arXiv subjects

Joydeep Bhattacharjee

Publications and source records attributed to Joydeep Bhattacharjee.

17 recordsLinked to original sources

arXiv2025

Robust electro-mechanical actuation in hydrogenated Xenes leading to reversible topological transition

We report from first principles, the possibility of reversible onset of topological insulator(TI) phase in heavier hydrogenated Xenes (Xane), namely, germanane and stanane, exclusively through in-plane electro-mechanical actuation. It is found possible to systematically induce robust uniaxial strain through non-uniform application of electric field in the plane of monolayers, as possible through application of in-homogeneous bias at gates of realizable length-scales embedded underneath. Electrically induced strain causes substantial lowering of band-gap across all Xanes, eventually evolving through weak followed by strong topologically insulating phases beyond a threshold degree of bias in-homogeneity in heavier Xanes, promisingly within the range of bias sustained by the monolayers. In case of nano-ribbons of these Xanes, bias applied in-homogeneously across width promises switchable emergence of TI phase over a fraction of width and topologically protected interface states localizable anywhere across the half-width of the ribbon. The demonstrated electro-mechanical actuation and the associated topological tuning of band-structure, thematically verified in gapped graphene based representative systems within the Kane-Mele model at half-filling, should be possible in the broader class of two dimensional covalent networks made of elements of the p-block.

cond-mat.mtrl-sci↗

arXiv2024

Partitioning of total charge in matter from geometric phases of electrons

Based on geometric phases of Bloch electrons computed from first-principles, we propose a scheme for unambiguous partitioning of charge in matter, derivable directly from the Kohn-Sham states. Generalizing the fact that geometric phases acquired by electrons due to evolution of their crystal momentum $\vec k$ in a direction through out the Brillouin zone(BZ), provide position of their localization with net minimum spread along the corresponding direction in real space. We find that the total charge can be meaningfully distributed into charge centres simultaneously contributed by triads of electrons with their crystal momentum evolving linearly independently through each unique $\vec k$ across the BZ. The resultant map of charge centres readily renders not only the qualitative nature of inter-atomic as well as intra-atomic hybridization of electrons, but also unbiased quantitative estimates of electrons on atoms or shared between them, as demonstrated in a select variety of isolated and periodic systems with varying degree of sharing of valence electrons among atoms, including variants of multi-centered bonds.

cond-mat.other↗

arXiv2023

Optical excitation from anti-causally corrected real-time dynamics in a minimal basis

Here we demonstrate workably accurate estimation of optical excitation threshold for large systems comprising of hundreds of atoms through an anti-causally corrected(ACC) real-time dynamics(RTD) approach implemented in a minimal tight-binding basis constituted by the directed hybrid atomic Wannier orbitals. A correction to the Hamiltonian is applied anti-causally at all time steps to account for electron-hole interaction using the density-density response function. Minimality of basis and ease of transferability of parameters to large systems arises from the directed nature of the Wannierized hybrid basis orbitals used.With self-energy corrected TB parameters evaluated at the DFT + G 0 W 0 level, the proposed ACC-RTD scheme can be systematically parametrized to render optical excitation threshold for systems of experimentally realizable length-scales through inexpensive computation.

cond-mat.mtrl-sci↗

arXiv2022

BLOOM: A 176B-Parameter Open-Access Multilingual Language Model

Large language models (LLMs) have been shown to be able to perform new tasks based on a few demonstrations or natural language instructions. While these capabilities have led to widespread adoption, most LLMs are developed by resource-rich organizations and are frequently kept from the public. As a step towards democratizing this powerful technology, we present BLOOM, a 176B-parameter open-access language model designed and built thanks to a collaboration of hundreds of researchers. BLOOM is a decoder-only Transformer language model that was trained on the ROOTS corpus, a dataset comprising hundreds of sources in 46 natural and 13 programming languages (59 in total). We find that BLOOM achieves competitive performance on a wide variety of benchmarks, with stronger results after undergoing multitask prompted finetuning. To facilitate future research and applications using LLMs, we publicly release our models and code under the Responsible AI License.

cs.CL↗

arXiv2022

Maximally valent orbitals in systems with non-ideal bond-angles

In pursuit of a minimal basis for systems with non-ideal bond angles, in this work we try to pinpoint the exact orientation of the major overlapping orbitals along the nearest neighbouring coordination segments in a given system such that they maximally represent the covalent interactions through out the system. We compute Mayer's bond order, akin to the Wiberg's bond index, in the basis of atomic Wannier orbitals with customizable non-degenerate hybridization constructed from first principles, in a representative variety of molecules and layered systems. We put them in perspective with unbiased maximally localized descriptions of bonding and non-bonding orbitals, and energetics to tunneling of electrons through them between nearest neighbours, to describe the different physical aspects of covalent interactions, which are not necessarily represented by a single unique set of atomic or bonding orbitals.

cond-mat.mtrl-sci↗

arXiv2021

Self-energy corrected tight binding parameters for few p-block semiconductors in the hybridized atomic orbital basis constructed from first principles

We present self-energy corrected tight-binging(TB) parameters in the basis of the directed hybridised atomic orbitals constructed from first principles, for nano-diamonds as well as bulk diamond and zinc blende structures made of elements of group 13, 14 and 15 in the 2p, 3p and 4p blocks. With increasing principal quantum number of frontier orbitals, the lowering of self-energy corrections(SEC) to the band-gap and consequently to the dominant inter-atomic TB parameters, is much faster in bulk than in nano-diamonds and hence not transferable from bulks to nano-structures. However, TB parameters transfered from smaller nano-diamonds to much larger ones exclusively through mapping of neighbourhoods of atoms not limited to nearest neighbours, are found to render HOMO-LUMO gaps of the larger nano-diamonds with few hundreds of atoms in good agreement with their explicitly computed values at the DFT as well as DFT+G0W0 levels. TB parameters and their SEC are found to vary significantly from 2p to 3p block but negligibly from 3p to 4p, while varying rather slowly within each block, implying the possibility of transfer of SEC across block with increasing principal quantum number. The demonstrated easy transferability of self-energy corrected TB parameters in the hybrid orbital basis thus promises computationally inexpensive estimation of quasi-particle electronic structure of large finite systems with thousands of atoms.

cond-mat.mtrl-sci↗

arXiv2021

Hybrid atomic orbital basis from first principles: Bottom-up mapping of self-energy correction to large covalent systems

Construction of hybrid atomic orbitals is proposed as the approximate common eigen states of finite first moment matrices. Their hybridization and orientation can be a-priori tunned as per their anticipated neighbourhood. Their Wannier function counterparts constructed from the Kohn-Sham(KS) single particle states constitute an orthonormal multi-orbital tight-binding(TB) basis resembling hybrid atomic-orbitals locked to their immediate atomic neighborhood, while spanning the subs-space of KS states. The proposed basis thus not only renders predominantly single TB parameters from first-principles for each nearest neighbour bonds involving no more than two orbitals irrespective of their orientation, but also facilitate an easy route for transfer of such TB parameters across isostructural systems exclusively through mapping of neighbourhoods and projection of orbital charge centres. With hybridized 2s,2p and 3s,3p valence electrons, the spatial extent of self-energy correction(SEC) to TB parameters in the proposed basis are found to be localized mostly within the third nearest neighbourhood, thus allowing effective transfer of self-energy corrected TB parameters from smaller reference systems to much larger target systems, with nominal additional computational cost beyond that required for explicit computation of SEC in the reference systems. The proposed approach promises inexpensive estimation of quasi-particle structure of large covalent systems with workable accuracy.

cond-mat.mtrl-sci↗

arXiv2020

Transferability of self-energy correction in tight-binding basis constructed from first principles

We demonstrate in this work the transferability of self-energy(SE) correction(SEC) of Kohn-Sham(KS) single particle states from smaller to larger systems, when mapped through localized orbitals constructed from the KS states. The approach results in a SE corrected TB framework, within which, the mapping of SEC of TB parameters is found to be transferable from smaller to larger systems of similar morphology, leading to a computationally inexpensive approach for estimation of SEC in large systems with reasonably high accuracy. The scheme has been demonstrated in insulating, semiconducting and magnetic nanoribbons of graphene and hexagonal boron nitride, where SEC tends to strengthen the individual pi bonds, leading to transfer of charge from edge to bulk. Additionally in magnetic bipartite systems SEC tends to enhance inter-sublattice spin separation. The proposed scheme thus promises to enable estimation of SEC of band-gaps of large systems without needing to explicitly calculate SEC of KS single particle levels which can be computationally prohibitively expensive.

cond-mat.mtrl-sci↗

arXiv2019

Hybrid superlattices of graphene and hexagonal boron nitride: A ferromagnetic semiconductor at room temperature

Carbon (C) doped hexagonal boron nitride (hBN) has been experimentally reported to be ferromagnetic at room temperature. Substitution by C in hBN has been also reported to form islands of graphene. In this work we derive a mechanistic understanding of ferromagnetism with graphene islands in hBN from first principles and mean-field Hubbard model. We find a general property, that in bipartite lattices where the sublattices differ in on-site energies, as in hBN, the ordering between local magnetic moments can be substantial and predominantly anti-ferromagnetic (AFM) if they are embedded in the same sublattice, unless dominated by Mott like inter-sublattice spin separation due to strong localization. The dominant AFM order is rooted at spin resolved spatial separation of lone pairs of nitrogen (N) and back transferred electrons on boron (B) due to Coulomb repulsion thus essentially implying a super-exchange pathway. Subsequently we propose a class of ferri-magnetically ordered inter-penetrating super-lattices of magnetic graphene islands in hBN, which can be chosen to be a ferromagnetic semiconductor or a half-metal, and retain a net non-zero magnetic moment at room temperature.

cond-mat.mes-hall↗

arXiv2019

A synergistic view of magnetism, chemical activation, and ORR as well as OER catalysis of carbon doped hexagonal boron nitride from first-principles

Carbon(C) doped hexagonal boron nitride(hBN) has been experimentally reported in recent years to be a possible catalytic host to oxygen reduction reaction(ORR), as well as a possible ferromagnet at room temperature. Substitution by C in hBN has been also reported to form islands of graphene. In this work, we explore from first principles, the connection between these different aspects of C doped hBN. We find formation of graphene islands covering unequal number of B and N sites in hBN to be energetically plausible. They possess a net non-zero magnetic moment and are also found to be substantially more chemically active than their non-magnetic counterparts covering equal number of B and N sites. On-site Coulomb repulsion between electrons, known to be responsible for magnetism in bipartite lattices like graphene and hBN, is also found to play a central role in chemical activation of not only the C atoms at the zigzag interface of magnetic graphene islands and hBN, but also of boron(B) sites in the immediate hBN neighborhood. However, such activated B or C due to substitution at B site, which is energetically more favorable than at N site, has been reported to be unfavorable for ORR. Advantageously, we find that the activation of C at B sites moderates systematically with increasing size of graphene islands, paving the way for abundance of efficient catalytic sites at the edges of magnetic graphene islands covering more B sites than N sites. Accordingly, as an alternate to precious metals for electrodes, we propose a class of graphene-hBN hybrids with lattices of magnetic graphene islands embedded in hBN, which can be metallic.

cond-mat.mtrl-sci↗

arXiv2017

Bias induced ferromagnetism and half-metallicity in graphene nano-ribbons

Towards spin selective electronics made of three coordinated carbon atoms, here we computationally propose robust and reversibly bias driven evolution of pristine undoped graphene nano-ribbons(GNR) into ferromagnetic-semiconductor, metal or a half metal, irrespective of their edge configurations. The evolution is a result of a rare ferromagnetic(FM) order emerging among nearest neighbouring(n-n) sites, in positively biased regions in their in-homogeneous bias unit-cells, in attempt to cooperatively minimise on-site Coulomb repulsion and kinetic energy, while maximising localization of electrons at the positively biased sites. The phenomenon appears to be a general property of in-homogeneously biased Coulomb correlated bipartite systems. Consequences are particularly rich in zigzag edged graphene nano-ribbons(ZGNR) due to the contest of bias driven n-n FM order and the inter-edge antiferromagnetic order inherent to ZGNRs, leading to systematic closing of gap for one of the spins, amounting to bias controlled unmissable opening of window for FM-semiconducting and half-metallic transport.

cond-mat.mes-hall↗

arXiv2015

Activation of Graphenic Carbon due to Substitutional Doping by Nitrogen: Mechanistic Understanding from First-principles

Nitrogen doped graphene and carbon nanotubes are popularly in focus as metal-free electro-catalysts for oxygen reduction reactions (ORR) central to fuel-cells. N doped CNTs have been also reported to chemisorb mutually, promising a route to their robust pre-determined assembly into devices and mechanical reinforcements. We propose from first-principles a common mechanistic understanding of these two aspects pointing further to a generic chemical activation of carbon atoms due to substitution by nitrogen in experimentally observed configurations. Wannier-function based orbital resolved study of mechanisms suggests increase in C-N bond-orders in attempt to retain $π$-conjugation among carbon atoms, causing mechanical stress and loss of charge neutrality of nitrogen and carbon atoms, which remedially facilitate chemical activation of N coordinated C atoms, enhancing sharply with increasing coordination to N and proximity to zigzag edges. Activated C atoms facilitate covalent adsorption of radicals in general, diradicals like O$_2$ relevant to ORR, and also other similarly activated C atoms leading to self-assembly of graphenic nano-structures, while remaining inert to ordinary graphenic C atoms.

cond-mat.mtrl-sci↗

arXiv2010

Quantum dot from chiral metallic single walled nanotubes

We propose a simple approach to construct a quantum-dot and it's electrodes using chiral metallic single walled carbon nanotube (CM-SWCNT) segments of exactly opposite chiralities $(m,n)$ and $(n,m)$. The degree and energetics of electron confinement crucially depends on the choice of $n$ and $m$, and collinearity of the SWCNT segments constituting the quantum-dot and the electrodes. All the segments can in principle be obtained through simple manipulations of fragments of a single nanotube of chirality either $(m,n)$ or $(n,m)$.

cond-mat.mes-hall↗

arXiv2008

Wannier Orbital Overlap Population (WOOP), Wannier Orbital Position Population (WOPP) and the Origin of Anomalous Dynamical Charges

Most d^0 transition metal (TM) oxides exhibit anomalously large Born dynamical charges associated with off-centering or motion of atoms along the TM-O chains. To understand their chemical origin, we introduce "Wannier Orbital Overlap Population" (WOOP) and "Wannier ond Orbital Position Population" (WOPP) based on the Wannier function based description of electronic structure obtained within first-principles density functional theory. We apply these concepts in a precise analysis of anomalous dynamical charges in PbTiO_3, BaTiO_3 and BaZrO_3 in the cubic perovskite structure. Determining contributions of different atomic orbitals to the dynamical charge and their break-up into local polarizability, charge transfer and covalency, we find that p orbitals of oxygen perpendicular to the -TM-O- chain contribute most prominently to the anomalous charge, by facilitating a transfer of tiny electronic charge through one unit cell from one TM atom to the next. Our results explain why the corner-shared linkage of TMO_6 octahedra, as in the perovskite structure, is ideal for large dynamical charges and hence for ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2006

Distribution of Electron Charge Centres: A Picture of Bonding Based on Geometric Phases

In the past two decades, geometric phases have provided a powerful new way of looking at quantum mechanical systems, manifesting themselves in subtle but observable ways. Here, we use them to define a versatile function ("distribution of electron charge centres" or DECC) which can be easily evaluated and interpreted, providing information about electronic structure in real space. Its utility is illustrated by application to a large variety of insulators, metals and molecules, treated here within the framework of density functional theory. The DECC is shown to provide a precise and compact description of bonding. Unshared-electron (ionic) and shared-electron (covalent, metallic) bonds are shown to present clearly distinct signatures: the former are uni-centred while the latter are $n$-centred (n > 1). Moreover, the charge contained in the DECC peaks gives either the ionic charge or the number of shared electrons, which is an even integer for covalent bonds. One obtains revealing insight into the microscopic chemical origins of macroscopic phenomena such as ferroelectricity in PbTi0$_3$ and the anomalous mechanical behaviour of bulk Al relative to that of Cu.

cond-mat.mtrl-sci↗

arXiv2005

Localized Orbital Description of Electronic Structure

We present a simple and general method for construction of localized orbitals to describe electronic structure of extended periodic metals and insulators as well as confined systems. Spatial decay of these orbitals is found to exhibit exponential behavior for insulators and power law for metals. While these orbitals provide a clear description of bonding, they can be also used to determine polarization of insulators. Within density functional theory, we illustrate applications of this method to crystalline Aluminium, Copper, Silicon, PbTiO$_3$ and molecules such as ethane and diborane.

cond-mat.mtrl-sci↗

arXiv2004

Geometric phases and Wannier functions of Bloch electrons in 1-dimension

We present a formal expression for Wannier functions of composite bands of 1-D Bloch electrons in terms of parallel-transported Bloch functions and their non-Abelian geometric phases. Spatial decay properties of these Wannier functions are studied in the case of simple bands of 1-D model insulator and metal. Within first-principles density functional theory, we illustrate the formalism through the construction of Wannier functions of polyethylene and polyacetylene.

cond-mat.mtrl-sci↗