arXiv subjects

Pratibha Dev

Publications and source records attributed to Pratibha Dev.

At least 19 recordsLinked to original sources

arXiv2026

Hidden in Plain Sight: Aromaticity of Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) and graphene are similar in many ways - they are isoelectronic, have the same structure, are chemically inert and show persistence. All of these properties are indicators of a deeper connection that has, thus far, been overlooked. Unlike graphene, which has been shown to be aromatic, it is not known if hBN is aromatic. In this density functional theory-based work, we investigate the aromaticity (or lack thereof ) of hBN. By employing the magnetic criterion, supported by group theoretic and energetic considerations, we show that hexagonal boron nitride is indeed aromatic, even if weakly so, as compared to graphene. Since aromaticity is used to understand physical and chemical properties of planar compounds, the picture developed in this work is important to bridging the gap between the physical and chemical understanding of hBN's properties.

cond-mat.mtrl-sci↗

arXiv2025

Tuning spin currents in collinear antiferromagnets and altermagnets

Spin current generation through non-relativistic spin splittings, found in uncompensated magnets and d-wave altermagnets, is desirable for low-power spintronics. Such spin currents, however, are symmetry forbidden in conventional collinear antiferromagnets and higher-order altermagnets. Using spin point group analysis, we demonstrate that finite spin currents can be induced in these materials via magnetoelectric, piezomagnetic, and piezomagnetoelectric-like couplings. We utilize electric fields, strain, and their combinations to drive symmetry-lowering phase transitions into uncompensated magnetic or d-wave altermagnetic states, thereby enabling finite spin conductivity in a broader class of magnetic materials. We further substantiate this framework using density functional theory and Boltzmann transport calculations on representative magnetic materials - KV2Se2O, RuF4 , Cr2O3 , FeS2 , and MnPSe3 - spanning these different cases. The charge-to-spin conversion ratio reaches up to almost 100% via uncompensated magnetism and about 40% via d-wave altermagnetism under realistic conditions, highlighting the effectiveness of this approach for efficient spin current generation.

cond-mat.mtrl-sci↗

arXiv2025

The influence of nitrogen doping and annealing on the silicon vacancy in 4H-SiC

The silicon vacancy ($V_{Si}$) in 4H-SiC at its cubic site (V2-center) has shown significant promise for quantum technologies, due to coherent spin states, the mature material system, and stable optical emission. In these SiC-based applications, doping plays a crucial role. It can be used to control the charge state of $V_{Si}$ and formation of different types of defects. Despite its importance, there has been little research on the effects of doping. In this work, we perform a study of the effects of nitrogen doping and annealing on the photoluminescence (PL), optically-detected magnetic resonance (ODMR) contrast, and dephasing times of ensembles of V2 in epilayers of 4H-SiC. The results show an enhancement of PL that depends on the electron irradiation dose for a given electron concentration, supported by theoretical modeling of the charge state of $V_{Si}$ in the presence of nitrogen. Nitrogen substituted for carbon is shown to very efficiently donate one electron to $V_{Si}$. We also observe that the ODMR contrast can be increased from 0.5% in low doped SiC to 1.5% by nitrogen doping of $10^{17}$ to $10^{18}$ cm$^{-3}$ and annealing at 500-600 $^{\circ}$C for 1 hour, with only a 20% decrease in PL compared to unannealed. Some of the improvement in contrast is offset by a reduction in $T_2^*$ at these doping levels, but the estimated cw ODMR shot-noise limited sensitivity is still 1.6 times higher than that of undoped, unannealed SiC.

cond-mat.other↗

arXiv2025

Spontaneous Anomalous Hall Effect in Two-Dimensional Altermagnets

The anomalous Hall effect (AHE) is an efficient tool for detecting the N\'eel vector in collinear compensated magnets with spin-split bands, known as altermagnets (AMs). Here, we establish design principles for obtaining non-zero anomalous Hall conductivity in the recently proposed two-dimensional (2D) AMs using spin and magnetic group symmetry analysis. We show that only two of the seven nontrivial spin layer groups exhibit an unconventional in-plane AHE in which the N\'eel vector lies within the plane of the Hall current. Through first-principles simulations on bilayers of MnPSe$_3$ and MnSe, we demonstrate the validity of our group theoretic framework for obtaining AHE with $d$ and $i$-wave altermagnetic orders, depending on the stacking of the bilayers. We find that the spin group symmetry is successful in determining the linear and cubic dependence of anomalous Hall conductivity in N\'eel vector space, although AHE is a relativistic effect. This work shows that the AHE in 2D AMs can probe the altermagnetic order and N\'eel vector reversal, thereby facilitating the miniaturization of altermagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Robust Aluminum Nitride Passivation of Silicon Carbide with Near-Surface Quantum Emitters for Quantum Computing and Sensing Applications

Silicon carbide (SiC) hosts a number of point defects that are being explored as single-photon emitters for quantum applications. Unfortunately, these quantum emitters lose their photostability when placed in proximity to the surface of the host semiconductor. In principle, a uniform passivation of the surface's dangling bonds by simple adsorbates, such as hydrogen or mixed hydrogen/hydroxyl groups, should remove detrimental surface effects. However, the usefulness of atomic and molecular passivation schemes is limited by their lack of long-term chemical and/or thermal stability. In this first principles work, we use aluminum nitride (AlN) to passivate SiC surfaces in a core-shell nanowire model. By using a negatively charged silicon vacancy in SiC as the proof-of-principle quantum emitter, we show that AlN-passivation is effective in removing SiC surface states from the band gap and in restoring the defect's optical properties. We also report the existence of a silicon vacancy-based defect at the SiC-AlN interface, which displays distinct spin and optical properties as compared to the other well-studied defects in SiC.

cond-mat.mtrl-sci↗

arXiv2024

Optimum surface-passivation schemes for near-surface spin defects in silicon carbide

Spin-active defects in silicon carbide (SiC) are promising quantum light sources for realizing scalable quantum technologies. In different applications, these photoluminescent defects are often placed in a nanostructured host or close to surfaces in order to enhance the signal from the defects. However, proximity to the surface not only modifies frequencies of the quantum emission from the defect, but also adversely affects their photo-stability, resulting in blinking and/or photobleaching of the defect. These effects can be ameliorated by passivating surfaces with optimal adsorbates. In this work, we explore different passivation schemes using density functional theory-based calculations. We show that a uniform surface passivation with either hydrogen or with mixed hydrogen/hydroxyl groups completely removes surface states from the SiC band gap, restoring the optical properties of the defects.

cond-mat.mtrl-sci↗

arXiv2023

Spintronic Quantum Phase Transition in a $Graphene/Pb_{0.24}Sn_{0.76}Te$ Heterostructure with Giant Rashba Spin-Orbit Coupling

Mechanical stacking of two dissimilar materials often has surprising consequences for heterostructure behavior. In particular, a two-dimensional electron gas (2DEG) is formed in the heterostructure of the topological crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar with a nonpolar surface and the resulting changes in electronic structure needed to avoid polar catastrophe. We study the spintronic properties of this heterostructure with non-local spin valve devices. We observe spin-momentum locking at lower temperatures that transitions to regular spin channel transport only at ~40 K. Hanle spin precession measurements show a spin relaxation time as high as 2.18 ns. Density functional theory calculations confirm that the spin-momentum locking is due to a giant Rashba effect in the material and that the phase transition is a Lifshitz transition. The theoretically predicted Lifshitz transition is further evident in the phase transition-like behavior in the Land\'e g-factor and spin relaxation time.

cond-mat.mes-hall↗

arXiv2022

Magnetotransport in graphene/Pb0.24Sn0.76Te heterostructures: finding a way to avoid catastrophe

While heterostructures are ubiquitous tools enabling new physics and device functionalities, the palette of available materials has never been richer. Combinations of two emerging material classes, two-dimensional materials and topological materials, are particularly promising because of the wide range of possible permutations that are easily accessible. Individually, both graphene and Pb0.24Sn0.76Te (PST) are widely investigated for spintronic applications because graphene's high carrier mobility and PST's topologically protected surface states are attractive platforms for spin transport. Here, we combine monolayer graphene with PST and demonstrate a hybrid system with properties enhanced relative to the constituent parts. Using magnetotransport measurements, we find carrier mobilities up to 20,000 cm2/Vs and a magnetoresistance approaching 100 percent, greater than either material prior to stacking. We also establish that there are two distinct transport channels and determine a lower bound on the spin relaxation time of 4.5 ps. The results can be explained using the polar catastrophe model, whereby a high mobility interface state results from a reconfiguration of charge due to a polar/non-polar interface interaction. Our results suggest that proximity induced interface states with hybrid properties can be added to the still growing list of remarkable behaviors in these novel materials.

cond-mat.mtrl-sci↗

arXiv2022

Non-linear hybrid surface-defect states in defective Bi$_2$Se$_3$

Surface-states of topological insulators are assumed to be robust against non-magnetic defects in the crystal. However, recent theoretical models and experiments indicate that even non-magnetic defects can perturb these states. Our first-principles calculations demonstrate that the presence of Se vacancies in Bi$_2$Se$_3$, has a greater impact than a mere n-doping of the structure, which would just shift the Fermi level relative to the Dirac point. We observe the emergence of a non-linear band pinned near the Fermi level, while the Dirac cone shifts deeper into the valence band. We attribute these features in the bandstructure to the interaction between the surface and defect states, with the resulting hybridization between these states itself depending on the position and symmetry of the Se vacancy relative to the surfaces. Our results bring us a step closer to understanding the exotic physics emerging from defects in Bi$_2$Se$_3$ that remained unexplored in prior studies.

cond-mat.mtrl-sci↗

arXiv2021

Site-dependent properties of quantum emitters in nanostructured silicon carbide

Deep defects in silicon carbide (SiC) possess atom-like electronic, spin and optical properties, making them ideal for quantum-computing and -sensing applications. In these applications, deep defects are often placed within fabricated nanostructures that modify defect properties due to surface and quantum confinement effects. Thus far, theoretical studies exploring deep defects in SiC have ignored these effects. Using density functional theory, this work demonstrates site-dependence of properties of bright, negatively-charged silicon monovacancies within a SiC nanowire. It is shown that the optical properties of defects depend strongly on the hybridization of the defect states with the surface states and on the structural changes allowed by proximity to the surfaces. Additionally, the analysis of the first principles results indicates that the charge-state conversion and/or migration to thermodynamically-favorable undercoordinated surface sites can deteriorate deep-defect properties. These results illustrate the importance of considering how finite-size effects tune defect properties, and of creating mitigating protocols to ensure a defect's charge-state stability within nanostructured hosts.

cond-mat.mtrl-sci↗

arXiv2021

Doping-limitations of cubic boron nitride: effects of unintentional defects on shallow doping

Cubic boron nitride (cBN) is an ultra-wide bandgap, super-hard material with potential for extreme-temperature and -pressure applications. A proof-of-principle p-n junction using cBN was demonstrated almost three decades ago. However, to date, there remain two unresolved challenges that prevent its practical use in technologies: (i) it is difficult to produce high-quality cBN films and (ii) it is difficult to controllably n- and p-dope its matrix. In this theoretical work, we study the reasons for doping-limitations, which is an acute issue in realizing cBN-based electronics. In particular, we find that different unintentionally-present intrinsic and extrinsic defects act as compensating defects and/or introduce trap states. In turn, the presence of these defects and their complexes affect the incorporation, as well as the electronic structure properties, of shallow dopants [silicon and beryllium], which are introduced intentionally to n- and p-dope cBN. Our analysis of doping-limitations provides a path towards finding solutions for controllably n- and p-doping cBN.

cond-mat.mtrl-sci↗

arXiv2021

Predicting phase preferences of two-dimensional transition metal dichalcogenides using machine learning

Two-dimensional transition metal dichalcogenides (TMDs) can adopt one of several possible structures, with the most common being the trigonal prismatic and octahedral symmetry phases. Since the structure determines the electronic properties, being able to predict phase-preferences of TMDs from just the knowledge of the constituent atoms is highly desired, but has remained a long-standing problem. In this study, we applied high-throughput quantum mechanical computations with machine learning algorithms to solve this old problem. Our analysis provides insights into determining physiochemical factors that dictate the phase-preference of a TMD, identifying and going beyond the attributes considered by earlier researchers in predicting crystal structures. A knowledge of these underlying physiochemical factors not only helps us to rationalize, but also to accurately predict structural preferences. We show that machine learning algorithms are powerful tools that can be used not only to find new materials with targeted properties, but also to find connections between elemental attributes and the target property/properties that were not previously obvious.

cond-mat.mtrl-sci↗

arXiv2020

Defect-induced $4p$-magnetism in layered platinum diselenide

Platinum diselenide (PtSe$_{2}$) is a recently-discovered extrinsic magnet, with its magnetism attributed to the presence of Pt-vacancies. The host material to these defects itself displays interesting structural and electronic properties, some of which stem from an unusually strong interaction between its layers. To date, it is not clear how the unique intrinsic properties of PtSe$_2$ will affect its induced magnetism. In this theoretical work, we show that the defect-induced magnetism in PtSe$_{2}$ thin films is highly sensitive to: (i) the layer-thickness (ii) defect density, and (iii) substrate choice. These different factors dramatically modify all magnetic properties, including the magnitude of local moments, strength of the coupling, and even nature of the coupling between the moments. We further show that the strong inter-layer interactions are key to understanding these effects. A better understanding of the various influences on magnetism, can enable controllable tuning of the magnetic properties in Pt-based dichalcogenides, which can be used to design novel devices for magnetoelectric and magneto-optic applications.

cond-mat.mtrl-sci↗

arXiv2020

A new class of intrinsic magnet: two-dimensional yttrium sulphur selenide

Exploring and controlling magnetism in two-dimensional (2D) layered magnetic crystals, as well as their inclusion in heterogeneous assemblies, provide an unprecedented opportunity for fundamental science and technology. To date, however, there are only a few known intrinsic 2D magnets. Here we predict a novel 2D intrinsic magnet, yttrium sulfur selenide (YSSe), using first-principles calculations. The magnetism of this transition metal dichalcogenide originates from the partially-filled $3p$- and $4p$-orbitals of the chalcogens, unlike other known intrinsic magnets where magnetism arises from the partially-filled $3d$- and $4f$-orbitals. The unconventional magnetism in YSSe is a result of a unique combination of its structural and electronic properties. We further show that a lack of mirror symmetry results in piezoelectric properties, while the broken space- and time-symmetry ensures valley polarization. YSSe is a rare magnetic-piezoelectric material that can enable novel spintronics, valleytronics, and quantum technologies.

cond-mat.mtrl-sci↗

arXiv2020

A multiconfigurational study of the negatively charged nitrogen-vacancy center in diamond

Deep defects in wide band gap semiconductors have emerged as leading qubit candidates for realizing quantum sensing and information applications. Due to the spatial localization of the defect states, these deep defects can be considered as artificial atoms/molecules in a solid state matrix. Here we show that unlike single-particle treatments, the multiconfigurational quantum chemistry methods, traditionally reserved for atoms/molecules, accurately describe the many-body characteristics of the electronic states of these defect centers and correctly predict properties that single-particle treatments fail to obtain. We choose the negatively charged nitrogen-vacancy (NV$^-$) center in diamond as the prototype defect to study with these techniques due to its importance for quantum information applications and because its properties are well-known, which makes it an ideal benchmark system. By properly accounting for electron correlations and including spin-orbit coupling and dipolar spin-spin coupling in the quantum chemistry calculations, for the NV$^-$ center in diamond clusters, we are able to: (i) show the correct splitting of the ground (first-excited) triplet state into two levels (four levels), (ii) calculate zero-field splitting values of the ground and excited triplet states, in good agreement with experiment, and (iii) calculate the energy differences between ground and exited spin-triplet and spin-singlet states, as well as their ordering, which are also found to be in good agreement with recent experimental data. The numerical procedure we have developed is general and it can screen other color centers whose properties are not well known but promising for applications.

quant-ph↗

arXiv2020

Thickness-dependence of hydrogen-induced phase transition in MoTe$_{2}$

Two-dimensional transition metal dichalcogenides (TMDs) usually exist in two or more structural phases with different physical properties, and can be repeatedly switched between these phases via different stimuli, making them potentially useful for memory devices. An understanding of the physics of interfaces between the TMDs and conventional semiconductors, or other 2D-crystals forming heterogenous or homogeneous assemblies is central to their successful application in technologies. However, to date, most theoretical works have explored phase-change properties of isolated TMD monolayers in vacuum. Using \textit{ab-initio} calculations, we show how interfacial effects modify the thermodynamics and kinetics of the phase transition by studying hydrogen-induced transitions in monolayers and bilayers of MoTe$_{2}$. The phase-change properties of MoTe$_{2}$ show substantial thickness-dependence, with the timescale for a transition in the hydrogenated bilayer being about $10^7$-times longer than that in a monolayer at room temperature. Our study highlights the importance of taking effects of immediate environment into account when predicting properties of 2D crystals.

cond-mat.mtrl-sci↗

arXiv2020

Quantum materials interfaces: graphene/Bismuth (111) heterostructures

Heterostructures involving graphene and bismuth, with their ability to absorb light over a very wide energy range, are of interest for engineering next-generation opto-electronics. Critical to the technological application of such heterostructures is an understanding of the underlying physics governing their properties. Here, using first-principles calculations, we study the interfacial interactions between graphene and bismuth thin-films. Our study reveals non-intuitive phenomena associated with the moiré-physics of these superlattices. We show a preservation of graphene-derived Dirac cones in spite of proximity to a substrate with large spin-orbit coupling, a greater influence of graphene on the electronic structure properties of bismuth, and the surprising presence of a magnetic solution, only slightly higher in energy (by several meV) than the non-magnetic structure, possibly validating experiments. Such subtle and unanticipated phenomena associated with the moiré-physics are expected to play key roles in the practical applications of heterogeneous assemblies of two-dimensional quantum systems.

cond-mat.mtrl-sci↗

arXiv2020

Fingerprinting quantum emitters in hexagonal boron nitride using strain

Two dimensional van der Waals crystals and their heterostructures provide an exciting alternative to bulk wide bandgap semiconductors as hosts of single photon emitters. Amongst different layered materials, bright and robust defect-based single photon emitters have been observed within hexagonal boron nitride, a layered wide-bandgap semiconductor. Despite research efforts to date, the identities of the deep defects responsible for quantum emissions in hexagonal boron nitride remain unknown. In this theoretical work, I demonstrate that the strain-induced changes in emission frequencies depend on: (i) the detailed nature of the defect states involved in the optical excitations, and (ii) the rich boron chemistry that results in complex interactions between boron atoms. As each defect shows a distinct response to the strain, it can be used not only to tune emission frequencies, but also to identify the quantum emitters in hexagonal boron nitride.

cond-mat.mtrl-sci↗