Coupling of electronic and magnetic properties in Fe_1+y(Te _1-xSe_x)

We have studied the coupling of electronic and magnetic properties in Fe_1+y(Te_1-xSe_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 K2), 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 K2 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∼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 K2 for 0.1<x<0.3 and drops to ∼34 mJ/mol K2 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 (π,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.