Lower critical field, anisotropy, and two-gap features of LiFeAs

The magnetic properties of LiFeAs, as single crystalline and polycrystalline samples, were investigated. The lower critical field deduced from the vortex penetration of two single crystals appears to be almost isotropic with a temperature dependence closer to that of two-gap superconductors. The parameters extracted from the reversible magnetizations of sintered polycrystalline samples are in good agreement with those from the single-crystal data.

Figure 1

(a) The virgin isothermal $M\left(H\right)$ under zero-field-cooled condition of a sheetlike single crystal (crystal A) at (from bottom up) 2 K, 3 K, 4 K, 5 K, 6 K, 7 K, and 8 K, respectively. Inset: the $M\text{−}H$ loop at 10 K. The sample volume is calculated using the low-field susceptibilities. (b) The calculated $\sqrt{M-{\chi }_{H=0}H}$ of crystal A along the $c$ axis based on the ${\chi }_{H=0}$ calculated below 10 Oe. The straight line is the model expectation. Inset: the zero-field-cooled magnetization at 10 Oe with $H\parallel c$.

Figure 2

a) The observed $dM/dH$ vs $H$ for crystal A at 3 K and $H\parallel c$. The symbols are the data and the lines are the linear fits at lower and higher fields, respectively. The thick dashed line corresponds to the “baseline” $M_0$ used (see text). (b) The deduced $\text{sign}\left(M\text{−}{M}_0\right)\cdot {\left|M\text{−}{M}_0\right|}^{1/2}$ (circles). The line is the best fit.

Figure 3

The lower critical fields of LiFeAs crystal A at $H\parallel c$. Solid triangles: deduced $H_{\text{p}2}$; open triangles: deduced $H_{\text{p}1}$; open squares: deduced from the reversible magnetization of the ceramic; solid line: two-BCS-gap fit; dashed line: one-gap fit.

Figure 4

The anisotropy of $H_{\text{c}1}$. Open triangles: crystal A. Solid squares: crystal B.

Figure 5

The average moment of the $H$-increase and the $H$-decrease branches along the isothermal $M\text{−}H$ loops after subtracting those at $18\text{K}>T_{\text{c}}$. Circles: 10 K; triangles: 12 K; and squares: 14 K. Inset: the raw $M\text{−}H$ loops. The line is the moment at 18 K.