We have demonstrated that a semimetallic phase emerges in a
TCI thin film by applying in-plane magnetic field. The semimetallic
phase is characterized by the existence of a pair of gapless Weyl
cones as in the case of the 3D Weyl semimetal.
We
argue what type of perturbation may open the gap in gen-
eral.
The terms which contribute to the band gap opening appear
in the coefficient of σ
z
in the 2 ×2 Hamiltonian (14). The effective
2 ×2 Hamiltonian is derived from the 4 ×4 Hamiltonian. Hence it
is enough to identify the terms in the 4 ×4theory which result in
the σ
and σ
z
z
term in the 2 ×2 Hamiltonian (14). Only the σ
τ
z
terms produce the σ
z
z
, σ
z
τ
x
term in the 2 ×2 Hamiltonian (14),
and open the gap. Without these external perturbations, the gap
never opens. On the other hands, all other terms produce the σ
and σ
terms in the 2 × 2 Hamiltonian, and renormalize the positions
and
the
Fermi
velocities
of
the
Weyl
cones.
Since
lattice
scale
y
disorders do not produce the mass term, the semimetallic phase is
robust for such disorders.
In
conclusion, we have proposed a magnetic-field induced
semimetal–semiconductor transition in 2D material. This is a
giant-magnetoresistance, where resistivity is controlled by mag-
netic
field. The transition between the insulator phase and the
semimetallic phase will be experimentally detectable by electric
transport measurement. The TCI thin film has already been
manufactured [15]. Our finding will open a way to magnetonanoelectronics
based
on
the
TCI
thin
film.