arXiv subjects

Priyamvada Jadaun

Publications and source records attributed to Priyamvada Jadaun.

16 recordsLinked to original sources

arXiv2022

Band Alignment in Black Phosphorus/Transition Metal Dichalcogenide Heterolayers: Impact of Charge Redistribution, Electric Field, Strain and Layer Engineering

The objective of this work is to study the effects of charge redistribution, applied layer-normal electric fields, applied strain, and layer engineering on the band alignment of Black Phosphorus (BP)/Molybdenum disulphide (MoS2) heterostructure through Density Functional Theory (DFT) simulations. Black phosphorus works as a p-type material with high mobility, mechanical flexibility, and sensitivity to number of layers. Combining it with the more electronegative material, MoS2 results in strong carrier confinement and a Type II heterostructure. Charge redistribution among the layers shifts the band alignment expected from the Electron Affinity Rule. Applied external fields, strain and multiple BP layers provide band-alignment tunability within the Type II range and/or, transition to Type I and Type III heterostructures. The tunability in BP/MoS2 heterostructure may be useful as tunnel field effect transistors, rectifier diodes with tunable barrier height, reconfigurable FETs, and electro-optical modulators. Furthermore, considering heterostructures of monolayer BP with other monolayer Transitional Metal Dichalcogenides (TMD) suggests the ability to achieve different band alignment types. In our simulations, a Type I alignment is found with Tungsten diselenide (WSe2), Molybdenum diselenide (MoSe2), and Tungsten disulphide (WS2), and a Type III for Hafnium disulphide (HfS2) and Hafnium diselenide (HfSe2).

cond-mat.mtrl-sci↗

arXiv2021

Adaptive cognition implemented with a context-aware and flexible neuron for next-generation artificial intelligence

Neuromorphic computing mimics the organizational principles of the brain in its quest to replicate the brain's intellectual abilities. An impressive ability of the brain is its adaptive intelligence, which allows the brain to regulate its functions "on the fly" to cope with myriad and ever-changing situations. In particular, the brain displays three adaptive and advanced intelligence abilities of context-awareness, cross frequency coupling and feature binding. To mimic these adaptive cognitive abilities, we design and simulate a novel, hardware-based adaptive oscillatory neuron using a lattice of magnetic skyrmions. Charge current fed to the neuron reconfigures the skyrmion lattice, thereby modulating the neuron's state, its dynamics and its transfer function "on the fly". This adaptive neuron is used to demonstrate the three cognitive abilities, of which context-awareness and cross-frequency coupling have not been previously realized in hardware neurons. Additionally, the neuron is used to construct an adaptive artificial neural network (ANN) and perform context-aware diagnosis of breast cancer. Simulations show that the adaptive ANN diagnoses cancer with higher accuracy while learning faster from smaller amounts of data and using a more compact and energy-efficient network than the state-of-the-art non-adaptive ANNs used for cancer diagnosis. The work further describes how hardware-based adaptive neurons can mitigate several critical challenges facing contemporary ANNs. Modern ANNs require large amounts of training data, energy and chip area and are highly task-specific; conversely, hardware-based ANNs built with adaptive neurons show faster learning from smaller datasets, compact architectures, energy-efficiency, fault-tolerance and can lead to the realization of general artificial intelligence.

physics.app-ph↗

arXiv2020

Large spin Hall effect in 5d-transition metal anti-perovskites

The spin Hall effect (SHE) is highly promising for spintronic applications, and the design of materials with large SHE can enable ultra-low power memory technology. Recently, 5d-transition metal oxides have been shown to demonstrate a large SHE. Here we report large values of SHE in four 5d-transition metal anti-perovskites which makes these anti-perovskites promising spintronic materials. We demonstrate that these effects originate in the mixing of dx2-y2 and dxy orbitals caused by spin orbit coupling.

cond-mat.mtrl-sci↗

arXiv2020

Tunnel magnetoresistance in scandium nitride magnetic tunnel junctions using first principles

The magnetic tunnel junction is a cornerstone of spintronic devices and circuits, providing the main way to convert between magnetic and electrical information. In state-of-the-art magnetic tunnel junctions, magnesium oxide is used as the tunnel barrier between magnetic electrodes, providing a uniquely large tunnel magnetoresistance at room temperature. However, the wide bandgap and band alignment of magnesium oxide-iron systems increases the resistance-area product and causes challenges of device-to-device variability and tunnel barrier degradation under high current. Here, we study using first principles narrower-bandgap scandium nitride tunneling properties and transport in magnetic tunnel junctions in comparison to magnesium oxide. These simulations demonstrate a high tunnel magnetoresistance in Fe/ScN/Fe MTJs via Δ_1 and Δ_2' symmetry filtering with low wavefunction decay rates, allowing a low resistance-area product. The results show that scandium nitride could be a new tunnel barrier material for magnetic tunnel junction devices to overcome variability and current-injection challenges.

cond-mat.mtrl-sci↗

arXiv2019

Rational design principles for giant spin Hall effect in 5d-transition metal oxides

Spin Hall effect (SHE), a mechanism by which materials convert a \textit{charge} current into a \textit{spin} current, invokes interesting physics and promises to empower transformative, energy-efficient memory technology. However, fundamental questions remain about the essential factors that determine SHE. Here we solve this open problem, presenting a comprehensive theory of five \textit{foundational factors} that control the value of intrinsic SHE in transition metal oxides. Arising from our key insight regarding the inherently geometric nature of SHE, we demonstrate that two of these factors are crystal field strength and structural distortions. Moreover, we discover that a new class of materials (anti-perovskites) promises to demonstrate \textit{giant} SHE, that is an order of magnitude larger than that reported for any oxide. We derive three other factors that control SHE and demonstrate the nuanced role of electron correlations. Our findings bring deeper insight into the physics driving SHE, and could help enhance, as well as, externally control SHE values.

cond-mat.mtrl-sci↗

arXiv2019

The microscopic origin of DMI in magnetic bilayers and prediction of giant DMI in new bilayers

Skyrmions are widely regarded as promising candidates for emergent spintronic devices. Dzyaloshinskii-Moriya interaction (DMI) is often critical to the generation and manipulation of skyrmions. However, there is a fundamental lack of understanding of the origin of DMI or the mechanism by which DMI generates skyrmions in magnetic bilayers. Very little is known of the material parameters that determine the value of DMI. This knowledge is vital for rational design of skyrmion materials and further development of skyrmion technology. To address this important problem, we investigate DMI in magnetic bilayers using first-principles. We present a new theoretical model that explains the microscopic origin of DMI in magnetic bilayers. We demonstrate that DMI depends on two parameters, interfacial hybridization and orbital contributions of the heavy metal. Using these parameters, we explain the trend of DMI observed. We also report four new materials systems with giant DMI and new designs for magnetic multilayers that are expected to outperform the best materials known so far. Our results present a notably new understanding of DMI, uncover highly promising materials and put forth novel pathways for the controlled generation of skyrmions.

cond-mat.mtrl-sci↗

arXiv2017

Room temperature zero field skyrmions in Fe-based thin film stacks

A new paradigm is required to facilitate the demand for the huge data storage capacity and faster data processing in the future. Nano structures such as magnetic skyrmions have been predicted to address this issues as these vortex structures are the smallest particle-like magnetic features and are topologically protected from crystallographic defects or magnetic disorder. We report here stable skyrmions at room temperature and with zero applied field in ebeam evaporated Ir Fe Ir MgO thin film stacks. Micromagnetic simulations show that these skyrmions are induced by interfacial Dzyaloshinskii Moriya interactions between the ferromagnetic and heavy metal ultra thin layers values in the range between 3.1 to 3.6. We also show the field dependencies of the skyrmion width and density. The room temperature zero field width is 110 nm, approximately comparable to the value showed by Fert et al and Boulle et al for the multi stacks thin film. DFT calculations corroborate our experimental results by predicting a DMI value of 3.67.

cond-mat.mtrl-sci↗

arXiv2016

Spin-Hall Torques Generated by Rare-Earth (Lanthanide) Thin Films

We report an initial experimental survey of spin-Hall torques generated by the rare-earth metals Gd, Dy, Ho, and Lu, along with comparisons to first-principles calculations of their spin Hall conductivities. Using spin torque ferromagnetic resonance (ST-FMR) measurements and DC-biased ST-FMR, we estimate lower bounds for the spin-Hall torque ratio, $ξ_{SH}$, of $\approx$ 0.04 for Gd, $\approx$ 0.05 for Dy, $\approx$ 0.14 for Ho, and $\approx$ 0.014 for Lu. The variations among these elements are qualitatively consistent with results from first principles (density functional theory, DFT, in the local density approximation with a Hubbard-U correction). The DFT calculations indicate that the spin Hall conductivity is enhanced by the presence of the partially-filled $f$ orbitals in Dy and Ho, which suggests a strategy to further strengthen the contribution of the $f$ orbitals to the spin Hall effect by shifting the electron chemical potential.

cond-mat.mes-hall↗

arXiv2014

Electronic and optical properties of GaSb:N from first principles

GaSb:N displays promise towards realization of optoelectronic devices accessing the mid-infrared wavelength regime. Theoretical and experimental results on its electronic and optical properties are however few. To address this, we present a first principles, density functional theory study using the hybrid HSE06 exchange-correlation functional of GaSb doped with 1.6$\%$ nitrogen. To study dilute-nitrides with small band gaps, the local density approximation (LDA) is insufficient and more accurate techniques such as HSE06 are needed. We conduct a comparative study on GaAs:N, also with 1.6$\%$ nitrogen mole fraction, and find that GaSb:N has a smaller band gap and displays more band gap bowing than GaAs:N. In addition we examine the orbital character of the bands, finding the lowest conduction band to be quasi-delocalized, with a large N-$3s$ contribution. At high concentrations, the N atoms interact via the host matrix, forming a dispersive band of their own which governs optoelectronic properties and dominates band gap bowing. While this band drives the optical and electronic properties of GaSb:N, its physics is not captured by traditional models for dilute-nitrides. We thus propose that a complete theory of dilute-nitrides should incorporate orbital character examination, especially at high N concentrations.

cond-mat.mtrl-sci↗

arXiv2013

Theory and synthesis of bilayer graphene intercalated with ICl and IBr for low power device applications

Graphene intercalation materials are potentially promising for the implementation of the ultra-low power, excitonic-condensate-based Bilayer pseudoSpin Field-Effect Transistor (BiSFET) concept, as well as other novel device concepts requiring a graphene interlayer dielectric. Using density functional theory (DFT) we study the structural and electronic properties of bilayer graphene intercalated with iodine monochloride (ICl) and iodine monobromide (IBr). We determine the structural configuration of ICl and IBr graphene intercalation compounds (GICs). We also conduct an in-depth exploration of inter-layer electronic coupling, using \textit{ab initio} calculations. The presence of intercalants dopes the graphene layer. It also reduces, but does not eliminate, the electronic coupling between graphene layers, which may enable BiSFET operation. In addition, we present experimental results for ICl-GIC synthesis and characterization.

cond-mat.mes-hall↗

arXiv2012

Topological Classification of Crystalline Insulators with Point Group Symmetry

We show that in crystalline insulators point group symmetry alone gives rise to a topological classification based on the quantization of electric polarization. Using C3 rotational symmetry as an example, we first prove that the polarization is quantized and can only take three inequivalent values. Therefore, a Z3 topological classification exists. A concrete tight-binding model is derived to demonstrate the Z3 topological phase transition. Using first-principles calculations, we identify graphene on BN substrate as a possible candidate to realize the Z3 topological states. To complete our analysis we extend the classification of band structures to all 17 two-dimensional space groups. This work will contribute to a complete theory of symmetry conserved topological phases and also elucidate topological properties of graphene like systems.

cond-mat.str-el↗

arXiv2012

Density functional theory studies of interactions of graphene with its environment: substrate, gate dielectric and edge effects

This paper reviews the theoretical work undertaken using density functional theory (DFT) to explore graphene's interactions with its surroundings. We look at the impact of substrates, gate dielectrics and edge effects on the properties of graphene. In particular, we focus on graphene-on-quartz and graphene-on-alumina systems, exploring their energy spectrum and charge distribution. Silicon-terminated quartz is found to not perturb the linear graphene spectrum. On the other hand, oxygen-terminated quartz and both terminations of alumina bond with graphene, leading to the opening of a band gap. Significant charge transfer is seen between the graphene layer and the oxide in the latter cases. Additionally, we review the work of others regarding the effect of various substrates on the electronic properties of graphene. Confining graphene to form nanoribbons also results in the opening of a band gap. The value of the gap is dependent on the edge properties as well as width of the nanoribbon.

cond-mat.mes-hall↗

arXiv2011

Dielectric capping effects on binary and ternary topological insulator surface states

Using a density functional based electronic structure method, we study the effect of crystalline dielectrics on the metallic surface states of Bismuth- and chalcogen-based binary and ternary three dimensional topological insulator (TI) thin films. Crystalline quartz (SiO2) and boron nitride (BN) dielectrics were considered. Crystalline approximation to the amorphous quartz allows to study the effect of oxygen coverage or environmental effects on the surface states degradation which has gained attention recently in the experimental community. We considered both symmetric and asymmetric dielectric cappings to the sufaces of TI thin films. Our studies suggest that BN and quartz cappings have negligible effects on the Dirac cone surface states of both binary and ternary TIs, except in the case of an oxygen-terminated quartz surface. Dangling bond states of oxygens in oxygen-terminated quartz dominate the region close to Fermi level, thereby distorting the TI Dirac cone feature and burying the Dirac point in the quartz valence band region. Passivating the oxygen-terminated surface with atomic hydrogen removes these dangling bond states from the Fermi surface region, and consequently the clear Dirac cone is recovered. Our results are consistent with recent experimental studies of TI surface degradation in the presence of oxygen coverage.

cond-mat.mtrl-sci↗

arXiv2011

Density functional theory based study of graphene and dielectric oxide interfaces

We study the effects of insulating oxides in their crystalline forms on the energy band structure of monolayer and bilayer graphene using a \textit{first principles} density functional theory based electronic structure method and a local density approximation. We consider the dielectric oxides, SiO$_{2}$ ($α$-quartz) and Al$_{2}$O$_{3}$ (alumina or $α$-sapphire) each with two surface terminations. Our study suggests that atomic relaxations and resulting equilibrium separations play a critical role in perturbing the linear band structure of graphene in contrast to the less critical role played by dangling bonds that result from cleaving the crystal in a particular direction. We also see that with the addition of a second graphene layer, the Dirac cone is restored for the quartz surface terminations. Alumina needs more than two graphene layers to preserve the Dirac cone. Our results are at best semi-quantitative for the common amorphous forms of the oxides considered. However, crystalline oxides for which our results are quantitative provide an interesting option for graphene based electronics, particularly in light of recent experiments on graphene with crystalline dielectrics (hexagonal BN) that find considerable improvements in transport properties as compared to the those with amorphous dielectrics.

cond-mat.mtrl-sci↗

arXiv2010

Intrinsic and extrinsic perturbations on the topological insulator Bi2Se3 surface states

Using a density functional based electronic structure method, we study the effect of perturbations on the surface state Dirac cone of a strong topological insulator Bi$_2$Se$_3$ from both the intrinsic and extrinsic sources. We consider atomic relaxations, and film thickness as intrinsic and interfacial thin dielectric films as an extrinsic source of perturbation to the surface states. We find that atomic relaxations has no effect on the degeneracy of the Dirac cone whereas film thickness has considerable effect on the surface states inducing a gap which increases monotonically with decrease in film thickness. We consider two insulating substrates BN and quartz as dielectric films and show that surface terminations of quartz with or without passivation plays critical role in preserving Dirac cone degeneracy whereas BN is more inert to the TI surface states. The relative orbital contribution with respect to bulk is mapped out using a simple algorithm, and with the help of it we demonstrate the bulk band inversion when spin-orbit coupling is switched on. The layer projected charge density distributions of the surface states shows that these states are not strictly confined to the surface. The spatial confinement of these states extends up to two to three quintuple layers, a quintuple layer consists of five atomic layers of Bi and Se

cond-mat.mtrl-sci↗

arXiv2010

Graphene on Insulating Oxide Substrates: Role of Surface Dangling States

We study the effect of insulating oxide substrates on the energy band structure of monolayer and bilayer graphene using a first principles density functional based electronic structure method and a local exchange correlation approximation. We consider two crystalline substrates, SiO2 (or alpha-quartz) and Al2O3 (alpha-alumina or sapphire), each with two surface terminations. We focus on the role of substrate surface dangling states and their passivation in perturbing the linear energy spectrum of graphene. On non-passivated surface terminations, with the relaxation of top surface layers, only Si-terminated quartz retains the linear band structure of graphene due to relatively large equilibrium separation from the graphene layer whereas the other three surface terminations considerably distort it. However, without relaxations of the top surface layer atoms, linear bands appear in the electronic spectrum but with the Dirac point shifted away from the Fermi level. Interestingly, with a second carbon layer on non-passivated oxygen terminated Quartz, with top surface layers relaxation, graphene features appear in the spectrum but sapphire with both surface terminations shows perturbed features even with two carbon layers. By passivating the surface dangling states with hydrogen atoms and without top layer atomic relaxations, the electron-hole symmetry occurs exactly at the Fermi level. This suggests that surface dangling states play a less important role than the atomic relaxations of the top surface layers in distorting the linear spectrum. In all cases we find that the first layer of graphene forms ripples, much like in suspended graphene, but the strength of rippling is found to be weaker probably due to the presence of the substrate.

cond-mat.mtrl-sci↗