arXiv subjects

Shoaib Khalid

Publications and source records attributed to Shoaib Khalid.

18 recordsLinked to original sources

arXiv2026

Effects of uniaxial strain on monolayer transition-metal dichalcogenides revisited

Using hybrid density functional calculations including spin-orbit coupling, we compute the strain evolution of the band structure of monolayer 1H-phase transition-metal dichalcogenides, MX$_2$ (M= Mo, W; X= S, Se, Te), emphasizing an accurate reproduction of the quasiparticle band gap (as opposed to the excitonic optical gap). We show that tensile uniaxial strain applied along either the armchair or zigzag directions leads to a pronounced reduction of the fundamental gap, with the conduction-band edge generally exhibiting the stronger strain response. Both the conduction-band electron valleys (CBM) and the valence-band hole valleys (VBM) remain degenerate under uniaxial strain, while simultaneously drifting away from the high-symmetry $K$ point under strain ("valley drift"), such that the band extrema occur at nearby off-symmetry wave vectors. A minimal tight-binding model rationalizes the valley drift and the unequal electron- and hole-valley drift rates in the presence of strain, leading to indirect band gaps. In particular, for MoS$_2$ the indirectness increases with tensile strain, providing a natural explanation for the experimentally observed decrease in photoluminescence intensity under uniaxial deformation. These results provide quantitative guidance for tailoring band structures for optoelectronic and quantum-defect applications.

cond-mat.mtrl-sci↗

arXiv2026

Transition Metal Dichalcogenide MoS${}_2$: oxygen and fluorine functionalization for selective plasma processing

Low-temperature plasma processing is a promising technique for tailoring transition metal dichalcogenides (TMDs). For chalcogen substitution processing, a key challenge is to identify the ion energy window that enables selective chalcogen removal while preserving the metal lattice. Using ab-initio molecular dynamics (AIMD), we demonstrate that oxygen and fluorine functionalization widen the processing window by significantly lowering the sulfur sputtering energy threshold ($E_{\text{sputt,S}}$) of MoS${}_2$ from $\sim 30$ eV to $\sim 10$ eV via formation of sputtering products such as SO${}_2$ and SF${}_n$. Additionally, we show that experimentally relevant cryogenic temperatures strongly affect $E_{\text{sputt,S}}$. The dependence is confirmed via AIMD and also predicted by a mechanistic parameter-free theory, suggesting that $E_{\text{sputt}}(T)$ generalizes to other TMDs, functionalization, and surface impacts in general. Our results highlight oxygen/fluorine functionalization, ionic impact angle, and material temperature to be key control parameters for selective, damage-controlled chalcogen removal in TMD processing.

cond-mat.mtrl-sci↗

arXiv2025

Weyl semimetal phases and intrinsic spin-Hall conductivity in SbAs ordered alloys

Using density functional theory calculations we investigated possible Weyl semimetal (WSM) phases in antimony arsenide ordered alloys Sb1-xAsx (x=0, 1/6, 1/3, 1/2, 2/3, 5/6, 1). We find WSM phases for all As compositions of Sb1-xAsx with broken inversion symmetry, in contrast to Bi1-xSbx where only compositions x=1/2 and 5/6 were predicted to exhibit WSM phases. The WSM phases in Sb1-xAsx are characterized by the presence of 12 Weyl points, located within 55 meV from the Fermi level in the case of x=1/2. The robust spin-orbit coupling strength and Berry curvature in these alloys produce large spin-Hall conductivity in the range of 176-602 (hbar/e)(S/cm), comparable to that in the BiSb alloys. Finally, Sb0.5As0.5 is predicted to be almost lattice-matched to GaAs(111), with the Fermi level within the gap of the semiconductor, facilitating growth and characterization, and thus, offering promising integration with conventional semiconductors.

cond-mat.mtrl-sci↗

arXiv2024

Band-gap reduction and band alignments of dilute bismide III--V alloys

Adding a few atomic percent of Bi to III--V semiconductors leads to significant changes in their electronic structure and optical properties. Bismuth substitution on the pnictogen site leads to a large increase in spin-orbit splitting $\Delta_{\rm SO}$ at the top of the valence band ($\Gamma_{8v}-\Gamma_{7v}$) and a large reduction in the band gap, creating unique opportunities in semiconductor device applications. Quantifying these changes is key to the design and simulation of electronic and optoelectronic devices. Using hybrid functional calculations, we predict the band gap of III--Vs (III=Al, Ga, In and V=As, Sb) with low concentrations of Bi (3.125\% and 6.25\%), the effects of adding Bi on the valence- and conduction-band edges, and the band offset between these dilute alloys and their III--V parent compounds. As expected, adding Bi raises the valence-band maximum (VBM). However, contrary to previous assumptions, the conduction-band minimum (CBM) is also significantly lowered, and both effects contribute to the sizable band-gap reduction. Changes in band gap and $\Delta_{\rm SO}$ are notably larger in the arsenides than in the antimonides. We also predict cases of band-gap inversion ($\Gamma_{6c}$ below $\Gamma_{8v}$) and $\Delta_{\rm SO}$ larger than the band gap, which are key parameters for designing topological materials and for minimizing losses due to Auger recombination in infrared lasers.

cond-mat.mtrl-sci↗

arXiv2024

Role of chalcogen vacancies and hydrogen in the optical and electrical properties of bulk transition-metal dichalcogenides

Like in any other semiconductor, point defects in transition-metal dichalcogenides (TMDs) are expected to strongly impact their electronic and optical properties. However, identifying defects in these layered two-dimensional materials has been quite challenging with controversial conclusions despite the extensive literature in the past decade. Using first-principles calculations, we revisit the role of chalcogen vacancies and hydrogen impurity in bulk TMDs, reporting formation energies and thermodynamic and optical transition levels. We show that the S vacancy can explain recently observed cathodoluminescence spectra of MoS$_2$ flakes and predict similar optical levels in the other TMDs. In the case of the H impurity, we find it more stable sitting on an interstitial site in the Mo plane, acting as a shallow donor, and possibly explaining the often observed n-type conductivity in some TMDs. We also predict the frequencies of the local vibration modes for the H impurity, aiding its identification through Raman or infrared spectroscopy.

cond-mat.mtrl-sci↗

arXiv2023

Large Rashba splittings in bulk and monolayer of BiAs

Two-dimensional materials with Rashba split bands near the Fermi level are key to developing upcoming next-generation spintronics. They enable generating, detecting, and manipulating spin currents without an external magnetic field. Here, we propose BiAs as a novel layered semiconductor with large Rashba splitting in bulk and monolayer forms. Using first-principles calculations, we determined the lowest energy structure of BiAs and its basic electronic properties. Bulk BiAs has a layered crystal structure with two atoms in a rhombohedral primitive cell, similar to the parent Bi and As elemental phases. It is a semiconductor with a narrow and indirect band gap. The spin-orbit coupling leads to Rashba-Dresselhaus spin splitting and characteristic spin texture around the L-point in the Brillouin zone of the hexagonal conventional unit cell, with Rashba energy and Rashba coupling constant for valence (conduction) band of $E_R$= 137 meV (93 meV) and $α_R$= 6.05 eVÅ~(4.6 eVÅ). In monolayer form (i.e., composed of a BiAs bilayer), BiAs has a much larger and direct band gap at $Γ$, with a circular spin texture characteristic of a pure Rashba effect. The Rashba energy $E_R$= 18 meV and Rashba coupling constant $α_R$= 1.67 eVÅ of monolayer BiAs are quite large compared to other known 2D materials, and these values are shown to increase under tensile biaxial strain.

cond-mat.mtrl-sci↗

arXiv2023

The deep-acceptor nature of the chalcogen vacancies in 2D transition-metal dichalcogenides

Chalcogen vacancies in the semiconducting monolayer transition-metal dichalcogenides (TMDs) have frequently been invoked to explain a wide range of phenomena, including both unintentional p-type and n-type conductivity, as well as sub-band gap defect levels measured via tunneling or optical spectroscopy. These conflicting interpretations of the deep versus shallow nature of the chalcogen vacancies are due in part to shortcomings in prior first-principles calculations of defects in the semiconducting two-dimensional (2D) TMDs that have been used to explain experimental observations. Here we report results of hybrid density functional calculations for the chalcogen vacancy in a series of monolayer TMDs, correctly referencing the thermodynamic charge transition levels to the fundamental band gap (as opposed to the optical band gap). We find that the chalcogen vacancies are deep acceptors and cannot lead to n-type or p-type conductivity. Both the (0/$-1$) and ($-$1/$-$2) transition levels occur in the gap, leading to paramagnetic charge states S=1/2 and S=1, respectively, in a collinear-spin representation. We discuss trends in terms of the band alignments between the TMDs, which can serve as a guide to future experimental studies of vacancy behavior.

cond-mat.mtrl-sci↗

arXiv2023

Tuning the Band Topology of GdSb by Epitaxial Strain

Rare-earth monopnictide (RE-V) semimetal crystals subjected to hydrostatic pressure have shown interesting trends in magnetoresistance, magnetic ordering, and superconductivity, with theory predicting pressure-induced band inversion. Yet, thus far, there have been no direct experimental reports of interchanged band order in RE-Vs due to strain. This work studies the evolution of band topology in biaxially strained GdSb (001) epitaxial films using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). We find that biaxial strain continuously tunes the electronic structure from topologically trivial to nontrivial, reducing the gap between the hole and the electron bands dispersing along the [001] direction. The conduction and valence band shifts seen in DFT and ARPES measurements are explained by a tight-binding model that accounts for the orbital symmetry of each band. Finally, we discuss the effect of biaxial strain on carrier compensation and magnetic ordering temperature.

cond-mat.mtrl-sci↗

arXiv2023

Inversion Symmetry Breaking in Epitaxial Ultrathin Bi (111) Films

Bismuth (Bi) films hold potential for spintronic devices and topological one-dimensional edge transport. Large-area high-quality (111) Bi ultrathin films are grown on InSb (111)B substrates. Strong film-substrate interactions epitaxially stabilize the (111) orientation and lead to inversion symmetry breaking. We resolve the longstanding controversy over the Z_2 topological assignment of bismuth and show that the surface states are topologically trivial. Our results demonstrate that interfacial bonds prevent the semimetal-to-semiconductor transition predicted for freestanding bismuth layers, highlighting the importance of controlled functionalization and surface passivation in two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2022

Epitaxial growth, magnetoresistance, and electronic band structure of GdSb magnetic semimetal films

Motivated by observations of extreme magnetoresistance (XMR) in bulk crystals of rare-earth monopnictide (RE-V) compounds and emerging applications in novel spintronic and plasmonic devices based on thin-film semimetals, we have investigated the electronic band structure and transport behavior of epitaxial GdSb thin films grown on III-V semiconductor surfaces. The Gd3+ ion in GdSb has a high spin S=7/2 and no orbital angular momentum, serving as a model system for studying the effects of antiferromagnetic order and strong exchange coupling on the resulting Fermi surface and magnetotransport properties of RE-Vs. We present a surface and structural characterization study mapping the optimal synthesis window of thin epitaxial GdSb films grown on III-V lattice-matched buffer layers via molecular beam epitaxy. To determine the factors limiting XMR in RE-V thin films and provide a benchmark for band structure predictions of topological phases of RE-Vs, the electronic band structure of GdSb thin films is studied, comparing carrier densities extracted from magnetotransport, angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. ARPES shows hole-carrier rich topologically-trivial semi-metallic band structure close to complete electron-hole compensation, with quantum confinement effects in the thin films observed through the presence of quantum well states. DFT predicted Fermi wavevectors are in excellent agreement with values obtained from quantum oscillations observed in magnetic field-dependent resistivity measurements. An electron-rich Hall coefficient is measured despite the higher hole carrier density, attributed to the higher electron Hall mobility. The carrier mobilities are limited by surface and interface scattering, resulting in lower magnetoresistance than that measured for bulk crystals.

cond-mat.mtrl-sci↗

arXiv2022

Defect engineering and Fermi-level tuning in half-Heusler topological semimetals

Three-dimensional topological semimetals host a range of interesting quantum phenomena related to band crossing that give rise to Dirac or Weyl fermions, and can be potentially engineered into novel quantum devices. Harvesting the full potential of these materials will depend on our ability to position the Fermi level near the symmetry-protected band crossings so that their exotic spin and charge transport properties become prominent in the devices. Recent experiments on bulk and thin films of topological half-Heuslers show that the Fermi level is far from the symmetry-protected crossings, leading to strong interference from bulk bands in the observation of topologically protected surface states. Using density functional theory calculations we explore how intrinsic defects can be used to tune the Fermi level in the two representative half-Heusler topological semimetals PtLuSb and PtLuBi. Our results explain recent results of Hall and angle-resolved photoemission measurements. The calculations show that Pt vacancies are the most abundant intrinsic defects in these materials grown under typical growth conditions, and that these defects lead to excess hole densities that place the Fermi level significantly below the expected position in the pristine material. Directions for tuning the Fermi level by tuning chemical potentials are addressed.

cond-mat.mtrl-sci↗

arXiv2022

Electron Identification at CMS detector using LHC data

The Standard Model of particle physics is extremely well tested and yet is not believed to be a theory of everything. Many extensions of the Standard Model predict the existence of new particles. The Large Hadron Collider (LHC) is a high energy proton-proton collider which may be able to produce such particles. In this work strategies have been developed to search for new physics by identifying high energy electrons produced by the decay of W boson using 2.8 pb-1 of real data. The electron identification for CMS has been presented. Simple cut based selections and a complete set of variables to distinguish between real electrons and background electrons are described. The detection of electrons is of particular importance at LHC as these particles intervene in the flagship H -> ZZ* -> 4e and H -> WW* -> 2e2v channels for the search of the Standard Model (SM) Higgs. Electrons are also important in many SUSY scenarios as produced in the leptonic decays of charginos and neutralinos. Electrons also appears in searches for TeV resonances that may come from new symmetries or as consequences of scenarios involving extra spatial dimensions. Last but not the least, electrons appear in the final state of many Standard Model processes involving top quarks or electroweak bosons, that constitute backgrounds to new signals or are intended to be used as calibration processes. In second part of my thesis background sources for electrons coming from W bosons have been studied. Electron identification variables are plotted in different regions of background, to study the background as deeply as possible. This work has been done for the first time using LHC's real data.

physics.ins-det↗

arXiv2022

Controlling magnetoresistance by tuning semimetallicity through dimensional confinement and heteroepitaxy

Controlling the electronic properties via bandstructure engineering is at the heart of modern semiconductor devices. Here, we extend this concept to semimetals where, utilizing LuSb as a model system, we show that quantum confinement lifts carrier compensation and differentially affects the mobility of the electron and hole-like carriers resulting in a strong modification in its large, non-saturating magnetoresistance behavior. Bonding mismatch at the heteroepitaxial interface of a semimetal (LuSb) and a semiconductor (GaSb) leads to the emergence of a novel, two-dimensional, interfacial hole gas and is accompanied by a charge transfer across the interface that provides another avenue to modify the electronic structure and magnetotransport properties in the ultra-thin limit. Our work lays out a general strategy of utilizing confined thin film geometries and heteroepitaxial interfaces to engineer electronic structure in semimetallic systems, which allows control over their magnetoresistance behavior and simultaneously, provides insights into its origin.

cond-mat.mtrl-sci↗

arXiv2022

Identifying the fingerprints of topological states by tuning magnetoresistance in a semimetal: the case of topological half-Heusler Pt1-xAuxLuSb

Topological materials often exhibit remarkably linear, non-saturating magnetoresistance (LMR), which is both of scientific and technological importance. However, the role of topologically non-trivial states in the emergence of such a behaviour has eluded clear demonstration in experiments. Here, by reducing the coupling between the topological surface states (TSS) and the bulk carriers we controllably tune the LMR behavior in Pt1-xAuxLuSb into distinct plateaus in Hall resistance, which we show arise from a quantum Hall phase. This allowed us to reveal how smearing of the Landau levels, which otherwise give rise to a quantum Hall phase, results in an LMR behavior due to strong interaction between the TSS with a positive g-factor and the bulk carriers. We establish that controlling the coupling strength between the surface and the bulk carriers in topological materials can bring about dramatic changes in their magnetotransport behavior. In addition, our work outlines a strategy to reveal macroscopic physical observables of TSS in compounds with a semi-metallic bulk band structure, as is the case in multi-functional Heusler compounds, thereby opening up opportunities for their utilization in hybrid quantum structures.

cond-mat.mtrl-sci↗

arXiv2021

Electronic properties of the Weyl semimetals Co$_2$MnX (X=Si, Ge, Sn)

Using first-principles electronic structure calculations, we show that ferromagnetic Heusler compounds Co$_2$MnX (X= Si, Ge, Sn) present non-trivial topological characteristics and belong to the category of Weyl semimetals. These materials exhibit two topologically interesting band crossings near the Fermi level. These band crossings have complex 3D geometries in the Brillouin zone and are characterized by non-trivial topology as Hopf links and chain-like nodal lines, that are protected by the perpendicular mirror planes. The spin-orbit interaction split these nodal lines into several zero-dimensional Weyl band crossings. Unlike previously known topologically non-trivial Heusler materials, these majority-spin band crossings lie in the band gap of minority spin bands, potentially facilitating its experimental realization.

cond-mat.mtrl-sci↗

arXiv2020

Trivial to nontrivial topology transition in rare-earth pnictides with epitaxial strain

The combination of magneto-transport and topological properties has brought great attention to rare-earth mono-pnictides semimetals. For some of them, like LaSb, it is unclear whether they show non-trivial topology or not based on density functional theory calculations and angular resolved photoemission spectroscopy measurements. Here, we use hybrid density functional theory to demonstrate that LaSb is in fact a trivial topological semimetal, in agreement with experiments, but on the verge of a transition to a topological phase. We show that under compressive epitaxial strain, the La $d$ band crosses the Sb $p$ band near the X$_3$ point in the Brillouin zone, stabilizing a topologically non-trivial phase, opening unique opportunities to probe the inter-relation between magneto-transport properties and the effects of band topology by examining epitaxially strained and unstrained thin films of the same material.

cond-mat.mtrl-sci↗

arXiv2019

Hybrid functional calculations of electronic structure and carrier densities in rare-earth monopnictides

The structural parameters and electronic structure of rare-earth pnictides are calculated using density functional theory (DFT) with the Heyd, Scuseria, and Ernzerhof (HSE06) screened hybrid functional. We focus on RE-V compounds, with RE=La, Gd, Er, and Lu, and V=As, Sb, and Bi, and analyze the effects of spin-orbit coupling and treating the RE 4$f$ electrons as valence electrons in the projector augmented wave approach. The results of HSE06 calculations are compared with DFT within the generalized gradient approximation (DFT-GGA) and other previous calculations. We find that all these RE-V compounds are semimetals with electron pockets at the $X$ point and hole pockets at $Γ$. Whereas in DFT-GGA the carrier density is significantly overestimated, the computed carrier densities using HSE06 is in good agreement with the available experimental data.

cond-mat.mtrl-sci↗

arXiv2019

Weak antilocalization in quasi-two-dimensional electronic states of epitaxial LuSb thin films

Observation of large non-saturating magnetoresistance in rare-earth monopnictides has raised enormous interest in understanding the role of its electronic structure. Here, by a combination of molecular-beam epitaxy, low-temperature transport, angle-resolved photoemssion spectroscopy, and hybrid density functional theory we have unveiled the bandstructure of LuSb, where electron-hole compensation is identified as a mechanism responsible for large magnetoresistance in this topologically trivial compound. In contrast to bulk single crystal analogues, quasi-two-dimensional behavior is observed in our thin films for both electron and holelike carriers, indicative of dimensional confinement of the electronic states. Introduction of defects through growth parameter tuning results in the appearance of quantum interference effects at low temperatures, which has allowed us to identify the dominant inelastic scattering processes and elucidate the role of spin-orbit coupling. Our findings open up new possibilities of band structure engineering and control of transport properties in rare-earth monopnictides via epitaxial synthesis.

cond-mat.mtrl-sci↗