  Superconductors are influenced by high-entropy alloys (HEAs); these have been
investigated in various functional materials. REOBiS$_2$ (RE = La, Ce, Pr, Nd,
Sm, and Gd in different combinations) single crystals with HEAs at the RE-site
were successfully grown using the flux method. The obtained crystals were
plate-shaped (1 mm$^2$) with a well-developed c-plane. Ce was present in both
trivalent (Ce$^{3+}$) and tetravalent (Ce$^{4+}$) electronic configurations;
the concentration of Ce$^{4+}$ at the RE-site was approximately 10 at% in all
single crystals. The single crystals showed superconducting transition
temperature with zero resistivity within 1.2-4.2 K. The superconducting
transition temperature, superconducting anisotropy, electronic specific heat
coefficient, and Debye temperature of the crystals were not correlated with the
mixed entropy at the RE-site. Except for the electronic specific heat
coefficient, the variation of these parameters as a function of mixed entropy
showed different trends for equivalent and non-equivalent RE element
compositions. Thus, the configuration of RE elements influences the
superconducting properties of REOBiS$_2$ single crystals, alluding to a method
of modulating transition temperatures.
