  In this work, we measure two important phenomenological parameters of corona
(and hot accretion flow) in black hole X-ray binaries: the photon index
$\Gamma$ and the electron temperature $\te$. Thanks to the capability of {\it
NuSTAR} in hard X-rays, we measure these two parameters over six orders of
magnitude in the 0.1-100 keV X-ray luminosity $\lx$, from $\sim 5\times10^{38}
\ergs$ down to as low as $\sim 5\times10^{32} \ergs$. We confirm the existence
of a "V"-shaped correlation between $\Gamma$ and $\lx$. Surprisingly, we
observe a "$\Lambda$"-shaped correlation between $\te$ and $\lx$. The "cooler
when brighter" branch in the high luminosity regime ($\lx \ga 3\times 10^{36}
\ergs$) agrees with previous results and can be understood under the existing
model of Compton scattering in the corona. On the other hand, the apparent
"cooler when fainter" (positive $\te$-$\lx$ correlation) branch in the
low-luminosity regime ($\lx \la 3\times 10^{36} \ergs$) is unexpected, thus it
puts a new challenge to existing models of hot accretion flow/corona.
