  The pion nucleon vertex function at finite temperature is studied in the
framework of: (a) the thermal (linear) sigma model to leading (one-loop) order,
and (b) a thermal QCD-Finite Energy Sum Rule. Results from both methods
indicate that the strength of the pion-nucleon coupling decreases with
increasing T, vanishing at a critical temperature. The associated mean-square
radius is a monotonically increasing function of T, diverging at the critical
temperature. This is interpreted as (analytical) evidence for the quark-gluon
deconfinement phase transition.
