Fig. 4. Sommerfeld coefficients γ with different Li dopings and applied magnetic fields.

(A) Each point is acquired by fitting the data with the formula C_p/T = γ + βT² between 2 and 4 K (C_p is the specific heat and βT² is the lattice contribution part). The raw data are shown in fig. S7. All the γ is normalized into fundamental NiAs structure, namely, 1/8(Li_xFe₇Se₈). The γ of metallic Fe₇Se₈ is 9.86 mJ/mol-K², lying in the typical value range of transitional metals (5 to 10 mJ/mol-K²) and agreeing with the data from the literature (45). Then, an MIT will ensue in the double-phase region. The γ values at x = 0.53 and 0.63 decrease slightly compared with those of Fe₇Se₈. The Sommerfeld coefficient continuously declines to 3.53 mJ/mol-K² at x = 0.89. Single crystal at x > 0.89 has not yet been obtained. (B) Sommerfeld coefficient γ of Li_0.89Fe₇Se₈ with applied magnetic field of 0, 2, and 8 T, respectively. All the fields are applied at room temperature. The purpose is to exclude the possible linear specific heat contribution of spin glass. As the figure shows, the Sommerfeld coefficient remains around 3.5 mJ/mol-K² under external fields.