Incoherent-coherent crossover and the pseudogap in Te-annealed superconducting ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ revealed by magnetotransport measurements

In this paper, we conducted various magnetotransport measurements on ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ single crystals from which excess iron was sufficiently removed. Our results revealed that crossover from the incoherent to the coherent electronic state and the opening of the pseudogap occur at high temperatures ($\approx 150\mathrm{K}$ for $x=0.2$). This is accompanied by a more substantial pseudogap and the emergence of a phase with a multiband nature at lower temperatures (below $\approx 50\mathrm{K}$ for $x=0.2$) before superconductivity sets in. A comparison of these results with those of the as-grown (nonsuperconducting) samples implies that the coherent state accompanied by the pseudogap is needed for the occurrence of superconductivity in this system.

Figure 1

Temperature dependence of the zero-field-cooled (ZFC) normalized magnetic susceptibilities $\chi$ for Te-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ single crystals with (a) $x=0.15$, (b) $x=0.2$, (c) $x=0.3$, and (d) $x=0.4$. The insets show the field-cooled (FC) data. Data were recorded by applying a magnetic field of 30 Oe parallel to the $c$ axis.

Figure 2

Normalized in-plane resistivity ${\rho }_{ab}\left(T\right)$ for the (a) as-grown and (b) Te-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x(0\le x\le 0.4)$ single crystals, and normalized out-of-plane resistivity ${\rho }_c\left(T\right)$ for the (c) as-grown and (d) Te-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x(0.2\le x\le 0.4)$ single crystals. The resistivity data are shifted vertically for clarity. The arrows indicate the superconducting, AFM transitions, and characteristic temperatures $T_{{\rho }_{ab}}^{*}$ and $T_{{\rho }_c}^{*}$.

Figure 3

Hall resistivity ${\rho }_{yx}$ at several temperatures for the (a) as-grown, (b) 400-h-annealed, and (c) 600-h-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystals. (d) Temperature dependence of Hall coefficients $R_H$ for the as-grown, 400-h-annealed, and 600-h-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystals. The arrow indicates the characteristic temperature $T_{R_H}^{*}$.

Figure 4

MR, at several temperatures for the (a) as-grown, (b) 400-h-annealed, and (c) 600-h-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystals. (d) Temperature dependence of coefficient $A$ for the as-grown, 400-h-annealed, and 600-h-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystals. The arrows indicate the characteristic temperatures $T_{R_H}^{**}$ and $T_{R_H}^{*}$.

Figure 5

MR as a function of ${\tan }^2{\mathrm{\Theta }}_H$ at several temperatures for the (a) as-grown, (b) 400-h-annealed, and (c) 600-h-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystals.

Figure 6

(a) Temperature dependence of Hall coefficient $R_H$ and the hole number $n$ for the as-grown ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystal. (b) In-plane resistivities ${\rho }_{ab}\left(T\right)$ for the as-grown, 400-h-annealed, and 600-h-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystals. (c) Temperature dependence of hole mobilities ${\mu }_h$ for the as-grown, 400-h-annealed, and 600-h-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystals. (d) Temperature dependence of electron mobilities ${\mu }_e$ for the as-grown, 400-h-annealed, and 600-h-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.8}{\mathrm{Se}}_{0.2}$ single crystals. The arrows indicate the characteristic temperatures $T_{{\rho }_{ab}}^{*},T_{R_H}^{**}$, and $T_{R_H}^{*}$.

Figure 7

Characteristic temperatures vs Se concentration $x$ for Te-annealed ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$. Closed symbols represent the characteristic temperatures obtained in this paper. The open circles of $T_{R_H}^{*}$ are replotted from Ref. [4].