shifts in the transmission eigenstates of the defective MoS2 sheets. Our simulations show a 2-4 folds decrease in carrier conductance of MoS2 sheets in the presence of line defects as compared to that for the perfect sheet.

cond-mat.mtrl-sci↗

arXiv2013

Implementation of the Cluster Based Tunable Sleep Transistor Cell Power Gating Technique for a 4x4 Multiplier Circuit

A modular, programmable, and high performance Power Gating strategy, called cluster based tunable sleep transistor cell Power Gating, has been introduced in the present paper with a few modifications. Furthermore, a detailed comparison of its performance with some of the other conventional Power Gating schemes; such as Cluster Based Sleep Transistor Design (CBSTD), Distributed Sleep Transistor Network (DSTN) etc.; has also been presented here. Considering the constraints of power consumption, performance, and the area overhead, while doing the actual implementation of any Power Gating scheme, it becomes important to deal with the various design issues like the proper sizing of the sleep transistors (STs), controlling the voltage drop (IR drop) across the STs, and obviously maintaining a desired performance with lower amount of delay degradation. With this notion, we tried to find out an efficient Power Gating strategy which can reduce the overall power consumption of any CMOS circuit by virtue of reducing the standby mode leakage current. Taking the different performance parameters into account, for an example circuit, which is actually the conventional 4x4 multiplier design, we found that the modified tunable sleep transistor cell Power Gating gives very much promising results. The reported architecture of the 4x4 multiplier with the tunable sleep transistor cell Power Gating, is designed using 45 nm technology and it consumes 1.3638x10-5 Watt of Average Power while being operated with the nominal case of the bit configuration word, that is, 1000. ...........

cs.OH↗