The effect of periodic potential on phonon transport in MoS2, topological insulator and graphene – possible implications for electron transport in these materials.

Aalto Quantum Physics Seminar (Nanotalo). Speaker: Dr. Maciej Wiesner (Adam Mickiewicz University, Poland).

The effect of periodic potential on electron transport in graphene was discussed in many theoretical papers. The papers were an inspiration for investigations of the effect on other 2D materials: MoS2, graphene and a topological insulator Bi2Te3 (TI). Most models presented in papers refer to periodic magnetic, electric fields or their combination. Still one, not explored field left – periodic strain. To produce the periodic strain we used natural ability of sapphire crystals to  reconstruct their surface subjected to a special heat-treatment procedure – fig.1. Investigations of the strain on optical phonon transport was realized using Raman and photoluminescence spectroscopy.

It is well known fact that the 2D peak of graphene is sensitive to DOS and doping. In case of MoS2 PL peak reflects the energy gap. Reducing the TI layer thickness, and thereby enhancing the surface-to-volume ratio, is an efficient way to reduce bulk conductance and to significantly enhance the relative contribution of topological surface states to the measured conductance and consequently reduce electron scattering on defects. Thickness reduction results in symmetry breaking in z-direction of the TI. So the broken symmetry is a main factor leading to emergence of 
Our recent experiments revealed that the periodic strain varies the energy gap of MoS2 by about 0.4% when comparing its value measured along and across the corrugated surface. In case of graphene the Raman shift of the 2D peak also revealed an anisotropic behavior.
The most interesting was, however, the topological insulator. Comparison of optical phonons propagating in the 50nm thick TI grown on a flat and corrugated sapphire revealed emergence of Raman-forbidden in the TI/corrugated sapphire. The phonons were not observed on the TI/flat sapphire system. This is a clear evidence that strain breaks the TI’s symmetry along z-direction leading to optical phonons characteristic to thin layers of TI’s only.
Briefly described above results need verification. In case of Bi2Te3 we expect that the symmetry breaking along z-direction reduces contribution of bulk electrons to surface states, what may be observed as decrease of noise level due to lack of scattering of surface electrons. The DOS anisotropy in graphene and the energy gap in MoS2 can be measured directly in transport measurements using suggested in fig.2 method.