Strain-tuned spin polarization and optical conductivity in MoS2/EuS heterostructures

We investigate strain-tuned spin transport and optical conductivity in a monolayer (ML) $n$-type ${\mathrm{MoS}}_{2}/\mathrm{EuS}$ heterostructure under linearly polarized terahertz radiation using a low-energy effective Hamiltonian, which takes displacement of the Dirac point induced by strain into account. Through modifying the strain modulus, we find an adjustable relationship between the band edge energy and the strain. Thus the threshold and range of allowed angles are strongly dependent on the strength of strain $\ensuremath{\epsilon}$, whereas the angle $\ensuremath{\theta}$ along which the strain is applied only acts on the former. Due to spin-flipped scattering generated by Rashba spin-orbit coupling, the in-plane spin polarizations (i.e., ${P}_{X}$ and ${P}_{Y}$) can be achieved, and their intensity can be enhanced by increasing $\ensuremath{\epsilon}$. Interestingly, we find that the out-of-plane spin polarization (i.e., ${P}_{Z}$) is related to the synergy of $\ensuremath{\epsilon}$ and the Rashba parameter ${\ensuremath{\lambda}}_{R}$. Especially, ${P}_{Z}$ can approach $100%$ when $\ensuremath{\epsilon}=9%$ and ${\ensuremath{\lambda}}_{R}=2$ meV. Furthermore, the optical transition between the spin-splitting subbands of the conduction band under the strain is studied. It is shown that by varying $\ensuremath{\epsilon}$ we can control the magnitude of both absorption peaks and absorption valleys in optical conductivity. Moreover, the presence of ${\ensuremath{\lambda}}_{R}$ can be used to further enhance the absorption peaks and valleys because ${\ensuremath{\lambda}}_{R}$ can tune the energy difference between the spin-splitting subbands within a specific valley and thus affect the optical transition channels. Our findings demonstrate that the strain can influence the tunneling behavior and valley Hall conductivity in a ML $n$-type ${\mathrm{MoS}}_{2}/\mathrm{EuS}$ heterostructure, and these features are applicable to other two-dimensional ML transition metal dichalcogenides, which is promising for terahertz and valleytronic devices.