Recently, BCS superconductivity at 203 K has been observed in a highly compressed hydrogen sulfide. We use first-principles calculations to systematically examine the effects of partially substituting chalcogen atoms on the superconductivity of hydrogen chalcogenides under high pressures. We find detailed trends of how the critical temperature changes upon increasing the V-, VI- or VII-substitution rate. These trends highlight the key roles played by low atomic mass and metallized covalent bonds. In particular, a possible record high critical temperature of 280 K is predicted in stable ${\mathrm{H}}_3{\mathrm{S}}_{0.925}{\mathrm{P}}_{0.075}$ with the $Im\overline{3}m$ structure under 250 GPa.

The doping and strain effects on the electron transport of monolayer ${\mathrm{MoS}}_{2}$ are systematically investigated using the first-principles calculations with Boltzmann transport theory. We estimate the mobility has a maximum 275 ${\mathrm{cm}}^{2}$/(V$\phantom{\rule{0.16em}{0ex}}$s) in the low doping level under the strain-free condition. Applying a small strain ($$\sim${}3%$) can improve the maximum mobility to 1150 ${\mathrm{cm}}^{2}$/(V$\phantom{\rule{0.16em}{0ex}}$s) and the strain effect is more significant in the high doping level. We demonstrate that the electric resistance mainly due to the electron transitions between $K$ and $Q$ valleys scattered by the $M$ momentum phonons. However, the strain can effectively suppress this type of electron-phonon coupling by changing the energy difference between the $K$ and $Q$ valleys. This sensitivity of mobility to the external strain may direct the improving electron transport of ${\mathrm{MoS}}_{2}$.