Efthimios Kaxiras

In topological materials, dissipationless edge currents are protected against local defect scattering by the bulk inverted band structure and band gap. We propose that large uniaxial strain can effectively switch a 2D Chern insulator to a topologically trivial state. Further, we suggest that the boundary between strained and unstrained regions of a sample can act as a new edge for dissipationless current flow. Using density functional theory (DFT) calculations we demonstrate the strain-tunability of the monolayer MnBi2S2Te2 band structure and the switching of the Chern number. We combine uniaxial and biaxial strain results to map out the strain-tuned topological phase diagram.

Topological insulators host Dirac surface states (SS) protected by time-reversal symmetry. Inter-surface hybridization can gap the SS and give rise to the quantum spin Hall effect in films that are sufficiently thin compared to the SS penetration depth. However, quantifying the SS penetration depth typically requires painstaking synthesis of multiple films with varying thickness. Here we introduce a direct method to probe the SS penetration depth in bulk crystals, by studying the interplay between SS and magnetic impurities in \SVT. Using scanning tunneling microscopy and spectroscopy, we find that even sparse magnetic impurities ($\lesssim0.25\%$ vanadium) can gap the Dirac SS. However, a single V impurity induces only localized states, and does not form an impurity band, so the gapped Dirac dispersion is preserved away from the impurity. In high magnetic fields, we observe an energy shift of the $0^\text{th}$ Landau level and a suppression of quasiparticle lifetime at the Dirac point, indicating \newtext{magnetic} scattering of the SS. Crucially, by employing V impurities at different depths as precise scattering probes, we reveal the SS penetration depth on the sub-nanometer scale in a bulk crystal.

Topological insulators host Dirac surface states (SS) protected by time-reversal symmetry. Inter-surface hybridization can gap the SS and give rise to the quantum spin Hall effect in films that are sufficiently thin compared to the SS penetration depth. However, quantifying the SS penetration depth typically requires painstaking synthesis of multiple films with varying thickness. Here we introduce a direct method to probe the SS penetration depth in bulk crystals, by studying the interplay between SS and magnetic impurities in \SVT. Using scanning tunneling microscopy and spectroscopy, we find that even sparse magnetic impurities ($\lesssim0.25\%$ vanadium) can gap the Dirac SS. However, a single V impurity induces only localized states, and does not form an impurity band, so the gapped Dirac dispersion is preserved away from the impurity. In high magnetic fields, we observe an energy shift of the $0^\text{th}$ Landau level and a suppression of quasiparticle lifetime at the Dirac point, indicating \newtext{magnetic} scattering of the SS. Crucially, by employing V impurities at different depths as precise scattering probes, we reveal the SS penetration depth on the sub-nanometer scale in a bulk crystal.

We present a comprehensive first-principles study of twisted bilayer graphene (tBLG) for a wide range of twist angles, with a focus on structural and electronic properties. By employing density functional theory (DFT) with an optimized local basis set, we simulate tBLG, obtaining fully relaxed commensurate structures for twist angles down to 0.987°. For all angles the lattice relaxation agrees well with continuum elastic models. For angles accessible to plane-wave DFT (VASP), we provide a detailed comparison with our local basis DFT (SIESTA) calculations, demonstrating excellent agreement in both the atomic and electronic structure. The dependence of the Fermi velocity and band width on the twist angle shows qualitative agreement with results from an `exact' $\mathbf{k \cdot p}$ continuum model, but reveals a small twist angle offset. Additionally, we provide details of the low-energy wavefunction character, band inversion and symmetries. Our results provide an ab initio reference point for the microscopic structure and electronic properties of tBLG which will serve as the foundation for future studies incorporating many-body effects.

We present a comprehensive first-principles study of twisted bilayer graphene (tBLG) for a wide range of twist angles, with a focus on structural and electronic properties. By employing density functional theory (DFT) with an optimized local basis set, we simulate tBLG, obtaining fully relaxed commensurate structures for twist angles down to 0.987°. For all angles the lattice relaxation agrees well with continuum elastic models. For angles accessible to plane-wave DFT (VASP), we provide a detailed comparison with our local basis DFT (SIESTA) calculations, demonstrating excellent agreement in both the atomic and electronic structure. The dependence of the Fermi velocity and band width on the twist angle shows qualitative agreement with results from an `exact' $\mathbf{k \cdot p}$ continuum model, but reveals a small twist angle offset. Additionally, we provide details of the low-energy wavefunction character, band inversion and symmetries. Our results provide an ab initio reference point for the microscopic structure and electronic properties of tBLG which will serve as the foundation for future studies incorporating many-body effects.

Motivated by recent experiments probing the wavefunctions of magic-angle twisted bilayer graphene (tBLG), we perform large-scale first-principles calculations of tBLG with full atomic relaxation across a wide range of twist angles down to $0.99^\circ$. Focusing on the magic angle, we compute wavefunctions of the low energy bands, resolving atomic-scale details and moiré-scale patterns that form triangular, honeycomb, and Kagome lattices. By tuning the interlayer interactions, we illustrate the formation of the flat bands from isolated monolayers and the emergence of the band inversion and fragile topology at a sufficiently large interaction strength. We identify strong indicators of a new phase transition with increasing interlayer interaction strength, achievable with external pressure or a decrease in the twist angle. When this transition occurs, the upper and lower flat bands exchange their wavefunction character and symmetry eigenvalues, which may be correlated with the appearance of superconductivity with electron doping below the magic angle. Our study demonstrates the feasibility of using first-principles wavefunctions to help interpret experimental signatures of topological and correlated phases in tBLG.

Abstract Highly doped semiconductor quantum wells (QWs) exhibit strong intersubband transitions resulting from nanoscale electron confinement. Coupling photons to these collective dipoles in this anisotropic quantum structure enables intersubband polaritons with strong nonlinear optical response and hyperbolicity. Analogous to epitaxially grown multi-quantum wells, two-dimensional (2D) van der Waals (vdW) semiconductor heterostructures provide a compelling alternative platform, offering additional degrees of freedom and exceptional optoelectronic properties. Here we report intersubband polaritons in multilayer vdW WSe 2 QWs with broadband tunability. By oxidizing the top WSe 2 layer into a self-limiting native oxide, we activate charge transfer–induced efficient, high-density doping, enabling strong intersubband transitions and directly visualized polariton propagation. Lithographically defined nanostructures reveal their hyperbolic nature and sub-diffractional confinement, while electrostatic gating offers dynamic dispersion control. These results position vdW multilayers as a highly adaptable platform for tunable mid-infrared nanophotonics and integrated polaritonic circuits, detectors, and light sources.

Two-dimensional multi-layer materials with an induced moiré pattern, either due to strain or relative twist between layers, provide a versatile platform for exploring strongly correlated and topological electronic phenomena. While these systems offer unprecedented tunability, their theoretical description remains challenging due to their complex atomic structures and large unit cells. A notable example is twisted bilayer graphene, where even the relevant symmetry group remains unsettled despite its critical role in constructing effective theories. Here, we focus on twisted bilayer graphene and use a combination of analytical methods, molecular dynamics simulations, and first-principles calculations to show that twisted atomic configurations with distinct microscopic symmetries converge to a universal interlayer structure that governs the low-energy physics. This emergent universality provides a robust foundation for symmetry-respecting models and offers insight into the role of commensurability in real twisted moiré systems.

Two-dimensional metals generically support gapless plasmons with wavelengths well below the wavelength of free-space radiation at the same frequency. Typically, however, this substantial confinement of electromagnetic energy is associated with commensurately high losses, and mitigating such losses may only be achieved through judicious band structure engineering near the Fermi level. In a clean system, an isolated, moderately flat, band at the Fermi level with sufficiently high carrier density can support a plasmon that is immune to propagation losses up to some order in the electron-phonon interaction. However, proposed materials that satisfy these criteria have been ferromagnetic, structurally unstable, or otherwise difficult to fabricate. Here, we propose a class of band structure engineered materials that evade these typical pitfalls -- Moire heterostructures of hexagonal boron nitride intercalated with alkali atoms. We find that only sodium atoms engender a sufficiently isolated band with plasmons lossless at first order in the electron-phonon interaction. We calculate higher order electron-phonon losses and find that at frequencies of about $1$eV the electron-phonon decay mechanism is negligible -- leading to a contribution to the decay rate of about 10^7 Hz in a small frequency range. We next calculate losses from the electron-electron interaction and find that this is the dominant process -- leading plasmons to decay to lower frequency plasmons at a rate of around 10^14 Hz.

In consecutively twisted trialyer material platforms, the complex atomic landscape of coexisting moir\'e superlattices gives rise to rich electronic properties. Near band-insulator gaps of each moir\'e superlattices, the authors report here transport signatures of strong Berry curvatures in twisted triple bilayer graphene, enhanced by tunable inter-moir\'e lattice reconstruction. This work shed new insights in understanding the atomic and electronic properties of twisted multilayers, and paves a path towards engineering the band structure and its topology for designer material functionalities.

Linear tight-binding models with long-range interactions and small-world geometry have a broad energy spectrum in the nearest neighbor coupling limit, while the spectrum becomes narrow in the fully connected limit due to the emergence of flat bands. A transition to a Wigner-like density of states appears at a low fraction of long-range bonds. Adding nonlinearity to the model introduces correlations among the stationary states, while multiple new states are generated as a result of the nonlinearity. In this work, we study the effect of band correlations on the local density of states for small-world networks as a function of the number of long-range bonds. We find that close to the nearest neighbor limit, the onset of correlations shifts the nonlinear density of states towards the band edge of the spectrum. Close to the opposite limit of the fully connected model, the band collapses in the band center, accompanied by a large increase in the new states induced by the nonlinearity. While in both limits the effect of correlations is to flatten the band, close to the mean field fully connected limit, the states are correlated and generally have distinct localized features. These effects may have implications for the dynamics of electrons in two-dimensional moiré structures and the onset of superconductivity in these systems.

High Resolution Image Download MS PowerPoint Slide Nitrogen core-doping of graphene nanoribbons (GNRs) allows trigonal planar carbon atoms along the backbone of GNRs to be substituted by higher-valency nitrogen atoms. The excess valence electrons are injected into the π-orbital system of the GNR, thereby changing not only its electronic occupation but also its topological properties. We have observed this topological change by synthesizing dilute nitrogen core-doped armchair GNRs with a width of five atoms (N 2 -5-AGNRs). The incorporation of pairs of trigonal planar nitrogen atoms results in the emergence of topological boundary states at the interface between doped and undoped segments of the GNR. These topological boundary states are offset in energy by approximately Δ E = 300 meV relative to the topological end states at the termini of finite 5-AGNRs. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that for finite GNRs the two types of topological states can interact through a linear combination of orbitals, resulting in a pair of asymmetric hybridized states. This behavior is captured by an effective Hamiltonian of nondegenerate diatomic molecules, where the analogous interatomic hybridization interaction strength is tuned by the distance between GNR topological modes.

Twistronics has received much attention as a method to manipulate the properties of two-dimensional van der Waals structures by introducing moir\'e patterns through a relative rotation between two layers. Here, we begin a theoretical exploration of twistronics beyond the realm of van der Waals materials by developing a first-principles description of the electronic structure and interlayer interactions of ultrathin perovskite bilayers. We construct both an ab initio tight-binding model as well as a minimal three-band effective model for the valence bands of monolayers and bilayers of oxides derived from the Ruddlesden-Popper phase of perovskites, which is amenable to thin-layer formation. We illustrate the approach with the specific example of ${\mathrm{Sr}}_{2}{\mathrm{TiO}}_{4}$ layers but also provide model parameters for ${\mathrm{Ca}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Ba}}_{2}{\mathrm{TiO}}_{4}$.

First-principles density functional theory (DFT) codes which employ a localized basis offer advantages over those which use plane-wave bases, such as better scaling with system size and better suitability to low-dimensional systems. The trade-off is that care must be taken in order to generate a good localized basis set which is efficient and accurate in a variety of environments. Here we develop and make freely available optimized local basis sets for two common two-dimensional materials, graphene and hexagonal boron nitride, for the DFT code. Each basis set is benchmarked against the plane-wave code, using the same pseudopotentials and exchange-correlation functionals. We find that a significant improvement is obtained by including the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>l</a:mi><a:mo>+</a:mo><a:mn>2</a:mn></a:mrow></a:math> polarization orbitals (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mn>4</b:mn><b:mi>f</b:mi></b:mrow></b:math>) in the basis set, which greatly improves angular flexibility. The optimized basis sets yield much better agreement with plane-wave calculations for key features of the physical system, including total energy, lattice constant, and cohesive energy. The optimized basis sets also result in a speedup of the calculations with respect to the nonoptimized, native choices.

Two-dimensional (2D) semiconductors are likely to dominate next-generation electronics due to their advantages in compactness and low power consumption. However, challenges such as high contact resistance and inefficient doping hinder their applicability. Here, we investigate work-function-mediated charge transfer (modulation doping) as a pathway for achieving high-performance p-type 2D transistors. Focusing on type-III band alignment, we explore the doping capabilities of 27 candidate materials, including transition metal oxides, oxyhalides, and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, on channel materials such as transition metal dichalcogenides (TMDs) and group-III nitrides. Our extensive first-principles density functional theory (DFT) reveal p-type doping capabilities of high electron affinity materials, including $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, ${\mathrm{MoO}}_{3}$, and ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$. We predict significant reductions in contact resistance and enhanced channel mobility through efficient hole transfer without introducing detrimental defects. We analyze transistor geometries and identify promising material combinations beyond the current focus on ${\mathrm{WSe}}_{2}$ doping, suggesting new avenues for hBN, AlN, GaN, and ${\mathrm{MoS}}_{2}$. This comprehensive investigation provides a roadmap for developing high-performance p-type monolayer transistors toward the realization of 2D electronics.

Spin moiré superlattices (SMSs) have been proposed as a magnetic analog of crystallographic moiré systems and a source of electron minibands offering vector-field moiré tunability and Berry curvature effects. However, it has proven challenging to realize an SMS in which a large exchange coupling J is transmitted between conduction electrons and localized spins. Furthermore, most systems have carrier mean free paths l mfp shorter than their spin moiré lattice constant a spin , inhibiting miniband formation. Here, we discover that the layered magnetic semimetal EuAg 4 Sb 2 overcomes these challenges by forming an interface with J ~ 100 milli–electron volts transferred between a Eu triangular lattice and anionic Ag 2 Sb bilayers hosting a two-dimensional electron band in the ballistic regime ( l mfp &gt;&gt; a spin ). The system realizes an SMS with a spin commensurate with the Fermi momentum, leading to a marked quenching of the transport response from miniband formation. Our findings demonstrate an approach to magnetically engineering moiré superlattices and a potential route to an emergent spin-driven quantum Hall state.

Plasmon polaritons are formed by coupling light with delocalized electrons. The half-light and half-matter nature of plasmon polaritons endows them with unparalleled tunability via a range of parameters, such as dielectric environments and carrier density. Therefore, plasmon polaritons are expected to be tuned when in proximity to polar materials since the carrier density is tuned by an electrostatic potential; conversely, the plasmon polariton response might enable the sensing of polarization. Here, we use infrared nanoimaging and nanophotocurrent measurements to investigate heterostructures composed of graphene and twisted hexagonal boron nitride (t-BN), with alternating polarization in a triangular network of moiré stacking domains. We observe that the carrier density and the corresponding plasmonic response of graphene are modulated by polar domains in t-BN. In addition, we demonstrate that the nanometer-wide domain walls of graphene moirés superlattices, created by the polar domains of t-BN, provide momenta to assist the plasmonic excitations. Furthermore, our study establishes that the plasmon of graphene could function as a delicate sensor for polarization textures. The evolution of polarization textures in t-BN under uniform electric fields is tomographically examined via plasmonic imaging. Strikingly, no noticeable polarization switching is observed under applied electric fields up to <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mn>0.21</a:mn> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi mathvariant="normal">V</a:mi> <a:mo stretchy="false">/</a:mo> <a:mi>nm</a:mi> </a:mrow> </a:math> , at variance with transport reports. Our nanoimages unambiguously reveal that t-BN with triangular domains acts like a ferrielectric rather than a ferroelectric as claimed by many previous studies.

Applying long wavelength periodic potentials on quantum materials has recently been demonstrated to be a promising pathway for engineering novel quantum phases of matter. Here, we utilize twisted bilayer boron nitride (BN) as a moiré substrate for band structure engineering. Small-angle-twisted bilayer BN is endowed with periodically arranged up and down polar domains, which imprints a periodic electrostatic potential on a target two-dimensional (2D) material placed on top. As a proof of concept, we use Bernal bilayer graphene as the target material. The resulting modulation of the band structure appears as superlattice resistance peaks, tunable by varying the twist angle, and Hofstadter butterfly physics under a magnetic field. Additionally, we demonstrate the tunability of the moiré potential by altering the dielectric thickness underneath the twisted BN. Finally, we find that near-60°-twisted bilayer BN also leads to moiré band features in bilayer graphene, which may come from the in-plane piezoelectric effect or out-of-plane corrugation effect. Tunable twisted BN substrate may serve as versatile platforms to engineer the electronic, optical, and mechanical properties of 2D materials and van der Waals heterostructures.

We predict that intercalation of Moire heterostructures with alkali atoms can enable plasmons that are lossless to first-order in electron-phonon coupling. This results from the formation of an isolated metallic flat-band.

Two-dimensional (2D) materials that exhibit spontaneous magnetization, polarization, or strain (referred to as ferroics) have the potential to revolutionize nanotechnology by enhancing the multifunctionality of nanoscale devices. However, multiferroic order is difficult to achieve, requiring complicated coupling between electron and spin degrees of freedom. We propose a universal method to engineer multiferroics from van der Waals magnets by taking advantage of the fact that changing the stacking between 2D layers can break inversion symmetry, resulting in ferroelectricity as well as magnetoelectric coupling. We illustrate this concept using first-principles calculations in bilayer NiI_{2}, which can be made ferroelectric upon rotating two adjacent layers by 180° with respect to the bulk stacking. Furthermore, we discover a novel strong magnetoelectric coupling between the interlayer spin order and interfacial electronic polarization. Our approach is not only general but also systematic and can enable the discovery of a wide variety of 2D multiferroics with strong magnetoelectric coupling.

Ultrawide bandgap (UWBG) semiconductors are poised to transform power electronics by surpassing the capabilities of established wide bandgap materials, such as GaN and SiC, owing to their capability to operate at higher voltage, frequency, and temperature ranges. While bulk group-III nitrides and their alloys have been extensively studied in the UWBG realm, their two-dimensional counterparts remain largely unexplored. Here, we examine the stability and electronic properties of monolayers of ordered boron-based group-III nitride alloys with general formula ${\mathrm{B}}_{x}{M}_{1\ensuremath{-}x}\mathrm{N}$, where $M=\mathrm{Al}$, Ga. On the basis of ab initio calculations we identify a number of energetically and dynamically stable structures. Instrumental to their stability is a previously overlooked out-of-plane displacement (puckering) of atoms, which induces a polar ordering and antiferroelectric ground state. Our findings reveal the energy barrier between metastable ferroelectric states is lowered by successive switching of out-of-plane displacements through an antiferroelectric state.

Twistronics has received much attention as a new method to manipulate the properties of 2D van der Waals structures by introducing moiré patterns through a relative rotation between two layers. Here we begin a theoretical exploration of twistronics beyond the realm of van der Waals materials by developing a first-principles description of the electronic structure and interlayer interactions of ultrathin perovskite bilayers. We construct both an ab initio tight-binding model as well as a minimal 3-band effective model for the valence bands of monolayers and bilayers of oxides derived from the Ruddlesden-Popper phase of perovskites, which is amenable to thin-layer formation. We illustrate the approach with the specific example of Sr$_2$TiO$_4$ layers but also provide model parameters for Ca$_2$TiO$_4$ and Ba$_2$TiO$_4$ .

Two-dimensional (2D) semiconductors are likely to dominate next-generation electronics due to their advantages in compactness and low power consumption. However, challenges such as high contact resistance and inefficient doping hinder their applicability. Here, we investigate workfunction-mediated charge transfer (modulation doping) as a pathway for achieving high-performance p-type 2D transistors. Focusing on type-III band alignment, we explore the doping capabilities of 27 candidate materials, including transition metal oxides, oxyhalides, and α-RuCl3, on channel materials such as transition metal dichalcogenides (TMDs) and group-III nitrides. Our extensive first-principles density functional theory (DFT) reveal p-type doping capabilities of high electron affinity materials, including α-RuCl3, MoO3, and V2O5. We predict significant reductions in contact resistance and enhanced channel mobility through efficient hole transfer without introducing detrimental defects. We analyze transistor geometries and identify promising material combinations beyond the current focus on WSe2 doping, suggesting new avenues for hBN, AlN, GaN, and MoS2. This comprehensive investigation provides a roadmap for developing high-performance p-type monolayer transistors toward the realization of 2D electronics.

Electron collimation via a graphene p-n junction allows electrostatic control of ballistic electron trajectories akin to that of an optical circuit. Similar manipulation of novel correlated electronic phases in twisted-bilayer graphene (tBLG) can provide additional probes to the underlying physics and device components toward advanced quantum electronics. In this work, we demonstrate collimation of the electron flow via gate-defined moiré barriers in a tBLG device, utilizing the band-insulator gap of the moiré superlattice. A single junction can be tuned to host a chosen combination of conventional pseudo barrier and moiré tunnel barriers, from which we demonstrate improved collimation efficiency. By measuring transport through two consecutive moiré collimators separated by 1 μm, we demonstrate evidence of electron collimation in tBLG in the presence of realistic twist-angle inhomogeneity.

By electrostatically tuning the strength and hierarchy of the inter-moir\'e interaction in a consecutively twisted trilayer graphene (tTLG) device with two distinct moir\'e superlattices, the authors observe here a new type of inter-moir\'e Hofstadter butterfly. The periodical pattern of the butterfly corresponds to one of the intermediate quasicrystal length scales of the reconstructed moir\'e of moir\'e (MoM) superlattice. This study sheds new light on emergent physics from competing atomic orders in twisted multilayer 2D material platforms.

We report a twisted triple bilayer graphene platform consisting of three units of Bernal bilayer graphene consecutively twisted at 1.49° and 1.68°. We demonstrate the atomic reconstruction between the two competing moiré superlattices strongly enhances the Berry curvature of each moiré band insulator state, characterized by measured strong nonlocal valley Hall effect that sensitively depends on the inter-moiré competition strength, tunable by manipulating the out-of-plane carrier distribution. Our study sheds light on the microscopic mechanism of atomic and electronic reconstruction in twisted multilayer systems, by systematically investigating transport signatures of moiré Berry curvature and its enhancement from moiré-of-moiré lattice reconstruction. We show that the reconstructed electronic band can be versatilely tuned by electrostatics, providing an approach toward engineering the band structure and its topology for a quantum material platform with designer electrical and optical properties.

Material characterization in nano-mechanical tests may provide information on the potential heterogeneity of mechanical properties. Here, we develop a robust neural-network interatomic potential (NNIP), and we provide a test for the example of molecular dynamics (MD) nanoindentation, and the case of body-centered cubic crystalline molybdenum (Mo). We employ a similarity measurement protocol, using standard local environment descriptors, to select ab initio configurations for the training dataset that capture the behavior of the indented sample. We find that it is critical to include generalized stacking fault (GSF) configurations, featuring a dumbbell self-interstitial on the surface, to capture dislocation cores, and also high-temperature configurations with frozen atom layers for the indenter tip contact. We develop a NNIP with distinct dislocation nucleation mechanisms, realistic generalized stacking fault energy (GSFE) curves, and an informative energy landscape for the atoms on the sample surface during nanoindentation. We compare our NNIP results with nanoindentation simulations, performed with three existing potentials – an embedded atom method (EAM) potential, a gaussian approximation potential (GAP), and a tabulated GAP (tabGAP) potential – that predict different dislocation nucleation mechanisms, and display the absence of essential information on the shear stress at the sample surface in the elastic region. Finally, we compared our NNIP nanoindentation results with experiments, showing reliable predictions for reduced Young’s modulus and observable slip traces.

Plasmon polaritons are formed by coupling light with delocalized electrons. The half-light and half-matter nature of plasmon polaritons endows them with unparalleled tunability via a range of parameters, such as dielectric environments and carrier density. Therefore, plasmon polaritons are expected to be tuned when in proximity to polar materials since the carrier density is tuned by an electrostatic potential; conversely, the plasmon polariton response might enable the sensing of polarization. Here, we use infrared nano-imaging and nano-photocurrent measurements to investigate heterostructures composed of graphene and twisted hexagonal boron nitride (t-BN), with alternating polarization in a triangular network of moiré stacking domains. We observe that the carrier density and the corresponding plasmonic response of graphene are modulated by polar domains in t-BN. In addition, we demonstrate that the nanometer-wide domain walls of graphene moirés superlattices, created by the polar domains of t-BN, provide momenta to assist the plasmonic excitations. Furthermore, our studies establish that the plasmon of graphene could function as a delicate sensor for polarization textures. The evolution of polarization textures in t-BN under uniform electric fields is tomographically examined via plasmonic imaging. Strikingly, no noticeable polarization switching is observed under applied electric fields up to 0.23 V/nm, at variance with transport reports. Our nano-images unambiguously reveal that t-BN with triangular domains acts like a ferrielectric, rather than ferroelectric claimed by many previous studies.

The persistence of voltage-switchable collective electronic phenomena down to the atomic scale has extensive implications for area- and energy-efficient electronics, especially in emerging nonvolatile memory technology. We investigate the performance of a ferroelectric field-effect transistor (FeFET) based on sliding ferroelectricity in bilayer boron nitride at room temperature. Sliding ferroelectricity represents a different form of atomically thin two-dimensional (2D) ferroelectrics, characterized by the switching of out-of-plane polarization through interlayer sliding motion. We examined the FeFET device employing monolayer graphene as the channel layer, which demonstrated ultrafast switching speeds on the nanosecond scale and high endurance exceeding 10 11 switching cycles, comparable to state-of-the-art FeFET devices. These characteristics highlight the potential of 2D sliding ferroelectrics for inspiring next-generation nonvolatile memory technology.

In this work, we develop a twist-dependent electrochemical activity map, combining a low-energy continuum electronic structure model with modified Marcus-Hush-Chidsey kinetics in trilayer graphene. We identify a counterintuitive rate enhancement region spanning the magic angle curve and incommensurate twists in the system geometry. We find a broad activity peak with a ruthenium hexamine redox couple in regions corresponding to both magic angles and incommensurate angles, a result qualitatively distinct from the twisted bilayer case. Flat bands and incommensurability offer new avenues for reaction rate enhancements in electrochemical transformations.

Two-dimensional (2D) materials that exhibit spontaneous magnetization, polarization or strain (referred to as ferroics) have the potential to revolutionize nanotechnology by enhancing the multifunctionality of nanoscale devices. However, multiferroic order is difficult to achieve, requiring complicated coupling between electron and spin degrees of freedom. We propose a universal method to engineer multiferroics from van der Waals magnets by taking advantage of the fact that changing the stacking between 2D layers can break inversion symmetry, resulting in ferroelectricity and possibly magnetoelectric coupling. We illustrate this concept using first-principles calculations in bilayer NiI$_2$, which can be made ferroelectric upon rotating two adjacent layers by $180^{\circ}$ with respect to the bulk stacking. Furthermore, we discover a novel multiferroic order induced by interlayer charge transfer which couples the interlayer spin order and electronic polarization. Our approach is not only general but also systematic, and can enable the discovery of a wide variety of 2D multiferroics.

We examine the electronic structure of recently fabricated in-plane heterojunctions of zigzag graphene nanoribbons embedded in hexagonal boron nitride. We focus on hitherto unexplored interface configurations in which both edges of the nanoribbon are bonded to the same chemical species, either boron or nitrogen atoms. Using ab initio and mean-field Hubbard model calculations, we reveal the emergence of one-dimensional magnetic conducting channels at these interfaces. These channels originate from the energy shift of the magnetic interface states that is induced by charge transfer between the nanoribbon and hexagonal boron nitride. We further address the response of these heterojunctions to external electric and magnetic fields, demonstrating the tunability of energy and spin splittings in the electronic structure. Our findings establish that zigzag graphene nanoribbon/hexagonal boron nitride heterojunctions are a suitable platform for exploring and engineering spin transport in the atomically thin limit, with potential applications in integrated spintronic devices.

Received 25 March 2024DOI:https://doi.org/10.1103/PhysRevB.109.199902©2024 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFlat bandsPhysical SystemsHoneycomb latticeKagome latticeTransition metal dichalcogenidesTwisted heterostructuresCondensed Matter, Materials & Applied Physics

Ultra-wide bandgap (UWBG) semiconductors are poised to transform power electronics by surpassing the capabilities of established wide bandgap materials, such as GaN and SiC, owing to their capability to operate at higher voltage, frequency, and temperature ranges. While bulk group-III nitrides and their alloys have been extensively studied in the UWBG realm, their two-dimensional counterparts remain unexplored. Here, we examine the stability and electronic properties of monolayers of ordered boron-based group-III nitride alloys with general formula BxM1-xN, where M = Al, Ga. On the basis of ab initio calculations we identify a number of energetically and dynamically stable structures. Instrumental to their stability is a previously overlooked out-of-plane displacement (puckering) of atoms, which induces a polar ordering and antiferroelectric ground state. Our findings reveal the energy barrier between metastable ferroelectric states is lowered by successive switching of out-of-plane displacements through an antiferroelectric state.

Applying long wavelength periodic potentials on quantum materials has recently been demonstrated to be a promising pathway for engineering novel quantum phases of matter. Here, we utilize twisted bilayer boron nitride (BN) as a moiré substrate for band structure engineering. Small-angle-twisted bilayer BN is endowed with periodically arranged up and down polar domains, which imprints a periodic electrostatic potential on a target two-dimensional (2D) material placed on top. As a proof of concept, we use Bernal bilayer graphene as the target material. The resulting modulation of the band structure appears as superlattice resistance peaks, tunable by varying the twist angle, and Hofstadter butterfly physics under a magnetic field. Additionally, we demonstrate the tunability of the moiré potential by altering the dielectric thickness underneath the twisted BN. Finally, we find that near-60°-twisted bilayer BN provides a unique platform for studying the moiré structural effect without the contribution from electrostatic moiré potentials. Tunable moiré polar substrates may serve as versatile platforms to engineer the electronic, optical, and mechanical properties of 2D materials and van der Waals heterostructures.

An accurate description of the low-energy electronic bands in twisted bilayer graphene (tBLG) is of great interest due to their relation to correlated electron phases such as superconductivity and Mott-insulator behavior at half-filling. The paradigmatic model of Bistritzer and MacDonald [Proc. Natl. Acad. Sci. USA 108, 12233 (2011)], based on the moir\'e pattern formed by tBLG, predicts the existence of ``magic angles'' at which the Fermi velocity of the low-energy bands goes to zero, and the bands themselves become dispersionless. Here, we reexamine the low-energy bands of tBLG from the ab initio electronic structure perspective, motivated by features related to the atomic relaxation in the moir\'e pattern, namely, circular regions of AA stacking, triangular regions of AB/BA stacking and domain walls separating the latter. We find that the bands are never perfectly flat and the Fermi velocity never vanishes, but rather a ``magic range'' exists where the lower band becomes extremely flat and the Fermi velocity attains a nonzero minimum value. We propose a simple $(2+2)$-band model, comprised of two different pairs of orbitals, both on a honeycomb lattice: the first pair represents the low-energy bands with high localization at the AA sites, while the second pair represents highly dispersive bands associated with domain-wall states. This model gives an accurate description of the low-energy bands with few (13) parameters that are physically motivated and vary smoothly in the magic range. In addition, we derive an effective two-band Hamiltonian which also gives an accurate description of the low-energy bands. This minimal two-band model affords a connection to a Hubbard-like description of the occupancy of subbands and can be used a basis for exploring correlated states.

Electron collimation via a graphene pn-junction allows electrostatic control of ballistic electron trajectories akin to that of an optical circuit. Similar manipulation of novel correlated electronic phases in twisted-bilayer graphene (tBLG) can provide additional probes to the underlying physics and device components towards advanced quantum electronics. In this work, we demonstrate collimation of the electron flow via gate-defined moiré barriers in a tBLG device, utilizing the band-insulator gap of the moiré superlattice. A single junction can be tuned to host a chosen combination of conventional pseudo barrier and moiré tunnel barriers, from which we demonstrate improved collimation efficiency. By measuring transport through two consecutive moiré collimators separated by 1 um, we demonstrate evidence of electron collimation in tBLG in the presence of realistic twist-angle inhomogeneity.

Materials design has traditionally evolved through trial-error approaches, mainly due to the non-local relationship between microstructures and properties such as strength and toughness. We propose ‘alloy informatics’ as a machine learning based prototype predictive approach for alloys and compounds, using electron charge density profiles derived from first-principle calculations. We demonstrate this framework in the case of hydrogen interstitials in face-centred cubic crystals, showing that their differential electron charge density profiles capture crystal properties and defect-crystal interaction properties. Radial Distribution Functions (RDFs) of defect-induced differential charge density perturbations highlight the resulting screening effect, and, together with hydrogen Bader charges, strongly correlate to a large set of atomic properties of the metal species forming the bulk crystal. We observe the spontaneous emergence of classes of charge responses while coarse-graining over crystal compositions. Nudge-Elastic-Band calculations show that RDFs and charge features also connect to hydrogen migration energy barriers between interstitial sites. Unsupervised machine-learning on RDFs supports classification, unveiling compositional and configurational non-localities in the similarities of the perturbed densities. Electron charge density perturbations may be considered as bias-free descriptors for a large variety of defects.

CoSn is a prototypical kagome compound showing lattice-born flat bands with suppressed bandwidth over large parts of the Brillouin zone. Here, by means of time- and angle-resolved photoelectron spectroscopy, we provide direct evidence of the response to photoexcitation of the flat bands, which underlie information about localization in real space. In particular, we detect a sudden shift and broadening of the flat bands, while after one picosecond only a broadening survives. We ascribe both these effects to an ultrafast disruption of electron localization, which renormalizes the effect of electron-electron interaction and affects the flat band dispersion. Since both variations are in the order of a few meV for an excitation fluence of a few hundreds of $\ensuremath{\mu}\mathrm{J}/{\mathrm{cm}}^{2}$, our measurements suggest that the flat bands are resilient to near-infrared photoexcitation in a moderate fluence re\'egime.