Pressure effects on superconductivity and magnetism in FeSe$_{0.88}$ and FeSe$_{1-x}$Te$_{x}$

We have performed the pressure ($P)$ dependence of ac, dc susceptibility and resistivity measurements on iron chalcogenides FeSe$_{0.88}$, FeSe$_{0.5}$Te$_{0.5}$, FeTe and FeTe$_{0.9}$. The superconducting transition temperature ($T_{c})$ of FeSe$_{0.88}$ is found to increase with $P$ linearly at a rate of d$T_{c}$/d$P \sim $ 3.09$\times $10$^{-2}$ K/kbar, while the $T_{c}$ of FeSe$_{0.5}$Te$_{0.5}$ increases nonlinearly with an initial rate of 1.47K/kbar then saturates at $P \sim $ 18 kbar. Such the enhancement of $T_{c}$ might be attributed to an increase of density of states. There is no indication of superconductivity observed in FeTe and FeTe$_{0.9}$, which is inconsistent with the result of theoretical calculation, and it might be due to the existence of long range magnetic spin coupling that inhibits the formation of superconductivity mediated by spin fluctuations, which is applied to describe the mechanism in Fe-based superconductors.