Dirac Materials Research Articles

Generating extreme quantum scattering in graphene with machine learning

Graphene quantum dots provide a platform for manipulating electron behaviors in two-dimensional (2D) Dirac materials. Most previous works were of the ``forward'' type in that the objective was to solve various confinement, transport, and scattering problems with given structures that can be generated by, e.g., applying an external electrical field. There are applications such as cloaking or superscattering where the challenging problem of inverse design needs to be solved: Finding a quantum-dot structure according to certain desired functional characteristics. A brute-force search of the system configuration based directly on the solutions of the Dirac equation is computationally infeasible. We articulate a machine-learning approach to addressing the inverse-design problem where artificial neural networks subject to physical constraints are exploited to replace the rigorous Dirac equation solver. In particular, we focus on the problem of designing a quantum-dot structure to generate both cloaking and superscattering in terms of the scattering efficiency as a function of the energy. We construct a physical loss function that enables accurate prediction of the scattering characteristics. We demonstrate that, in the regime of Klein tunneling, the scattering efficiency can be designed to vary over two orders of magnitude, allowing any scattering curve to be generated from a proper combination of the gate potentials. Our physics-based machine-learning approach can be a powerful design tool for 2D Dirac material-based electronics.

Effects of tilted Dirac cones and in-plane electric field on the valley-dependent magneto-optical absorption spectra in monolayer 8–Pmmn borophene

We investigate the optical absorption properties in a 8-Pmmn borophene monolayer with anisotropic tilted Dirac cones, subjected to a perpendicular magnetic field and in-plane dc electric field (EF). Numerical results for the magneto-optical absorption coefficient (MOAC) show that the tilt of the Dirac cones causes the redshift of absorption peaks. More interestingly, there appears minor absorption peaks corresponding to the transitions between the Landau levels (LLs) n and n±2 beside the principal maxima corresponding to the transitions between LLs n and n±1. These minor peaks are absent in other two-dimensional Dirac materials with non-titled Dirac cones. The simultaneous presence of in-plane EF and the tilt parameter in 8–Pmmn borophene lifts the valley degeneracy in the LLs and causes the valley polarization in the MOAC, that do not occur in the absence of in-plane EF and/or the tilt parameter. The valley-polarized MOAC opens up potential applications in valleytronics.

Bonding and Electronic Nature of the Anionic Framework in LaPd3S4

Double Dirac materials are a topological phase of matter in which a non-symmorphic symmetry enforces greater electronic degeneracy than normally expected – up to eightfold. The cubic palladium bronzes NaPd3O4 and LaPd3S4 are built of Pd3X4 (X = O, S) anionic frameworks that are ionically bonded to A cations (A = Na, La). These materials were recently identified computationally as harboring eightfold fermions. Here we report the preparation of single crystals and electronic properties of LaPd3S4. Measurements down to T = 0.45 K and in magnetic fields up to μ0H = 65 T are consistent with normal Fermi liquid physics of a Dirac metal in the presence of dilute magnetic impurities. This interpretation is further confirmed by analysis of specific heat, magnetization measurements and comparison to density functional theory (DFT) calculations. Through a bonding analysis of the DFT electronic structure of NaPd3O4 and LaPd3S4, we identify the origin of the stability of the anionic Pd3X4 framework at higher electron counts for X = S than X = O, and propose chemical tuning strategies to enable shifting the 8-fold fermion points to the Fermi level.

InBi: A Ferroelastic Monolayer with Strain Tunable Spin-Orbit Dirac Points and Carrier Self-Doping Effect.

Semimetallic two-dimensional (2D) Dirac materials beyond graphene, especially 2D materials with robust Dirac points against the spin-orbit coupling (SOC), are still highly sought. Herein, we theoretically demonstrate the InBi monolayer as a long-sought 2D Dirac material whose exotic Dirac Fermionic states cannot be gapped out by SOC. The InBi monolayer with the litharge crystal structure possesses not only 4-fold band degeneracy, linear energy dispersion, and ultrahigh Fermi velocity in the order of 105 m/s, but also spontaneous ferroelasticity that can lead to the orthorhombic lattice deformation and semimetallic electronic structure. Specifically, the symmetry protected spin-orbit Dirac points in 2D InBi are located at the Brillouin Zone (BZ) boundary and near the Fermi level in energy. More importantly, with coexisting spin-orbit Dirac points and spontaneous ferroelasticity, the InBi monolayer exhibits an additional advantage for engineering Dirac Fermionic states by ferroelastic (FE) strain. Energy levels of Dirac points are strongly coupled to FE strain, and the semimetallic electronic structure of the InBi monolayer is also susceptible to the FE strain induced carrier self-doping effect. Depending on the strain orientation within the InBi monolayer, electron and hole Fermi pockets will develop along the two planar directions, leading to the characteristic transport coefficients (as evidenced by our transport simulations based on Boltzmann formalism) for future experimental detection. FE strain tunable Dirac Fermionic states together with the carrier self-doping effect will benefit future development of ultrathin electronic devices with both high carrier mobility and controllable charge conductivities.

Synergetic Ferroelectricity and Superconductivity in Zero-Density Dirac Semimetals near Quantum Criticality.

We study superconductivity in a three-dimensional zero-density Dirac semimetal in proximity to a ferroelectric quantum critical point. We find that the interplay of criticality, inversion-symmetry breaking, and Dirac dispersion gives rise to a robust superconducting state at the charge-neutrality point, where no Fermi surface is present. Using Eliashberg theory, we show that the ferroelectric quantum critical point is unstable against the formation of a ferroelectric density wave (FDW), whose fluctuations, in turn, lead to a first-order superconducting transition. Surprisingly, long-range superconducting and FDW orders are found to cooperate with each other, in contrast to the more usual scenario of phase competition. Therefore, we suggest that driving charge neutral Dirac materials, e.g., Pb_{x}Sn_{1-x}Te, through a ferroelectric quantum critical point may lead to superconductivity intertwined with FDW order.

Role of electronic correlations in room-temperature ferromagnetism of monolayer MnSe2

The electronic structure of a two-dimensional ferromagnetic $\mathrm{Mn}{\mathrm{Se}}_{2}$ monolayer was investigated using density functional theory. It was found that the computed Curie temperature (${T}_{\mathrm{C}}$), which is fairly high but lower than room temperature, is sensitive to the on-site Coulomb repulsion $U$, indicating that electronic correlations have an important role in the magnetism exhibited in the system. The primary exchange mechanism responsible for the large ${T}_{\mathrm{C}}$ of monolayer $\mathrm{Mn}{\mathrm{Se}}_{2}$ was also clarified. Furthermore, it was demonstrated that the ${T}_{\mathrm{C}}$ can be enhanced up to nearly room temperature through charge doping and heterostructure engineering. These findings not only provide a fundamental understanding of the mechanism of magnetic ordering in $\mathrm{Mn}{\mathrm{Se}}_{2}$ monolayers, but also suggest a practical remedy for the enhancement of the ${T}_{\mathrm{C}}$ of $\mathrm{Mn}{\mathrm{Se}}_{2}$ by using various substrates, including Dirac materials.

Goos–Hänchen shift of electron waves reflected by 8-Pmmn borophene np junctions

Electron optics introducing the optical concepts into the electronic system unifies the propagation and interference behaviors of different waves and promises nontrivial electronic applications. Due to the high mobility of massless Dirac fermions (MDFs), Dirac materials are suitable for the exploration of optics-like phenomena, in which the Goos–Hänchen shift is an outstanding example. The Goos–Hänchen shift has been studied very well for isotropic MDFs, e.g., in graphene, the relevant generalization to anisotropic MDFs is interesting but lacking. Here, we study the Goos–Hänchen shift of the reflected MDFs by np junctions based on 8-Pmmn borophene, in which MDFs are tilted. The Goos–Hänchen shift of tilted MDFs has a strong dependence on the junction direction, providing an additional tunability. Accounting for two valleys coupled by the time-reversal symmetry, the valley-contrasting Goos–Hänchen shift is demonstrated, this feature favors the use of 8-Pmmn borophene np junctions in valleytronics. This study is helpful to understand the anomalous shift of tilted MDFs along the longitudinal interface of np junctions, and implies the potential applications in valleytronics of 8-Pmmn borophene junctions.

Ultrasensitive Self-Driven Terahertz Photodetectors Based on Low-Energy Type-II Dirac Fermions and Related Van der Waals Heterojunctions.

The exotic electronic properties of topological semimetals (TSs) have opened new pathways for innovative photonic and optoelectronic devices, especially in the highly pursuit terahertz (THz) band. However, in most cases Dirac fermions lay far above or below the Fermi level, thus hindering their successful exploitation for the low-energy photonics. Here, low-energy type-II Dirac fermions in kitkaite (NiTeSe) for ultrasensitive THz detection through metal-topological semimetal-metal heterostructures are exploited. Furthermore, a heterostructure combining two Dirac materials, namely, graphene and NiTeSe, is implemented for a novel photodetector exhibiting a responsivity as high as 1.22AW-1 , with a response time of 0.6µs, a noise-equivalent power of 18pWHz-0.5 , with outstanding stability in the ambient conditions. This work brings to fruition of Dirac fermiology in THz technology, enabling self-powered, low-power, room-temperature, and ultrafast THz detection.

Graphene plasmons-enhanced terahertz response assisted by metallic gratings

Abstract Terahertz detectors based on two-dimensional Dirac materials offer a new approach for room-temperature terahertz detection with high response and low noise. However, these devices can hardly show high response over a broad frequency range, mainly due to the poor absorption caused by their ultrathin nature. Here we apply metallic gratings to enhance the excitation efficiency of graphene plasmons. When nonzero source-drain bias is applied, graphene plasmons can generate terahertz response orthogonal to the polarization of the incidence. The response is attributed to the orthogonal overdamped plasmon rectification effect, and graphene plasmons-enhanced photo-thermoelectric effect. By comparing the normalized on/off ratio, the metallic gratings are found to effectively enhance the coupling efficiency between graphene plasmons and THz incidence, and thus the absorption and responsivity. The results are beneficial for improving the response of room temperature THz detectors.

Planar MN4 (M = Zn, Cd) Monolayers: 2D Anisotropic Dirac Semimetals of Ultrahigh Fermi Velocities

After the exfoliation of graphene, it is appealing to seek more 2D Dirac materials with unique structures, rich physics, and intriguing properties. Herein, by means of density functional theory (DFT) computations, two planar 2D Dirac transition‐metal nitrides are reported, namely, MN4 (M = Zn, Cd) monolayers. The basal planes of these monolayers can be regarded as tiled by hexagons and pentagons, with tetracoordinated metal and tricoordinated nitrogen as vertices. Besides the demonstrated mechanical, dynamic, and thermal stabilities, the two MN4 monolayers both exhibit an anisotropic Dirac cone at the Fermi level, thus featuring anisotropic Dirac fermions. Particularly, these Dirac fermions present ultrahigh Fermi velocities, which are in the same order of magnitude as that of graphene (106 m s−1). Moreover, the Dirac cone is robust against external strains and electric fields due to the protection of σv(x) and C2z symmetry. Above all, the remarkable electronic properties suggest that MN4 (M = Zn, Cd) monolayers have potential applications as nanoelectronic devices.

Ultrahigh Carrier Mobility in Cd3As2 Nanowires

Magnetotransport measurements are carried out on nanowires of the Dirac semimetal . Weak antilocalization is observed at 1.9 K, consistent with the presence of strong spin–orbit interaction. With decreasing temperature, Shubnikov–de Haas oscillations are seen, revealing an ultrahigh mobility of 57 000 cm2 Vs at 1.9 K. The strong oscillations display a linear dependence of the Landau‐level index on the inverse of the magnetic field, yielding an intercept that is consistent with a π Berry phase—the signature feature of Dirac fermions. By studying the fundamental properties of Dirac materials, new avenues can be explored by exploiting their unique properties for spintronics and magnetoelectronic devices.

Topology and applications of 2D Dirac and semi-Dirac materials

Abstract Two dimensional (2D) Dirac materials, such as graphene, hold promise of being useful in energy storage, and thus have merged as candidates that are worth exploring through the last couple of decades. In this chapter, we mainly focus on three aspects of these materials, namely, the electronic properties, via computing the band structure, the topological properties through the topological invariants, and the prospects of these 2D materials for spintronic applications, via studying the spin polarized transport. All of these properties are correlated, and hence warrant a thorough discussion. Further, in order to ascertain whether a band deformation induces noticeable effects on the electronic, topological and spintronic properties, we have considered a 2D semi-Dirac system, that does not have Dirac cones, however the conduction and the valence bands touch at an intermediate to the Dirac points in the Brillouin zone. From our studies, we infer that the behaviour of these semi-Dirac systems is quite distinct from their Dirac counterpart. Finally, in order to have noticeable spin polarized transport, we use heavy adatoms (such as, Au) on the graphene matrix which enhances the spin–orbit coupling, and thereby propose a mechanism that will ramify on the spintronic applications.

Nontrivial Berry's phase and exotic quantum phenomena in the vicinity of the quantum limit in the three-dimensional Dirac system Bi0.97Sb0.03

Quantum oscillations are known as the manifestation of the Fermi surface of metals in the presence of a strong magnetic field and of associated geometrical phases such as Berry's phase, playing a critical role in topological quantum materials. This study reports quantum oscillations on the three-dimensional (3D) Dirac material Bi0.97Sb0.03 single crystals in pulsed magnetic fields up to 50 T with three possible orientations. Under a magnetic field, we observe very pronounced Shubnikov-de Haas (SdH) oscillations originating from the Landau quantization of T-point holes. The SdH oscillations were well indexed with a large Landé g factor. The anisotropic behaviors of the Landé g factor were also deduced based on a detailed analysis of SdH oscillations. The Landau fan diagram revealed a nontrivial Berry's phase. Beyond the quantum limit (QL) under an extremely strong magnetic field, unusual quantum oscillations (UQO) were found. The possible origin of this phenomenon may be electronic instability caused by interactions. These results offer insights into the unusual electronic properties under the extremely strong fields caused by the unique band topology in this case.

Observation of Surface Superconductivity in a 3D Dirac Material

AbstractSuperconductivity becomes more interesting when it encounters dimensional constraint or topology because it is of importance for exploring exotic quantum phenomena or developing superconducting electronics. Here, the coexistence of naturally formed surface superconducting state and 3D topological Dirac state in single crystals of BaMg2Bi2 is reported. The electronic structure obtained from the first‐principles calculations demonstrates that BaMg2Bi2 is an ideal Dirac material, in which the Dirac point is very close to the Fermi level and no other energy band crosses the Fermi level. Superconductivity up to 4.77−−5.17 K can be observed under ambient pressure in the measurements of resistivity. The anisotropic upper critical field and angle dependent magnetoresistance reveals the 2D characteristic of superconductivity, indicating that superconductivity occurs on the surface of the sample and is absent in the bulk state. The study not only provides BaMg2Bi2 as a suitable platform to study the interplay between superconductivity and topological Dirac state but also indicates that MgBi‐based materials may be a promising system for exploring new superconductors.

Signatures of Lifshitz transition in the optical conductivity of two-dimensional tilted Dirac materials

Lifshitz transition is a kind of topological phase transition in which the Fermi surface is reconstructed. It can occur in the two-dimensional (2D) tilted Dirac materials when the energy bands change between the type-I phase ($0<t<1$) and the type-II phase ($t>1$) through the type-III phase ($t=1$), where different tilts are parametrized by the values of $t$. In order to characterize the Lifshitz transition therein, we theoretically investigate the longitudinal optical conductivities (LOCs) in type-I, type-II, and type-III Dirac materials within linear response theory. In the undoped case, the LOCs are constants either independent of the tilt parameter in both type-I and type-III phases or determined by the tilt parameter in the type-II phase. In the doped case, the LOCs are anisotropic and possess two critical frequencies determined by $\omega=\omega_1(t)$ and $\omega=\omega_2(t)$, which are also confirmed by the joint density of state. The tilt parameter and chemical potential can be extracted from optical experiments by measuring the positions of these two critical boundaries and their separation $\Delta\omega(t)=\omega_2(t)-\omega_1(t)$. With increasing the tilting, the separation becomes larger in the type-I phase whereas smaller in the type-II phase. The LOCs in the regime of large photon energy are exactly the same as that in the undoped case. The type of 2D tilted Dirac bands can be determined by the asymptotic background values, critical boundaries and their separation in the LOCs. These can therefore be taken as signatures of Lifshitz transition therein. The results of this work are expected to be qualitatively valid for a large number of 2D tilted Dirac materials, such as 8-\emph{Pmmn} borophene monolayer, $\alpha$-SnS$_2$, TaCoTe$_2$, TaIrTe$_4$, and $1T^\prime$ transition metal dichalcogenides, due to the underlying intrinsic similarities of 2D tilted Dirac bands.

Momentum Alignment and the Optical Valley Hall Effect in Low-Dimensional Dirac Materials

Momentum Alignment and the Optical Valley Hall Effect in Low-Dimensional Dirac Materials

Self-Induced Dirac Boundary State and Digitization in a Nonlinear Resonator Chain.

The low-energy excitations in many condensed matter and metamaterial systems can be well described by the Dirac equation. The mass term associated with these collective excitations, also known as the Dirac mass, can take any value and is directly responsible for determining whether the resultant band structure exhibits a band gap or a Dirac point with linear dispersion. Manipulation of this Dirac mass has inspired new methods of band structure engineering and electron confinement. Notably, it has been shown that a massless state necessarily localizes at any domain wall that divides regions with Dirac masses of different signs. These localized states are known as Jackiw-Rebbi-type Dirac boundary modes and their tunability and localization features have valuable technological potential. In this study, we experimentally demonstrate that nonlinearity within a 1D Dirac material can result in a self-induced domain boundary for the Dirac mass. Our experiments are performed in a dimerized magnetomechanical metamaterial that allows complete control of both the magnitude and sign of the local material nonlinearity, as well as the sign of the Dirac mass. We find that the massless bound state that emerges at the self-induced domain boundary acts similarly to a dopant site within an insulator, causing the material to exhibit a dramatic binary switch in its conductivity when driven above an excitation threshold.

Spin-polarized and possible pseudospin-polarized scanning tunneling microscopy in kagome metal FeSn

A kagome lattice (KL) is a two-dimensional atomic network comprising hexagons interspersed with triangles, which provides a fascinating platform for studying competing quantum ground states. The KL contains three atoms in a unit cell, and their degrees of freedom combine to yield Dirac bands and a flat band. Despite many studies to understand the flat band in KL, exploring the pseudospin of Dirac bands in KL has been scarce. In this paper, we suggest pseudospin-polarized scanning tunneling microscopy that is analogous to spin-polarized scanning tunneling microscopy. Using a pseudospin-polarized tip, we possibly observed the pseudospin texture of kagome metal FeSn in real space. Based on a simple tight-binding calculation, we further simulated the pseudospin texture of KL, confirming the geometric origin of pseudospin. This work potentially deepens our understanding of the lattice symmetry-preserving tunneling process in Dirac materials.

High switching ratio and inorganic gas sensing performance in BeN4 based nanodevice: a first-principles study

Recently, Dirac material BeN4 has been synthesized by using laser-heated diamond anvil-groove technology (Bykov et al 2021 Phys. Rev. Lett. 126 175501). BeN4 layer, i.e. beryllonitrene, represents a qualitatively class of two-dimensional (2D) materials that can be built of a metal atom and polymeric nitrogen chains, and hosts anisotropic Dirac fermions. Enlighten by this discovered material, we study the electronic structure, anisotropic transport properties and gas sensitivity of 2D BeN4 using the density functional theory combined with non-equilibrium Green’s function method. The results manifest that the 2D BeN4 shows a typical semi-metallic property. The electronic transport properties of the intrinsic BeN4 devices show a strong anisotropic behavior since electrons transmitting along the armchair direction is much easier than that along the zigzag direction. It directly results in an obvious switching characteristic with the switching ratio up to 105. Then the adsorption characteristics indicate that H2S, CO, CO2 and H2 molecules are physisorption, while the NH3, NO, NO2, SO2 molecules are chemisorption. Among these chemisorption cases, the 2D gas sensor devices show an extremely high response for SO2 recognition, and the high anisotropy of the original 2D BeN4 device still maintains after adsorbing gas molecules. Finally, high switching ratio and inorganic gas sensing performance of BeN4 monolayer could be clearly understood with local density of states, bias-dependent spectra, scattered state distribution. In general, the results indicate that the designed BeN4 devices have potential practical application in high-ratio switching devices and high gas-sensing molecular devices.

Ultrafast electronic heat dissipation through surface-to-bulk Coulomb coupling in quantum materials

The timescale of electronic cooling is an important parameter controlling the performance of devices based on quantum materials for optoelectronic, thermoelectric and thermal management applications. In most conventional materials, cooling proceeds via the emission of phonons, a relatively slow process that can bottleneck the carrier relaxation dynamics, thus degrading the device performance. Here we present the theory of near-field radiative heat transfer, that occurs when a two-dimensional electron system is coupled via the non-retarded Coulomb interaction to a three-dimensional bulk that can behave as a very efficient electronic heat sink. We apply our theory to study the cooling dynamics of surface states of three dimensional topological insulators, and of graphene in proximity to small-gap bulk materials. The ``Coulomb cooling'' we introduce is alternative to the conventional phonon-mediated cooling, can be very efficient and dominate the cooling dynamics under certain circumstances. We show that this cooling mechanism can lead to a sub-picosecond time scale, significantly faster than the cooling dynamics normally observed in Dirac materials.