1,546 research outputs found
Lattice model of three-dimensional topological singlet superconductor with time-reversal symmetry
We study topological phases of time-reversal invariant singlet
superconductors in three spatial dimensions. In these particle-hole symmetric
systems the topological phases are characterized by an even-numbered winding
number . At a two-dimensional (2D) surface the topological properties of
this quantum state manifest themselves through the presence of flavors of
gapless Dirac fermion surface states, which are robust against localization
from random impurities. We construct a tight-binding model on the diamond
lattice that realizes a topologically nontrivial phase, in which the winding
number takes the value . Disorder corresponds to a (non-localizing)
random SU(2) gauge potential for the surface Dirac fermions, leading to a
power-law density of states . The bulk
effective field theory is proposed to be the (3+1) dimensional SU(2) Yang-Mills
theory with a theta-term at .Comment: 5 pages, 3 figure
Nonsymmorphic symmetry-required band crossings in topological semimetals
We show that for two-band systems nonsymmorphic symmetries may enforce the
existence of band crossings in the bulk, which realize Fermi surfaces of
reduced dimensionality. We find that these unavoidable crossings originate from
the momentum dependence of the nonsymmorphic symmetry, which puts strong
restrictions on the global structure of the band configurations. Three
different types of nonsymmorphic symmetries are considered: (i) a unitary
nonsymmorphic symmetry, (ii) a nonsymmorphic magnetic symmetry, and (iii) a
nonsymmorphic symmetry combined with inversion. For nonsymmorphic symmetries of
the latter two types, the band crossings are located at high-symmetry points of
the Brillouin zone, with their exact positions being determined by the algebra
of the symmetry operators. To characterize these band degeneracies we introduce
a \emph{global} topological charge and show that it is of type,
which is in contrast to the \emph{local} topological charge of Fermi points in,
say, Weyl semimetals. To illustrate these concepts, we discuss the -flux
state as well as the SSH model at its critical point and show that these two
models fit nicely into our general framework of nonsymmorphic two-band systems.Comment: 6.5 pages, 4 figure
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