Superconducting critical fields of K$_{0.8}$Fe$_{2}$Se$_2$ single crystal

We report the results of an experimental study of dc and low frequencies magnetic properties of K$_{0.8}$Fe$_{2}$Se$_2$ single crystal when the dc magnetic field is applied parallel to the $\bf{ab}$ plane. From the data obtained, we deduce the full H-T phase diagram which consists of all three H$_{c1}$(T), H$_{c2}(T)$ and H$_{c3}(T)$ critical magnetic field plots. The two H$_{c1}$(T) and H$_{c2}$(T) curves were obtained from dc magnetic measurements, whereas the surface critical field H$_{c3}$(T) line was extracted by ac susceptibility studies. It appears that near T$_c$, the H$_{c3}$(T)/H$_{c2}$(T) ratio is $\approx 4.4$ which is much larger than expected.