  By applying pressures up to 42 GPa on the nitrogen-doped lutetium hydride
(LuH$_{2\pm\text{x}}$N$_y$), we have found a gradual change of color from
dark-blue to pink-violet in the pressure region of about 12 GPa to 21 GPa. The
temperature dependence of resistivity under pressures up to 50.5 GPa shows
progressively optimized metallic behavior with pressure. Interestingly, in the
pressure region for the color change, a clear decrease of resistivity is
observed with the increase of pressure, which is accompanied by a clear
increase of the residual resistivity ratio (RRR). Fitting to the low
temperature resistivity gives exponents of about 2, suggesting a Fermi liquid
behavior in low temperature region. The general behavior in wide temperature
region suggests that the electron-phonon scattering is still the dominant one.
The magnetoresistance up to 9 tesla in the state under a pressure of 50.5 GPa
shows an almost negligible effect, which suggests that the electric conduction
in the pink-violet state is dominated by a single band. It is highly desired to
have theoretical efforts in understanding the evolution of color and
resistivity in this interesting system.

Superconductivity