Breakup of a Stoner model for the 2D ferromagnetic quantum critical point

Re-interpretation of the results by [A. V. Chubukov et. al., Phys. Rev. Lett. 90, 077002 (2003)] leads to the conclusion that ferromagnetic quantum critical point (FQCP) cannot be described by a Stoner model because of a strong interplay between the paramagnetic fluctuations and the Cooper channel, at least in two dimensions.Comment: 5 pages, 2 EPS figures, RevTeX

Sum rule for the optical scattering rates

An important quantity in electronic systems is the quasiparticle scattering rate (QPSR). A related optical scattering rate (OSR) is routinely extracted from optical data, and, while it is not the same as the QPSR, it nevertheless displays many of the same features. We consider a sum rule which applies to the area under a closely related quantity, almost equal to the OSR in the low energy region. We focus on the readjustment caused by, for example, a quasiparticle density of states change due to the superconducting transition. Unfortunately, no general statement about mechanism can be made solely on the energy scale in which the spectral weight readjustment on the OSR occurs.Comment: 22 pages, 7 figures accepted for publication by Phys. Rev.

Proximity of Iron Pnictide Superconductors to a Quantum Tricritical Point

We determine the nature of the magnetic quantum critical point in the doped LaFeAsO using a set of constrained density functional calculations that provide ab initio coefficients for a Landau order parameter analysis. The system turns out to be remarkably close to a quantum tricritical point, where the nature of the phase transition changes from first to second order. We compare with the effective field theory and discuss the experimental consequences.Comment: 4 pages, 4 figure