Charged Fixed Point Found in Superconductor Below Tc
Hagen M. Kleinert, Flavio S. Nogueira
Abstract
We present a perturbative approach which yields, for the first time, an infrared-stable fixed point in the Ginzburg-Landau (GL) model at the oneloop level. The calculations are done in d = 3 dimensions and below Tc, where the renormalization group functions can be expressed directly as functions of the Ginzburg parameter κ which is the ratio between the two fundamental scales of the problem, the penetration depth λ and the correlation length ξ. We find a charged fixed point for κ> 1 / √ 2, that is, in the type II regime, where ∆κ ≡ κ − 1 / √ 2 is shown to be a natural expansion parameter. This parameter controls a momentum space instability in the two-point correlation function of the order field. This instability appears at a nonzero wave-vector p0 whose magnitude scales like ∼ ∆κ ¯ β, with a critical exponent ¯ β = 1/2 in the one-loop approximation, a behavior known from magnetic systems with a Lifshitz point in the phase diagram. This momentum space instability is argued to be the origin of the negative η-exponent of the order field.