In general, it yields Q = 0, ±1, ±2, ···, because φ is defined only
modulo 2π. For the specific field configuration given in (15), we
find Q =±1for the Weyl cones at X
. These values are topologically
stable
because
any
perturbation
cannot
change
the
quantized
±
value of the topological charge Q . We may also evaluate the topological
number for the spin configuration at the phase transition
point [Fig. 4(a)] and also before the phase transition point (i.e. in
the insulator phase) to find that Q = 0. We may interpret that a
pair creation of Weyl cones with Q =±1 occurs from the topological
trivial
Dirac
cone
with
Q = 0at the phase transition point.
It is interesting to note that a flat edge mode appear to connect
the two X
points in a nanoribbon [Fig. 1(c)]. Note that the total
topological charge (20) is zero ( Q = 0) both before and after
±
the phase transition. Nevertheless, the semimetallic phase is stable
topologically
due
to
the
presence
of
a
pair
of
two
Weyl
cones
with Q =±1 generated by the in-plane field, precisely as in the
3D Weyl semimetal.
The
same analysis is carried out for the Dirac cone at the Y
point,
from which a pair of Weyl cones emerge located at the Y
point under in-plane magnetic field.
In general, it yields Q = 0, ±1, ±2, ···, because φ is defined only modulo 2π. For the specific field configuration given in (15), we find Q =±1for the Weyl cones at X. These values are topologicallystablebecauseanyperturbationcannotchangethequantized±value of the topological charge Q . We may also evaluate the topologicalnumber for the spin configuration at the phase transition point [Fig. 4(a)] and also before the phase transition point (i.e. in the insulator phase) to find that Q = 0. We may interpret that a pair creation of Weyl cones with Q =±1 occurs from the topologicaltrivialDiracconewithQ = 0at the phase transition point. It is interesting to note that a flat edge mode appear to connect the two Xpoints in a nanoribbon [Fig. 1(c)]. Note that the totaltopological charge (20) is zero ( Q = 0) both before and after ±the phase transition. Nevertheless, the semimetallic phase is stabletopologicallyduetothepresenceofapairoftwoWeylconeswith Q =±1 generated by the in-plane field, precisely as in the 3D Weyl semimetal.The same analysis is carried out for the Dirac cone at the Ypoint, from which a pair of Weyl cones emerge located at the Ypoint under in-plane magnetic field.
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