Calculation of Particle-Number Susceptibility of QED₃ at Finite Temperature

Based on the external field approach and the differential form of Ward identity, we derive a more compact formula for the particle-number susceptibility in QED₃ at finite temperature. Using the zero frequency approximation the numerical value of the particle-number susceptibility is calculated in the Dyson--Schwinger approach for the case that the number of fermion flavours equals one and two, respectively. An enhanced fluctuation of the particle-number density is observed across the transition temperature, which should be an essential characteristic of chiral phase transition in QED₃.