A. Kondrat

Commensurate Spin Density Wave in LaFeAsO: A Local Probe Study

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe Mössbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5)muB at the iron site below T(N)=138 K, well separated from a structural phase transition at T(S)=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below T(N) are reproduced.

Field and Temperature Dependence of the Superfluid Density in LaFeAsO1 xFx Superconductors: A Muon Spin Relaxation Study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda ab(0)=254(2) nm for x=0.1 and lambda ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.

The intrinsic electronic phase diagram of iron-oxypnictide superconductors

We present the first comprehensive derivation of the intrinsic electronic phase diagram of the iron-oxypnictide superconductors in the normal state based on the analysis of the electrical resistivity ρ of both LaFeAsO1- xFx and SmF eAsO1- xFx for a wide range of doping. Our data give clear-cut evidence for unusual normal-state properties in these new materials. In particular, the emergence of superconductivity at low doping levels is accompanied by distinct anomalous transport behavior in ρ of the normal state which is reminiscent of the spin-density wave (SDW) signature in the parent material. At higher doping levels ρ of LaFeAsO1- xFx shows a clear transition from this pseudogap-like behavior to Fermi-liquid–like behavior, mimicking the phase diagram of the cuprates. Moreover, our data reveal a correlation between the strength of the anomalous features and the stability of the superconducting phase. The pseudogap-like features become stronger in SmF eAsO1- xFx where superconductivity is enhanced and vanish when superconductivity is reduced in the doping region with Fermi-liquid–like behavior.

High-Field Pauli-Limiting Behavior and Strongly Enhanced Upper Critical Magnetic Fields near the Transition Temperature of an Arsenic-Deficient LaO0.9F0.1FeAs1-δ Superconductor

We report upper critical field Bc2(T) data for disordered (arsenic-deficient) LaO0.9F0.1FeAs1-delta in a wide temperature and magnetic field range up to 47 T. Because of the large linear slope of Bc2 approximately -5.4 to -6.6 T/K near Tc approximately 28.5 K, the T dependence of the in-plane Bc2(T) shows a flattening near 23 K above 30 T which points to Pauli-limited behavior with Bc2(0) approximately 63-68 T. Our results are discussed in terms of disorder effects within [corrected] unconventional superconducting pairings.

Magnetic ordering and negative thermal expansion in PrFeAsO

We report the structure and magnetism of PrFeAsO, one of the parent phases of the Fe-As family of superconductors, as measured by neutron powder diffraction. In common with other REFeAsO materials, a tetragonal-orthorhombic phase transition is found on cooling below 136 K and striped Fe magnetism with

Thermal expansion of LaFeAsO1-xFx: evidence for high temperature fluctuations

k=(1,0,1)

Optical study of LaO0.9F0.1FeAs: evidence for a weakly coupled superconducting state.

is detected below

Upper critical field, penetration depth, and depinning frequency of the high-temperature superconductor LafeAsO0.9F0.1 studied by microwave surface impedance

\ensuremath{\sim}85\text{ }\text{K}

Unusual Nernst effect and spin density wave precursors in superconducting LaFeAsO1−xFx

. Our magnetic order-parameter measurements show that the ordered Fe moment along the

Gd3+ electron spin resonance spectroscopy on LaO1 − xFxFeAs superconductors

a