  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.
