Critical behaviour of finite-temperature U(1) lattice gauge model

With the variational-cumulant expansion (VCE), the expectation 〈L〉 of the Polyakov line of the finite-temperature U(1) lattice gauge model in (3 + 1)-dimensions (Nτ = 1) is expanded to the 4th order. Because the variational parameter is determined by the accumulation point scheme, the expansion rapidly converges and accurate critical behaviour and the critical exponent β are easily obtained.