19,665 research outputs found
(3+1) Massive Dirac Fermions with Ultracold Atoms in Optical Lattices
We propose the experimental realization of (3+1) relativistic Dirac fermions
using ultracold atoms in a rotating optical lattice or, alternatively, in a
synthetic magnetic field. This approach has the advantage to give mass to the
Dirac fermions by coupling the ultracold atoms to a Bragg pulse. A dimensional
crossover from (3+1) to (2+1) Dirac fermions can be obtained by varying the
anisotropy of the lattice. We also discuss under which conditions the
interatomic potentials give rise to relativistically invariant interactions
among the Dirac fermions
Renormalization Group Approach to Stability of Two-dimensional Interacting Type-II Dirac Fermions
The type-II Weyl/Dirac fermions are a generalization of conventional or
type-I Weyl/Dirac fermions, whose conic spectrum is tilted such that the Fermi
surface becomes lines in two dimensions, and surface in three dimensions rather
than discrete points of the conventional Weyl/Dirac fermions. The
mass-independent renormalization group calculations show that the tilting
parameter decreases monotonically with respect to the length scale, which leads
to a transition from two dimensional type-II Weyl/Dirac fermions to the type-I
ones. Because of the non-trivial Fermi surface, a photon gains a finite mass
partially via the chiral anomaly, leading to the strong screening effect of the
Weyl/Dirac fermions. Consequently, anisotropic type-II Dirac semimetals become
stable against the Coulomb interaction. This work provides deep insight into
the interplay between the geometry of Fermi surface and the Coulomb
interaction.Comment: Final pulished versio
Non-Abelian statistics of vortices with non-Abelian Dirac fermions
We extend our previous analysis on the exchange statistics of vortices having
a single Dirac fermion trapped in each core, to the case where vortices trap
two Dirac fermions with U(2) symmetry. Such a system of vortices with
non-Abelian Dirac fermions appears in color superconductors at extremely high
densities, and in supersymmetric QCD. We show that the exchange of two vortices
having doublet Dirac fermions in each core is expressed by non-Abelian
representations of a braid group, which is explicitly verified in the matrix
representation of the exchange operators when the number of vortices is up to
four. We find that the result contains the matrices previously obtained for the
vortices with a single Dirac fermion in each core as a special case. The whole
braid group does not immediately imply non-Abelian statistics of identical
particles because it also contains exchanges between vortices with different
numbers of Dirac fermions. However, we find that it does contain, as its
subgroup, a genuine non-Abelian statistics for the exchange of the identical
particles, that is, vortices with the same number of Dirac fermions. This
result is surprising compared with conventional understanding because all Dirac
fermions are defined locally at each vortex, unlike the case of Majorana
fermions for which Dirac fermions are defined non-locally by Majorana fermions
located at two spatially separated vortices.Comment: 32 pages, no figures, v3: published versio
First direct observation of Dirac fermions in graphite
Originating from relativistic quantum field theory, Dirac fermions have been
recently applied to study various peculiar phenomena in condensed matter
physics, including the novel quantum Hall effect in graphene, magnetic field
driven metal-insulator-like transition in graphite, superfluid in 3He, and the
exotic pseudogap phase of high temperature superconductors. Although Dirac
fermions are proposed to play a key role in these systems, so far direct
experimental evidence of Dirac fermions has been limited. Here we report the
first direct observation of massless Dirac fermions with linear dispersion near
the Brillouin zone (BZ) corner H in graphite, coexisting with quasiparticles
with parabolic dispersion near another BZ corner K. In addition, we report a
large electron pocket which we attribute to defect-induced localized states.
Thus, graphite presents a novel system where massless Dirac fermions,
quasiparticles with finite effective mass, and defect states all contribute to
the low energy electronic dynamics.Comment: Nature Physics, in pres
Simulation and detection of Dirac fermions with cold atoms in an optical lattice
We propose an experimental scheme to simulate and observe relativistic Dirac
fermions with cold atoms in a hexagonal optical lattice. By controlling the
lattice anisotropy, one can realize both massive and massless Dirac fermions
and observe the phase transition between them. Through explicit calculations,
we show that both the Bragg spectroscopy and the atomic density profile in a
trap can be used to demonstrate the Dirac fermions and the associated phase
transition.Comment: 4 pages; Published versio
Two dimensional Dirac fermions and quantum magnetoresistance in CaMnBi
We report two dimensional Dirac fermions and quantum magnetoresistance in
single crystals of CaMnBi. The non-zero Berry's phase, small cyclotron
resonant mass and first-principle band structure suggest the existence of the
Dirac fermions in the Bi square nets. The in-plane transverse magnetoresistance
exhibits a crossover at a critical field from semiclassical weak-field
dependence to the high-field unsaturated linear magnetoresistance ( in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The
temperature dependence of satisfies quadratic behavior, which is
attributed to the splitting of linear energy dispersion in high field. Our
results demonstrate the existence of two dimensional Dirac fermions in
CaMnBi with Bi square nets.Comment: 5 pages, 4 figure
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