Upper Critical fields and Pauli Limiting Behavior in a FeSe$_{0.4}$Te$_{0.6}$ Single Crystal

We investigate temperature-dependence of the upper critical fields $\emph{H}_{c2}$($\emph{T}$) of a superconducting FeSe$_ {0.4}$Te$_{0.6}$ single crystal by measuring resistivity in static magnetic fields up to 45 T. $\emph{H}_{c2}$($\emph{T}$) along a planar $\emph{ab}$-direction, $\emph{H}_{c2}^{\emph{ab}} $($\emph{T}$), steeply increases near its superconducting transition temperature $\emph{T}_{c}$ $\sim$ 14.5 K, starts to saturate even around 10 K, and finally approaches $\emph{H}_{c2} ^{\emph{ab}}$(0) $\sim$ 48 T, a much smaller value than the expected orbital limiting field ($\sim$ 130 T), indicating the predominant Pauli limiting effect. Although $\emph{H}_{c2}^ {\emph{c}}$($\emph{T}$) increases with a smaller positive slope near $\emph{T}_{c}$, it shows a positive curvature at overall temperatures to reach $\emph{H}_{c2}^{\emph{c}}$(0) $\sim$ 48 T, suggesting the Pauli paramagnetic effect also exists even along the $\emph{c}$-direction. We include the spin-orbit coupling and the Pauli paramagnetic effect in the Werthamer- Helfand-Hohenberg (WHH) formula to explain the shape of $\emph {H}_{c2}$($\emph{T}$) for both directions and discuss enhanced local magnetism resulting from the excess iron or Se(Te) vacancies as a possible origin for the persistent Pauli limiting behavior.