Superconducting critical fields of single-crystalline K${}_{0.73}$Fe${}_{1.68}$Se${}_2$

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

Figure 1

ZFC and FC plots measured at 11.5 Oe. The FC branch in an extended scale is shown in the inset.

Figure 2

Ascending and descending branches of $M\left(H_0\right)$ curves at various temperatures. Lower inset: determination of $H_{c1}$. Upper inset: determination of $H_{c2}$.

Figure 3

H-T phase diagram. Inset: Temperature dependence of $H_{c1}$. All lines are to guide the eye.

Figure 4

Magnetization curves in a normal state, $T=35$ K. The inset shows the shifted hysteresis loop obtained at low $H_0$. See text.

Figure 5

(a) Magnetization curve at $T=29$ and 35 K. (b) Field dependencies of ${\chi }^{\prime }$ and ${\chi }^{\prime \prime }$ at $T=29$ K.

Figure 6

Temperature dependencies of ${\chi }^{\prime }$ and ${\chi }^{\prime \prime }$ at $H_0=0$ and 50 kOe.