Specific heat and angle-resolved photoemission spectroscopy study of the superconducting gaps in LiFeAs

We present specific-heat, $c_p$, and angle-resolved photoemission spectroscopy (ARPES) data on single crystals of the stoichiometric superconductor LiFeAs. A pronounced anomaly is found in $c_p$ at the superconducting transition. The electronic contribution can be described by two $s$-type energy gaps with magnitudes of approximately ${\Delta }_1=1.2$ meV and ${\Delta }_2=2.6$ meV and a normal-state $\gamma$ coefficient of 10 mJ/mol ${\mathrm{K}}^2$. All these values are in agreement with ARPES results.

Figure 1

The temperature dependence of the specific heat of LiFeAs is shown as $c_p/T$ vs $T$ in zero magnetic field and in a field $B\parallel c$ of 9 T. The phonon and electron contributions $c_{\mathrm{ph}}$ and $c_{\mathrm{el}-\mathrm{n}\hspace{0.5em}9\mathrm{\hspace{0.5em}}\mathrm{T}}$ have been determined from a fit of the 9-T data as explained in the text. In addition, the fitted total $c_p/T$ for 0 and 9 T are shown.

Figure 2

The main plot shows the electronic contribution to the specific heat of LiFeAs in zero field as $c_{\mathrm{el}}/T$ vs $T$. The dotted line corresponds to a single-gap BCS curve taking into account a residual nonsuperconducting contribution with ${\gamma }_{\mathrm{res}}/{\gamma }_n=0.09$. The red curve is a two-gap model of the data assuming likewise ${\gamma }_{\mathrm{res}}/{\gamma }_n=0.09$. The inset shows the low-$T$ data as $c_{\mathrm{el}}/{\gamma }_{n}T$ vs $T$ on a larger scale. In addition, an estimate for the low-$T$ $c_{\mathrm{el}}$ expected in case of a $d$-wave order parameter is plotted.

Figure 3

(a) and (b) Photoemission intensity in the high-symmetry cuts together with the fitted tight-binding dispersions. ${\mathrm{e}}_1$ and ${\mathrm{e}}_2$ are the two bands forming the electron pockets at the X point of the BZ. ${\mathrm{h}}_1$ and ${\mathrm{h}}_2$ are the outer and the inner hole bands centered at the $\Gamma$ point. (c) Band contribution to the DOS derived from the fitted quasiparticle dispersions. (d) Temperature-dependent Sommerfeld coefficient determined as $\gamma (T)=d{c}_{\mathrm{el}}/dT$.

Figure 4

(a) Integrated energy distribution curves (IEDCs) for the electron-like M barrel, measured in the temperature range from 0.9 to 23 K. (b) Normalized IEDCs with fits to the Dynes function. Inset: derived temperature dependence of the superconducting gap.