New QuantumATK Study Discovers Magnetic States on the Edges of 1T’ TMDs - impacting their utilization as quantum spin Hall insulators

Research project between the Synopsys QuantumATK Team and the Atomic-Scale Materials Design Group at Technical University of Denmark led by Prof. Karsten Jacobsen discovers new properties of the 1T’ phase monolayer (ML) transition metal dichalcogenides (TMDs), such as MoS2, MoTe2, and WTe2 [1]. The 1T’ phase of ML TMDs is known to host topologically protected conducting edge states, and thus has been considered as a potential Quantum spin Hall insulator (QSHI) [2]. TMD-based QSHIs could be used to construct topological field-effect transistors (TFETs) (see Figure 1) for low-power quantum electronics and spintronics applications. In such a device, the “on” state is due to conductance through topologically protected edge channels, which can be switched off by applying a vertical electric field (above 1.7 V/Å for the “X” edge of MoS2), as shown in Figure 2, top. This new study performed with QuantumATK demonstrates that even though edge “X” shows this behavior, there are some edge terminations, “m” and “c”, that host magnetic edge states which spontaneously break the time-reversal symmetry, rendering the material topologically trivial (see Figure 2, bottom). This means that these gapless magnetic edge states are no longer protected against impurity scattering and that they, in principle, could be removed by edge modifications. Furthermore, the investigated magnetic edge states are seen to be robust and cannot be switched off when applying an electric field as shown in Figure 2, bottom.  These findings, therefore, may have an impact on the prospects of utilizing WTe2 as a quantum spin Hall insulator.

 

 

Figure 1. Schematic vdW-topological field-effect transistor proposed in Ref [2] with the central component containing TMD-based QSHI. Ref [2] considered the X edge.

Figure 2. Surface bandstructure calculated for different edges (X - top and m-bottom) of the  1T’ phase of MoS2 with and without electric field [1].

Calculations with QuantumATK

MoS2, MoTe2, and WTe2 monolayers with different edges were constructed using QuantumATK NanoLab GUI. The novel surface Green’s function method [3] for studying truly semi-infinite systems and implementation of spin-orbit coupling (SOC) in QuantumATK made it possible to do fully self-consistent DFT calculations on a single isolated edge. This revealed breaking of the time-reversal symmetry in the band structure which would have been hidden by the spatial symmetry of a conventional calculation on a 2D slab. Response to electric fields was modelled by adding an external potential as a shift between the gate and the top of the cell, i.e., shifting the potential near the surface in the vacuum, and then running a self-consistent DFT calculation. This procedure of applying an electric field in QuantumATK resembles the real situation in which the device channel is modulated by external gates. Furthermore, the surface Green’s function method ensured that there was no change of the chemical potential of the surface in the presence of the electric field.

 

Learn more

The results will be presented on July 4th in the 2D materials session at the 21st IVC Conference hosted in Malmö, Sweden.

 

Relevant resources

The Surface Configuration method used in the paper is available in QuantumATK and is described in

·       Tutorial: Green’s function surface calculations

 

References

[1] L. Jelver, D. Stradi, T. Olsen, K. Stokbro, and K.W. Jacobsen, “Spontaneous breaking of time-reversal symmetry at the edges of 1T’ monolayer transition metal dichalcogenides”,  Phys. Rev. B 99, 155420 (2019).

[2] X. Qian, J. Liu, L. Fu, and J. Li, “Quantum spin Hall effect in two-dimensional transition metal dichalcogenides”, Science 346, 1344 (2014).

[3] S. Smidstrup, D. Stradi, J. Wellendorff, P.A. Khomyakov, U.G. Vej-Hansen, M.-E. Lee, E.J. T. Ghosh, H. Jónsson, and K. Stokbro, “First-principles Green’s function method for surface calculations: A pseudopotential localized basis sets approach”, Phys. Rev. B 96, 195309 (2017). arXiv:1707.02141 [cond-mat.mtrl-sci]

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