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Coupling of electronic and magnetic properties in Fe1+y(Te1-xSex)

We have studied the coupling of electronic and magnetic properties in Fe1+y(Te1-xSex) via systematic specific heat, magnetoresistivity, 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 large Sommerfeld coefficient Gamma (~55-65 mJ/mol K^2), positive Hall coefficient R_H and negative magnetoresistance MR at low temperature, in sharp contrast with the x=0.4-0.5 region where Gamma drops to ~ 26 mJ/mol K^2 and R_H / MR becomes negative/positive at low temperature. Dramatic changes of Gamma, as well as sign reversal in low-temperature RH 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 Gamma, R_H and MR with x are distinct from those seen in y = 0.02 system: Gamma 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 does 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 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.