Multi-band superconductivity in LaFeAsO_{0.9}F_{0.1} single crystals probed by high-field vortex torque magnetometry

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of the magnetic torque $\tau(\theta)$ in the mixed state of LaO$_{0.9}$F$_{0.1}$FeAs single crystals as a function of temperature $T$ and magnetic fields $H$ up to 18 T. The paramagnetic contribution of the Fe ions is properly treated in order to extract the effective mass anisotropy parameter $\gamma=(m_c/m_{ab})^{1/2}$ from $\tau(\theta)$. We show that $\gamma$ depends strongly on both $T$ and $H$, reaching a maximum value of $\sim$ 10 followed by a decrease towards values close to 1 as $T$ is lowered. The observed field dependencies of the London penetration depth $\lambda_{ab}$ and $\gamma$ suggest the onset of suppression of a superconducing gap at $H \approx H_{c2}/3$.Comment: 7 pages, 7 figure

Single crystal of superconducting SmFeAsO1-xFy grown at high pressure

Single crystals of SmFeAsO1-xFy of a size up to 120 micrometers have been grown from NaCl/KCl flux at a pressure of 30 kbar and temperature of 1350-1450 C using the cubic anvil high-pressure technique. The superconducting transition temperature of the obtained single crystals varies between 45 and 53 K.Obtained crystals are characterized by a full diamagnetic response in low magnetic fields and by a high critical current density in high magnetic fields. Structural refinement has been performed on single crystal. Differential thermal analysis investigations at 1 bar Ar pressure show decomposition of SmFeAsO1-xFy at 1302 C.Comment: 12 pages, 3 tables, 6 figure

Field Dependent Superfluid Density in the Optimally Doped SmFeAsO_(1-x)F_y Superconductor

The magnetic field dependence of the in-plane magnetic penetration depth for optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density as the field is increased up to 1.4 T. This behavior is analysed within a two-band model with self-consistently derived coupled gaps, where the superfluid density related to the larger gap is field independent and the superfluid density related to the smaller gap is strongly suppressed with increasing field.Comment: 7 pages, 5 figure

Bulk electronic structure of superconducting LaRu2P2 single crystals measured by soft x-ray angle-resolved photoemission spectroscopy

We present a soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the stoichiometric pnictide superconductor LaRu2P2. The observed electronic structure is in good agreement with density functional theory (DFT) calculations. However, it is significantly different from its counterpart in high-temperature superconducting Fe-pnictides. In particular the bandwidth renormalization present in the Fe-pnictides (~2 - 3) is negligible in LaRu2P2 even though the mass enhancement is similar in both systems. Our results suggest that the superconductivity in LaRu2P2 has a different origin with respect to the iron pnictides. Finally we demonstrate that the increased probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is essential in determining the bulk electronic structure in the experiment.Comment: 4 pages, 4 figures, 1 supplemental material. Accepted for publication in Physical Review Letter

Quasiparticle relaxation dynamics in spin-density-wave and superconducting SmFeAsO_{1-x}F_{x} single crystals

We investigate the quasiparticle relaxation and low-energy electronic structure in undoped SmFeAsO and near-optimally doped SmFeAsO_{0.8}F_{0.2} single crystals - exhibiting spin-density wave (SDW) ordering and superconductivity respectively - using pump-probe femtosecond spectroscopy. In the undoped single crystals a single relaxation process is observed, showing a remarkable critical slowing down of the QP relaxation dynamics at the SDW transition temperature T_{SDW}\simeq125{K}. In the superconducting (SC) crystals multiple relaxation processes are present, with distinct SC state quasiparticle recombination dynamics exhibiting a BCS-like T-dependent superconducting gap, and a pseudogap (PG)-like feature with an onset above 180K indicating the existence of a pseudogap of magnitude 2\Delta_{\mathrm{PG}}\simeq120 meV above T_{\mathrm{c}}. From the pump-photon energy dependence we conclude that the SC state and PG relaxation channels are independent, implying the presence of two separate electronic subsystems. We discuss the data in terms of spatial inhomogeneity and multi-band scenarios, finding that the latter is more consistent with the present data.Comment: Replaced by the correct versio

Evidence for Two Distinct Anisotropies in the Oxypnictide Superconductors SmFeAsO0.8F0.2 and NdFeAsO0.8F0.2

Single crystals of the oxypnictide superconductors SmFeAsO0.8F0.2 and NdFeAsO0.8F0.2 with T c in the range of 44 to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the magnetic penetration depth anisotropy γ λ differently than the upper critical field anisotropy γ H , provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result γ λ differs significantly in magnitude and in temperature dependence from γ H , analogous to MgB2 but with a reversed sign of slope. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductor

{Rearrangement of the antiferromagnetic ordering at high magnetic fields in SmFeAsO and SmFeAsO0.9_{0.9}F0.1_{0.1} single crystals

The low-temperature antiferromagnetic state of the Sm-ions in both nonsuperconducting SmFeAsO and superconducting SmFeAsO$_{0.9}$F$_{0.1}$ single crystals was studied by magnetic torque, magnetization, and magnetoresistance measurements in magnetic fields up to 60~T and temperatures down to 0.6~K. We uncover in both compounds a distinct rearrangement of the antiferromagnetically ordered Sm-moments near $35-40$~T. This is seen in both, static and pulsed magnetic fields, as a sharp change in the sign of the magnetic torque, which is sensitive to the magnetic anisotropy and hence to the magnetic moment in the $ab$-plane, ({\it i.e.} the FeAs-layers), and as a jump in the magnetization for magnetic fields perpendicular to the conducting planes. This rearrangement of magnetic ordering in $35-40$~T is essentially temperature independent and points towards a canted or a partially polarized magnetic state in high magnetic fields. However, the observed value for the saturation moment above this rearrangement, suggests that the complete suppression of the antiferromagnetism related to the Sm-moments would require fields in excess of 60~T. Such a large field value is particularly remarkable when compared to the relatively small N\'{e}el temperature $T_{\rm N}\simeq5$~K, suggesting very anisotropic magnetic exchange couplings. At the transition, magnetoresistivity measurements show a crossover from positive to negative field-dependence, indicating that the charge carriers in the FeAs planes are sensitive to the magnetic configuration of the rare-earth elements. This is indicates a finite magnetic/electronic coupling between the SmO and the FeAs layers which are likely to mediate the exchange interactions leading to the long range antiferromagnetic order of the Sm ions.Comment: 10 pages, 7 figures, accepted in Phys. Rev.