  Gamma-ray bursts (GRBs) show different behaviours and trends in their
spectral evolution. One of the methods used to understand the physical origin
of these behaviours is to study correlation between the spectral fit
parameters. In this work, we used a Bayesian analysis method to fit
time-resolved spectra of GRB pulses that were detected by the
\textit{Fermi}/GBM during its first 9 years of mission. We studied single
pulsed long bursts ($T_{90}\geq2$ s). Among all the parameter correlations, we
found that the correlation between the low-energy power-law index $\alpha$ and
the energy flux exhibited a systematic behaviour. We presented the properties
of the observed characteristics of this behaviour and interpreted it in the
context of the photospheric emission model.
