arXiv subjects

Darshana Wickramaratne

Publications and source records attributed to Darshana Wickramaratne.

At least 19 recordsLinked to original sources

arXiv2026

Charge-state control of carbon-related optical absorption in AlN

Sub-bandgap optical absorption in AlN between 2 eV and 4 eV is widely observed, but its microscopic origin remains contested. Using photo-induced electron paramagnetic resonance (photo-EPR) and optical absorption spectroscopy on the same samples, we demonstrate a correlation between this absorption band and the neutral charge state of substitutional carbon on the nitrogen site (C$_N$). Hybrid functional calculations of the optical absorption spectra show that a transition involving C$_N$ and the valence band occurs near 3.3 eV, which agrees well with a peak identified within the measured optical absorption between 2 eV and 4 eV. This conclusion requires the combined ability to manipulate the charge state of carbon using photo-EPR and to use first-principles calculations of the absorption line shape that account for the dispersion of the valence band and the energy dependence of the optical matrix elements.

cond-mat.mtrl-sci↗

arXiv2026

Self-compensation by silicon $DX$ centers in ultrawide-bandgap nitrides

\textit{DX} behavior limits $n$-type carrier concentrations in ultrawide-bandgap nitrides such as aluminum nitride (AlN) and cubic boron nitride ($c$-BN). Instead of acting as effective-mass donors, \textit{DX} centers capture two electrons, stabilizing a negative charge state that leads to self compensation. Silicon is the most effective $n$-type dopant in this class of materials; in AlN, its \textit{DX} level [(i.e., the (+/$-$) transition level] is $\sim$270 meV from the conduction-band minimum. This implies that many silicon impurities incorporated into AlN will be negatively charged and compensate the intended $n$-type doping. By combining density functional theory calculations of temperature-dependent band gaps and Si dopant transition levels, we show here that significant compensation occurs in silicon-doped AlN, even in the absence of any other defects. This compensation strongly limits free electron concentrations which become independent of doping concentration, and donor activation is only significant for light doping scenarios. Higher free carrier concentrations can be achieved in AlGaN alloys or in $c$-BN, where the \textit{DX} level sits closer to the conduction-band minimum.

cond-mat.mtrl-sci↗

arXiv2025

Effects of altermagnetic order, strain and doping in RuO$_2$

RuO$_2$, one of the most widely studied transition metal oxides, was recently predicted to host a novel form of collinear magnetic order referred to as altermagnetism. In this study we combine experiment (reflectance, transmittance, ellipsometry and Raman measurements) and first-principles calculations to elucidate the potential role of altermagnetic order, strain and doping on the optical and vibrational properties of RuO$_2$ grown on TiO$_2$ (001), (101) and (110) substrates. Our combined experimental and theoretical results point toward a nonmagnetic ground state as the most consistent description of the optical and vibrational properties of bulk RuO$_2$. RuO$_2$ strained to TiO$_2$ (001) remains nonmagnetic in our calculations. Straining to TiO$_2$ (110) stabilizes ferromagnetic and altermagnetic states that are nearly degenerate and both lower in energy than the nonmagnetic state. The relative energetic ordering of these states is highly sensitive to the level of strain and the choice of exchange-correlation approximation.

cond-mat.mtrl-sci↗

arXiv2024

Nonradiative quenching of EPR signals in germanium-doped AlGaN: evidence for DX-center formation

We present photo-electron paramagnetic resonance (EPR) measurements and first-principles calculations that indicate germanium (Ge) is a DX-center in AlGaN. Our photo-EPR measurements on Ge-doped AlGaN samples show no EPR spectra in the dark, while persistent EPR spectra is observed upon photoexcitation with photon energies greater than ~1.3 eV. Thermally annealing the samples decreased the EPR signal, with the critical temperature to quench the EPR signal being larger in the lower Al-content sample. Using detailed first-principles calculations of Ge in AlGaN, we show all of these observations can be explained by accounting for the DX configuration of Ge in AlGaN.

cond-mat.mtrl-sci↗

arXiv2024

Assessing the SCAN functional for deep defects and small polarons in wide-bandgap semiconductors and insulators

We find the recently developed strongly constrained and appropriately normed (SCAN) functional, now widely used in calculations of many materials, is not able to reliably describe the properties of deep defects and small polarons in a set of wide-bandgap semiconductors and insulators (ZnO, ZnSe, GaN, Ga$_2$O$_3$, and NaF). By comparing first-principles calculations using the SCAN functional against established experimental information and first-principles calculations using a hybrid functional, we find that the SCAN functional systematically underestimates the magnitude of the structural distortions at deep defects and tends to delocalize the charge density of these defect states.

cond-mat.mtrl-sci↗

arXiv2024

Direct evidence for carbon incorporation on the nitrogen site in AlN

We use photo-electron paramagnetic resonance (EPR) measurements and first-principles calculations to identify and explain the properties of carbon in AlN. We present clear evidence for carbon substitution on the nitrogen site (C$_{\rm N}$). We also clarify the origin of a widely observed EPR spectra in AlN that, although often attributed to a deep donor defect, we demonstrate is surprisingly due to C$_{\rm N}$. Finally, we show the presence of C$_{\rm N}$ is consistent with the absorption spectra at 4.7 eV observed in AlN.

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

Ising superconductivity: a first-principles perspective

The recent discovery of Ising superconductivity has garnered a lot of interest due in part to the resilience of these superconductors to large in-plane magnetic fields. In this Perspective we explain the basic concepts that define the behavior of Ising superconductors, provide an overview of the electronic structure and magnetic properties with a focus on NbSe$_2$, summarize key experimental observations that have been made in this class of superconductors, highlight the role that defects and proximity-induced effects at interfaces have on Ising superconductivity and finally discuss the prospects for observing Ising superconductivity in bulk materials.

cond-mat.supr-con↗

arXiv2023

Protection of Ising spin-orbit coupling in bulk misfit superconductors

Low-dimensional materials have remarkable properties that are distinct from their bulk counterparts. A paradigmatic example is Ising superconductivity that occurs in monolayer materials such as NbSe2 which show a strong violation of the Pauli limit. In monolayers, this occurs due to a combination of broken inversion symmetry and spin-orbit coupling that locks the spins of the electrons out-of-plane. Bulk NbSe2 is centrosymmetric and is therefore not an Ising superconductor. We show that bulk misfit compound superconductors, (LaSe)1.14(NbSe2) and (LaSe)1.14(NbSe2)2, comprised of monolayers and bilayers of NbSe2, exhibit unexpected Ising protection with a Pauli-limit violation comparable to monolayer NbSe2, despite formally having inversion symmetry. We study these misfit compounds using complementary experimental methods in combination with first-principles calculations. We propose theoretical mechanisms of how the Ising protection can survive in bulk materials. We show how some of these mechanisms operate in these bulk compounds due to a concerted effect of charge-transfer, defects, reduction of interlayer hopping, and stacking. This highlights how Ising superconductivity can, unexpectedly, arise in bulk materials, and possibly enable the design of bulk superconductors that are resilient to magnetic fields.

cond-mat.supr-con↗

arXiv2022

Nontrivial doping evolution of electronic properties in Ising-superconducting alloys

Transition metal dichalcogenides offer unprecedented versatility to engineer 2D materials with tailored properties to explore novel structural and electronic phase transitions. In this work, we present the atomic-scale evolution of the electronic ground state of a monolayer of Nb$_{1-δ}$Mo$_δ$Se$_2$ across the entire alloy composition range (0 < $δ$ < 1) using low-temperature (300 mK) scanning tunneling microscopy and spectroscopy (STM/STS). In particular, we investigate the atomic and electronic structure of this 2D alloy throughout the metal to semiconductor transition (monolayer NbSe$_2$ to MoSe$_2$). Our measurements let us extract the effective doping of Mo atoms, the bandgap evolution and the band shifts, which are monotonic with $δ$. Furthermore, we demonstrate that collective electronic phases (charge density wave and superconductivity) are remarkably robust against disorder. We further show that the superconducting TC changes non-monotonically with doping. This contrasting behavior in the normal and superconducting state is explained using first-principles calculations. We show that Mo doping decreases the density of states at the Fermi level and the magnitude of pair-breaking spin fluctuations as a function of Mo content. Our results paint a detailed picture of the electronic structure evolution in 2D TMD alloys, which is of utmost relevance for future 2D materials design.

cond-mat.mtrl-sci↗

arXiv2022

Laser-patterned submicron Bi2Se3-WS2 pixels with tunable circular polarization at room temperature

Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS2) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of x161), peak position (38.4meV range), circular polarization (39.4% range), and valley polarization of a Bi2Se3-WS2 2D heterostructure using a low-power laser (0.762uW) in ambient. Changes are spatially confined to the laser spot, enabling submicron (814nm) features, and are long-term stable (>334 days). PL and valley polarization changes can be controllably reversed through laser exposure in vacuum, allowing the material to be erased and reused. Atmospheric experiments and first-principles calculations indicate oxygen diffusion modulates the exciton radiative vs. non-radiative recombination pathways, where oxygen absorption leads to brightening, and desorption to darkening.

cond-mat.mtrl-sci↗

arXiv2022

Role of carbon and hydrogen in limiting $n$-type doping of monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$

We use hybrid density functional calculations to assess n-type doping in monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys. We focus on Si, the most promising donor dopant, and study the structural properties, formation energies and charge-state transition levels of its various configurations. We also explore the impact of C and H, which are common impurities in metal-organic chemical vapor deposition (MOCVD). In Ga$_2$O$_3$, Si$_{Ga}$ is an effective shallow donor, but in Al$_2O_3$ Si$_{Al}$ acts as a DX center with a (+/-) transition level in the band gap. Interstitial H acts as a shallow donor in Ga$_2$O$_3$, but behaves as a compensating acceptor in n-type Al$_2O_3$. Interpolation indicates that Si is an effective donor in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ up to 70% Al, but it can be compensated by H already at 1% Al. We also assess the diffusivity of H and study complex formation. Si$_{cation}$-H complexes have relatively low binding energies. Substitutional C on a cation site acts as a shallow donor in Ga$_2$O$_3$, but can be stable in a negative charge state in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ when x>5%. Substitutional C on an O site (C$_O$) always acts as an acceptor in n-type (Al$_x$Ga$_{1-x}$)$_2$O$_3$, but will incorporate only under relatively O-poor conditions. C$_O$-H complexes can actually incorporate more easily, explaining observations of C-related compensation in Ga$_2$O$_3$ grown by MOCVD. We also investigate C$_{cation}$-H complexes, finding they have high binding energies and act as compensating acceptors when x>56%; otherwise the H just passivates the unintentional C donors. C-H complex formation explains why MOCVD grown Ga$_2$O$_3$ can exhibit record-low free-carrier concentrations, in spite of the unavoidable incorporation of C. Our study highlights that, while Si is a suitable shallow donor in ALGO alloys, control of unintentional impurities is essential to avoid compensation.

cond-mat.mtrl-sci↗

arXiv2021

Metallic vs. Semiconducting Properties of Quasi-One-Dimensional Tantalum Selenide van der Waals Nanoribbons

We conducted a tip-enhanced Raman scattering spectroscopy (TERS) and photoluminescence (PL) study of quasi-1D TaSe3 nanoribbons exfoliated onto gold substrates. At a selenium deficiency of ~0.25 (Se/Ta=2.75,), the nanoribbons exhibit a strong, broad PL peak centered around ~920 nm (1.35 eV), suggesting their semiconducting behavior. Such nanoribbons revealed a strong TERS response under 785-nm laser excitation, allowing for their nanoscale spectroscopic imaging. Nanoribbons with a smaller selenium deficiency of ~0.15 (Se/Ta=2.85) did not show any PL or TERS response. The confocal Raman spectra of these samples agree with the previously-reported spectra of metallic TaSe3. The differences in the optical response of the nanoribbons examined in this study suggest that even small variations in Se content can induce changes in electronic structure, causing samples to exhibit either metallic or semiconducting character. The temperature-dependent electrical measurements of devices fabricated with both types of materials corroborate these observations. The density-functional-theory calculations revealed that incorporation of an oxygen atom in a Se vacancy can result in band gap opening and thus enable the transition from a metal to a semiconductor. However, the predicted bandgap is substantially smaller than that derived from PL data. These results indicate that the properties of van der Waals materials can vary significantly depending on stoichiometry, defect types and concentration, and possibly environmental and substrate effects. In view of this finding, local probing of nanoribbon properties with TERS becomes essential to understanding such low-dimensional systems.

cond-mat.mtrl-sci↗

arXiv2021

Stacking-Dependent Optical Properties in Bilayer WSe2

The twist angle between the monolayers in van der Waals heterostructures provides a new degree of freedom in tuning material properties. We compare the optical properties of WSe2 homobilayers with 2H and 3R stacking using photoluminescence, Raman spectroscopy, and reflectance contrast measurements under ambient and cryogenic temperatures. Clear stacking- dependent differences are evident for all temperatures, with both photoluminescence and reflectance contrast spectra exhibiting a blue shift in spectral features in 2H compared to 3R bilayers. Density functional theory (DFT) calculations elucidate the source of the variations and the fundamental differences between 2H and 3R stackings. DFT finds larger energies for both A and B excitonic features in 2H than in 3R, consistent with experimental results. In both stacking geometries, the intensity of the dominant A1g Raman mode exhibits significant changes as a function of laser excitation wavelength. These variations in intensity are intimately linked to the stacking- and temperature-dependent optical absorption through resonant enhancement effects. The strongest enhancement is achieved when the laser excitation coincides with the C excitonic feature, leading to the largest Raman intensity under 514 nm excitation in 2H stacking and at 520 nm in 3R stacked WSe2 bilayers.

cond-mat.mtrl-sci↗

arXiv2021

Ising superconductivity in monolayer niobium dichalcogenide alloys

NbSe$_{2}$ and NbS$_{2}$ are isostructural two-dimensional materials that exhibit contrasting superconducting properties when reduced to the single monolayer limit. Monolayer NbSe$_{2}$ is an Ising superconductor, while there have been no reports of superconductivity in monolayer NbS$_{2}$. NbS$_{x}$Se$_{2-x}$ alloys exhibit an intriguing non-monotonic dependence of the superconducting transition temperature with sulfur content, which has been interpreted as a manifestation of fractal superconductivity. However, several key questions about this result are not known: (1) Does the electronic structure of the alloy differ from the parent compounds, (2) Are spin fluctuations which have been shown to be prominent in monolayer NbSe$_{2}$ also present in the alloys? Using first-principles calculations, we show that the density of states at the Fermi level and the proximity to magnetism in NbS$_{x}$Se$_{2-x}$ alloys are both reduced compared to the parent compound; the former would decrease the transition temperature while the latter would increase it. We also show that Se vacancies, which are likely magnetic pair-breaking defects, may form in large concentrations in NbSe$_{2}$. Based on our results, we suggest an alternative explanation of the non-monotonic behavior the superconducting transition temperature in NbS$_{x}$Se$_{2-x}$ alloys, which does not require the conjecture of multifractality.

cond-mat.supr-con↗

arXiv2021

Magnetism-driven unconventional effects in Ising superconductors: role of proximity, tunneling, and nematicity

Hybrid Ising superconductor-ferromagnetic insulator heterostructures provide a unique opportunity to explore the interplay between proximity-induced magnetism, spin-orbit coupling and superconductivity. Here we use a combination of first-principles calculations of NbSe$_{2}$/CrBr$_{3}$ heterostructures and an analytical theory of Ising superconductivity to analyze the existing experiments and provide a complete explanation of highly nontrivial and largely counterintuitive effects: an increase in the magnitude of the superconducting gap accompanied by the broadening of the tunneling peaks; hysteretic behavior of the tunneling conductance that sets in $\approx 2$ K below $T_c$; and nematic symmetry breaking in the superconducting state. The microscopic reason in all three cases appears to be the interplay between the proximity-induced exchange splitting and intrinsic defects. Finally, we predict additional interesting effects that at the moment cannot be addressed experimentally: spin-filtering when tunneling across CrBr$_{3}$ and tunneling ``hot spots'' in momentum space that are anticorrelated with regions where the spin-orbit splitting is maximum.

cond-mat.supr-con↗

arXiv2020

Nonrad: Computing Nonradiative Capture Coefficients from First Principles

Point defects in semiconductor crystals provide a means for carriers to recombine nonradiatively. This recombination process impacts the performance of devices. We present the Nonrad code that implements the first-principles approach of Alkauskas et al. [Phys. Rev. B 90, 075202 (2014)] for the evaluation of nonradiative capture coefficients based on a quantum-mechanical description of the capture process. An approach for evaluating electron-phonon coupling within the projector augmented wave formalism is presented. We also show that the common procedure of replacing Dirac delta functions with Gaussians can introduce errors into the resulting capture rate, and implement an alternative scheme to properly account for vibrational broadening. Lastly, we assess the accuracy of using an analytic approximation to the Sommerfeld parameter by comparing with direct numerical evaluation.

cond-mat.mtrl-sci↗

arXiv2020

Chiral charge-density wave in TiSe$_2$ due to photo-induced structural distortions

A variety of experiments have been carried out to establish the origin of the chiral charge-density wave transition in 1T-TiSe$_2$, which in turn has led to contradictory conclusions on the origin of this transition. Some studies suggest the transition is a phonon-driven structural distortion while other studies suggest it is an excitonic insulator phase transition that is accompanied by a lattice distortion. First, we propose these interpretations can be reconciled if one analyzes the available experimental and theoretical data within a formal definition of what constitutes an excitonic insulator as initially proposed by Keldysh and Kopaev. Next, we present pump-probe measurements of circularly polarized optical transitions and first-principles calculations where we highlight the importance of accounting for structural distortions to explain the finite chirality of optical transitions in the CDW phase. We show that at the elevated electronic temperature that occurs upon photoexcitation, there is a non-centrosymmetric structure that is near-degenerate in energy with the centrosymmetric charge density wave structure, which explains the finite chirality of the optical transitions observed in the CDW phase of TiSe$_2$.

cond-mat.mtrl-sci↗