arXiv subjects

Sharat Chandra

Publications and source records attributed to Sharat Chandra.

At least 19 recordsLinked to original sources

arXiv2026

Effect of Vacancies on Hydrogen Mobility and Trapping in Elemental Fe and Cr: A DFT and kMC Study

Hydrogen-vacancy interactions play an important role in governing hydrogen transport and embrittlement in body-centered cubic (BCC) metals. In this study, a multiscale approach combining density functional theory (DFT) and kinetic Monte Carlo (kMC) simulations is employed to investigate hydrogen behavior in BCC Fe and Cr. The DFT-calculated binding energies and Bader charge analysis indicate stronger hydrogen trapping in Cr than in Fe. Migration and detrapping energy barriers are determined using the climbing-image nudged elastic band method, showing that the detrapping energy generally decreases with increasing hydrogen occupancy. However, the sixth hydrogen atom in Fe exhibits a finite barrier, contrary to some previous reports. kMC simulations are then used to evaluate hydrogen diffusion over extended time and length scales. The results demonstrate that vacancy defects significantly reduce hydrogen mobility and increase the effective activation energy, with a more pronounced effect observed in Cr due to stronger trapping. The combined DFT-kMC framework provides detailed insight into the mechanisms of hydrogen trapping, detrapping, and diffusion in BCC metals, offering important implications for understanding hydrogen embrittlement in structural materials.

cond-mat.mtrl-sci↗

arXiv2025

Ab-initio investigation of the interfacial structural, electronic, and magnetic properties of Co$_{2}$MnAl/X (X = MgO and GaAs) heterostructures

The structural, electronic, and magnetic properties of (100)-oriented Co$_{2}$MnAl/MgO and Co$_{2}$MnAl/GaAs heterostructures are investigated using plane-wave pseudopotential density functional theory. For the Co$_{2}$MnAl/MgO, CoCo-MgMg, CoCo-OO, MnAl-MgMg, and MnAl-OO interfaces in top-to-top configurations are studied, while for Co$_{2}$MnAl/GaAs, both top-to-top (Co-Ga, Co-As, Mn-Ga, Mn-As, Al-Ga, Al-As) and bridge-site (CoCo-Ga, CoCo-As, MnAl-Ga, MnAl-As) interfaces are considered. The interfacial geometries featuring Co- or CoCo-atomic terminations for the Co2MnAl slab exhibit larger adhesion energies compared to those terminated with Mn-, Al-, or MnAl-atomic terminations. This indicates their greater interfacial stability. In contrast, MnAl-, Mn-, or Al-terminated interfaces preserve near half-metallicity, whereas Co- and CoCo-terminated geometries display a strongly metallic character. All studied interfaces show enhanced magnetic moments relative to their bulk counterparts, primarily arising from interfacial atoms and their nearest neighbours. These findings offer valuable insights for optimizing Co2MnAl-based heterostructures in spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Spin-dependent Transport Studies of Fe/Mo$_x$Cr$_{1-x}$S$_2$/Fe Magnetic Tunnel Junction

Using Density functional theory and non-equilibrium Green's function formalism, spin dependent electron transport in Fe/Mo$_x$Cr$_{1-x}$S$_2$/Fe magnetic tunnel junction is studied. Spin transport for different thicknesses (1, 3, 5, and 7 layers) of the spacer MoS2 and for two different surface orientations of the Fe electrode, the Fe(001) and Fe(111) surface and with substitutional doping of 3d transition metal Cr at Mo site in MoS2 is investigated. The electronic structure of the heterostructure shows the presence of metal-induced states in the semiconducting MoS2 at the Fe/MoS2/Fe interface. The I-V characteristics of the junctions for the monolayer and three layer MoS2 spacer show linear behaviour due to the metallic nature of the junction. The tunnelling nature of the junction is observed for the thicker junctions with five-layer and seven layer spacers. With the introduction of magnetic impurity Cr, the tunnelling magnetoresistance for 7-layer junction is reduced. The Cr-defect states are observed below the conduction band, and the Cr-doped devices are stable up to a bias of 0.5V. Spin-transport through close-packed Fe(111) surface as electrode show low GMR value.

cond-mat.mtrl-sci↗

arXiv2025

Spin-dependent electronic transport in NiMnSb/MoS2(001)/NiMnSb magnetic tunnel junction

Half-metallic Heusler alloy compounds with Curie temperatures above room temperature are suitable candidate electrode materials for injecting large spin-polarised charge carriers into the semiconducting barriers at the ferromagnet semiconductor junction to obtain highly spin-polarised current. Combining the density functional theory and non-equilibrium Green's function method, the electronic structure, spin dependent electron transport in NiMnSb/MoS2(001)/NiMnSb is studied. The possibilities of injecting 100% spin-polarised electron into MoS2 using half metallic NiMnSb as an electrode, the layer dependent, and the effect of the type of interface on electronic structure and spin-transport properties in magnetic tunnel junction devices are studied. We show that the half-metallicity of NiMnSb(111) is preserved at the interface between the half-Heusler alloy NiMnSb and MoS2. NiMnSb keeps a fully spin-polarised state in the majority spin channel at the interface between NiMnSb and MoS2, injecting fully spin-polarised electrons into the semiconductor. The device based on NiMnSb/MoS2(single layer)/NiMnSb has a metallic interface. Metal-induced states in the spin-majority channel of MoS2 are seen after making an interface with half metallic NiMnSb. In contrast, the NiMnSb/MoS2(three layers)/NiMnSb interface with a multilayer of MoS2 has a band gap region, and electrons can tunnel through the junction. The Mn-S interface is more conducting than the Sb-S interface due to the strong bonding of Mn and S atoms at the Mn-S interface.

cond-mat.mtrl-sci↗

arXiv2025

Surface properties of Co$_2$MnAl Heusler alloy

Using the plane-wave pseudopotential method within the framework of density functional theory, Co$_2$MnAl (100), (110), and (111) surfaces with different atomic terminations have been studied in the context of some key spintronics properties, viz., surface energy, half-metallicity, magnetization, and magnetic anisotropy. The present study reveals that the MnAl-(100), Co-Al-(111), and Al-(111) surfaces exhibit negative surface energies over a wide range of chemical potentials, indicating their strong structural stability. The MnAl-(100), CoCoMnAl-(110), and Co-Mn-(111) surfaces maintain the nearly half-metallic nature like the bulk-Co$_2$MnAl, while this nearly half-metallic nature even improved for the Al-(111) surface. In contrast, the rest of the considered surfaces, CoCo-(100), Co-Al-(111) and Mn-(111) surfaces, display the strong metallic nature. Magnetization is enhanced for most surface configurations, except for Al-(111), where it decreases due to reduced moments of the exterior atoms. Regarding magnetic anisotropy, only the MnAl-(100) and Co-Mn-(111) surfaces exhibit the positive magneto-crystalline anisotropy of $\sim$0.23 and $\sim$0.33 mJ/m2, respectively. All these findings suggest that the Co-Mn-(111) and MnAl-(100) surfaces are quite appealing for spintronics applications, considering the structural stability, electronic properties, and magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2024

Effect of the Lattice-distortion on the Electronic Structure, Magnetic Anisotropy, and Hall Conductivities of the CoFeCrGa Spin Gapless Semiconductor: A First-Principles Study

Spin gapless semiconductors (SGSs), novel quantum materials, are notable for their tunable spin-transport properties. Considering that the SGS materials might have an invariably deformed lattice upon integration into devices, and given that the SGS nature is highly sensitive to external factors, the impact of lattice distortions on the different physical properties of CoFeCrGa SGS alloy has been investigated using density functional theory calculations. For lattice distortions, the uniform strain corresponding to $-6\% \leq \Delta V / V_0 \leq 6\% \quad (a: 5.60\text{-}5.83~\text\r{A})$, and the tetragonal distortion corresponding to $0.8 \leq c/a \leq 1.2 \quad (a: 5.38\text{-}6.16~\text\r{A},~c: 4.92\text{-}6.45~\text\r{A})$ are modelled. All uniformly strained CoFeCrGa structures are found to display SGS character, magnetic isotropy, small anomalous Hall conductivity (AHC), and small spin Hall conductivity (SHC) - closely resembling those of the ideal CoFeCrGa structure. In contrast, the tetragonally deformed structures display nearly half-metallic behavior with very high spin polarization, very large magnetic anisotropy ($ \sim 10^6~\mathrm{J/m^3}$), and very large AHC ranging from ($ -215 \text{ to } 250~\mathrm{S/cm} $) depending on the axial ratio of the distorted structure. The SHC, however, does not change significantly under tetragonal distortion and remains nearly of the same order as that of the Y-I ordered structure. In summary, these findings demonstrate that CoFeCrGa displays favorable spintronic properties even under lattice distortions, underscoring its potential for next-generation spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Iron phosphate glass structure at different length scale with emphasis on the medium range: a classical molecular dynamic study

Glasses are known to have medium range order (MRO) but its link to any experimentally measurable quantity is still ambiguous. The first sharp diffraction peak (FSDP) in structure-factor S(q) obtained from diffraction experiments on glasses has been associated with this MRO but understanding the fundamental origin of this universal peak is still an open problem. We have addressed this issue for a complex glass i.e. iron phosphate glass (IPG) through atomistic models generated from a hybrid approach (our in-house developed MC code with MD simulation). We have performed a comparative study by generating glass models from different initial configurations and randomization techniques. The developed IPG models were first validated with existing data on short range order (SRO) and MRO, through study of pair correlation functions, bond angle distributions and coordination number (CN) for SRO and rings distribution, FSDP in structure factor and void size distribution for MRO. The study of coordination environment of oxygen is specifically shown to aide in understanding glass formation through topological constraint theory. Thereafter to understand the fundamental origin of FSDP in S(q), structure factors were calculated corresponding to individual ring sizes present in the model. The relative contribution of these individual S(q)s in the total experimental S(q) is estimated using an inverse fitting approach. The contributions thus obtained directly correlated with rings size percentages in the models for the considered q-range. Through this exercise we can connect the rings distribution of an atomistic glass model with an experimentally measurable quantity like FSDP in S(q) for a complex glass like IPG.

cond-mat.mtrl-sci↗

arXiv2024

An investigation on electronic and magnetic properties of Cr substituted MoS$_2$ monolayer and multilayers-Hybrid functional calculations

With help of ab initio density functional theory calculation, DFT+U, and hybrid functional HSE06, we revisit the layer dependent electronic structure and magnetic properties of pristine and 3d transition metal Cr doped MoS$_2$ monolayer and multilayers. Our results show that the dopant Cr atoms prefer to stay at nearest neighbor distances. In the multilayers, they prefer to remain in the outermost surface layers. Matching with the experimental band gap, the optimized U parameter we report is 4 eV. The band gap of the Cr doped monolayer is indirect, confirming the experimental observation from photoluminescence experiments. The HSE06 calculation for Cr doped monolayer shows that the band gap of doped Cr MoS$_2$ monolayer is indirect and no magnetism is observed. From the DFT studies, the band gap for the multilayers is indirect, and doping with Cr does not induce magnetic moments in MoS$_2$ layers. The band gap is observed to decrease with the multilayer thickness. The strain induced by substitutional Cr doping at the Mo site transforms the band gap in monolayer MoS$_2$ from direct to indirect. The defect states are produced within the band gap region close to the conduction band minimum.

cond-mat.mtrl-sci↗

arXiv2023

Effect of Point Defects and Lattice Distortions on the Structural, Electronic, and Magnetic properties of Co$_2$MnAl Heusler alloy

The effects of various point defects and lattice distortions on the structural, electronic, and magnetic properties of Co$_2$MnAl alloy are investigated using density functional theory calculations. For the point defects, six types of binary antisites, three types of ternary antisites, and three kinds of vacancies have been simulated with different disorder degrees, up to a maximum of 12.50%. For the lattice distortions, cubic strain within -10% $\leq$$\Delta{V/V_0}$$\leq$ 10% (corresponding to 5.50\r{A} $\leq$ a $\leq$5.88\r{A}) and tetragonal distortions with 0.5$\leq$$\textit{c/a}$$\leq$1.5 at three different unit-cell volumes - $\textit{V}_0$ and ($\textit{V}_0$$\pm5$%$\textit{V}_0$) have been considered. The Co$_{Al}$ and Mn$_{Al}$ binary antisite disordered structures (namely, Co$_{2.0625}$MnAl$_{0.9375}$, Co$_{2.125}$MnAl$_{0.875}$, Co$_2$Mn$_{1.0625}$Al$_{0.9375}$ and Co$_2$Mn$_{1.125}$Al$_{0.875}$) and (Co$_{Al}$+Mn$_{Al}$) ternary antisite disordered structure (Co$_{2.0625}$Mn$_{1.0625}$Al$_{0.875}$) exhibit perfect half-metallicity. The rest of the antisite disorders have a marginal effect on the half-metallic properties of Co$_2$MnAl, along with high spin polarization ($\textit{P}$ $\geq$ 70%) and nearly same magnetization ($\textit{M$_s$}$) as that for ideal structure. Conversely, the vacancy defects significantly affect the electronic and magnetic properties. The cubic strained structures exhibit high $\textit{P}$ and constant $\textit{M$_s$}$. Under negative strain within -10% $\leq$$\Delta{V/V_0}$$\leq$ -7% (for 5.50\r{A} $\leq$ a $\leq$ 5.58\r{A}), the strained structures have perfect half-metallicity. On the other hand, tetragonal distortions lead to significant degradation in half-metallic behavior, except for small distortion values $\Delta{c/a}$, irrespective of their volume.

cond-mat.mtrl-sci↗

arXiv2023

Towards atomistic understanding of Iron phosphate glass: a first-principles based DFT modeling and study of its physical properties

Iron phosphate glasses (IPG) have been proposed as futuristic glass material for nuclear waste immobilization, anode material for lithium batteries and also as bioactive glass. In the last decade, there have been attempts to propose atomistic models of IPG to explain their properties from atomistic viewpoint and to predict their behavior in radioactive environment. In this paper, we seek to produce small scale models of IPG that can be handled within the scheme of Density Functional Theory (DFT) to study the electronic structure of this material. The starting models generated using Monte Carlo (MC) method [S. Singh and S. Chandra, Comp. Mat. Sci., 202, 110943, (2022)] were subsequently annealed (at 1000 K) using ab-initio molecular dynamics (AIMD). This removes coordination defects present in the MC models. The equilibrated structure at this temperature was then force-relaxed using conjugate-gradient (CG) optimization. This hybrid approach (MC + AIMD + 0K DFT-CG optimization) produced good atomistic models of IPG which can reproduce experimentally observed electronic band-gap, vibrational density of states (VDOS), magnetic moment of Fe, the elastic constants as well as optical and dielectric properties. Computationally expensive melt-quench simulation can be avoided using present approach allowing the use of DFT for accurate calculations of properties of complex glass like IPG.

cond-mat.mtrl-sci↗

arXiv2023

Characterizing MRO in atomistic models of vitreous SiO$_2$ generated using ab-initio molecular dynamics

Vitreous silica is the most versatile material for scientific and commercial applications. Although large-scale atomistic models of vitreous-SiO$_2$ (v-SiO$_2$) having medium-range order (MRO) have been successfully developed by melt-quench through classical molecular dynamics, the MRO is not well studied for the smaller-scale models developed by melt-quench using ab-initio molecular dynamics (AIMD). In this study, we obtain atomistic models of v-SiO$_2$ by performing melt-quench simulation using AIMD. The final structure is compared with the experimental data and some recent atomistic models, on the basis of the structural properties. Since AIMD allows for the estimation of electronic structure, a detailed study of electronic properties is also done. It shows the presence of defect states mainly due to dangling bonds in the band-gap region of electronic density of states, whereas the edge-shared type of defective structures in the glassy models are found to contribute mainly in the valence band. In addition, Oxygen and Silicon vacancies as well as bridging Oxygen type of defects were created and their contributions to the band-gap were studied.

cond-mat.mtrl-sci↗

arXiv2022

Self-healing in unpassivated and passivated CdTe nanostructures: structural stability and optical properties

We report the effects of passivation on the various properties like electronic structure, structural stability and optical properties of CdTe in the different nanostructure forms such as ultra-thin slabs, monolayers, nanorods and nanotubes. Further, based on these properties, the self-healing ability of each nanostructure has been predicted. The optical properties suggest that all of the passivated and specific unpassivated nanostructures are suitable for optoelectronic applications. The 2D system in <110> orientation and nanotube derived from the <111> monolayer show significant self-healing in the pristine structures.

cond-mat.mes-hall↗

arXiv2022

Structural stability, electronic structure and optical properties of dimension controlled self-assembled structures from clusters of cadmium telluride

We report the first principle theory-based study of stability, electronic structure and optical properties of cluster assembled materials in various 1D, 2D and 3D nanostructures using a cage-like Cd9Te9 cluster as the super-atom. The bulk 3D self-assemblies form in 2D stacked structures for different cubic lattices. The face centered stacking is the most stable as compared to the simple cubic, body centered and zinc blende type stackings. The 2D stacks are formed as cluster assembled monolayers and the monolayer derived from the face centered structure is most stable. Further, the cluster chains (or wires) with more number of inter-cluster bonds are also seen to be dynamically stable. The electronic structure, bandgap, dielectric constant and absorption spectra along with the phonon dispersions are discussed for these self-assembled nanostructures.

cond-mat.mes-hall↗

arXiv2022

Tunnel Barrier to Spin Filter: Electronic Transport Characteristics of Transition Metal Atom Encapsulated in Smallest Cadmium Telluride Cage

We report first principles theory-based comparative electronic transport studies performed for an atomic chain of Au, bare Cd9Te9 cage-like cluster and single transition metal (TM) (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd) atom encapsulated within the Cd9Te9 using Au(111) as electrodes. The bare cluster is semiconducting and acts as a tunnel barrier up to a particular applied bias and beyond that, the device has a linear current-voltage relationship. Several TM (Ti, V, Cr, Mn, Fe) encapsulated in the cage show half-metallic behavior and spin filtering effect in the I-V characteristics of the device. A detailed qualitative and quantitative analysis of I-V characteristics for metallic, semiconducting, and half-metallic nanostructures has been carried out.

cond-mat.mes-hall↗

arXiv2021

Strain Engineering of 2D-C3N5 Monolayer and its Application in Overall Water-Splitting: A Hybrid Density Functional Study

The recent experimental synthesis of 2D graphitic C3N5 has attracted lot of interests in its electronic and optical properties and its comparison with other graphitic C3N4 and C3N3. To this end, we performed DFT calculations using more accurate HSE06 functional and estimated the corresponding electronic properties. From a comparative study of the band structures of C3N3, C3N4, and C3N5, we found that, the electronic band-gap decreases as 3.24 eV (C3N3) > 2.81 eV (C3N4) > 2.19 eV (C3N5) with increase in the number of nitrogen atoms in the unit cell of these graphitic carbon nitrides. Further, the strain dependency of the band structure of 2D g-C3N5 under uniaxial and biaxial strain is performed using the same HSE- 06 functional. We found a systematic decrease of band-gap as strain increases. Out of the two types of strain, the biaxial strain has been found to be more efficient in modulating the band-gap. The effect of strain on the structure is also explored by analyzing the bond lengths and bond-angles as well as the charge density plots. Furthermore, we found that at a biaxial strain of 20% strain an interesting structural rearrangement occurs in 2D g-C3N5, which reults in a finite magnetic moment arising from the loss of spin-degeneracy of electronic levels. Finally, by studying the evolution of band-gap, band-alignments and optical absorption as a function of strain we are able to predict that biaxially compressed C3N5 with strain in the range 12-14% can be a promising photocatalyst in overall water-splitting with an excellent optical absorption in the visible light spectrum.

cond-mat.mtrl-sci↗

arXiv2021

Half-Metallicity in Smallest Cage-like Cluster of CdTe with Doping of Transition Metal Atoms

We report first principles theory based electronic structure studies of a semiconducting stoichiometric cage-like Cd9Te9 cluster. Substantial changes are observed in the electronic structure of the cluster on passivation with fictitious hydrogen atoms, in particular, widening of the energy gap between highest occupied molecular orbital and lowest unoccupied molecular orbital and enhancement in stability of cluster is seen. The cluster, when substitutionally mono-doped for a Cd by a set of 3d and 4d transition metal atoms (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh and Pd), is found to acquire polarization as seen from spin resolved density of states near Fermi level. Further, such mono-doping in passivated cluster shows half-metallic behavior. Mapping of partial density of states of each system on that of undoped cluster reveals additional levels caused by doping each TM atom separately. In the 3d elemental doping, Ti and Mn doping result into electron type doping whereas all other cases result into hole doped systems. For all the 4d elements studied, it is akin to the doping with holes for Cd substitution in the outer ring, whereas for Ru and Rh, there is electron type doping in case of substitution for Cd in central ring upon passivation. A comparison of partial density of states plots for bare and passivated clusters, on doping with transition metal atoms, suggests suitability of the cage-like cluster for spintronics applications.

cond-mat.mtrl-sci↗

arXiv2021

Developing atomistic glass models using potential-free Monte Carlo method: From simple to complex structures

We propose here a method to generate random networked amorphous structure using only readily available short-range properties like bond lengths, bond angles and connectivity of the constituents. This method is a variant of Monte-Carlo (MC) method wherein the basic constituents of an amorphous network i.e. rigid polyhedral units are connected randomly obeying certain steric constraints. The algorithm is designed to reproduce the medium-range order universally observed in glasses. The method somewhat resembles the reverse MC (RMC) method where a random move of an atom inside a box is accepted or rejected depending upon whether it decreases or increases the deviation from the experimentally observed features. However unlike RMC, this method does not demand large experimental sets of scattering data which in most cases is a priori not available for glasses. It rather relies on the stochasticity of MC method to produce glassy structures. The algorithm is first validated against SiO2 glass structure by comparing with the available structures from other methods and experimental data. The method is then extended for developing more complex Iron Phosphate Glass (IPG) structures and a comparison with existing models of IPG developed using quench-from-melt scheme implemented in classical Molecular Dynamics (MD) reveals that the method is extensible to complex glasses also. This study addresses the often-neglected issue of non-availability of correct starting structures in simulating glasses using MD or Density Functional Theory (DFT).

cond-mat.mtrl-sci↗

arXiv2011

Efficacy of surface error corrections to density functional theory calculations of vacancy formation energy in transition metals

We calculate properties like equilibrium lattice parameter, bulk modulus and monovacancy formation energy for nickel (Ni), iron (Fe) and chromium (Cr) using Kohn-Sham density functional theory (DFT). We compare relative performance of local density approximation (LDA) and generalized gradient approximation (GGA) for predicting such physical properties for these metals. We also make a relative study between two different flavors of GGA exchange correlation functional, namely, PW91 and PBE. These calculations show that there is a discrepancy between DFT calculations and experimental data. In order to understand this discrepancy in the calculation of vacancy formation energy, we introduce a correction for the surface intrinsic error corresponding to an exchange correlation functional using the scheme implemented by Mattsson et al. [Phys. Rev. B 73, 195123 (2006)] and compare the effectiveness of the correction scheme for Al and the 3d-transition metals.

cond-mat.mtrl-sci↗