Nernst Effect of Iron Pnictide and Cuprate Superconductors: Signatures of Spin Density Wave and Stripe Order

摘要

The Nernst effect has recently proven to be a sensitive probe for detecting unusual normal state properties of unconventional superconductors. In particular, it may sensitively detect Fermi surface reconstructions which are connected to a charge or spin density wave (SDW) ordered state, and even fluctuating forms of such a state. Here we summarize recent results for the Nernst effect of the iron pnictide superconductor LaO_(1-x)F_xFeAs, whose ground state evolves upon doping from an itinerant SDW to a superconducting state, and the cuprate superconductor La_(1.8-x)Eu_(0.2)Sr_xCuO_4 which exhibits static stripe order as a ground state competing with the superconductivity. In LaFeASO_(1-x)F_x, the SDW order leads to a huge Nernst response, which allows to detect even fluctuating SDW precursors at superconducting doping levels where long range SDW order is suppressed. This is in contrast to the impact of stripe order on the normal state Nernst effect in La_(1.8-x)Eu_(0.2)Sr_xCuO_4. Here, though signatures of the stripe order are detectable in the temperature dependence of the Nernst coefficient, its overall temperature dependence is very similar to that of La_(2-x)Sr_xCuO_4, where stripe order is absent. The anomalies which are induced by the stripe order are very subtle and the enhancement of the Nernst response due to static stripe order in La_(1.8-x)Eu_(0.2)Sr_xCuO_4 as compared to that of the pseudogap phase in La_(2-x)Sr_xCuO_4, if any, is very small.