Hexagonal and Square Flux Line Lattices in ${\mathrm{C}\mathrm{e}\mathrm{C}\mathrm{o}\mathrm{I}\mathrm{n}}_5$

Using small-angle neutron scattering, we have imaged the magnetic flux line lattice (FLL) in the $d$-wave heavy-fermion superconductor ${\mathrm{C}\mathrm{e}\mathrm{C}\mathrm{o}\mathrm{I}\mathrm{n}}_5$. At low fields we find a hexagonal FLL. Around 0.6 T this undergoes what is most likely a first-order transition to square symmetry, with the nearest neighbors oriented along the gap node directions. This orientation of the square FLL is consistent with theoretical predictions based on the $d$-wave order parameter symmetry.

Figure 1

FLL diffraction patterns for ${\mathrm{C}\mathrm{e}\mathrm{C}\mathrm{o}\mathrm{I}\mathrm{n}}_5$ obtained at 50 mK and applied fields of 0.3 T (a), 0.6 T (b), and 2 T (c), after subtraction of background measurement. In addition the data are smoothed by a $3\times 3$ box car average, and the center of the image is masked off. The arrow in panel (a) shows the orientation of the crystalline axes.

Figure 2

Symmetry and orientation of the hexagonal (a) and square (b) FLL. The orientation of the nodes in the superconducting gap is shown in (c).

Figure 3

Rocking curve for the ${\mathrm{C}\mathrm{e}\mathrm{C}\mathrm{o}\mathrm{I}\mathrm{n}}_5$ $(1,0)$ FLL reflection at 50 mK and 2 T. The curve is a fit to a Gaussian.