Metamagnetic Behavior and Kondo Breakdown in Heavy-Fermion CeFePO

We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional XY-type anisotropy shows a metamagnetic behavior at the metamagnetic field H_M \simeq 4 T perpendicular to the c-axis and that a critical behavior is observed around H_M. Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, H_M in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptibility shows a peak. This finding suggests that H_M is a magnetic field breaking the local Kondo singlet, and the critical behavior around H_M is driven by the Kondo breakdown accompanied by the Fermi-surface instability.Comment: 5pages, 6 figures published in Phys. Rev. Let

Nernst effect of the new iron-based superconductor LaO1−x_{1-x}Fx_{x}FeAs

We report the first Nernst effect measurement on the new iron-based superconductor LaO$_{1-x}$F$_{x}$FeAs $(x=0.1)$. In the normal state, the Nernst signal is negative and very small. Below $T_{c}$ a large positive peak caused by vortex motion is observed. The flux flowing regime is quite large compared to conventional type-II superconductors. However, a clear deviation of the Nernst signal from normal state background and an anomalous depression of off-diagonal thermoelectric current in the normal state between $T_{c}$ and 50 K are observed. We propose that this anomaly in the normal state Nernst effect could correlate with the SDW fluctuations.Comment: 8 pages, 4 figures; Latex file changed, references adde

Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictides

Weak-coupling approaches to the pairing problem in the iron pnictide superconductors have predicted a wide variety of superconducting ground states. We argue here that this is due both to the inadequacy of certain approximations to the effective low-energy band structure, and to the natural near-degeneracy of different pairing channels in superconductors with many distinct Fermi surface sheets. In particular, we review attempts to construct two-orbital effective band models, the argument for their fundamental inconsistency with the symmetry of these materials, and the comparison of the dynamical susceptibilities in two- and five-orbital models. We then present results for the magnetic properties, pairing interactions, and pairing instabilities within a five-orbital Random Phase Approximation model. We discuss the robustness of these results for different dopings, interaction strengths, and variations in band structure. Within the parameter space explored, an anisotropic, sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to the near nesting of Fermi surface sheets.Comment: 17 pages, 23 figure

Spin-singlet superconductivity with multiple gaps in PrO0.89F0.11FeAs

Since the discovery of high transition-temperature (Tc) superconductivity in copper oxides two decades ago, continuous efforts have been devoted to searching for similar phenomenon in other compounds. With the exception of MgB2 (Tc =39 K), however, Tc is generally far lower than desired. Recently, breakthrough has been made in a new class of oxypnictide compounds. Following the initial discovery of superconductivity in LaO1-x FxFeAs (Tc =26 K), Tc onset has been raised to 55 K in ReO1-xFxFeAs (Re: Ce, Pr, Nd, Sm). Meanwhile, unravelling the nature of the energy associated with the formation of current-carrying pairs (Cooper pairs), referred to as the superconducting energy gap, is the first and vital step towards understanding why the superconductivity occurs at such high temperature and is also important for finding superconductors with still higher Tc. Here we show that, on the basis of the nuclear magnetic resonance (NMR) measurements in PrO0.89F0.11FeAs (Tc =45 K), the Cooper pair is in the spin-singlet state (two spins are anti-paralleled), with two energy gaps opening below Tc. The results strongly suggest the existence of nodes (zeros) in the gap. None of superconductors known to date has such unique gap features, although copper-oxides and MgB2 share part of them.Comment: submitted on May 1

Point contact Andreev reflection spectroscopy of NdFeAsO_0.85

The newly discovered oxypnictide family of superconductors show very high critical temperatures of up to 55K. Whilst there is growing evidence that suggests a nodal order parameter, point contact Andreev reflection spectroscopy can provide crucial information such as the gap value and possibly the number of energy gaps involved. For the oxygen deficient NdFeAsO0.85 with a Tc of 45.5K, we show that there is clearly a gap value at 4.2K that is of the order of 7meV, consistent with previous studies on oxypnictides with lower Tc. Additionally, taking the spectra as a function of gold tip contact pressure reveals important changes in the spectra which may be indicative of more complex physics underlying this structure.Comment: 11 pages, 3 figures. New references included, extra discussion. This version is accepted in Superconductor Science and Technolog