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Brillouin zone in graphene
Electronic band structure of monolayer graphene, with a zoomed inset showing the Dirac cones. There are 6 cones corresponding to the 6 vertices of the hexagonal first Brillouin zone.

In physics, Dirac cones are features that occur in some electronic band structures that describe unusual electron transport properties of materials like graphene and topological insulators.[1][2][3] In these materials, at energies near the Fermi level, the valence band and conduction band take the shape of the upper and lower halves of a conical surface, meeting at what are called Dirac points.

Typical examples include graphene, topological insulators, bismuth antimony thin films and some other novel nanomaterials,[1][4][5] in which the electronic energy and momentum have a linear dispersion relation such that the electronic band structure near the Fermi level takes the shape of an upper conical surface for the electrons and a lower conical surface for the holes. The two conical surfaces touch each other and form a zero-band gap semimetal.

The name of Dirac cone comes from the Dirac equation that can describe relativistic particles in quantum mechanics, proposed by Paul Dirac. Isotropic Dirac cones in graphene were first predicted by P. R. Wallace in 1947[6] and experimentally observed by the Nobel Prize laureates Andre Geim and Konstantin Novoselov in 2005.[7]

Description

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Tilted Dirac cones in momentum space. From left to right, the tilt increases, from no tilt in the first cone to overtilt in the last. The three first are Type-I Weyl semimetals, the last one is a Type-II Weyl semimetal.

In quantum mechanics, Dirac cones are a kind of crossing-point which electrons avoid,[8] where the energy of the valence and conduction bands are not equal anywhere in two dimensional lattice k-space, except at the zero dimensional Dirac points. As a result of the cones, electrical conduction can be described by the movement of charge carriers which are massless fermions, a situation which is handled theoretically by the relativistic Dirac equation.[9] The massless fermions lead to various quantum Hall effects, magnetoelectric effects in topological materials, and ultra high carrier mobility.[10][11] Dirac cones were observed in 2008-2009, using angle-resolved photoemission spectroscopy (ARPES) on the potassium-graphite intercalation compound KC8[12] and on several bismuth-based alloys.[13][14][11]

As an object with three dimensions, Dirac cones are a feature of two-dimensional materials or surface states, based on a linear dispersion relation between energy and the two components of the crystal momentum kx and ky. However, this concept can be extended to three dimensions, where Dirac semimetals are defined by a linear dispersion relation between energy and kx, ky, and kz. In k-space, this shows up as a hypercone, which have doubly degenerate bands which also meet at Dirac points.[11] Dirac semimetals contain both time reversal and spatial inversion symmetry; when one of these is broken, the Dirac points are split into two constituent Weyl points, and the material becomes a Weyl semimetal.[15][16][17][18][19][20][21][22][23][24][25][excessive citations] In 2014, direct observation of the Dirac semimetal band structure using ARPES was conducted on the Dirac semimetal cadmium arsenide.[26][27][28]

Analog systems

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Dirac points have been realized in many physical areas such as plasmonics, phononics, or nanophotonics (microcavities,[29] photonic crystals[30]).

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Dirac cone

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A Dirac cone is a distinctive feature in the of certain materials, characterized by a linear where the conduction and valence bands touch at isolated points in momentum space, called Dirac points, leading to quasiparticles that mimic massless relativistic particles described by the . This conical shape arises from the symmetry of the lattice, resulting in a constant Fermi velocity vF≈106v_F \approx 10^6 m/s, independent of energy near the Dirac points, and a vanishing at zero energy. The prototypical example of a Dirac cone occurs in , a single atomic layer of carbon arranged in a honeycomb lattice, where the cones are located at the K and K' points of the . The existence of these cones in was first theoretically predicted in 1947 using tight-binding approximations to model the band structure of , revealing a semi-metallic nature with touching bands. Experimental confirmation came in 2005, when isolated sheets were studied, demonstrating the linear dispersion and massless Dirac fermion behavior through measurements of the and cyclotron resonance. The Dirac cone imparts remarkable electronic properties to , including ultrahigh carrier mobility exceeding 200,000 cm²/V·s , minimal backscattering due to the pseudospin conservation, and the ability to tune the across the Dirac point via gating, enabling both and conduction in a single device. These features underpin phenomena like the Klein tunneling effect, where electrons transmit perfectly through potential barriers, and the anomalous with plateaus , observed even at ambient conditions. Such properties position as a for applications in high-speed electronics, flexible devices, and quantum technologies. Beyond , Dirac cones have been identified in other two-dimensional materials, such as , germanene, and dichalcogenides under strain, as well as on the surface states of three-dimensional topological insulators like Bi₂Se₃, where they are protected by time-reversal symmetry and host spin-momentum locking. In these systems, the cones often exhibit slight warping or tilting but retain the linear dispersion essential for topological protection against backscattering, enabling robust edge states and potential uses in and dissipationless transport.

Introduction and Historical Context

Definition

A Dirac cone is a linear in the electronic band structure of certain materials, characterized by the conduction and valence bands touching at discrete points called Dirac points, which results in charge carriers behaving as massless Dirac fermions. This feature arises in systems where the low-energy excitations mimic relativistic particles, with energy proportional to momentum near these points. The key characteristics of a Dirac cone include its distinctive conical shape in momentum-energy space, where the bands linearly disperse from the touching point, leading to relativistic-like dynamics at low energies and an effective mass of zero for the quasiparticles. This zero effective mass imparts unique transport properties to the charge carriers, distinguishing them from conventional parabolic band structures in semiconductors. The name "Dirac cone" derives from its analogy to the Dirac equation, which governs the linear spectrum of massless relativistic fermions in quantum field theory. Such structures occur notably in two-dimensional materials like .

History

The concept of the Dirac cone emerged from early theoretical studies of 's electronic structure. In 1947, Philip R. Wallace applied the tight-binding approximation to model the π-electron bands in , the single-layer form of , predicting a linear near the that forms conical points where the conduction and valence bands touch. This semimetallic behavior, with massless Dirac fermions, laid the groundwork for understanding Dirac cones, though experimental verification awaited decades due to challenges in isolating . The experimental realization of Dirac cones occurred with the isolation of in 2004 by and at the , using mechanical exfoliation from . Their subsequent work demonstrated that graphene's charge carriers behave as massless Dirac fermions, with linear dispersion confirmed through transport measurements showing the half-integer . This breakthrough, detailed in their 2005 paper, sparked intense research into two-dimensional materials exhibiting Dirac-like spectra. For these pioneering experiments on graphene, Geim and Novoselov were awarded the 2010 . Following the graphene discovery, Dirac cones were observed in other systems using angle-resolved photoemission spectroscopy (ARPES). In 2008, ARPES measurements on the potassium-graphite intercalation compound KC₈ revealed a full, undistorted Dirac cone from doped layers with minimal interlayer coupling. By 2009, surface states in bismuth-based topological insulators like Bi₂Te₃ and Sb₂Te₃ exhibited a single, time-reversal-protected Dirac cone, marking the first observation on the surfaces of three-dimensional topological insulators. These findings expanded the Dirac cone beyond to topologically protected . The evolution toward three-dimensional Dirac semimetals accelerated in , with ARPES confirming bulk Dirac cones in Na₃Bi, where linear dispersions extend along all momentum directions, analogous to a three-dimensional . Concurrently, Cd₃As₂ was identified as a 3D Dirac through transport and ARPES studies showing robust Weyl-like nodes protected by symmetry. These milestones established Dirac cones as a versatile feature in condensed matter, bridging relativistic physics with solid-state realizations.

Theoretical Description

Mathematical Formulation

The Dirac cone is characterized by a linear dispersion relation near the Dirac point, where the EE of quasiparticles scales linearly with the magnitude of the wavevector k\mathbf{k} measured from the Dirac point: E=±ℏvF∣k∣,E = \pm \hbar v_F |\mathbf{k}|, with vFv_F denoting the Fermi velocity, a material-specific analogous to the in relativistic systems. This relation emerges from the low-energy approximation of the tight-binding model for lattices, capturing the conical band touching at the corners. In two dimensions, the low-energy effective Hamiltonian describing electrons around a single Dirac point takes the form of a massless Dirac Hamiltonian: H=ℏvF(σxkx+σyky),H = \hbar v_F (\sigma_x k_x + \sigma_y k_y), where σx\sigma_x and σy\sigma_y are the acting on the sublattice pseudospin degree of freedom, and k=(kx,ky)\mathbf{k} = (k_x, k_y) is the in-plane wavevector relative to the Dirac point. This 2×2 Hamiltonian yields the linear dispersion upon diagonalization, with eigenstates resembling spinors and eigenvalues E=±ℏvF∣k∣E = \pm \hbar v_F |\mathbf{k}|. The full description for systems like includes valley degeneracy at the two inequivalent Dirac points K\mathbf{K} and K′\mathbf{K}', where the Hamiltonian at K′\mathbf{K}' involves the complex conjugate Pauli matrices. For three-dimensional realizations, the bulk Dirac cone extends the dispersion to a point-like band crossing in the 3D , described by an effective Hamiltonian H=ℏvF(σ⋅k),H = \hbar v_F (\boldsymbol{\sigma} \cdot \mathbf{k}), now with k=(kx,ky,kz)\mathbf{k} = (k_x, k_y, k_z) and σ=(σx,σy,σz)\boldsymbol{\sigma} = (\sigma_x, \sigma_y, \sigma_z), though in practice, this minimal 2×2 form applies to Weyl points, while full 3D Dirac points require a 4×4 Hamiltonian incorporating spin and orbital to respect time-reversal . In materials with cubic , such as certain topological semimetals, additional terms may arise from lattice anisotropy, but the linear σ⋅k\boldsymbol{\sigma} \cdot \mathbf{k} term dominates the low-energy physics near the Dirac point, preserving the conical dispersion E=±ℏvF∣k∣E = \pm \hbar v_F |\mathbf{k}|. A key topological feature of the Dirac cone is the Berry phase acquired by quasiparticles encircling the cone in momentum space, which equals π\pi (modulo 2π2\pi) due to the pseudospin winding. This π\pi Berry phase shifts the Landau level , manifesting as the anomalous quantum Hall effect with plateaus at half-integer fillings σxy=±4(n+1/2)e2/h\sigma_{xy} = \pm 4(n + 1/2) e^2 / h, where n=0,1,2,…n = 0, 1, 2, \dots.

Relation to the Dirac Equation

The , originally formulated to describe relativistic electrons, provides a foundational analogy for understanding Dirac cones in condensed matter systems. For massless fermions, the time-dependent takes the form iℏ∂ψ∂t=cα⋅p ψ,i \hbar \frac{\partial \psi}{\partial t} = c \boldsymbol{\alpha} \cdot \mathbf{p} \, \psi, where ψ\psi is a multi-component , cc is the , p\mathbf{p} is the , and α\boldsymbol{\alpha} are matrices generalizing the to higher dimensions. This equation yields a linear energy-momentum dispersion E=±c∣p∣E = \pm c |\mathbf{p}|, characteristic of massless particles like photons or neutrinos, with the linear dependence in momentum mirroring the conical band structure near Dirac points in materials. In , charge carriers near Dirac cones in two-dimensional (2D) or three-dimensional (3D) systems effectively behave as massless Dirac particles, described by an analogous low-energy effective Hamiltonian. For 2D cases, such as those arising from sublattice , the Hamiltonian simplifies to H=vFσ⋅kH = v_F \boldsymbol{\sigma} \cdot \mathbf{k}, where vFv_F is the Fermi , σ\boldsymbol{\sigma} are , and k\mathbf{k} is the crystal momentum measured from the Dirac point; this maps directly to a 2D massless Dirac equation. The pseudospin in these systems originates from orbital or sublattice (e.g., the two carbon sublattices in lattices), playing a role akin to the real spin in the relativistic case and endowing quasiparticles with . In 3D realizations, such as , the description extends to helical Dirac fermions with similar linear dispersion. Key differences distinguish these material realizations from the fundamental relativistic . The effective particle speeds are non-relativistic, with vF≈c/300v_F \approx c/300 (e.g., vF∼106v_F \sim 10^6 m/s), breaking Lorentz invariance and arising instead from the periodicity of the underlying crystal lattice via tight-binding or k⋅p\mathbf{k} \cdot \mathbf{p} models. This linear dispersion E=ℏvF∣k∣E = \hbar v_F |k| emerges at isolated points in the due to band-touching symmetries. Nonetheless, the holds for low-energy excitations, enabling the study of relativistic quantum effects in tabletop experiments. These parallels have profound implications, particularly in topological contexts where the Dirac-like structure enforces spin-momentum locking: the spin (or pseudospin) aligns perpendicular to its momentum, suppressing backscattering and ensuring robustness against perturbations. This locking, a direct consequence of the chiral nature of the massless Dirac description, underpins the protected edge or in topological materials.

Realizations in Electronic Materials

Graphene and Carbon-Based Systems

Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, serves as the prototypical realization of the Dirac cone in electronic materials. The two-dimensional honeycomb structure results in Dirac points located at the and corners of the hexagonal , where the conduction and valence bands touch linearly. This linear dispersion arises from the equivalence of the A and B sublattices in the tight-binding model, leading to massless Dirac fermions at low energies. The charge carriers in graphene exhibit a constant Fermi velocity of approximately vF≈106v_F \approx 10^6 m/s near the Dirac points, remarkably independent of carrier density. This velocity, about 300 times smaller than the speed of light, governs the relativistic-like behavior of electrons and remains robust across a wide range of doping levels, enabling unique transport phenomena such as the half-integer quantum Hall effect. External perturbations like strain and doping allow for tunability of the Dirac cone structure in . Uniaxial strain shifts the positions of the Dirac points in the and can induce pseudo-magnetic fields, effectively modifying the cone's shape and opening small bandgaps under specific conditions, such as or armchair alignments. Doping, achieved through chemical or electrostatic means, primarily shifts the [Fermi level](/page/Fermi level) relative to the Dirac point without altering the linear dispersion, enabling control over carrier type ( or ) and density while preserving the cone's integrity. In with Bernal stacking, interlayer coupling transforms the low-energy dispersion from linear to parabolic, resulting in massive Dirac fermions with quadratic band touching at the K and K' points. Recent advancements in carbon-based systems have explored beyond pristine graphene. Propylenidene (PPD), a predicted metallic carbon monolayer with a rectangular lattice incorporating 3-, 8-, and 10-membered rings, exhibits enhanced absorption in the infrared and visible spectra, suggesting potential applications in optoelectronics.

Topological Insulators and Semimetals

In three-dimensional topological insulators, such as Bi₂Se₃ and Bi₂Te₃, the surface states host a single Dirac cone at the Γ point of the surface Brillouin zone, featuring massless Dirac fermions with linear dispersion. These surface states exhibit a spin-helical texture, where the electron spin is locked perpendicular to the momentum due to strong spin-orbit coupling, suppressing backscattering and enhancing robustness against nonmagnetic impurities. The Dirac cone is protected by time-reversal symmetry, ensuring an odd number of such cones and topological nontriviality, as confirmed by angle-resolved photoemission spectroscopy (ARPES) with the Dirac point typically approximately 0.2–0.3 eV below the Fermi level in Bi₂Te₃ and a Fermi velocity of about 5 × 10⁵ m/s. In thinner films of Bi₂Se₃, quantum tunneling between top and bottom surfaces induces a gap in the Dirac cone for thicknesses below six quintuple layers, while the states remain topologically protected in bulk samples. Bulk Dirac and Weyl semimetals, exemplified by Na₃Bi and Cd₃As₂, feature three-dimensional linear band crossings forming Dirac points in the bulk band structure, distinct from the surface-localized cones in insulators. In Na₃Bi, the Dirac points are located along the (001) direction at wave vectors K± = (0, 0, ±k_D) with k_D ≈ 0.1 Å⁻¹, arising from band inversion between Na-3s and Bi-6p orbitals under strong spin-orbit coupling, and exhibit linear dispersion E(k) = ħv|k| with Fermi velocity v, as evidenced by Shubnikov–de Haas oscillations. Similarly, Cd₃As₂ hosts bulk Dirac cones along the direction at ±k_D, each composed of two degenerate Weyl nodes of opposite , displaying linear dispersion confirmed by ARPES and leading to exotic transport like giant positive . These materials manifest the through a negative longitudinal when the electric and magnetic fields are parallel, arising from charge pumping between Weyl nodes of opposite , with the effect aligning sensitively with the field direction in Na₃Bi. In Weyl semimetals derived from such Dirac systems, breaking time-reversal or inversion symmetry splits the Dirac points into Weyl nodes, enabling surface Fermi arcs connecting nodes of opposite . The bulk topology in these materials gives rise to magnetoelectric effects described by electrodynamics, where an field θ = π modulates the electromagnetic response, inducing a topological magnetoelectric polarization. This manifests as a quantized magnetoelectric in time-reversal-invariant topological insulators, with electric fields generating magnetic dipoles and vice versa, enhanced in dynamical insulators through magnetic ordering. In Weyl semimetals, the term captures the and anomalous , linking bulk topology to observable responses like the chiral magnetic effect. Semi-Dirac points, representing a hybrid dispersion intermediate between Dirac and conventional parabolic bands, appear in TiO₂/VO₂ heterostructures due to quantum confinement of VO₂ slabs within insulating TiO₂ layers. In configurations like (TiO₂)₅/(VO₂)₃ multilayers, the band crossing at the occurs at points such as k_{sD} = (±0.37, ±0.37) π/a, with linear dispersion along one direction (e.g., (1,1)) and quadratic along the perpendicular, resulting in extreme in effective mass and velocity (up to 1.5 × 10⁷ cm/s in one direction). This half-metallic state signals a topological transition from to insulator, robust against interface disorder and VO₂ thickness variations beyond three layers, and implies highly anisotropic transport and thermodynamic properties.

Recent Materials and Developments

Recent advancements in Dirac cone materials have focused on novel electronic systems exhibiting non-trivial topological features, expanding the scope beyond traditional realizations. In 2025, studies on the Dirac semimetal YNiSn₂ revealed putative non-trivial through investigations of its single-crystalline form, including x-ray powder diffraction, electrical resistivity, and measurements that confirm low-energy excitations akin to massless Dirac fermions. In the realm of superconductors, the Laves phase compound CeRu₂, featuring a pyrochlore sublattice, has been identified as hosting both flat bands and Dirac cones in its electronic structure. (ARPES) experiments conducted in 2024 demonstrated flat bands originating from Ce 4f orbitals and additional three-dimensional flat bands from Ru 4d orbitals, alongside Dirac cones at deeper binding energies (∼1.7 eV below the ), suggesting a link between these topological features and the material's . Theoretical modeling of electron transfer dynamics at graphene defects has advanced in 2025 by treating the graphene surface as a perturbed Dirac cone with localized defect states. This framework incorporates fractional statistics to describe processes, highlighting how topological defects couple to the Dirac cone dispersion and influence charge dynamics in these systems.

Physical Properties

Electronic and Transport Properties

The electronic properties of materials featuring Dirac cones are dominated by the behavior of massless Dirac fermions, which exhibit a linear energy-momentum near the Dirac point. This results in ambipolar conduction, where the charge carriers can be continuously tuned between electrons and holes by applying a gate voltage, allowing the to cross the Dirac point and control the carrier type and electrostatically. In , this gate-tunable property enables the realization of bipolar field-effect transistors with conductivity that increases linearly with carrier on either side of the neutrality point. Transport characteristics in Dirac cone systems reveal unique quantum phenomena arising from the pseudospin structure of the wavefunctions. At the Dirac point, where the carrier density vanishes, the minimal conductivity is universally quantized at σ=4e2πh\sigma = \frac{4 e^2}{\pi h}, with hh denoting the reduced Planck's constant; this value stems from the evanescent nature of the wavefunctions in the absence of propagating states, leading to a finite residual conductance despite the gapless spectrum. Additionally, the chiral nature of Dirac fermions enables Klein tunneling, manifesting as perfect transmission of charge carriers through potential barriers at normal incidence, which suppresses backscattering and enhances overall transport efficiency. These effects contribute to exceptionally high carrier mobilities, reaching up to 200,000 cm²/Vs in suspended graphene samples at room temperature, limited primarily by phonon scattering rather than impurities. In moiré superlattices formed by twisted , electron-electron interactions introduce further complexity to the transport landscape. Recent nanoscale imaging has revealed that these interactions cause the flat-band Dirac cones to migrate toward the center of the mini-Brillouin zone in semimetallic phases, spontaneously breaking the threefold and altering the local . This migration, observed through quantum oscillations in 2025 experiments, highlights how long-range effects can renormalize the band structure, influencing dynamics in correlated moiré systems.

Topological Features

The topological features of Dirac cones arise from their association with Berry curvature in momentum space, which encodes the acquired by electrons during adiabatic evolution. In three-dimensional (3D) systems, Weyl points—linear band crossings analogous to but distinct from 2D Dirac cones—serve as sources or sinks of Berry curvature, manifesting as monopoles with topological charge ±1. These monopoles induce nontrivial Berry flux, leading to phenomena like the , where the charge is determined by the of the Weyl . In 3D Dirac semimetals, a Dirac point emerges as a superposition of two Weyl points of opposite , resulting in a net-zero monopole charge due to protection, yet preserving linear dispersion and topological robustness. Time-reversal symmetry plays a crucial role in enforcing the twofold degeneracy at Dirac points, particularly in 2D materials like . This maps the wavefunction under time inversion, ensuring that the two sublattice components remain degenerate at the corners (K and K' points), preventing gap opening unless both time-reversal and inversion symmetries are broken. The protection arises because time-reversal pairs the Dirac cones, making them stable against perturbations that preserve the , as detailed in the tight-binding model of where the low-energy effective Hamiltonian exhibits this degeneracy. In gapped variants of Dirac systems, such as those induced by sublattice imbalance or magnetic fields, the Chern number emerges as a key topological invariant quantifying the Berry curvature flux through the Brillouin zone. For a single gapped Dirac cone, the Chern number is typically ±1/2 per spin-valley flavor, leading to a half-integer contribution to the quantum Hall conductivity of σ_xy = (n + 1/2) e²/h, where n is an integer. In the Haldane model on a honeycomb lattice, breaking time-reversal symmetry via complex next-nearest-neighbor hopping gaps the Dirac points, with each acquiring a Chern number of ±1/2, yielding a total Chern insulator phase with quantized Hall conductivity σ_xy = ± e²/h. Recent theoretical advancements in 2025 have explored non-Hermitian Dirac cones, where gain and loss introduce complex spectra and valley-dependent lifetimes. In lattice models incorporating non-reciprocal hopping, one Dirac cone exhibits amplification (positive imaginary part, longer lifetime) while the other shows decay (negative imaginary part, shorter lifetime), enabling valley-selective control without breaking Hermitian symmetries. This contrast in lifetimes, quantified by the imaginary part of the eigenvalue, enhances filtering efficiency, with applications in robust topological under .

Experimental Observations

Techniques and Early Discoveries

The initial experimental confirmation of Dirac cones in relied heavily on (ARPES), which directly maps the . In 2007, high-resolution ARPES measurements on exfoliated sheets revealed the characteristic linear dispersion of the π and π* bands touching at the Dirac point near the , providing the first direct visualization of the conical band structure predicted for massless Dirac fermions. These observations confirmed the relativistic-like energy-momentum relation, with Fermi velocities on the order of 10^6 m/s, distinguishing graphene's electronic properties from conventional parabolic bands. ARPES was also pivotal in early studies of topological insulators, particularly Bi-based alloys. In 2009, ARPES on single crystals of Bi₂Te₃ demonstrated a single Dirac cone on the surface, protected by time-reversal symmetry, with the cone located at the Γ point of the and exhibiting a helical spin texture. Similar measurements on Sb₂Te₃ confirmed an analogous surface state, though with a slightly larger bulk , establishing these materials as model systems for 2D topological surface states hosting Dirac fermions. Scanning tunneling microscopy (STM) provided complementary real-space insights into Dirac cone physics by visualizing quantum interference patterns. Early STM experiments on in 2009 imaged standing wave patterns near the Dirac points, arising from backscattering of quasiparticles off impurities or edges, which highlighted the suppression of intervalley due to the pseudospin conservation in Dirac systems. These interference fringes, with wavelengths scaling linearly with from the Dirac point, offered microscopic evidence of the linear dispersion and the Berry phase associated with the cone topology. Transport measurements further corroborated the Dirac nature through magnetoconductance oscillations. Between 2005 and 2007, Shubnikov-de Haas (SdH) oscillations in under perpendicular magnetic fields exhibited a phase shift indicative of a nontrivial phase of π, distinct from the conventional 0 or 2π in massive systems, confirming the massless Dirac description. These oscillations, observed at low temperatures and high mobilities, showed half-integer quantum Hall plateaus, linking the transport anomalies directly to the conical band structure. Early evidence for three-dimensional Dirac cones emerged in 2014 through magnetotransport in Cd₃As₂ crystals. Low-temperature measurements revealed a linear quantum persisting up to , attributed to the in Weyl-like nodes, alongside SdH oscillations consistent with bulk Dirac dispersion along the crystal axis. This provided the first transport signature of protected 3D Dirac semimetal states in a non-magnetic material, with the cones tunable by gating.

Modern Probes and Findings

Recent advancements in angle-resolved photoemission spectroscopy (ARPES) have enabled nanoscale spatial resolution and time-resolved probing of Dirac cone dynamics in moiré graphene systems. Nano-ARPES, with its sub-micrometer beam focus, has been instrumental in mapping the electronic structure of rhombohedral pentalayer aligned with hexagonal , revealing moiré-enhanced topological flat bands near the charge neutrality point. These measurements extract hopping parameters and demonstrate how moiré potentials flatten bands, providing direct visualization of symmetry-protected features in twisted structures. Complementing this, time-resolved ARPES (TR-ARPES) captures ultrafast electron dynamics in , including moiré superlattices, by resolving non-equilibrium states on timescales following optical excitation. Defects in , such as vacancies or substitutions induced by strain or , scatter electrons and alter local band structures, inducing pseudo-magnetic fields that warp the linear Dirac dispersion into . Terahertz has emerged as a powerful tool for probing tilted Dirac cones in novel two-dimensional materials, exploiting their unique optoelectronic responses. In type-II Dirac semimetals like PtSe₂, terahertz emission under femtosecond laser excitation reveals anisotropic carrier dynamics driven by tilted cones at electron-hole pocket junctions. The photon drag effect generates both linear and circularly polarized terahertz signals, with emission intensity modulated by orientation and pump polarization, confirming the role of tilt in enhancing separation and nonlinear responses. Theoretical analyses of optical reflectivity spectra predict signatures of tilt parameters, such as Fermi velocity anisotropy, in materials like dichalcogenides. Key findings from these probes include the observation of migrating Dirac cones under Coulomb interactions in twisted graphene systems. In alternating-twist trilayer graphene near charge neutrality, exchange Coulomb interactions cause flat-band Dirac cones to migrate toward the mini-Brillouin zone center, spontaneously breaking threefold rotational symmetry and forming a nematic semimetal phase. Local quantum oscillations at low fields (56 mT) image this migration, with no gap opening (<1 meV) at the neutrality point, while Hartree terms renormalize band widths up to 49.4 meV. Additionally, in pyrochlore lattice superconductors like CeRu₂, ARPES has confirmed the coexistence of flat bands and Dirac cones, with destructive interference yielding flat Ru 4d bands at -0.5 to -0.6 eV and nonsymmorphic symmetry-protected Dirac cones at the X point around -1.7 eV. This 2024 discovery highlights the interplay of topology and superconductivity in three-dimensional realizations.

Analog Systems

Photonic and Optical Analogs

Photonic crystals engineered with lattices provide a direct optical analog to the electronic Dirac cones in , where the lattice geometry induces linear dispersion relations for electromagnetic waves at the corners, mimicking the massless Dirac fermions. These structures, often fabricated using materials like or , exhibit Dirac points where the photonic bands touch, enabling phenomena such as pseudodiffusive transport and enhanced light-matter interactions. Experimental realizations in two-dimensional photonic crystals have confirmed these Dirac cones through angle-resolved transmission measurements, demonstrating isotropic wave propagation akin to graphene's electronic states. In topological photonic insulators, surface states manifest as Dirac cones, protected by symmetries that ensure robust edge propagation immune to backscattering. A notable 2025 advancement involves a three-dimensional photonic antiferromagnetic topological insulator, where a single surface Dirac cone emerges due to the interplay of antiferromagnetic ordering and spin-orbit coupling analogs in photonic lattices composed of magneto-optical materials. This configuration, experimentally verified via near-field scanning, supports unidirectional surface waves with linear dispersion, offering potential for low-loss photonic routing in integrated circuits. Dirac-vortex modes arise in Kekulé-distorted photonic lattices, where a modulated introduces vortex-like defects that bind topologically protected states. In a 2025 study, three-dimensional photonic topological insulators with such distortions were proposed and demonstrated, hosting Dirac-vortex modes that enable robust waveguiding along defect lines with minimal dissipation. These modes, characterized by their zero-energy bound states and helical field profiles, were observed in dielectric stack experiments, facilitating applications in topological photonic fibers for high-fidelity signal transmission. Semi-Dirac photonic metamaterials exhibit hybrid dispersions—linear in and quadratic in another—leading to unique zero-refractive-index effects that permit anomalous and phase uniformity. A 2025 investigation into semi-Dirac photonic crystals revealed highly robust anisotropic zero , where incoming waves bend without deviation due to the effective approaching zero at the semi-Dirac point. These metamaterials show potential in superlensing and devices.

Phononic and Mechanical Analogs

Phononic analogs of Dirac cones have been realized in sonic crystals, where mimic the linear electronic dispersion of . In these structures, such as plates with honeycomb-patterned boreholes, surface exhibit Dirac cones at the edges, characterized by linear dispersion relations around the cone vertices. These cones emerge from the effective massless governing wave propagation, with the Dirac velocity determined by borehole geometry and material properties. Near the Dirac points, van Hove singularities in the lead to enhanced wave interactions, analogous to those in electronic . Mechanical analogs extend this concept to classical elastic systems, particularly vibrating thin plates or lattices supporting flexural waves with pseudo-relativistic behavior. A honeycomb array of spring-mass resonators attached to a thin elastic plate creates an effective graphene-like , where flexural waves display linear dispersion at Dirac points in the . These waves obey a Dirac-like , enabling massless propagation and topological edge states protected against backscattering. Such systems demonstrate chiral selection and unidirectional transport, mirroring relativistic particle dynamics in curved analogs. Valley-dependent phononic transport arises in these analogs from the inequivalent Dirac cones at K and K' points, enabling selective waveguiding based on valley pseudospin. In graphene-like phononic crystals with modulated air cavities, band inversion at the valleys opens a topological gap, supporting dual-band edge states with valley-protected robustness. These states exhibit suppressed backscattering and immunity to defects, facilitating one-way acoustic propagation and beam splitting via valley vortex beams of opposite chirality. This valley Hall effect in phonons parallels electronic valleytronics, with applications in noise-immune acoustic devices. Anisotropic semi-Dirac phonons, featuring linear dispersion in and quadratic in the perpendicular, have been observed in multi-component phononic , extending electronic semi-Dirac cases to acoustics. In square lattices of core-shell elliptical cylinders embedded in , accidental degeneracy at the zone center yields semi-Dirac points, leading to directionally dependent . Along the linear axis, waves experience zero phase shift and total transmission at the semi-Dirac , mimicking zero-index behavior, while the quadratic direction suppresses transmission, enabling acoustic diodes and control. Recent developments, building on these foundations, explore enhanced robustness in deformed lattices for applications like unidirectional routing.

Other Artificial Systems

In artificial systems beyond photonic and phononic analogs, spin-orbit superlattices in -based moiré patterns enable the engineering of designer Dirac cones with enhanced tunability. By laterally patterning proximity-induced spin-orbit coupling in -transition metal dichalcogenide heterostructures, such as on WSe₂, periodic modulations create symmetry-protected Dirac points where the spin-orbit interaction strength can be spatially varied, leading to relativistic band structures with adjustable cone velocities and spin Berry curvature effects. This approach, demonstrated theoretically in 2024, allows for the realization of flat bands and topological phases by controlling the periodicity, offering a platform for exploring spin-valley locking without intrinsic material limitations. Cold atomic gases loaded into optical lattices provide a versatile for Dirac physics through synthetic gauge fields that realize highly tunable Dirac points. Ultracold s, such as ⁴⁰K atoms, in a lattice subjected to Raman-induced synthetic vector potentials mimic the low-energy Dirac Hamiltonian, where the Dirac cone position, velocity, and can be adjusted via lattice parameters and gauge . Experimental realizations have achieved Dirac points with velocities tunable over an order of magnitude, enabling studies of massless dynamics and interactions under controlled conditions. These systems also facilitate the observation of Berry phase effects in artificial gauge fields, analogous to those in solid-state Dirac materials. Non-Hermitian analogs of Dirac cones emerge in PT-symmetric lattices, where gain-loss balances introduce exceptional points that mimic valley-dependent lifetimes. In a proposed 2025 lattice model, two Dirac cones exhibit contrasting decay rates between s due to non-reciprocal hopping modulated by imaginary potentials, transforming Hermitian Dirac points into exceptional rings under increasing non-Hermiticity. This valley lifetime contrast, exceeding 10:1 in simulations, enables ultra-strong valley selection rules and unidirectional , with applications in non-reciprocal devices. The PT symmetry ensures real spectra below the exceptional point threshold, providing a benchmark for probing in topological systems. Electrical circuit networks using LC resonators serve as topolectrical analogs for topological Dirac transport, replicating Dirac cone dispersions through impedance matrices. In LC-based honeycomb lattices, inductors and capacitors tuned to form a tight-binding model yield Dirac points at the Brillouin zone corners, with topological edge states protected by valley Chern numbers up to 1. Experimental platforms have demonstrated robust chiral edge currents near these Dirac points, with transport robust against disorder via modular resistor additions. These circuits offer scalability for simulating higher-order topology and non-equilibrium dynamics at room temperature.

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