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.

Spin-current generation through nonrelativistic spin splittings, found in uncompensated magnets and d -wave altermagnets (AMs), is desirable for low-power spintronics. Such spin currents, however, are symmetry forbidden in conventional collinear antiferromagnets and higher-order AMs. 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 AM 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— KV 2 Se 2 O , RuF 4 , Cr 2 O 3 , FeS 2 , and MnPSe 3 —spanning these different cases. The charge-to-spin conversion and spin-splitter ratio reaches up to almost 100% via uncompensated magnetism and about 40% via d -wave AM under realistic conditions, highlighting the effectiveness of this approach for efficient spin-current generation.

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 the proximity of 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 bandgap 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.

Intercalated group-V transition metal dichalcogenides (TMD) exhibit a variety of magnetotransport phenomena arising from diverse nuclear and magnetic structures. However, a comprehensive mapping of the magnetic, thermodynamic, and structural phases is lacking. Here, we investigate Co x NbSe2 for x = 0.29-0.36, using quenching and controlled cooling to tune structural order and map the resulting magnetic phases. We find that the altermagnetic states (x = 1/4) and the spin density wave (x = 1/3) remain robust under both synthesis conditions. In contrast, intermediate compositions give rise to spin-glass-like behavior or magnetic phase coexistence, producing two distinct phase diagrams depending on the thermal history of the synthesis process. To identify the underlying drivers of each ground state, we applied reverse Monte Carlo modeling to diffuse neutron scattering data, revealing the presence of local 3 a h x 3 a h sublattice domains in all samples independently of the synthesis conditions. As the intercalant concentration decreases, the Co ions preferentially adopt the 2a h x 2a h sublattice, where dominant magnetic interactions emerge, stabilizing the altermagnetic phase. We evaluated these interactions through first-principles calculations for x = 1/4 and 1/3, where each domain interaction type is structurally and magnetically well-defined. The resulting exchange reveals spin-glass and spin density wave phases driven by frustration from tighter intercalant arrangement and altermagnetism from 2a h x 2a h domains. Overall, we present local structure-driven magnetic phases across Co x NbSe2 compositions, demonstrating how synthesis impacts local order of the Co atoms and thereby the overall magnetic properties.

Emerging quantum information technologies demand robust, tunable, single photon sources. Solid-state single photon emitters (SPEs) in the two-dimensional material hexagonal boron nitride (hBN) offer unique advantages, including stability and integration potential, yet current fabrication methods lack precise control over the emitter placement and properties. In this work, we demonstrate a high-yield approach to patterning SPE arrays in hBN by combining focused ion beam (FIB) milling with chemical vapor deposition (CVD) of nanocrystalline graphitic carbon. Using statistical design and analysis of experiments, we systematically map a high-dimensional parameter space─spanning FIB exposure and CVD conditions─to identify the optimal regimes for SPE formation and tunability. Our method leverages widely available fabrication tools and provides critical insights into defect activation mechanisms, offering a scalable, reproducible path toward controllable quantum emitter synthesis. Beyond hBN, this approach opens the door to generating defect-based SPEs in other low-defect solid-state materials. The result is a practical and versatile platform for creating quantum light sources tailored for applications in communication, sensing, and computation.

Intercalated group-V transition metal dichalcogenides (TMD) exhibit a variety of magnetotransport phenomena arising from diverse nuclear and magnetic structures. However, a comprehensive mapping of the magnetic, thermodynamic, and structural phases is lacking. Here, we investigate CoxNbSe2 for x = 0.29-0.36, using quenching and controlled cooling to tune structural order and map the resulting magnetic phases. We find that the altermagnetic states (x = 1/4) and the spin density wave (x = 1/3) remain robust under both synthesis conditions. In contrast, intermediate compositions give rise to spin-glass-like behavior or magnetic phase coexistence, producing two distinct phase diagrams depending on the thermal history of the synthesis process. To identify the underlying drivers of each ground state, we applied reverse Monte Carlo modeling to diffuse neutron scattering data, revealing the presence of local 3ah×3ah sublattice domains in all samples independently of the synthesis conditions. As the intercalant concentration decreases, the Co ions preferentially adopt the 2ah× 2ah sublattice, where dominant magnetic interactions emerge, stabilizing the altermagnetic phase. We evaluated these interactions through first-principles calculations for x = 1/4 and 1/3, where each domain interaction type is structurally and magnetically well-defined. The resulting exchange reveals spin-glass and spin density wave phases driven by frustration from tighter intercalant arrangement and altermagnetism from 2ah× 2ah domains. Overall, we present local structure-driven magnetic phases across CoxNbSe2 compositions, demonstrating how synthesis impacts local order of the Co atoms and thereby the overall magnetic properties.

The anomalous Hall effect (AHE) is an efficient tool for detecting the N & eacute;el vector in collinear compensated magnets with spin-split bands, known as altermagnets (AMs). Here, we establish design principles for obtaining nonzero 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 & eacute;el vector lies within the plane of the Hall current. Through first-principles simulations on bilayers of MnPSe3 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 & eacute;el vector space, although AHE is a relativistic effect. This work shows that the AHE in 2D AMs can probe the altermagnetic order and N & eacute;el vector reversal, thereby facilitating the miniaturization of altermagnetic spintronics.

The silicon vacancy (VSi) 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 SiCbased applications, doping plays a crucial role. It can be used to control the charge state of VSi 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 VSi in the presence of nitrogen. Nitrogen substituted for carbon is shown to very efficiently donate one electron to VSi. We also observe that the ODMR contrast can be increased from 0.5% in low doped SiC to 1.5% by nitrogen doping of 1017 to 1018 cm-3 and annealing at 500-600 degrees C for one hour, with only a 20% decrease in PL compared to unannealed. Some of the improvement in contrast is offset by a reduction in T2 & lowast; 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.

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 Landé g-factor and spin relaxation time.

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 the frequencies of the quantum emission from the defect, but also adversely affects their photostability, 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.

Discoveries of low-dimensional quantum materials have renewed interest in some of the fundamental phe-nomena, such as magnetism and Rashba-type spin orbit coupling. In particular, exploring these phenomena by themselves and/or in combination within one-dimensional (1D) systems is of interest for fields as disparate as spintronics and biology. For example, a better understanding of Rashba-type spin orbit coupling in 1D systems may be used to explain spin-selective electron transport in long helical molecules. In this paper, using first principle calculations, we show that each edge of a zigzag nanoribbon composed of a bismuth (Bi) bilayer is a truly 1D structure that naturally combines both 1D magnetism and Rashba-type spin orbit coupling in a single system. In particular, we study the combined effects of exchange and spin-orbit coupling in nanoribbons that are: (i) ideal freestanding, (ii) placed on a hexagonal boron nitride substrate, and (iii) decorated with N atoms. The edges of the Bi zigzag NRs can display ferromagnetic, antiferromagnetic, or noncollinear ordering, resulting in a broken quantum spin Hall state. The interplay of Rashba and exchange effects in different magnetic phases can result in different spin-dependent transport regimes.

Controllable and reliable doping of cubic boron nitride (cBN) is a critical challenge in its widespread application in power electronics. Recently, progress was made in this regard when an experiment reported successful n-doping of cBN with carbon, reaching carbon concentrations as high as 5%, which is beyond the thermodynamic solubility limit. However, the nature of carbon based defects introduced within the cBN matrix remains unknown thus far. Here, we explore the electronic structure of carbon doped cBN under nitrogen-rich conditions, which are conducive to the formation of donor-type defects (such as carbon substituents replacing boron), and predict several possible arrangements of carbon in clusters at experimentally relevant concentrations of 1.5-7.8%. Our theoretical calculations show that carbon dopants prefer to aggregate into small clusters with local ordering rather than distributing randomly in the bulk. Along with defects that result in easily ionizable defect states, we also report a distribution of carbon dopants, where the structure exhibits a defect-bound small electron polaron. The polaron can be identified via a split-off localized dopant band near the valence band edge. These findings not only shed light on identities of possible carbon-based defects but also predict additional carriers-defect-bound small polarons.

Quantum emitters (QEs) based on deep-level defects in hexagonal boron nitride (hBN) layers are promising alternatives to other qubit-candidates in three-dimensional wide bandgap semiconductors. The two-dimensional (2D) form factor of hBN allows the possibility of near-deterministic placement of quantum emitters and an ease of property-tuning via different means, such as application of strain. However, the 2D nature of hBN also results in a unique set of challenges, including a sensitivity of the QEs to their environment that can influence their different properties, such as their emission frequencies and brightness. In particular, although observed experimentally, theoretical works thus far have ignored substrate-induced modulation of hBN's QE properties. As a result, to date, the magnitude of substrate effects and the underlying mechanism(s) involved in the modulation of QE properties remain unknown. In our density functional theory-based work, we use silicon dioxide as a prototype substrate to demonstrate that the substrate effects can indeed have a significant impact on ground- and excited-state properties of defects responsible for quantum emission. Our analysis shows large structural distortions at the defect sites due to substrate interactions, resulting in significant changes in quantum emission frequencies. These calculations reveal that accounting for substrate effects is critical to the successful use of hBN in quantum sensing and quantum computing.

Chemical modifications and/or simple vertical stacking of disparate van der Waals layered crystals can be used as a materials design approach for creating novel phases of matter. Here, using ab initio computations, we demonstrate the realization of an unusual state in a bismuth nanoribbon decorated with nitrogen atoms along one of the edges. In this phase, the quantum spin Hall state on one edge of the nanoribbon coexists with the ferromagnetism on the other edge. Such a coexistence is made possible by the short-range nature of the exchange interactions on the magnetic edge. As a result, the quantum spin Hall state on the opposite edge of the nanoribbon does not feel the local breaking of time-reversal symmetry on the magnetic edge. While the edge with quantum spin Hall state exhibits the typical spin-helical texture associated with the state, the magnetic edge displays ±k-asymmetry due to the interplay of Rashba and exchange effects. The latter is also a half-metal and can generate a fully spin-polarized current. We demonstrate that this coexistence of states is robust and that it is exhibited even when the nitrogen-decorated nanoribbon is placed on a substrate. In addition, with a proof-of-principle heterostructure, composed of an undecorated bismuth nanoribbon on hexagonal boron nitride, we show that this mixture of states can potentially exist even without passivation with nitrogen-atoms. In the heterostructure, an unequal relaxation along the two edges of the nanoribbon is found to be responsible for the coexistence of two states.

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 Pb1-xSnxTe (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/(V s) and a magnetoresistance approaching 100%, 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/nonpolar interface interaction. Our results suggest that proximity induced interface states with hybrid properties can be added to the still growing list of behaviors in these materials.

Charge transfer (CT) cocrystals, molecular crystals composed of electron donating and accepting species, are being developed for applications in optoelectronics. Here we present optical and electronic characterization of the CT cocrystal phenothiazine-tetracyanoquinodimethane (PTZ-TCNQ). This material has a broad NIR absorption peak with an optical band edge less than 0.6 eV. We used density functional theory calculations to identify the origin of the low energy CT states and changes in the Raman spectra. We also demonstrate the fabrication of long, ribbon-like oriented cocrystals using an evaporative alignment method. Cocrystals grown on Si substrates were fabricated into organic field effect transistors. Despite theoretical predictions of ambipolarity, only electron conduction was observed, with mobilities on the order of 10-4 cm2 V-1 s-1. Measurements of the temperature dependence of the mobility indicated a superexchange mediated hopping mechanism for charge transport, with a characteristic scale of 0.19 eV.

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.

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.

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.

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.