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Register a new userSmall-angle neutron scattering study of the vortex lattice in superconducting LuNi2B2C
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Morten Ring Eskildsen
Publication date
2009
fields
Physics
and research's language is
English
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We present studies of the magnetic field distribution around the vortices in LuNi2B2C. Small-angle neutron scattering measurements of the vortex lattice (VL) in this material were extended to unprecedentedly large values of the scattering vector q, obtained both by using high magnetic fields to decrease the VL spacing and by using higher order reflections. A square VL, oriented with the nearest neighbor direction along the crystalline [110] direction, was observed up to the highest measured field. The first-order VL form factor, |F(q10)|, was found to decrease exponentially with increasing magnetic field. Measurements of the higher order form factors, |F(qhk)|, reveal a significant in-plane anisotropy and also allow for a real-space reconstruction of the VL field distribution.
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The influence of Twin Boundaries (TB) on the Flux Line Lattice(FLL) structure was investigated by Small Angle Neutron Scattering (SANS). YBaCuO single crystals possessing different TB densities were studied. The SANS experiments show that the TB strongly modify the structure of the FLL. The flux lines meander as soon as the magnetic field makes an angle with the TB direction. According to the value of this angle but also to the ratio of the flux lines density over the TB density, one observes that the FLL exhibits two different unit cells in the plane perpendicular to the magnetic field. One is the classical hexagonal and anisotropic cell while the other is affected by an additional deformation induced by the TB. We discuss a possible relation between this deformation and the increase of the critical current usually observed in heavily twinned samples.
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.
We present point-contact (PC) Andreev-reflection measurements of a superconducting epitaxial c-axis oriented nickel borocarbide film LuNi2B2C (Tc=15.9 K). The averaged value of the superconducting gap is found to be 2.6 +/-0.2 meV in the one-gap approach, whereas the two-gap approach results in 2.14+/-0.36 meV and 3.0+/-0.27 meV. The better fit of the Andreev-reflection spectra for the LuNi2B2C - Cu PC obtained by the two-gap approach provides evidence for multiband superconductivity in LuNi2B2C. For the first time, PC electron-phonon interaction (EPI) spectra have been measured for this compound. They demonstrate pronounced phonon maximum at 8.5+/-0.4meV and a second shallow one at 15.8+/-0.6 meV. The electron-phonon coupling constant estimated from the PC EPI spectra turned out to be small (~ 0.1), like in other superconducting rare-earth nickel borocarbides. Possible reasons for this are discussed.
We study the intrinsic anisotropy of the superconducting state in KFe$_2$As$_2$, using small-angle neutron scattering to image the vortex lattice as the applied magnetic field is rotated towards the FeAs crystalline planes. The anisotropy is found to be strongly field dependent, indicating multiband superconductivity. Furthermore, the high field anisotropy significantly exceeds that of the upper critical field, providing further support for Pauli limiting in KFe$_2$As$_2$ for field applied in the basal plane. The effect of Pauli paramagnetism on the unpaired quasiparticles in the vortex cores is directly evident from the ratio of scattered intensities due to the longitudinal and transverse vortex lattice field modulation.
Flux Lines Lattice (FLL) states have been studied using transport measurements and Small Angle Neutron Scattering in low T$_c$ materials. In Pb-In, the bulk dislocations in the FLL do not influence the transport properties. In Fe doped NbSe$_{2}$, transport properties can differ after a Field Cooling (FC) or a Zero Field Cooling (ZFC) procedure, as previously reported. The ZFC FLL is found ordered with narrow Bragg Peaks and is linked to a linear V(I) curve and to a superficial critical current. The FC FLL pattern exhibits two Bragg peaks and the corresponding V(I) curve shows a S-shape. This can be explained by the coexistence of two ordered FLL slightly tilted from the applied field direction by different superficial currents. These currents are wiped out when the transport current is increased.
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