  The spin-fluctuation effect in the Se-substituted system
Fe$_{1+\delta}$Te$_{1-x}$Se$_x$ ($x$ = 0, 0.05, 0.12, 0.20, 0.28, 0.33, 0.45,
0.48 and 1.00; $0<\delta < 0.12$) has been studied by the measurements of the
X-ray diffraction, the magnetic susceptibility under high magnetic fields and
the electrical resistivity under magnetic fields up to 14 T. The samples with
$x$ = 0.05, 0.12, 0.20, 0.28, 0.33, 0.45 and 0.48 show superconducting
transition temperatures in the ranger of 10 K$\sim$14 K. We obtained their
intrinsic susceptibilities by the Honda-Owen method. A nearly linear-in-$T$
behavior in magnetic susceptibility of superconducting samples was observed,
indicating the antiferromagnetic spin fluctuations have a strong link with the
superconductivity in this series. The upper critical field $\mu_0H_{c2}^{orb}$
for $T\to$ 0 was estimated to exceed the Pauli paramagnetic limit. The
Kadowaki-Woods and Wilson ratios indicate that electrons are strongly
correlated in this system. Furthermore, the superconducting coherence length
and the electron mean free path were also discussed. These superconducting
parameters indicate that the superconductivity in the
Fe$_{1+\delta}$Te$_{1-x}$Se$_x$ system is unconventional.
