  Superconducting iron chalcogenide FeSe has the simplest crystal structure
among all the Fe-based superconductors. Unlike other iron pnictides, FeSe
exhibits no long range magnetic order accompanying the
tetragonal-to-orthorhombic structural distortion, which raises the fundamental
question about the role of magnetism and its associated spin fluctuations in
mediating both nematicity and superconductivity. The extreme sensitivity of
FeSe to external pressure suggests that chemical pressure, induced by
substitution of Se by the smaller ion S, could also a be good tuning parameter
to further study the coupling between superconductivity and nematicity and to
obtain information on both the Fermi-surface changes and the symmetry of the
superconducting state. Here we study the thermodynamic properties of
Fe(Se$_{1-x}$S$_{x}$) for 3 compositions, $x=0$, 0.08 and 0.15, using
heat-capacity and thermal-expansion measurements. With increasing S content we
observe a significant reduction of the tetragonal-to-orthorhombic transition
temperature T$_{s}$. However, this suppression of T$_{s}$ is counterintuitively
accompanied by an enhancement of the orthorhombic distortion $\delta$ below
T$_c$, which clearly indicates that superconductivity favors the nematic state.
In parallel, the superconducting transition temperature T$_{c}$ is sizeably
enhanced, whereas the increase of the Sommerfeld coefficient $\gamma_{n}$ is
quite moderate. In the T$\to$ 0 limit, an unusually large residual density of
states is found for $x>0$ indicative of significant substitution-induced
disorder. We discuss these observations in the context of $s+d$
superconducting-state symmetry.
