Presence Of Dirac Cones Research Articles

Holographic description of an anisotropic Dirac semimetal

Holographic quantum matter exploits the AdS/CFT correspondence to study systems in condensed matter physics. An example of these systems are strongly correlated semimetals, which feature a rich phase diagram structure. In this work, we present a holographic model for a Dirac semimetal in 2 + 1 dimensions that features a topological phase transition. Our construction relies on deforming a relativistic UV fixed point with some relevant operators that explicitly break rotations and some internal symmetries. The phase diagram for different values of the relevant coupling constants is obtained. The different phases are characterized by distinct dispersion relations for probe fermionic modes in the AdS geometry. We find semi-metallic phases characterized by the presence of Dirac cones and an insulating phase featuring a mass gap with a mild anisotropy. Remarkably, we find as well an anisotropic semi-Dirac phase characterized by a massless a fermionic excitation dispersing linearly in one direction while quadratically in the other.

Layered van der Waals Topological Metals of TaTMTe4 (TM = Ir, Rh, Ru) Family

Layered van der Waals materials of the family TaTMTe4 (TM = Ir, Rh, Ru) are showing interesting electronic properties. We report the growth and characterization of TaIrTe4, TaRhTe4, TaIr1-xRhxTe4 (x = 0.06, 0.14, 0.78, 0.92), Ta1+xRu1-xTe4 single crystals. X-ray powder diffraction confirms that TaRhTe4 is isostructural to TaIrTe4. All these compounds are metallic with diamagnetic behavior. Below T ≈ 4 K we observed signatures of the superconductivity in the TaIr1-xRhxTe4 compounds for x = 0.92. All samples show weak quadratic-in-field magnetoresistance (MR). However, for TaIr1-xRhxTe4 with x ≈ 0.78, the MR has a linear term dominating in low fields that indicates the presence of Dirac cones in the vicinity of the Fermi energy. For TaRhTe4 series the MR is almost isotropic. Electronic structure calculations for TaIrTe4 and TaRhTe4 reveal appearance of the Rh band close to the Fermi level.

Band engineering of non-hexagonal 2D tetragonal-silicene sheet and nanoribbons: A theoretical approach

We report combined first-principle and tight-binding (TB) calculation to investigate the mechanical and electronic properties of non-hexagonal two-dimensional (2D) tetragonal-silicene (TS) sheet and nanoribbons (NRs). The results obtained by the TB method are consistent with the density functional theory (DFT) results. The elastic constants of the TS sheet are comparable to silicene. The existence of two Dirac cones is observed at different k→ points in the irreducible Brillouin Zone. A spin-orbit bandgap larger than silicene is observed in TS. The criteria for the stability and robustness of the Dirac Cones on the TB parameters are extensively investigated. Both the Dirac points are robust and stable under a wide range of hopping parameters. The degeneracy of the Dirac cones can be removed by introducing asymmetry in the on-site energy. Comparing the DFT and TB band structures, the values of the TB hopping parameters are estimated. The electronic properties of the TSNRs show a strong dependence on the width and the edge states. The symmetric armchair TSNRs show a width dependent multiple Dirac cones in the Brillouin zone. The number of Dirac cones of the NR is found to be dependent on the TB hopping parameters as well. On the other hand, asymmetric armchair TSNRs are semiconducting. The tuning of the bandgap for asymmetric armchair TSNRs by modulating the TB hopping parameters is also explored. Zigzag TSNR behaves as a degenerate semiconductor with the presence of Dirac cones just below and above EF. We subsequently expect that these theoretical findings will give a better comprehension of Dirac materials and their NRs with its potential application in nano-electronics.

Boosting the discovery of 3D topological materials: mixing chemistry with physics via a two-step computational screening strategy

Topological materials in crystal solids, including topological insulators (TIs), topological crystalline insulators (TCIs), topological Dirac semimetals (DSMs), topological Weyl semimetals (WSMs), topological Dirac or Weyl nodal line semimetals (NLSMs) and beyond, are mainly featured with topological, protected non-trivial surface states, and their bulk phases are insulators or semimetals with the proper presence of Dirac cones, Weyl nodes or Dirac nodal lines around the Fermi level. The author suggests a two-step computational screening strategy of 3D topological materials by mixing chemistry with physics with the considerations of fully filled bands and band inversion.

Topological metal of NaBi with ultralow lattice thermal conductivity and electron-phonon superconductivity.

By means of first-principles and ab initio tight-binding calculations, we found that the compound of NaBi is a three-dimensional non-trivial topological metal. Its topological feature can be confirmed by the presence of band inversion, the derived effective Z2 invariant and the non-trivial surface states with the presence of Dirac cones. Interestingly, our calculations further demonstrated that NaBi exhibits the uniquely combined properties between the electron-phonon coupling superconductivity in nice agreement with recent experimental measurements and the obviously anisotropic but extremely low thermal conductivity. The spin-orbit coupling effects greatly affect those properties. NaBi may provide a rich platform to study the relationship among metal, topology, superconductivity and thermal conductivity.

Graphene monolayers on GaN(0 0 0 1)

The epitaxial growth of graphene on GaN(0001) surfaces is studied by first principles total energy calculations. The idea is to understand how defect-free graphene can be grown on substrates. It is found that the most stable structures were a 4×4(0001) GaN/3√3×3√3 graphene and a 2√3×2√3(0001) GaN Northrup bilayer/√21×√21 graphene, grown under N- and Ga-rich conditions, respectively. In these structures, graphene maintains its hexagonal honeycomb structure with the CC bonds intact. Preservation of the π-network for the graphene layers was demonstrated by the presence of Dirac cones. The band structures for both the N- and Ga-rich configurations show metallic characteristics and the Ga-rich configuration is slightly magnetic. This demonstrates that GaN(0001) is an excellent substrate for supporting graphene layers.

Role of Dirac cones in magnetotransport properties of REFeAsO (RE = rare earth) oxypnictides

In this work we study the effect of the rare earth element in iron oxypnictides of composition REFeAsO (RE = rare earth). On one hand we carry out density functional theory calculations of the band structure, which evidence the multiband character of these compounds and the presence of Dirac cones along the Y-Γ and Z-R directions of the reciprocal space. On the other hand, we explore transport behavior by means of resistivity, Hall resistance and magnetoresistance measurements, which confirm the dominant role of Dirac cones. By combining our theoretical and experimental approaches, we extract information on effective masses, scattering rates and Fermi velocities for different rare earth elements.

Tilted elliptical Dirac cones at a half-metal surface

The surface electronic structure of the half-metallic alloy NiMnSb is investigated in detail, revealing the presence of Dirac cones in the minority-spin states. Surface-localized carriers of suitable energy will consequently behave as massless Dirac fermions, a remarkable situation for a half-metallic material and potentially of great significance in future spintronic device applications. Unlike related features observed previously in graphene and at the surfaces of topological insulators, the Dirac cones described here are of elliptical cross section and are tilted with respect to the energy axis. Such unique properties are permitted due to the lower symmetry in the present case.

Topological analysis of the quantum Hall effect in graphene: Dirac-Fermi transition across van Hove singularities and edge versus bulk quantum numbers

Inspired by a recent discovery of a peculiar integer quantum Hall effect (QHE) in graphene, we study QHE on a honeycomb lattice in terms of the topological quantum number, with two interests. First, how the zero-mass Dirac QHE around the center of the tight-binding band crosses over to the ordinary finite-mass fermion QHE around the band edges. Second, how the bulk QHE is related with the edge QHE for the entire spectrum including Dirac and ordinary behaviors. We find the following. (i) The zero-mass Dirac QHE [with ${\ensuremath{\sigma}}_{xy}=\ensuremath{\mp}(2N+1){e}^{2}∕h,N$: integer] persists, surprisingly, up to the van Hove singularities, at which the ordinary fermion behavior abruptly takes over. Here a technique developed in the lattice gauge theory enabled us to calculate the behavior of the topological number over the entire spectrum. This result indicates a robustness of the topological quantum number, and should be observable if the chemical potential can be varied over a wide range in graphene. (ii) To see if the honeycomb lattice is singular in producing the anomalous QHE, we have systematically surveyed over square $\ensuremath{\leftrightarrow}$ honeycomb $\ensuremath{\leftrightarrow}\ensuremath{\pi}$-flux lattices, which is scanned by introducing a diagonal transfer ${t}^{\ensuremath{'}}$. We find that the massless Dirac QHE [$\ensuremath{\propto}(2N+1)$] forms a critical line, that is, the presence of Dirac cones in the Brillouin zone is preserved by the inclusion of ${t}^{\ensuremath{'}}$ and the Dirac region sits side by side with ordinary one persists all through the transformation. (iii) We have compared the bulk QHE number obtained by an adiabatic continuity of the Chern number across the square $\ensuremath{\leftrightarrow}$ honeycomb $\ensuremath{\leftrightarrow}\ensuremath{\pi}$-flux transformation and numerically obtained edge QHE number calculated from the whole energy spectra for sample with edges, which shows that the bulk QHE number coincides, as in ordinary lattices, with the edge QHE number throughout the lattice transformation.