Upper critical magnetic field of unconventional superconductors from chemical equilibrium

The fraction of bosons, f(t), is obtained from a standard equilibrium theory in terms of the chemical constant phi(t) by f(t) = 1 - [1 + phi(t)](-1/2) Here t = T/T* and T* is the scaling temperature connected with the binding energy of bosons 2 Delta by Delta = k(B) T*, independent of magnetic field together with f(t). The transition temperature t(c)(H) is obtained from n(0)f(t(c)(H)) = n(c)(H) = n(c)(0)[1 + gamma H-mu], where n(c)(H), the critical density for superconductivity is assumed to be a power law. In the cases where t(0) = t(c)(0) = T-c(0)/T* is small the chemical constant can be approximated by phi(t) = 1/(beta t) and t(c)(H) call be calculated. This is equivalent of knowing H-c2(y), given by H-c2(y) = H-c2(0)[(f(t(0)y) - f(t(0)))/ (1 - f(t(0)))](1/mu), y = t(c)(H)/t(c)(0). For several heavy fermions (beta = 13.7 and T* = 150K) data on the quantity mu(-1) ln[H-c2(y)/H-c2(0)] fall on the same curve. For T12201 we obtain mu = 2/3. (C) 2000 Elsevier Science B.V. All rights reserved.