arXiv subjects

Somnath Bhowmick

Publications and source records attributed to Somnath Bhowmick.

At least 37 records · Page 2Linked to original sources

arXiv2023

Accelerating microstructure modelling via machine learning: a new method combining Autoencoder and ConvLSTM

Phase-field modeling is an elegant and versatile computation tool to predict microstructure evolution in materials in the mesoscale regime. However, these simulations require rigorous numerical solutions of differential equations, which are accurate but computationally expensive. To overcome this difficulty, we combine two popular machine learning techniques, autoencoder and convolutional long short-term memory (ConvLSTM), to accelerate the study of microstructural evolution without compromising the resolution of the microstructural representation. After training with phase-field generated microstructures of ten known compositions, the model can accurately predict the microstructure for the future nth frames based on previous m frames for an unknown composition. Replacing n phase-field steps with machine-learned microstructures can significantly accelerate the in silico study of microstructure evolution.

cond-mat.mtrl-sci↗

arXiv2023

PdSe2 based field-effect transistors

Pentagonal PdSe2 is a promising candidate for layered electronic devices, owing to its high air-stability and anisotropic transport properties. Here, we investigate the performance of p-type FET based on PdSe$_2$ mono-layer using multi-scale simulation framework combining Density functional theory and quantum transport. We find that mono-layer PdSe$_2$ devices show excellent switching characteristics ($<$ 65 mV/decade) for the source-drain direction aligned along both [010] and [100] directions. Both directions also show good on-state current and large transconductance, though these are larger along the [010] direction for a 15 nm channel device. The channel length scaling study of these p-FETs indicates that channel length can be easily scaled down to 7 nm without any significance compromise in the performance. Going below 7 nm, we find that there is a severe degradation in the sub-threshold swing for 4 nm channel length. However, this degradation can be minimized by introducing an underlap structure. The length of underlap is determined by the trade-off between on-state current and the switching performance.

cond-mat.mes-hall↗

arXiv2022

Designing power efficient transistors using narrow bandwidth materials from the MA2Z4 monolayer series

The subthreshold leakage current in transistors has become a critical limiting factor for realizing ultra-low-power transistors. The leakage current is predominantly dictated by the long thermal tail of the charge carriers. We propose a solution to this problem by using narrow bandwidth semiconductors for limiting the thermionic leakage current by filtering out the high energy carriers. We specifically demonstrate this solution in transistors with laterally confined monolayer MoSi2N4 with different passivation serving as channel material. Remarkably, we find that the proposed narrow bandwidth devices can achieve a large ON/OFF current ratio with an ultra-low-power supply voltage of 0.1 V, even for devices with a 5 nm gate length. We also show that several other materials share the unique electronic properties of narrow bandwidth conduction and valance bands in the same series.

cond-mat.mes-hall↗

arXiv2022

Prediction of Magnetoelectric Multiferroic Janus Monolayers VOXY(X/Y = F, Cl, Br, or I, and X$\not=$Y) with in-plane ferroelectricity and out-of-plane piezoelectricity

Multifunctional two-dimensional (2D) multiferroics are promising materials for designing low-dimensional multipurpose devices. The key to multifunctionality in these materials is breaking the space-inversion and the time-reversal symmetry, which results in spontaneous electric polarization and magnetization in the same phase. A new class of 2D materials, Janus 2D materials, has emerged, which works on a similar principle of breaking out-of-plane symmetry to invoke new exciting functionalities in the 2D materials, such as an out-of-plane piezoelectric polarization. In this work, a new group of 2D multiferroic Janus monolayers VOXY (X/Y = F, Cl, Br, or I, and X$\not=$Y) is derived by breaking the out-of-plane symmetry in the parent multiferroics VOX$_2$ (X = F, Cl, Br, or I). The structural, magnetic, and ferroelectric properties of multiferroics VOX$_2$ are compared with their Janus derivatives. We calculated in-plane and out-of-plane piezoelectric polarization for VOX$_2$ and VOXY series, where VOFCl, VOFBr, VOFI, and VOClI are found to have significant out-of-plane piezoelectric polarization. Our theoretical work predicts a new series of 2D multiferroic materials and encourages their further investigation for high-performance nanoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Two-dimensional MoSi2N4: An Excellent 2D Semiconductor for Transistors

We report the performance of field-effect transistors (FETs), comprised of mono-layer of recently synthesized layered two-dimensional MoSi2N_4 as channel material, using the first principles quantum transport simulations. The devices' performance is assessed as per the International Roadmap for Devices and Systems (IRDS) 2020 roadmap for the year 2034 and compared to advanced silicon-based FETs, carbon nanotube-based FETs, and other promising two-dimensional materials based FETs. Finally, we estimate the figure of merits of a combinational and a sequential logic circuit based on our double gate devices and benchmark against promising alternative logic technologies. The performance of our devices and circuits based on them are encouraging, and competitive to other logic alternatives.

cond-mat.mes-hall↗

arXiv2022

Strain-tunable in-plane ferroelectricity and lateral tunnel junction in monolayer group-IV monochalcogenides

2D Ferroelectric materials are promising for designing low-dimensional memory devices. Here, we explore strain tunable ferroelectric properties of group-IV monochalcogenides MX (M=Ge, Sn; X=S, Se) and their potential application in lateral field tunnel junction devices. We find that these monolayers have in-plane ferroelectricity, with their ferroelectric parameters being on par with other known 2D ferroelectric materials. Amongst SnSe, SnS, GeSe, and GeS, we find that GeS has the best ferroelectric parameters for device applications, which can be improved further by applying uniaxial tensile strain. We use the calculated ferroelectric properties of these materials to study the tunneling electroresistance (TER) of a 4 nm device based on lateral ferroelectric tunnel junction. We find a substantial TER ratio $10^3-10^5$ in the devices based on these materials, which can be further improved up to a factor of 40 on the application of tensile strain.

cond-mat.mtrl-sci↗

arXiv2021

Experimental and theoretical studies of the gas-phase reactions of O($^1$D) with H$_2$O and D$_2$O at low temperature

Here we report the results of an experimental and theoretical study of the gas-phase reactions between O($^1$D) and H$_2$O and O($^1$D) and D$_2$O at room temperature and below. On the experimental side, the kinetics of these reactions have been investigated over the 50-127 K range using a continuous flow Laval nozzle apparatus, coupled with pulsed laser photolysis and pulsed laser induced fluorescence for the production and detection of O($^1$D) atoms respectively. Experiments were also performed at 296 K in the absence of a Laval nozzle. On the theoretical side, the existing full-dimensional ground X$^1$A potential energy surface for the H$_2$O$_2$ system involved in this process has been reinvestigated and enhanced to provide a better description of the barrierless H-atom abstraction pathway. Based on this enhanced potential energy surface, quasiclassical trajectory calculations and ring polymer molecular dynamics simulations have been performed to obtain low temperature rate constants. The measured and calculated rate constants display similar behaviour above 100 K, showing little or no variation as a function of temperature. Below 100 K, the experimental rate constants increase dramatically, in contrast to the essentially temperature independent theoretical values. The possible origins of the divergence between experiment and theory at low temperatures are discussed.

physics.chem-ph↗

arXiv2021

Decoupled Strain Response of Ferroic Properties in Multiferroic VOCl2 Monolayer

Two-dimensional (2D) magnetoelectric multiferroics are promising multifunctional materials for miniaturized logic and memory devices. Herein, we explore the effectiveness of strain-engineering for tuning the properties of a recently predicted 2D antiferromagnetic-ferroelectric, VOCl2 monolayer. Interestingly, we find that magnetic-ordering and electric polarization can be tuned independently using uniaxial tensile strain along different in-plane lattice vectors. A 4% tensile strain along lattice vector b induces a transition from an antiferromagnetic (AFM) ground state with an out-of-plane magnetization to a ferromagnetic (FM) ground state with in-plane magnetization. On the other hand, tensile strain along lattice vector a enhances spontaneous electric polarization, without affecting the magnetic ordering. The monolayers remain dynamically stable under tensile strain, which further helps to raise the Curie temperature of ferromagnetism, as well as ferroelectricity. Such a strain-tunable multiferroic material holds great promises for future generation nanoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Isotopic Separation of Helium through Nanoporous Graphene Membranes: A Ring Polymer Molecular Dynamics Study

Microscopic-level understanding of the separation mechanism for two-dimensional (2D) membranes is an active area of research due to potential implications of this class of membranes for various technological processes. Helium (He) purification from the natural resources is of particular interest due to the shortfall in its production. In this work, we applied the ring polymer molecular dynamics (RPMD) method to graphdiyne (Gr2) and graphtriyne (Gr3) 2D membranes having variable pore sizes for the separation of He isotopes. We found that the transmission rate through Gr3 is many orders of magnitude greater than Gr2. The selectivity of either isotope at low temperatures is a consequence of a delicate balance between the zero-point energy effect and tunneling of $^4$He and $^3$He. RPMD provides an efficient approach for studying the separation of He isotopes, taking into account quantum effects of light nuclei motions at low temperatures, which classical methods fail to capture.

cond-mat.mes-hall↗

arXiv2021

Role of interface morphology on the martensitic transformation in pure Fe

Using classical molecular dynamics simulations, we study austenite to ferrite phase transformation in iron, focusing on the role of interface morphology. We compare two different morphologies; a \textit{flat} interface in which the two phases are joined according to Nishiyama-Wasserman orientation relationship vs. a \textit{ledged} one, having steps similar to the vicinal surface. We identify the atomic displacements along a misfit dislocation network at the interface leading to the phase transformation. In case of \textit{ledged} interface, stacking faults are nucleated at the steps, which hinder the interface motion, leading to a lower mobility of the inter-phase boundary, than that of flat interface. Interestingly, we also find the temperature dependence of the interface mobility to show opposite trends in case of \textit{flat} vs. \textit{ledged} boundary. We believe that our study is going to present a unified and comprehensive view of martensitic transformation in iron with different interface morphology, which is lacking at present, as \textit{flat} and \textit{ledged} interfaces are treated separately in the existing literature.

cond-mat.mtrl-sci↗

arXiv2020

K$_2$CoS$_2$: A new two-dimensional in-plane antiferromagnetic insulator

The recent discovery of two-dimensional (2D) magnetic materials has brought magnetism to the flatland. This has opened up exciting opportunities for the exploration of fundamental physics as well as for novel device applications. Here, we predict a new thermodynamically stable 2D magnetic material, K$_2$CoS$_2$, which retains its in-plane anti-ferromagnetic order down to the monolayer and bilayer limits. We find that the magnetic moments ($2.5 μ_B/$Co) are aligned along the intra-Co chains, from monolayer to bulk. The non-magnetic electronic spectrum of both the monolayer and bilayer films is found to host flat bands and van-Hove singularities, which in instrumental in giving rise to the the magnetic ground state. Based on classical Monte-Carlo simulations, we estimate the Neel temperature for the antiferromagnetic monolayer to be $\approx 15$K. Our study thus establishes that K$_2$CoS$_2$ hosts a robust antiferromagnetic state which persists from the monolayer limit to the bulk material.

cond-mat.mes-hall↗

arXiv2019

Electronic structure and transport in amorphous metal oxide and amorphous metal oxy-nitride semiconductors

Recently amorphous oxide semiconductors (AOS) have gained commercial interest due to their low-temperature processability, high mobility and areal uniformity for display backplanes and other large area applications. A multi-cation amorphous oxide (a-IGZO) has been researched extensively and is now being used in commercial applications. It is proposed in the literature that overlapping In-5s orbitals form the conduction path and the carrier mobility is limited due to the presence of multiple cations which create a potential barrier for the electronic transport in a-IGZO semiconductors. A multi-anion approach towards amorphous semiconductors has been suggested to overcome this limitation and has been shown to achieve hall mobilities up to an order of magnitude higher compared to multi-cation amorphous semiconductors. In the present work, we compare the electronic structure and electronic transport in a multi-cation amorphous semiconductor, a-IGZO and a multi-anion amorphous semiconductor, a-ZnON using computational methods. Our results show that in a-IGZO, the carrier transport path is through the overlap of outer s-orbitals of mixed cations and in a-ZnON, the transport path is formed by the overlap of Zn-4s orbitals, which is the only type of metal cation present. We also show that for multi-component ionic amorphous semiconductors, electron transport can be explained in terms of orbital overlap integral which can be calculated from structural information and has a direct correlation with the carrier effective mass which is calculated using computationally expensive first principle DFT methods.

cond-mat.mtrl-sci↗

arXiv2019

Intrinsic magnetism in monolayer transition metal trihalides: a comparative study

Two dimensional magnetic materials, with tunable electronic properties could lead to new spintronic, magnetic and magneto-optic applications. Here, we explore intrinsic magnetic ordering in two dimensional monolayers of transition metal tri-halides (MX$_3$, M = V, Cr, Mn, Fe and Ni, and X = F, Cl, Br and I), using density functional theory. We find that other than FeX$_3$ family which has an anti-ferromagnetic ground state, rest of the trihalides are ferromagnetic. Amongst these the VX$_3$ and NiX$_3$ family are found to have the highest magnetic transition temperature, beyond the room temperature. In terms of electronic properties, the tri-halides of Mn and Ni are either half metals or Dirac half metals, while the tri-halides of V, Fe and Cr are insulators. Among all the trihalides studied in this paper, we find the existence of very clean spin polarized Dirac half metallic state in MnF$_3$, MnCl$_3$, MnBr$_3$, NiF$_3$ and NiCl$_3$. These spin polarized Dirac half metals will be immensely useful for spin-current generation and other spintronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Interlayer decoupling in twisted bilayers of $β$-phosphorus and arsenic: a computational study

We investigate magnetism and band structure engineering in Moiré superlattice of blue phosphorus ($β$-P) and grey arsenene ($β$-As) bilayers, using \textit{ab initio} calculations. The electronic states near the valence and conduction band edges have significant $p_z$ character in both the bilayers. Thus, twisting the layers significantly reduce the interlayer orbital overlap, leading to a decrease in the binding energy (up to $\sim33\%$) and an increase in interlayer distance (up to $\sim10\%$), compared to the most stable AA-stacking. This interlayer decoupling also results in a notable increase (up to $\sim$25-50\%) of the bandgap of twisted bilayers, with the valance band edge becoming relatively flat with van-Hove singularities in the density of states. Thus, hole doping induces a Stoner instability, leading to ferromagnetic ground state, which is more robust in Moiré superlattices, than that of AA-stacked $β$-P and $β$-As.

cond-mat.mtrl-sci↗

arXiv2019

Role of disconnections in mobility of the austenite-ferrite inter-phase boundary in Fe

Austenite (γ-Fe, face centered cubic (FCC)) to ferrite (α-Fe, body centered cubic (BCC)) phase transformation in steel is of great significance from the point of view of industrial applications. In this work, using classical molecular dynamics simulations, we study the atomistic mechanisms involved during the nucleation and growth of the ferrite phase embedded in an austenite phase. We find that the disconnections present at the inter-phase boundary can act as the nucleation centers for the ferrite phase. Relatively small interface velocities (1.19 - 4.67 m/s) confirm a phase change via massive transformation mechanism. Boundary mobilities obtained in a temperature range of 1000 to 1400 K show an Arrhenius behavior, with activation energies ranging from 30 - 40 kJ/mol.

cond-mat.mtrl-sci↗

arXiv2018

The low temperature D$^+$ + H$_2$ $\rightarrow$ HD + H$^+$ reaction rate coefficient: a ring polymer molecular dynamics and quasi-classical trajectory study

The reaction between D$^+$ and H$_2$ plays an important role in astrochemistry at low temperatures and also serves as a prototype for simple ion-molecule reaction. Its ground $\tilde{X}~^1A^{\prime}$ state has a very small thermodynamic barrier (up to 1.8$\times 10^{-2}$ eV) and the reaction proceeds through the formation of an intermediate complex lying within the potential well of depth of at least 0.2 eV thus representing a challenge for dynamical studies. In the present work, we analyze the title reaction within the temperature range of 20 $-$ 100 K by means of ring polymer molecular dynamics (RPMD) and quasi-classical trajectory (QCT) methods over the full-dimensional global potential energy surface developed by Aguado et al. [A. Aguado, O. Roncero, C. Tablero, C. Sanz, and M. Paniagua, J. Chem. Phys., 2000, 112, 1240]. The computed thermal RPMD and QCT rate coefficients are found to be almost independent of temperature and fall within the range of 1.34 $-$ 2.01$\times$10$^{-9}$ cm$^3$ s$^{-1}$. They are also in a very good agreement with the previous time-independent quantum mechanical and statistical quantum method calculations. Furthermore, we observe that the choice of asymptotic separation distance between the reactants can markedly alter the rate coefficient in the low temperature regime (20 $-$ 50 K). Therefore it is of utmost importance to correctly assign the value of this parameter for dynamical studies, particularly at very low temperatures of astrochemical importance. We finally conclude that experimental rate measurements for the title reaction are highly desirable in future.

physics.chem-ph↗

arXiv2018

External-strain-induced semimetallic and metallic phase of chlorographene

To overcome the limitations of graphene due to lack of intrinsic band gap, it is generally functionalized with hydrogen or halogen atoms such as fluorine and chlorine. Generally, such functionalization yields a moderate- to high-band-gap material in case of 100% coverage, for example ~1.5 eV in graphene functionalized with chlorine atoms or chlorographene. In this paper, using ab initio calculations, we report very interesting transformations observed in chlorographene under external strain, driving it to a state with nearly vanishing band gap (under tensile strain) and even converting it to a metal (under compressive strain). We also show the importance of spin-orbit coupling, responsible for the few meV band gap of chlorographene observed under high tensile strain, which would have been a gapless semimetal otherwise.

cond-mat.mtrl-sci↗

arXiv2018

Strain-tunable charge carrier mobility of atomically thin phosphorus allotropes

We explore the impact of strain on charge carrier mobility of monolayer $α$, $β$, $γ$ and $δ$-P, the four well known atomically thin allotropes of phosphorus, using density functional theory. Owing to the highly anisotropic band dispersion, the charge carrier mobility of the pristine allotropes is significantly higher (more than 5 times in some cases) in one of the principal directions (zigzag or armchair) as compared to the other. Uniaxial strain (upto 6% compressive/tensile) leads to bandgap alteration in each of the allotropes, especially a direct to indirect bandgap semiconductor transition in $γ$-P and a complete closure of the bandgap in $γ$ and $δ$-P. We find that the charge carrier mobility is enhanced typically by a factor of $\approx 5-10$ in all the allotropes due to uniaxial strain; notably among them a $\approx 250$ (30) times increase of the hole (electron) mobility along the armchair (zigzag) direction is observed in $β$-P ($γ$-P) under a compressive strain, acting in the armchair direction. Interestingly, the preferred electronic conduction direction can also be changed in case of $α$ and $γ$-P, by applying strain.

cond-mat.mtrl-sci↗