M. Zolliker

Coupled Superconducting and Magnetic Order in CeCoIn5

Strong magnetic fluctuations can provide a coupling mechanism for electrons that leads to unconventional superconductivity. Magnetic order and superconductivity have been found to coexist in a number of magnetically mediated superconductors, but these order parameters generally compete. We report that close to the upper critical field, CeCoIn5 adopts a multicomponent ground state that simultaneously carries cooperating magnetic and superconducting orders. Suppressing superconductivity in a first-order transition at the upper critical field leads to the simultaneous collapse of the magnetic order, showing that superconductivity is necessary for the magnetic order. A symmetry analysis of the coupling between the magnetic order and the superconducting gap function suggests a form of superconductivity that is associated with a nonvanishing momentum.

Superconducting Vortices in CeCoIn5: Toward the Pauli-Limiting Field

Many superconducting materials allow the penetration of magnetic fields in a mixed state in which the superfluid is threaded by a regular lattice of Abrikosov vortices, each carrying one quantum of magnetic flux. The phenomenological Ginzburg-Landau theory, based on the concept of characteristic length scales, has generally provided a good description of the Abrikosov vortex lattice state. We conducted neutron-scattering measurements of the vortex lattice form factor in the heavy-fermion superconductor cerium-cobalt-indium (CeCoIn5) and found that this form factor increases with increasing field—opposite to the expectations within the Abrikosov-Ginzburg-Landau paradigm. We propose that the anomalous field dependence of the form factor arises from Pauli paramagnetic effects around the vortex cores and from the proximity of the superconducting state to a quantum critical point.

Gap in KFe2As2studied by small-angle neutron scattering observations of the magnetic vortex lattice

We report the observation, by small-angle-neutron-scattering (SANS), of magnetic flux lines “vortices” in super-clean KFe2As2 single crystals. The results show clear Bragg spots from a well ordered vortex lattice, for the first time in a FeAs superconductor. These measurements can give important information about the pairing state in this material, because the spatial variation of magnetic field in the vortex lattice reflects this pairing. With field parallel to the fourfold c-axis, nearly isotropic hexagonal packing of vortices was observed without VL-symmetry transitions up to high fields, indicating rather small anisotropy of the superconducting properties around this axis. This rules out gap nodes parallel to the c-axis, and thus d-wave and also anisotropic s-wave pairing. The strong temperature-dependence of the scattered intensity down to T Tc further indicates either widely different full gaps on different Fermi surface sheets, or nodal lines perpendicular to the axis. PACS numbers: 74.25.Uv, 74.70.Xa, 74.20.Rp, 74.25.-q

Observations of Pauli paramagnetic effects on the flux line lattice in CeCoIn5

From small-angle neutron scattering studies of the flux line lattice (FLL) in CeCoIn5, with magnetic field applied parallel to the crystal c-axis, we obtain the field and temperature dependence of the FLL form factor (FF), which is a measure of the spatial variation of the field in the mixed state. We extend our earlier work (Bianchi et al 2008 Science 319 177) to temperatures up to 1250u2009mK. Over the entire temperature range, paramagnetism in the flux line cores results in an increase of the FF with field. Near Hc2 the FF decreases again, and our results indicate that this fall-off extends outside the proposed Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) region. Instead, we attribute the decrease to a paramagnetic suppression of Cooper pairing. At higher temperatures, a gradual crossover toward more conventional mixed state behavior is observed.

Vortex lattice structure in BaFe2(As0.67P0.33)(2) via small-angle neutron scattering

We have observed a magnetic vortex lattice (VL) in BaFe2(As_{0.67}P_{0.33})2 (BFAP) single crystals by small-angle neutron scattering (SANS). With the field along the c-axis, a nearly isotropic hexagonal VL was formed in the field range from 1 to 16 T, which is a record for this technique in the pnictides, and no symmetry changes in the VL were observed. The temperature-dependence of the VL signal was measured and confirms the presence of (non d-wave) nodes in the superconducting gap structure for measurements at 5 T and below. The nodal effects were suppressed at high fields. At low fields, a VL reorientation transition was observed between 1 T and 3 T, with the VL orientation changing by 45{deg}. Below 1 T, the VL structure was strongly affected by pinning and the diffraction pattern had a fourfold symmetry. We suggest that this (and possibly also the VL reorientation) is due to pinning to defects aligned with the crystal structure, rather than being intrinsic.

Vortex lattice structure inBaFe2(As0.67P0.33)2via small-angle neutron scattering

We have observed a magnetic vortex lattice (VL) in BaFe2(As_{0.67}P_{0.33})2 (BFAP) single crystals by small-angle neutron scattering (SANS). With the field along the c-axis, a nearly isotropic hexagonal VL was formed in the field range from 1 to 16 T, which is a record for this technique in the pnictides, and no symmetry changes in the VL were observed. The temperature-dependence of the VL signal was measured and confirms the presence of (non d-wave) nodes in the superconducting gap structure for measurements at 5 T and below. The nodal effects were suppressed at high fields. At low fields, a VL reorientation transition was observed between 1 T and 3 T, with the VL orientation changing by 45{deg}. Below 1 T, the VL structure was strongly affected by pinning and the diffraction pattern had a fourfold symmetry. We suggest that this (and possibly also the VL reorientation) is due to pinning to defects aligned with the crystal structure, rather than being intrinsic.

High Magnetic Fields and Low Temperatures for Neutron Scattering Experiments at SINQ

The broad range of experiments carried out at SINQ requires an extensive suite of sample environment devices, which is maintained and operated by the Polarized Target & Sample Environment group of the Laboratory for Developments and Methods. The Sample Environment group consists of three people, one scientist and two technicians, who help scientific users with their sample environment needs. The available equipment allows experiments at temperatures between T = 100 mK to 1400 K, magnetic fields up to H = 15 T and pressures up to P = 10 GPa.