Charge-carrier localization induced by excess Fe in the superconductor ${\text{Fe}}_{1+y}{\text{Te}}_{1-x}{\text{Se}}_x$

We have investigated the effect of Fe nonstoichiometry on properties of the ${\text{Fe}}_{1+y}\left(\text{Te},\text{Se}\right)$ superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific-heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity but also results in a weakly localized electronic state. We argue that these effects originate from the magnetic coupling between the excess Fe and the adjacent Fe square-planar sheets, which favors a short-range magnetic order.

Figure 1

(Color online) Upper panel: schematic crystal structure of ${\text{Fe}}_{1+y}\left(\text{Te},\text{Se}\right)$. The iron on the square-planar sheet is denoted by Fe(1); the iron partially occupying at the interstitial sites of the (Te, Se) layers is the excess Fe, denoted by Fe(2). (a) Magnetic susceptibility as a function of temperature $\chi \left(T\right)$ measured under a magnetic field of 30 Oe (applied along the $c$ axis). (b) In-plane resistivity as a function of temperatures ${\rho }_{\text{ab}}\left(T\right)$. SC1 and SC2 represent two superconducting samples with 3% and 11% Fe(2).

Figure 2

(Color online) (a) In-plane resistivity as a function of temperature ${\rho }_{\text{ab}}\left(T\right)$ for superconducting samples SC1 [3% Fe(2)] and SC2 [11%Fe(2)]. (b) Resistivity along the $c$ axis ${\rho }_{\text{c}}\left(T\right)$ for samples SC1 and SC2.

Figure 3

(Color online) (a) Resistivity as a function of temperature plotted on the $\text{log}T$ scale for superconducting sample SC2 [11%Fe(2)]. (b) Resistivity as a function of temperature plotted on the $T^2$ scale for nonsuperconducting sample NSC1 [4% Fe(2)].

Figure 4

(Color online) (a) In-plane resistivity as a function of temperature ${\rho }_{\text{ab}}\left(T\right)$ for nonsuperconducting samples NSC1 [4% Fe(2)] and NSC2 [11% Fe(2)]. (b) Resistivity along the $c$ axis as a function of temperature ${\rho }_{\text{c}}\left(T\right)$ for NSC1 and NSC2.

Figure 5

(Color online) (a) Hall coefficient $R_{\text{H}}$ as a function of temperature for superconducting samples SC1 [3% Fe(2)] and SC2 [11%Fe(2)]. (b) Hall coefficient $R_{\text{H}}$ as a function of temperature for nonsuperconducting samples NSC1 [4% Fe(2)] and NSC2 [11% Fe(2)]. Inset in (a): Hall resistivity ${\rho }_{xy}$ vs magnetic field at various temperatures for sample SC2.

Figure 6

(Color online) Heat capacity $C/T$ as a function of temperature for superconducting samples (a) SC1 and SC2, and nonsuperconducting samples (b) NSC1 and NSC2. Inset in (b) shows $C/T$ versus $T^2$. The solid lines represent the fit to experimental data below 12 K (see the text).