We have studied the coupling of electronic and magnetic properties in Fe${}_{1+y}$(Te${}_{1-x}$Se${}_x$) via systematic specific heat, magnetoresistivity (MR), and Hall coefficient measurements on two groups of samples with $y=0.02$ and 0.1. In the $y=0.02$ series, we find that the $0.09<x<0.3$ composition region, where superconductivity is suppressed, has a large Sommerfeld coefficient γ (∼55–65 mJ/mol K${}^2$), positive Hall coefficient $R_H$, and negative MR at low temperature, in sharp contrast with the $x$ $=$ 0.4–0.5 region, where γ drops to ∼26 mJ/mol K${}^2$ and $R_H$ and MR become negative and positive, respectively, at low temperature. Dramatic changes of γ, as well as sign reversal in low-temperature $R_H$ and MR, are also observed across the $x\sim 0.1$ boundary, where the long-range antiferromagnetic order is suppressed. However, for the system with rich interstitial excess Fe ($y=0.1$), where bulk superconductivity is suppressed even for $x$ $=$ 0.4–0.5, the variations of γ, $R_H$, and MR with $x$ are distinct from those seen in $y=0.02$ system: γ is ∼40 mJ/mol K${}^2$ for $0.1<x<0.3$ and drops to ∼34 mJ/mol K${}^2$ for $x$ $=$ 0.4–0.5; $R_H$ and MR do not show any sign reversal as $x$ is increased above 0.3. We will show that all these results can be understood in light of the evolution of the incoherent magnetic scattering by ($\pi ,0$) magnetic fluctuations with Se concentration. In addition, with the suppression of magnetic scattering by the magnetic field, we observed the surprising effect of a remarkable increase in the superconducting volume fraction under moderate magnetic fields for $x$ $=$ 0.3–0.4 samples in the $y=0.02$ system.

• Received 25 June 2013

DOI:https://doi.org/10.1103/PhysRevB.88.094505