arXiv subjects

Rohit Babar

Publications and source records attributed to Rohit Babar.

15 recordsLinked to original sources

arXiv2024

Carbon-contaminated topological defects in hexagonal boron nitride for quantum photonics

Topological defects, such as Stone-Wales defects and grain boundaries, are common in 2D materials. In this study, we investigate the intricate interplay of topological defects and carbon contamination in hexagonal boron nitride revealing an intriguing class of color centers. We demonstrate that both carbon contamination and strain can stabilize Stone-Wales configurations and give rise to emitters with desirable optical properties in the visible spectral range. Inspired by these results, we further demonstrate that carbon atoms at grain boundaries can resolve energetic B-B and N-N bonds leading to highly favorable atomic structures that may facilitate the accumulation of carbon contamination at the boundaries. Similarly to contaminated Stone-Wales defects, carbon-doped grain boundaries can also give rise to color centers emitting in the visible spectral range with short radiative lifetime and high Debye-Waller factors. Our discoveries shed light on an exciting class of defects and pave the way toward the identification of color centers and single photon emitters in hBN.

cond-mat.mtrl-sci↗

arXiv2023

First principles theory of the nitrogen interstitial in hBN: a plausible model for the blue emitter

Color centers in hexagonal boron nitride (hBN) have attracted considerable attention due to their remarkable optical properties enabling robust room temperature photonics and quantum optics applications in the visible spectral range. On the other hand, identification of the microscopic origin of color centers in hBN has turned out to be a great challenge that hinders in-depth theoretical characterization, on-demand fabrication, and development of integrated photonic devices. This is also true for the blue emitter, which is an irradiation damage in hBN emitting at 436 nm wavelengths with desirable properties. Here, we propose the negatively charged nitrogen split interstitial defect in hBN as a plausible microscopic model for the blue emitter. To this end, we carry out a comprehensive first principles theoretical study of the nitrogen interstitial. We carefully analyze the accuracy of first principles methods and show that the commonly used HSE hybrid exchange-correlation functional fails to describe the electronic structure of this defect. Using the generalized Koopman's theorem, we fine tune the functional and obtain a zero-phonon photoluminescence (ZPL) energy in the blue spectral range. We show that the defect exhibits high emission rate in the ZPL line and features a characteristic phonon side band that resembles the blue emitter's spectrum. Furthermore, we study the electric field dependence of the ZPL and numerically show that the defect exhibits a quadratic Stark shift for perpendicular to plane electric fields, making the emitter insensitive to electric field fluctuations in first order. Our work emphasize the need for assessing the accuracy of common first principles methods in hBN and exemplifies a workaround methodology. Furthermore, our work is a step towards understanding the structure of the blue emitter and utilizing it in photonics applications.

cond-mat.mtrl-sci↗

arXiv2023

Symmetric carbon tetramers forming chemically stable spin qubits in hBN

Point defect quantum bits in semiconductors have the potential to revolutionize sensing at atomic scales. Currently, vacancy related defects, such as the NV center in diamond and the VB$^-$ in hexagonal boron nitride (hBN), are at the forefront of high spatial resolution and low dimensional sensing. On the other hand, vacancies' reactive nature and instability at the surface limit further developments. Here, we study the symmetric carbon tetramers in hBN and propose them as a chemically stable spin qubit for sensing in low dimensions. We utilize periodic-DFT and quantum chemistry approaches to reliably and accurately predict the electronic, optical, and spin properties of the studied defect. We show that the nitrogen centered symmetric carbon tetramer gives rise to spin state dependent optical signals with strain sensitive intersystem crossing rates. Furthermore, the weak hyperfine coupling of the defect to their spin environments results in a reduced electron spin resonance linewidth that may enhance sensitivity.

cond-mat.mtrl-sci↗

arXiv2022

Exhaustive characterization of modified Si vacancies in 4H-SiC

The negatively charged silicon vacancy ($\mathrm{V_{Si}^-}$) in silicon carbide is a well-studied point defect for quantum applications. At the same time, a closer inspection of ensemble photoluminescence and electron paramagnetic resonance measurements reveals an abundance of related but so far unidentified signals. In this study, we search for defects in 4H-SiC that explain the above magneto-optical signals in a defect database generated by Automatic Defect Analysis and Qualification (ADAQ) workflows. This search reveals only one class of atomic structures that exhibit silicon-vacancy-like properties in the data: a carbon antisite ($\mathrm{C_{Si}}$) within sub-nanometer distances from the silicon vacancy only slightly alters the latter without affecting the charge or spin state. Such a perturbation is energetically bound. We consider the formation of $\mathrm{V_{Si}^-+C_{Si}}$ up to 2 nm distance and report their zero phonon lines and zero field splitting values. In addition, we perform high-resolution photoluminescence experiments in the silicon vacancy region and find an abundance of lines. Comparing our computational and experimental results, several configurations show great agreement. Our work demonstrates the effectiveness of a database with high-throughput results in the search for defects in quantum applications.

cond-mat.mtrl-sci↗

arXiv2021

Quantum sensor in a single layer van der Waals material

Point defect qubits in semiconductors have demonstrated their outstanding high spatial resolution sensing capabilities of broad multidisciplinary interest. Two-dimensional (2D) semiconductors hosting such sensors have recently opened up new horizons for sensing in the subnanometer scales in 2D heterostructures. However, controlled creation of quantum sensor in a single layer 2D materials with high sensitivity has been elusive so far. Here, we report on a novel 2D quantum sensor, the VB2 centre in hexagonal boron nitride (hBN), with superior sensing capabilities. The centre's inherently low symmetry configuration gives rise to unique electronic and spin properties that implement a qubit in a 2D material with unprecedented sensitivity. The qubit is decoupled from its dense spin environment at low magnetic fields that gives rise to the reduction of the spin resonance linewidth and elongation of the coherence time. The VB2 centre is also equipped with a classical memory that can be utilized in storing population information. Using scanning transmission electron microscopy imaging, we confirm the presence of the point defect structure in free standing monolayer hBN created by electron beam irradiation. Our results provide a new material solution towards atomic-scale sensing in low dimensions.

cond-mat.mes-hall↗

arXiv2020

Non-van der Waals honeycomb antiferromagnet SrRu$_2$O$_6$ down to a few layers

The current family of experimentally realized two-dimensional magnetic materials consist of 3$d$ transition metals with very weak spin-orbit coupling. In contrast, we report a new platform in a chemically bonded and layered 4$d$ oxide, with strong electron correlations and competing spin-orbit coupling. We synthesize ultra-thin sheets of SrRu$_2$O$_6$ using scalable liquid exfoliation. These exfoliated sheets are characterized by complementary experimental and theoretical techniques. The thickness of the nano-sheets varies between three to five monolayers, and within the first-principles calculations, we show that antiferromagnetism survives in these ultra-thin layers. Experimental data suggest that exfoliation occurs from the planes perpendicular to the $c$-axis as the intervening hexagonal Sr-lattice separates the two-dimensional magnetic honeycomb Ru-layers. The high-resolution transmission electron microscope images indicate that the average inter-atomic spacing between the Ru-layers is slightly reduced, which agrees with the present calculations. The signatures of rotational stacking of the nanosheets are also observed. Such new two-dimensional platform offers enormous possibilities to explore emergent properties that appear due to the interplay between magnetism, strong correlations and spin-orbit coupling. Moreover, these effects can be further tuned as a function of layer thickness.

cond-mat.str-el↗

arXiv2019

van der Waals heterostructure for photocatalysis: Graphitic carbon nitride and Janus transition-metal dichalcogenides

Converting solar energy into chemical energy by splitting water is a promising means to generate a sustainable and renewable solution without detrimental environmental impact. The two-dimensional semiconductors serve as potential catalysts in this regard, and here we combine Janus transition-metal dichalcogenides (MoXY, X/Y = S, Se, Te) and graphitic carbon nitride in a van der Waals heterostructure. Within the first-principles calculations, we investigate the electronic, optical and excitonic properties that determine the photocatalytic activity. Due to the internal electric field, the photogenerated electrons and holes are separated in the MoXY layers, and also generates high overpotentials for the redox reactions. The high optical absorptions span throughout the entire visible and near ultraviolet regime in these heterostructure nanocomposites. Further, the lower exciton binding, calculated within the two-dimensional hydrogenic model, indicates efficient charge separation. Enormous tunability of photocatalytic properties in such heterostructures should attract considerable theoretical and experimental attention in future.

cond-mat.mes-hall↗

arXiv2019

Mechanistic insights on the phosphorene degradation

The structural and chemical degradations of phosphorene severely limit its practical applications despite the enormous promise. In this regard, we investigate a plethora of microscopic kinetic mechanisms and develop a degradation phase diagram within the first-principles calculations. At 400 K, the degradation and the competing self-annealing proceeds through the merger and annihilation of vacancies, respectively, which are triggered via itinerant vacancy and adatom. A further increase in temperature beyond 650 K, the structural degradation results through the emission of the undercoordinated atoms from the defect and the concurrent pair-wise sublimation. The role of inter-layer vacancy diffusion is discarded in the context of structural degradation. The chemical degradation is routed through the dissociation of oxygen molecule that is either activated at the room-temperature on the pristine surface or spontaneous at the single-vacancy site. The present results are in agreement with the few available experimental conjectures and will motivate further efforts.

cond-mat.mes-hall↗

arXiv2018

Ferromagnetism in nitrogen doped graphene

Inducing a robust long-range magnetic order in diamagnetic graphene remains a challenge. While nitrogen-doped graphene is reported to be a promising candidate, the corresponding exchange mechanism endures unclear and is essential to tune further and manipulate magnetism. Within the first-principles calculations, we systematically investigate the local moment formation and the concurrent interaction between various defect complexes. The importance of adatom diffusion on the differential defect abundance is discussed. The individual nitrogen complexes that contribute toward itinerant and a local magnetic moment are identified. The magnetic interaction between the complexes is found to depend on the concentration, complex type, sublattice, distance, and orientation. We propose that the direct exchange mechanism between the delocalized magnetic moment originating from the itinerant $π$-electron at the prevalent graphitic complexes to be responsible for the observed ferromagnetism. We show that B co-doping further improves ferromagnetism. Present results will assist in the microscopic understanding of the current experimental results and motivate experiments to produce robust magnetism following the proposed synthesis strategy.

cond-mat.mes-hall↗

arXiv2018

Exciton in phosphorene: Strain, impurity, thickness and heterostructure

Reduced electron screening in two-dimension plays a fundamental role in determining exciton properties, which dictates optoelectronic and photonic device performances. Considering the explicit electron-hole interaction within the $GW-$Bethe-Salpeter formalism, we first study the excitonic properties of pristine phosphorene and investigate the effects of strain and impurity coverage. The calculations reveal strongly bound exciton in these systems with anisotropic spatial delocalization. Further, we present a simplified hydrogenic model with anisotropic exciton mass and effective electron screening as parameters, and the corresponding results are in excellent agreement with the present $GW-$BSE calculations. The simplified model is then used to investigate exciton renormalization in few-layer and heterostructure phosphorene. The changes in carrier effective mass along with increasing electron screening renormalizes the exciton binding in these systems. We establish that the present model, where the parameters are calculated within computationally less expensive first-principles calculations, can predict exciton properties with excellent accuracy for larger two-dimensional systems, where the many-body $GW-$BSE calculations are impossible.

cond-mat.mes-hall↗

arXiv2018

Nature of magnetism and transport in BxCyNz thin films: The intriguing role of nitrogen defects in the electronic structure

Boron carbonitride (BxCyNz) represents an interesting family of materials containing all light elements and two dimensional graphene like hybrid layers. Although rich literature exists on this peculiar material in chemically processed form, there are relatively fewer reports on device application-worthy thin film form. This form also offers a natural platform for basic studies on important aspects such as magnetism and transport, which have attracted attention in the context of ferromagnetism in doped and defect carbon systems. Thus, in this work we have grown thin films of this compound using Pulsed Laser Deposition (PLD) method, and investigated their magnetic and transport properties in their entirety along with the detailed electronic structure using various spectroscopic techniques like X-ray photoelectron spectroscopy (XPS), Valence Band Spectroscopy (VBS) and X-ray absorption near edge spectroscopy (XANES). In depth analysis of the typical role of dopants and defects, especially the prevalent nitrogen defects, is elucidated using Density Functional Theory (DFT) calculations to understand the experimental observations. A dramatic crossover in the transport mechanism of charge carriers is observed in this system with the change in doping level of specific nitrogen defects. A robust and high saturation magnetization is achieved in BCN films which is higher by almost hundred times as compared to that in similarly grown undoped carbon film. An anomalous transition and increase in coercive field is also observed at low temperature. A careful analysis brings out the intriguing role of specific nitrogen defects in defining the peculiar nature and concentration dependence of the physical properties of this system.

cond-mat.mtrl-sci↗

arXiv2018

Photocatalytic Activity of Phosphorene Derivatives: Coverage, Electronic, Optical and Excitonic properties

In the context of two-dimensional metal-free photocatalyst, we investigate the electronic, optical and excitonic properties of phosphorene derivatives within first-principles approach. While two-dimensional phosphorene does not catalyze the complete water splitting reactions, O, S, and N coverages improve the situation drastically, and become susceptible to catalyze the complete reaction at certain coverages. We find that for all these dopants, 0.25 -- 0.5 ML coverages are thermodynamically more stable, and does not introduce midgap defect states and the composite systems remain semiconducting along with properly aligned valance and conduction bands. Further, within visible light excitation, the optical absorption remain very high 10$^5$ cm$^{-1}$ in these composite systems, and the fundamental optical anisotropy of phosphorene remains intact. We also investigate the effect of layer thickness through bilayer phosphorene with oxygen coverages. Finally we investigate the excitonic properties in these composite materials that are conducive to both redox reactions. The present results will open up new avenues to take advantage of these metal-free phosphorene derivatives toward its outstanding potential in photocatalysis.

cond-mat.mtrl-sci↗

arXiv2018

Gate-dependent vacancy diffusion in graphene

Kinetics of vacancy defect in graphene drives structural modifications leading to disorder, multi-vacancy complex and edge reconstruction. Within the first-principles calculations, we study the dynamic Jahn-Teller distortion and diffusion of a vacancy defect. Further, the intricate dependence of carrier doping is systematically investigated. The experimental observation of dynamic Jahn-Teller distortion is argued to be blocked by defect functionalization and charge doping. We demonstrate that lattice relaxation perpendicular to the graphene sheet along with the in-plane strain relaxation plays predominant roles in predicting the correct microscopic mechanism for vacancy diffusion. The importance of quantum correction to the classical barrier is discussed. The calculated activation barrier increases upon both electron and hole doping and the observed trends are explained by the differential charge density distribution and hardening of the responsible low-energy phonon modes. Electron doping essentially freezes the vacancy motion, and thus any degradation mediated by it. While tracking and analyzing the vacancy diffusion experimentally in graphene is a difficult task, the present results will motivate new experimental efforts and assist interpretation of the results.

cond-mat.mes-hall↗

arXiv2018

Engineering Kondo state in two-dimensional semiconducting phosphorene

Correlated interaction between dilute localized impurity electrons with the itinerant host conduction electrons in metals gives rise to the conventional many-body Kondo effect below sufficiently low temperature. In sharp contrast to these conventional Kondo systems, we report an intrinsic, robust and high-temperature Kondo state in two-dimensional semiconducting phosphorene. While absorbed at a thermodynamically stable lattice defect, Cr impurity triggers an electronic phase transition in phosphorene to provide conduction electrons, which strongly interact with the localized moment generated at the Cr site. These manifests into intrinsic Kondo state, where the impurity moment is quenched at multi-stage and at temperatures in the 40-200 K range. Further, along with a much smaller extension of Kondo cloud, the predicted Kondo state is shown to be robust under uniaxial strain and layer thickness, which greatly simplifies its future experimental realization. We predict the present study will open up new avenues in Kondo physics and trigger further theoretical and experimental studies.

cond-mat.mes-hall↗

arXiv2016

Transition Metal and Vacancy Defect Complexes in Phosphorene: A Spintronic Perspective

Inducing magnetic moment in otherwise nonmagnetic two-dimensional semiconducting materials is the key first step to design spintronic materials. Here, we study the absorption of transition-metals on pristine and defected single-layer phosphorene, within density functional theory. We predict that increased transition-metal diffusivity on pristine phosphorene would hinder any possibility of controlled magnetism, and thus any application. In contrast, the point-defects will anchor metals, and exponentially reduce the diffusivity. Similar to other two-dimensional materials, metals bind strongly on both pristine and defected phosphorene, and we provide a microscopic description of bonding, which explain the qualitative trend with increasing number of valence electrons. We further argue that the divacancy complex is imperative in any practical purpose due to their increased thermodynamic stability over monovacancy. For most cases, the defect-transition metal complexes retain the intrinsic semiconduction properties, and also induce a local magnetic moment with large exchange-splitting and spin-flip energies, which are necessary for spintronic applications. Specifically, the V/Mn/Fe absorbed at the divacancy have tremendous promise in this regard. Further, we provide a simple microscopic model to describe the local moment formation in these transition metal and defect complexes.

cond-mat.mes-hall↗