  A large and high-quality single crystal (Li0.84Fe0.16)OHFe0.98Se, the optimal
superconductor of newly reported (Li1-xFex)OHFe1-ySe system, has been
successfully synthesized via a hydrothermal ion-exchange technique. The
superconducting transition temperature (Tc) of 42 K is determined by magnetic
susceptibility and electric resistivity measurements, and the zero-temperature
upper critical magnetic fields are evaluated as 79 and 313 Tesla for the field
along the c-axis and the ab-plane, respectively. The ratio of out-of-plane to
in-plane electric resistivity,\r{ho}c/\r{ho}ab, is found to increases with
decreasing temperature and to reach a high value of 2500 at 50 K, with an
evident kink occurring at a characteristic temperature T*=120 K. The negative
in-plane Hall coefficient indicates that electron carriers dominate in the
charge transport, and the hole contribution is significantly reduced as the
temperature is lowered to approach T*. From T* down to Tc, we observe the
linear temperature dependences of the in-plane electric resistivity and the
magnetic susceptibility for the FeSe layers. Our findings thus reveal that the
normal state of (Li0.84Fe0.16)OHFe0.98Se becomes highly two-dimensional and
anomalous prior to the superconducting transition, providing a new insight into
the mechanism of high-Tc superconductivity.
