  We investigated several FeSe single crystals grown by two different methods
by utilizing experimental techniques namely, resistivity, magnetoresistance,
specific heat, scanning tunneling microscopy, and spectroscopy. The residual
resistivity ratio (RRR) shows systematic differences between samples grown by
chemical vapor transport and flux vapor transport, indicating variance in the
amount of scattering centers. Although the superconducting transition
temperature $T_c$ is not directly related to RRR, our study evidences subtle
differences in the features of an incipient ordering mode related to a
depletion of density of states at the Fermi level. For instance, the onset
temperature of anisotropic spin-fluctuations at $T^* \approx 75$ K, and the
temperature of the opening-up of a partial gap in the density of states at
$T^{**} \approx 30$ K are not discernible in the samples with lower RRR.
Further, we show that the functional dependence of the electronic specific heat
below 2 K, which allows to determine the nodal features as well as the small
superconducting gap, differs significantly in crystals grown by these two
different methods. Our investigation suggests that some of the controversies
about the driving mechanism for the superconducting gap or its structure and
symmetry is related to minute differences in the crystals arising due to the
growth techniques used and the total amount of scattering centers present in
the sample.
