91,024 research outputs found
Symmetric Real Dirac Fermions and Semimetals
Recently Weyl fermions have attracted increasing interest in condensed matter
physics due to their rich phenomenology originated from their nontrivial
monopole charges. Here we present a theory of real Dirac points that can be
understood as real monopoles in momentum space, serving as a real
generalization of Weyl fermions with the reality being endowed by the
symmetry. The real counterparts of topological features of Weyl semimetals,
such as Nielsen-Ninomiya no-go theorem, D sub topological insulators and
Fermi arcs, are studied in the symmetric Dirac semimetals, and the
underlying reality-dependent topological structures are discussed. In
particular, we construct a minimal model of the real Dirac semimetals based on
recently proposed cold atom experiments and quantum materials about
symmetric Dirac nodal line semimetals.Comment: 7.5 pages, 5 figures. Accepted by Phys. Rev. Let
General response theory of topologically stable Fermi points and its implications for disordered cases
We develop a general response theory of gapless Fermi points with nontrivial
topological charges for gauge and nonlinear sigma fields, which asserts that
the topological character of the Fermi points is embodied as the terms with
discrete coefficients proportional to the corresponding topological charges.
Applying the theory to the effective non-linear sigma models for topological
Fermi points with disorders in the framework of replica approach, we derive
rigorously the Wess-Zumino terms with the topological charges being their
levels in the two complex symmetry classes of A and AIII. Intriguingly, two
nontrivial examples of quadratic Fermi points with the topological charge `2'
are respectively illustrated for the classes A and AIII. We also address a
qualitative connection of topological charges of Fermi points in the real
symmetry classes to the topological terms in the non-linear sigma models, based
on the one-to-one classification correspondence.Comment: 8 pages and 2 figures, revised version with appendi
Topological Classification and Stability of Fermi Surfaces
In the framework of the Cartan classification of Hamiltonians, a kind of
topological classification of Fermi surfaces is established in terms of
topological charges. The topological charge of a Fermi surface depends on its
codimension and the class to which its Hamiltonian belongs. It is revealed that
six types of topological charges exist, and they form two groups with respect
to the chiral symmetry, with each group consisting of one original charge and
two descendants. It is these nontrivial topological charges which lead to the
robust topological protection of the corresponding Fermi surfaces against
perturbations that preserve discrete symmetries.Comment: 5 pages, published version in PR
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