Magnetic fields penetrate a type-II superconductor via quantized flux lines. Why these flux lines sometimes form square arrays, as in ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$ with the field parallel to the c axis, rather than the expected hexagonal ordering, has long fascinated physicists and has eluded a simple explanation. Our latest measurements on ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$ prove conclusively that the London penetration depth within the a-b plane is not isotropic. This anisotropy of the London depths implies that the cross section of an individual flux line has square symmetry, which makes a square lattice energetically favored over the more prosaic hexagonal ordering.

• Received 24 August 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R14767