arXiv subjects

Vivekanand Shukla

Publications and source records attributed to Vivekanand Shukla.

9 recordsLinked to original sources

arXiv2026

Element-Specific Visualization of Layer-Parity and Twist-Dependent Magnetism in CrSBr

Van der Waals (vdW) based antiferromagnets (AFMs) are an ideal platform for probing and understanding thickness- and twist-angle-dependent emergent spin phenomena. However, element-specific nanoscale characterization of the spin structure in atomically thin vdW-based AFMs systems and layer-parity effects remain elusive, making them crucial for both fundamental insight into low-dimensional magnetism and the rational design of spintronic devices based on these materials. Here, we utilize X-ray magnetic circular and linear dichroisms paired with photoemission electron microscopy to resolve the magnetic order in atomically thin CrSBr. Our comprehensive measurements reveal CrSBr magnetic structure at the nanoscale and its dependence on the layer number, surface encapsulation, temperature, and applied field. Moreover, in the orthogonally twisted bilayer configuration, obtained by twisting two CrSBr ferromagnetic monolayers by 90$^\circ$, the magnetic easy axis fundamentally differs from the individual monolayers, unlocking a new pathway for moiré magnetism.

cond-mat.mes-hall↗

arXiv2022

Accurate non-empirical range-separated hybrid van der Waals density functional for complex molecular problems, solids, and surfaces

We introduce a new, general-purpose, range-separated hybrid van der Waals density \ph{functional, termed vdW-DF-ahbr,} within the non-empirical vdW-DF method [JPCM 32, 393001 (2020)]. It combines correlation from vdW-DF2 with a screened Fock exchange that is fixed by \ph{a new model of exchange effects} in the density-explicit vdW-DF2-b86r functional [PRB 89, 121103(R) (2014)]. The new vdW-DF2-ahbr prevents spurious exchange binding and has a small-density-gradient form set from many-body perturbation analysis. It is accurate for \ph{bulk as well as layered materials} and it systematically and significantly improves the performance of present vdW-DFs for molecular problems. Importantly, vdW-DF2-ahbr also outperforms present-standard (dispersion-corrected) range-separated hybrids on a broad collection of noncovalent-interaction benchmark sets, while at the same time successfully mitigating the density-driven errors that often affect the description of molecular transition states and isomerization calculations. vdW-DF2-ahbr furthermore improves on state of the art density functional theory approaches by 1) correctly predicting both the substrate structure and the site preference for CO adsorption on Pt(111), 2) outperforming existing non-empirical vdW-DFs for the description of CO$_2$ adsorption in both a functionalized and in a simple metal-organic framework, and 3) being highly accurate \ph{for the} set of base-pair interactions in a model of DNA assembly.

physics.chem-ph↗

arXiv2022

Stability of and conduction in single-walled Si$_2$BN nanotubes

We explore the possibility and potential benefit of rolling a Si2BN sheet into single-walled nanotubes (NTs). Using density functional theory (DFT), we consider both structural stability and the impact on the nature of chemical bonding and conduction. The structure is similar to carbon NTs and hexagonal boron-nitride (hBN) NTs and we consider both armchair and zigzag Si2BN configurations with varying diameters. The stability of these Si$_2$BN NTs is confirmed by first-principles molecular dynamics calculations, by an exothermal formation, an absence of imaginary modes in the phonon spectra. Also, we find the nature of conduction varies semiconducting, from semi-metallic to metallic, reflecting differences in armchair/zigzag-type structures, curvature effects, and the effect of quantum confinement. We present the detailed characterization of how these properties lead to differences in both the bonding nature and electronic structures

cond-mat.mes-hall↗

arXiv2022

vdW-DF-ahcx: a range-separated van der Waals density functional hybrid

Hybrid density functionals replace a fraction of an underlying generalized-gradient approximation (GGA) exchange description with a Fock-exchange component. Range-separated hybrids (RSHs) also effectively screen the Fock-exchange component and thus open the door for characterizations of metals and adsorption at metal surfaces. The RSHs are traditionally based on a robust GGA, such as PBE [PRL $\textbf{77}$, 3865 (1996)], for example, as implemented in the HSE design [JPC $\textbf{118}$, 8207 (2003)]. Here we define a RSH extension to the van der Waals density functional (vdW-DF) method [ROPP $\textbf{78}$, 066501 (2015)], launching vdW-DF-ahcx. We use an analytical-hole (AH) framework [JCP $\textbf{128}$, 194105 (2008)] to characterize the GGA-type exchange in the vdW-DF-cx version [PRB $\textbf{89}$, 075148 (2014)], isolate the short-ranged exchange component, and define the new RSH. We find that the performance vdW-DF-ahcx compares favorably to (dispersion-corrected) HSE for descriptions of bulk (broad molecular) properties. We also find that it provides accurate descriptions of noble-metal surface properties, including CO adsorption.

cond-mat.mtrl-sci↗

arXiv2021

Screening nature of the van der Waals density functional method: A review and analysis of the many-body physics foundation

We review the screening nature and many-body physics foundation of the van der Waals density functional (vdW-DF) method, a systematic approach to construct truly nonlocal exchange-correlation energy density functionals. To that end we define and focus on a class of consistent vdW-DF versions that adhere to the Lindhard screening logic of the full method formulation. The consistent-exchange vdW-DF-cx version and its spin extension represent the first examples of this class; In general, consistent vdW-DFs reflect a concerted expansion of a formal recast of the adiabatic-connection formula, an exponential summation of contributions to the local-field response, and the Dyson equation. We argue that the screening emphasis is essential because the exchange-correlation energy reflects an effective electrodynamics set by a long-range interaction. Two consequences are that 1) there are, in principle, no wiggle room in how one balances exchange and correlation, for example, in vdW-DF-cx, and that 2) consistent vdW-DFs have a formal structure that allows them to incorporate vertex-correction effects, at least in the case of levels that experience recoil-less interactions (for example, near the Fermi surface). We explore the extent to which the strictly nonempirical vdW-DF-cx formulation can serve as a systematic extension of the constraint-based semilocal functionals. For validation, we provide a complete survey of vdW-DF-cx performance for broad molecular processes and comparing to the quantum-chemistry calculations that are summarized in that paper. We also provide new vdW-DF-cx results for metal surface energies and work functions that we compare to experiment. Finally, we use the screening insight to separate the vdW-DF nonlocal-correlation term and present tools to compute and map the binding signatures.

cond-mat.mtrl-sci↗

arXiv2021

Hard and soft materials: Putting consistent van der Waals density functionals to work

We present the idea and illustrate potential benefits of having a tool chain of closely related regular, unscreened and screened hybrid exchange-correlation (XC) functionals, all within the consistent formulation of the van der Waals density functional (vdW-DF) method [JPCM 32, 393001 (2020)]. Use of this chain of nonempirical XC functionals allows us to map when the inclusion of truly nonlocal exchange and of truly nonlocal correlation is important. Here we begin the mapping by addressing hard and soft material challenges: magnetic elements, perovskites, and biomolecular problems. We also predict the structure and polarization for a ferroelectric polymer. To facilitate this work and future broader explorations, we furthermore present a stress formulation for spin vdW-DF and illustrate use of a simple stability-modeling scheme to assert when the prediction of a soft mode (an imaginary-frequency vibrational mode, ubiquitous in perovskites and soft matter) implies a prediction of an actual low-temperature transformation.

cond-mat.mtrl-sci↗

arXiv2020

Optical excitations and thermoelectric properties of 2D holey graphene

Recently, holey graphene (HG) has successfully synthesized at atomic precision of hole size and shape. This shows interesting physical and chemical properties for energy and environmental applications. Shaping of the pores also transforms semimetallic graphene to semiconductor holey graphene, which opens new door for its use in electronic applications. We systematically investigated the structural, electronic, optical and thermoelectric properties of HG structure using first-principles calculations. HG was found to have a direct band gap with 0.65 eV (PBE functional), 0.95 eV (HSE06 functional) and HSE06 functional is in good agreement with experimental results. For the optical properties, we use single-shot G0W0 calculations by solving the Bethe-Salpeter equation to determining the intralayer excitonic effects. From the absorption spectrum, we obtained the optical gap of 1.28 eV and a week excitonic binding energy of 80 meV. We have found the large values of thermopower of 1662.59 $μ$V/K and better electronic figure of merit, ZT$_{e}$ as 1.13 from the investigated thermoelectric properties. Our investigations exhibit strong and broad optical absorption in the visible light region, which makes HG monolayer a promising candidate for optoelectronic and thermoelectric applications.

cond-mat.mes-hall↗

arXiv2018

Rectifying properties in 90° rotated bilayer black phosphorus nanojunction: A first principle study

We explore the possibility of using van dar Waals bonded heterostructures of stacked together 2D bilayer black phosphorus (BP) for nanoscale device applications. The electronic property of BP in AA stacking and 90° twisted is studied with density functional theory. Further, we study the homogeneous nanojunction architecture of BP to use its anisotropic properties. Using the first principle simulations along with NEGF approach, we calculate quantum transport properties of the nanojunction setup. The interlayer directionally dependent current characteristics are explained in different setups. Our result revealed that 90° twisted nanojucntion device would be a potential rectifier despite having no p-n junction characteristic only due to the intrinsic anisotropy of the material, making tunneling between armchair- and zigzag-directional BP sheets asymmetric.

physics.app-ph↗

arXiv2018

Structural, electronic and intrinsic transport in two-dimensional borophene sheets

Experimental realization of two-dimensional boron sheets was reported very recently by Feng et. al. using molecular beam epitaxy on silver (111) surface. These boron sheets possess promising electronic and transport properties. We performed the density functional theory (DFT) calculation to see the stability of two $β_{12}$ and $χ$ polymorphs of boron under strain and further studied electronic and transport properties. We verified the directional dependency in electron transport properties in these two boron polymorphs. Here we report tunable anisotropic transport properties of the borophenes. We also investigated current-voltage characteristics in low bias regime after applying strain on these systems to see how this external strain affects the anisotropy of current.

cond-mat.mes-hall↗