N. H. Andersen

Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in Superconducting LuNi2B2C

We have investigated the temperature dependence of the H parallel to c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi2B2C ( T(c) = 16.6 K). At temperatures below 10 K the transition onset field, H2(T), is only weakly temperature dependent. Above 10 K, H2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H2(T) merging with the upper critical field and suggests that just below the H(c2)(T) curve the flux line lattice might be hexagonal.

Non-locality and the flux line lattice square to hexagonal symmetry transition in the borocarbide superconductors

Abstract Using small angle neutron scattering we have studied the square to hexagonal flux line lattice symmetry transition in different members of the borocarbide superconductors. The studies were performed using samples of ErNi 2 B 2 C, Lu(Ni 1− x Co x ) 2 B 2 C with cobalt doping levels x =1.5–9% and Y 0.64 Lu 0.36 Ni 2 B 2 C. We find that the onset field of the symmetry transition can be shifted more than an order of magnitude due to changes in the range of the non-local electrodynamics. Comparing the results to transport measurements of the electronic mean free path and the superconducting coherence length we find that the transition onset follows a model by V. Kogan et al., which includes non-local corrections to the London model due to the Fermi surface anisotropy of the borocarbides.

Hysteresis in the field-induced magnetic structure in TmNi2B2C

Abstract Using small-angle neutron scattering we have studied the flux line lattice and the magnetic structure in TmNi2B2C. With an applied field parallel to the crystalline c-axis several magnetic and flux line lattice symmetry transitions have been reported. The subject of this paper is the field-induced magnetic structure and how it is influenced by the flux line lattice symmetry. We find that the domain splitting of the field-induced magnetic structure is hysteretic, and appears to be determined by the history of the flux line lattice symmetry.

Interwoven magnetic and flux line structures in single crystal (Tm,Er)Ni2B2C (invited)

We review studies of the interactions between magnetic order and the flux line lattice (FLL) in the (RE)Ni2B2C intermetallic borocarbides for (RE)=Tm and Er using small angle neutron scattering (SANS) and magneto-transport. For (RE)=Tm the magnetic order and the FLL assume a common symmetry, sharing a phase transition at ∼2 kOe, despite an order of magnitude difference in periodicity. For (RE)=Er, the penetration depth λ and the coherence length ξ, both of which are derived from the FLL form factor, are modified near TN=6 K by a theoretically predicted weakly divergent pairbreaking. Finally, below 2.3 K, (RE)=Er shows a coexistence of weak ferromagnetism and superconductivity. This state reveals a highly disordered FLL and a striking increase in the critical current, both arising from the strong ferromagnetic pairbreaking.

Flux Line Lattice Symmetry Transitions in the Borocarbide Superconductors

We review the results of small neutron scattering studies of the superconducting flux line lattice (FLL) in the borocarbide RNi2B2C superconductors. The first experiments on these compounds with magnetic field parallel to the c axis discovered a square FLL in the majority of the phase diagram, with a smooth transition to a hexagonal symmetry at field below ~ 1 kOe. These results proved to be in agreement with the predictions of a model by V. Kogan et al. which includes non-local corrections to the London model due to the Fermi surface anisotropy of the borocarbides. A very different transition is observed when the applied magnetic field is perpendicular to the c axis. In this case, a discontinuous reorientation between two hexagonal vortex arrangements is observed at a well defined line in the magnetic phase diagram.

Interwoven magnetic and flux line structures in single crystal (Tm,Er)Ni{sub 2}B{sub 2}C (invited)

We review studies of the interactions between magnetic order and the flux line lattice (FLL) in the (RE)Ni2B2C intermetallic borocarbides for (RE)=Tm and Er using small angle neutron scattering (SANS) and magneto-transport. For (RE)=Tm the magnetic order and the FLL assume a common symmetry, sharing a phase transition at ∼2 kOe, despite an order of magnitude difference in periodicity. For (RE)=Er, the penetration depth λ and the coherence length ξ, both of which are derived from the FLL form factor, are modified near TN=6 K by a theoretically predicted weakly divergent pairbreaking. Finally, below 2.3 K, (RE)=Er shows a coexistence of weak ferromagnetism and superconductivity. This state reveals a highly disordered FLL and a striking increase in the critical current, both arising from the strong ferromagnetic pairbreaking.

Interwoven magnetic and flux line structures in single crystal (Tm,Er)Ni2B2C

We review studies of the interactions between magnetic order and the flux line lattice (FLL) in the (RE)Ni2B2C intermetallic borocarbides for (RE)=Tm and Er using small angle neutron scattering (SANS) and magneto-transport. For (RE)=Tm the magnetic order and the FLL assume a common symmetry, sharing a phase transition at ∼2 kOe, despite an order of magnitude difference in periodicity. For (RE)=Er, the penetration depth λ and the coherence length ξ, both of which are derived from the FLL form factor, are modified near TN=6 K by a theoretically predicted weakly divergent pairbreaking. Finally, below 2.3 K, (RE)=Er shows a coexistence of weak ferromagnetism and superconductivity. This state reveals a highly disordered FLL and a striking increase in the critical current, both arising from the strong ferromagnetic pairbreaking.

Square to hexagonal symmetry transition of the flux line lattice in YNi2B2C for different field orientations

Abstract Using small-angle neutron scattering we have studied the magnetic flux line lattice in YNi2B2C with the field rotated 30° away from the crystalline c-axis. Previously we have reported on a square to hexagonal symmetry transition of the flux line lattice below 1 kOe for H|c . We find that the rotation of the field shifts the transition to higher fields in agreement with theoretical prediction. Two different directions in the ab-plane were chosen as axes of rotation. Rotating the field around [1,1,0] shifts the onset of the transition up to 3 kOe. Rotating the field around [1,0,0] stabilizes the hexagonal flux line lattice up to the highest measured field of 30 kOe. The difference between the two axes is also reflected in the flux line lattice distortion.