In physics, magnetic topological insulators are three dimensional magnetic materials with a non-trivial topological index protected by a symmetry other than time-reversal.
[1][2][3][4][5] This type of material conducts electricity on its outer surface, but its volume behaves like an insulator.
[6] In contrast with a non-magnetic topological insulator, a magnetic topological insulator can have naturally gapped surface states as long as the quantizing symmetry is broken at the surface.
These gapped surfaces exhibit a topologically protected half-quantized surface anomalous Hall conductivity (
The sign of the half-quantized surface anomalous Hall conductivity depends on the specific surface termination.
classification of a 3D crystalline topological insulator can be understood in terms of the axion coupling
A scalar quantity that is determined from the ground state wavefunction[8] where
is a shorthand notation for the Berry connection matrix where
is the cell-periodic part of the ground state Bloch wavefunction.
The topological nature of the axion coupling is evident if one considers gauge transformations.
Since a gauge choice is arbitrary, this property tells us that
classification based on the axion coupling comes from observing how crystalline symmetries act on
The consequence is that if time-reversal or inversion are symmetries of the crystal we need to have
Furthermore, we can combine inversion or time-reversal with other symmetries that do not affect
giving rise to crystalline topological insulators,[9] while the first intrinsic magnetic topological insulator MnBi
So far we have discussed the mathematical properties of the axion coupling.
) will result in a half-quantized surface anomalous Hall conductivity (
, a quantity that is determined from bulk considerations as we have seen, while the other is the Berry phase
which we wrap with a 2D quantum anomalous Hall insulator with Chern index
As long as we do this without closing the surface gap, we are able to increase
and time-reversal symmetry is present, i.e. non-magnetic topological insulator.
on every surface resulting in a Dirac cone (or more generally an odd number of Dirac cones) on every surface and therefore making the boundary of the material conducting.
A half quantized surface Hall conductivity and a related treatment is also valid to understand topological insulators in magnetic field [12] giving an effective axion description of the electrodynamics of these materials.
[13] This term leads to several interesting predictions including a quantized magnetoelectric effect.
[14] Evidence for this effect has recently been given in THz spectroscopy experiments performed at the Johns Hopkins University.
[15] Magnetic topological insulators have proven difficult to create experimentally.
In 2023 it was estimated that a magnetic topological insulator might be developed in 15 years' time.
[16] A compound made from manganese, bismuth, and tellurium (MnBi2Te4) has been predicted to be a magnetic topological insulator.
In 2024, scientists at the University of Chicago used MnBi2Te4 to develop a form of optical memory which is switched using lasers.
This memory storage device could store data more quickly and efficiently, including in quantum computing.