Flux line lattice structure in YNi2B2C.

Recently Nakai et at. reported a theoretical H-T phase diagram of flux line lattice (FLL) structure in which successive transitions from a triangular, a square (square(v)), a triangular and another square (square(g)) occur with increasing a magnetic field. Here square(v) and square(g) indicate the FLL structures reflecting anisotropies in the Fermi velocity and the superconducting gap, respectively. In the case of YNi2B2C, square(v) and square(g) should rotate by 45 degrees. The low field transition from triangular to square(v) is observed in RENi2B2C (RE = Er, Tm, Lu, and Y). However, there is no experimental evidence for the appearance of square(g) phase so far. We studied the FLL structure of YNi2B2C in the higher field region by small-angle neutron scattering. Our results show that a large area of the H-T phase diagram is occupied by square(v) phase and there is no evidence for the appearance of square(g) lattice. © 2008, Physical Society of Japa