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New Generation of Massless Dirac Fermions in Graphene under External Periodic Potentials  [PDF]
Cheol-Hwan Park,Li Yang,Young-Woo Son,Marvin L. Cohen,Steven G. Louie
Physics , 2008, DOI: 10.1103/PhysRevLett.101.126804
Abstract: We show that new massless Dirac fermions are generated when a slowly varying periodic potential is applied to graphene. These quasiparticles, generated near the supercell Brillouin zone boundaries with anisotropic group velocity, are different from the original massless Dirac fermions. The quasiparticle wavevector (measured from the new Dirac point), the generalized pseudospin vector, and the group velocity are not collinear. We further show that with an appropriate periodic potential of triangular symmetry, there exists an energy window over which the only available states are these quasiparticles, thus, providing a good system to probe experimentally the new massless Dirac fermions. The required parameters of external potentials are within the realm of laboratory conditions.
Massless Dirac fermions in epitaxial graphene on Fe(110)  [PDF]
A. Varykhalov,J. Sánchez-Barriga,P. Hlawenka,O. Rader
Physics , 2012,
Abstract: Graphene grown on Fe(110)by chemical vapor deposition using propylene is investigated by means of angle-resolved photoemission. The presence of massless Dirac fermions is clearly evidenced by the observation of a fully intact Dirac cone. Unlike Ni(111) and Co(0001), the Fe(110) imposes a strongly anisotropic quasi-one-dimensional structure on the graphene. Certain signatures of a superlattice effect appear in the dispersion of its \sigma-bands but the Dirac cone does not reveal any detectable superlattice or quantum-size effects although the graphene corrugation is twice as large as in the established two-dimensional graphene superlattice on Ir(111).
Magnetic confinement of massless Dirac fermions in graphene  [PDF]
A. De Martino,L. Dell'Anna,R. Egger
Physics , 2006, DOI: 10.1103/PhysRevLett.98.066802
Abstract: Due to Klein tunneling, electrostatic potentials are unable to confine Dirac electrons. We show that it is possible to confine massless Dirac fermions in a monolayer graphene sheet by inhomogeneous magnetic fields. This allows one to design mesoscopic structures in graphene by magnetic barriers, e.g. quantum dots or quantum point contacts.
Quantum electrodynamics with anisotropic scaling: Heisenberg-Euler action and Schwinger pair production in the bilayer graphene  [PDF]
M. I. Katsnelson,G. E. Volovik
Physics , 2012, DOI: 10.1134/S0021364012080061
Abstract: We discuss quantum electrodynamics emerging in the vacua with anisotropic scaling. Systems with anisotropic scaling were suggested by Horava in relation to the quantum theory of gravity. In such vacua the space and time are not equivalent, and moreover they obey different scaling laws, called the anisotropic scaling. Such anisotropic scaling takes place for fermions in bilayer graphene, where if one neglects the trigonal warping effects the massless Dirac fermions have quadratic dispersion. This results in the anisotropic quantum electrodynamics, in which electric and magnetic fields obey different scaling laws. Here we discuss the Heisenberg-Euler action and Schwinger pair production in such anisotropic QED
Anisotropic Weyl Fermions from Quasiparticle Excitation Spectrum of a 3D Fulde-Ferrell Superfluid  [PDF]
Yong Xu,Rui-Lin Chu,Chuanwei Zhang
Physics , 2013, DOI: 10.1103/PhysRevLett.112.136402
Abstract: Weyl fermions, first proposed for describing massless chiral Dirac fermions in particle physics, have not been observed yet in experiments. Recently, much effort has been devoted to explore Weyl fermions around band touching points of single particle energy dispersions in certain solid state materials (named \textit{Weyl semimetals}), similar as graphene for Dirac fermions. Here we show that such Weyl semimetals also exist in the quasiparticle excitation spectrum of a three-dimensional (3D) spin-orbit coupled Fulde-Ferrell (FF) superfluid. By varying Zeeman fields, the properties of Weyl fermions, such as their creation and annihilation, number and position, as well as anisotropic linear dispersions around band touching points, can be tuned. We study the manifestation of anisotropic Weyl fermions in sound speeds of FF fermionic superfluids, which are detectable in experiments.
Massless Dirac Fermions in a Square Optical Lattice  [PDF]
Jing-Min Hou,Wen-Xing Yang,Xiong-Jun Liu
Physics , 2009, DOI: 10.1103/PhysRevA.79.043621
Abstract: We propose a novel scheme to simulate and observe massless Dirac fermions with cold atoms in a square optical lattice. A U(1) adiabatic phase is created by two laser beams for the tunneling of atoms between neighbor lattice sites. Properly adjusting the tunneling phase, we find that the energy spectrum has conical points in per Brillouin zone where band crossing occurs. Near these crossing points the quasiparticles and quasiholes can be considered as massless Dirac fermions. Furthermore, the anisotropic effects of massless Dirac fermions are obtained in the present square lattice model. The Dirac fermions as well as the anisotropic behaviors realizeded in our system can be experimentally detected with the Bragg spectroscopy technique.
Massless Dirac Fermions in Graphene under an External Periodic Magnetic Field  [PDF]
Shuanglong Liu,Argo Nurbawono,Na Guo,Chun Zhang
Physics , 2013, DOI: 10.1088/0953-8984/25/39/395302
Abstract: By solving two-component spinor equation for massless Dirac Fermions, we show that graphene under a periodic external magnetic field exhibits a unique energy spectrum: At low energies, Dirac Fermions are localized inside the magnetic region with discrete Landau energy levels, while at higher energies, Dirac Fermions are mainly found in non-magnetic regions with continuous energy bands originating from wavefunctions analogous to particle-in-box states of electrons. These findings offer a new methodology for the control and tuning of massless Dirac Fermions in graphene.
Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride  [PDF]
Matthew Yankowitz,Jiamin Xue,Daniel Cormode,Javier D. Sanchez-Yamagishi,K. Watanabe,T. Taniguchi,Pablo Jarillo-Herrero,Philippe Jacquod,Brian J. LeRoy
Physics , 2012, DOI: 10.1038/nphys2272
Abstract: The Schr\"odinger equation dictates that the propagation of nearly free electrons through a weak periodic potential results in the opening of band gaps near points of the reciprocal lattice known as Brillouin zone boundaries. However, in the case of massless Dirac fermions, it has been predicted that the chirality of the charge carriers prevents the opening of a band gap and instead new Dirac points appear in the electronic structure of the material. Graphene on hexagonal boron nitride (hBN) exhibits a rotation dependent Moir\'e pattern. In this letter, we show experimentally and theoretically that this Moir\'e pattern acts as a weak periodic potential and thereby leads to the emergence of a new set of Dirac points at an energy determined by its wavelength. The new massless Dirac fermions generated at these superlattice Dirac points are characterized by a significantly reduced Fermi velocity. The local density of states near these Dirac cones exhibits hexagonal modulations indicating an anisotropic Fermi velocity.
Quantum transport of massless Dirac fermions in graphene  [PDF]
Kentaro Nomura,A. H. MacDonald
Physics , 2006, DOI: 10.1103/PhysRevLett.98.076602
Abstract: Motivated by recent graphene transport experiments, we have undertaken a numerical study of the conductivity of disordered two-dimensional massless Dirac fermions. Our results reveal distinct differences between the cases of short-range and Coulomb randomly distributed scatterers. We speculate that this behavior is related to the Boltzmann transport theory prediction of dirty-limit behavior for Coulomb scatterers.
Transport of Massless Dirac Fermions in Non-topological Type Edge States  [PDF]
Yu. I. Latyshev,A. P. Orlov,V. A. Volkov,V. V. Enaldiev,I. V. Zagorodnev,O. F. Vyvenko,Yu. V. Petrov,P. Monceau
Physics , 2015, DOI: 10.1038/srep07578
Abstract: There are two types of intrinsic surface states in solids. The first type is formed on the surface of topological insulators. Recently, transport of massless Dirac fermions in the band of "topological" states has been demonstrated. States of the second type were predicted by Tamm and Shockley long ago. They do not have a topological background and are therefore strongly dependent on the properties of the surface. We study the problem of the conductivity of Tamm-Shockley edge states through direct transport experiments. Aharonov-Bohm magneto-oscillations of resistance are found on graphene samples that contain a single nanohole. The effect is explained by the conductivity of the massless Dirac fermions in the edge states cycling around the nanohole. The results demonstrate the deep connection between topological and non-topological edge states in 2D systems of massless Dirac fermions.
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