  We have systematically investigated the annealing effect on the
superconductivity of iron chalcogenide Fe1+y(Te1-xSex). The atmospheres used
for annealing include O2, N2, I2 vapor, air and vacuum. We observed that
annealing in O2, I2 and air could enhance superconductivity for the underdoped
samples, consistent with the results reported in literatures. Interestingly, we
found that annealing in N2 also leads to superconductivity enhancement, similar
to the annealing effects of O2, I2 and air. However, vacuum annealing does not
enhance superconductivity, which indicates that the enhanced superconductivity
in O2-, N2-, I2- and air-annealed samples is not due to improved homogeneity.
In addition, we have treated the underdoped samples with nitric acid, which is
found to enhance superconductivity as well. Our analyses of these results
support the argument that the superconductivity enhancement, caused either by
annealing or nitric acid treatment, originates from the variation of
interstitial Fe. The interstitial Fe, which is destructive to superconducting
pairing, can be reduced by annealing in oxidation agents or nitric acid
treatment. We also find that although N2-, O2- and air-annealed samples exhibit
strong superconducting diamagnetism with -4pichi ~1 (chi, dc magnetic
susceptibility) for some samples, their actual superconducting volume fraction
probed by specific heat is low, ranging from 10% to 30% for 0.09 < x < 0.3,
indicating that the superconductivity suppression remains significant even in
annealed samples. The strong diamagnetism is associated with the
superconducting shielding effect on the non-superconducting phase. We have also
established the phase diagram of the annealed samples and compared it with that
of the as-grown samples. The effect of annealing on the interplay between
magnetism and superconductivity is discussed.
