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Modified Dirac Hamiltonian for Efficient Quantum Mechanical Simulations of Micron Sized Devices  [PDF]
K. M. Masum Habib,Redwan N. Sajjad,Avik W. Ghosh
Physics , 2015,
Abstract: Representing massless Dirac fermions on a spatial lattice poses a potential challenge known as the Fermion Doubling problem. Addition of a quadratic term to the Dirac Hamiltonian circumvents this problem. We show that the modified Hamiltonian with the additional term results in a very small Hamiltonian matrix when discretized on a real space square lattice. The resulting Hamiltonian matrix is considerably more efficient for numerical simulations without sacrificing on accuracy and is several orders of magnitude faster than the atomistic tight binding model. Using this Hamiltonian and the Non-Equilibrium Green's Function (NEGF) formalism, we show several transport phenomena in graphene, such as magnetic focusing, chiral tunneling in the ballistic limit and conductivity in the diffusive limit in micron sized graphene devices. The modified Hamiltonian can be used for any system with massless Dirac fermions such as Topological Insulators, opening up a simulation domain that is not readily accessible otherwise.
Intrinsic chiral magnetic effect in Dirac semimetals due to dislocations  [PDF]
M. N. Chernodub,Mikhail Zubkov
Physics , 2015,
Abstract: The dislocation in Dirac semimetal carries an emergent magnetic flux parallel to the dislocation axis. We show that due to the emergent magnetic field the dislocation accommodates a single fermion massless mode of the corresponding low-energy one-particle Hamiltonian. The mode is propagating along the dislocation with its spin directed parallel to the dislocation axis. In agreement with the chiral anomaly observed in Dirac semimetals, an external electric field results in the spectral flow of the one-particle Hamiltonian, in pumping of the fermionic quasiparticles out from vacuum, and in creating a nonzero axial (chiral) charge in the vicinity of the dislocation. In the presence of the chirality imbalance, the intrinsic magnetic field of the dislocation generates an electric current along the dislocation axis. We point out that this effect - which is an "intrinsic" analogue of the chiral magnetic effect - may experimentally reveal itself through transport phenomena in Dirac semimetals via the enhanced conductivity, when the external electric field is parallel to the dislocation axis.
Effective Dirac Hamiltonian for anisotropic honeycomb lattices: optical properties  [PDF]
M. Oliva-Leyva,Gerardo G. Naumis
Physics , 2015,
Abstract: We derive the low-energy Hamiltonian for a honeycomb lattice with anisotropy in the hopping parameters. Taking the reported Dirac Hamiltonian for the anisotropic honeycomb lattice, we obtain its optical conductivity tensor and its transmittance for normal incidence of linearly polarized light. Also, we characterize its dichroic character due to the anisotropic optical absorption. As an application of our general findings, which reproduce the case of uniformly strained graphene, we study the optical properties of graphene under a nonmechanical distortion.
Effective non-Hermitian Hamiltonian and continuum shell model  [PDF]
Alexander Volya,Vladimir Zelevinsky
Physics , 2002, DOI: 10.1103/PhysRevC.67.054322
Abstract: The intrinsic dynamics of a system with open decay channels is described by an effective non-Hermitian Hamiltonian which at the same time allows one to find the external dynamics, - reaction cross sections. We discuss ways of incorporating this approach into the shell model context. Several examples of increasing complexity, from schematic models to realistic nuclear calculations (chain of oxygen isotopes), are presented. The approach is capable of describing a multitude of phenomena in a unified way combining physics of structure and reactions. Self-consistency of calculations and threshold energy dependence of the coupling to the continuum are crucial for the description of loosely bound states.
Baryons in Effective Chiral Quark Models with Polarized Dirac Sea  [PDF]
Th. Meissner,E. Ruiz Arriola,A. Blotz,K. Goeke
Physics , 1994,
Abstract: We give a comprehensive review of concepts and numerical calculations for SU(2) and SU(3) baryonic systems which are considered as many body systems composed of three quarks coupled to a polarized Dirac sea by a Nambu--Jona-Lasinio like interaction. The general formalism as well as the construction and numerical solution of non-topological mean field solitons are exhibited. Systems with good spin/flavour/isospin/momentum quantum numbers are obtained by using the SU(2)/SU(3)-cranking and the pushing approach. The article contains also recent results for various extensions of the model, e.g. vector and axialvector couplings or the inclusion of scale invariance. Furthermore the relationship of this approach to topological models (Skyrme type) as well as to (valence-)quark-meson models (Gell-Mann--Levi chiral sigma model) is explained. (to be publ. in Rep.Prog.Phys.)
Redundancy of the off-shell parameters in chiral effective field theory with explicit spin-3/2 degrees of freedom  [PDF]
H. Krebs,E. Epelbaum,U. -G. Mei?ner
Physics , 2009, DOI: 10.1016/j.physletb.2009.12.023
Abstract: In this note we prove to all orders in the small scale expansion that all off-shell parameters which appear in the chiral effective Lagrangian with explicit Delta(1232) isobar degrees of freedom can be absorbed into redefinitions of certain low-energy constants and are therefore redundant.
Shell-model study of the N=82 isotonic chain with a realistic effective hamiltonian  [PDF]
L. Coraggio,A. Covello,A. Gargano,N. Itaco,T. T. S. Kuo
Physics , 2009, DOI: 10.1103/PhysRevC.80.044320
Abstract: We have performed shell-model calculations for the even- and odd-mass N=82 isotones, focusing attention on low-energy states. The single-particle energies and effective two-body interaction have been both determined within the framework of the time-dependent degenerate linked-diagram perturbation theory, starting from a low-momentum interaction derived from the CD-Bonn nucleon-nucleon potential. In this way, no phenomenological input enters our effective Hamiltonian, whose reliability is evidenced by the good agreement between theory and experiment.
Effective chiral-spin Hamiltonian for odd-numbered coupled Heisenberg chains  [PDF]
V. Subrahmanyam
Physics , 1994, DOI: 10.1103/PhysRevB.50.16109
Abstract: An $L \times \infty$ system of odd number of coupled Heisenberg spin chains is studied using a degenerate perturbation theory, where $L$ is the number of coupled chains. An effective chain Hamiltonian is derived explicitly in terms of two spin half degrees of freedom of a closed chain of $L$ sites, valid in the regime the inter-chain coupling is stronger than the intra-chain coupling. The spin gap has been calculated numerically using the effective Hamiltonian for $L=3,5,7,9$ for a finite chain up to ten sites. It is suggested that the ground state of the effective Hamiltonian is correlated, by examining variational states for the effective chiral-spin chain Hamiltonian.
Generalization of Chiral Symmetry for Tilted Dirac Cones  [PDF]
Tohru Kawarabayashi,Yasuhiro Hatsugai,Takahiro Morimoto,Hideo Aoki
Physics , 2012, DOI: 10.1142/S2010194512006046
Abstract: The notion of chiral symmetry for the conventional Dirac cone is generalized to include the tilted Dirac cones, where the generalized chiral operator turns out to be non-hermitian. It is shown that the generalized chiral symmetry generically protects the zero modes (n=0 Landau level) of the Dirac cone even when tilted. The present generalized symmetry is equivalent to the condition that the Dirac Hamiltonian is elliptic as a differential operator, which provides an explicit relevance to the index theorem.
Effective Magnetic Fields in Graphene Superlattices  [PDF]
Jianmin Sun,H. A. Fertig,L. Brey
Physics , 2010, DOI: 10.1103/PhysRevLett.105.156801
Abstract: We demonstrate that the electronic spectrum of graphene in a one-dimensional periodic potential will develop a Landau level spectrum when the potential magnitude varies slowly in space. The effect is related to extra Dirac points generated by the potential whose positions are sensitive to its magnitude. We develop an effective theory that exploits a chiral symmetry in the Dirac Hamiltonian description with a superlattice potential, to show that the low energy theory contains an effective magnetic field. Numerical diagonalization of the Dirac equation confirms the presence of Landau levels. Possible consequences for transport are discussed.
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