LuNi2B2C has the crystal structure illustrated in
Figure 3.1 with lattice parameters
leading to a calculated density of
. Table 3.1
lists the interatomic distances.
|Atom pair||Separation ( )|
|B - C||1.47|
|Lu - C||2.449|
|Lu - B||2.855|
|B - B||2.94|
|Ni - B||2.10|
|Ni - Ni (in Ni plane)||2.449|
The superconducting properties of LuNi2B2C are intriguing. There exists much experimental evidence both for and against an s-wave pairing state . Scanning tunneling microscopy  discloses a bulk energy gap of , and thermal conductivity measurements  detect a large gap anisotropy . The average out of plane upper critical field anisotropy holds constant with temperature at , as found from magnetisation studies . The slight basal plane anisotropy falls from at temperature to by the critical temperature Tc. The initial slope of the upper critical field gives an estimated coherence length . On the other hand small angle neutron scattering (SANS)  extracts a coherence length and a penetration depth at temperature .
One of the most fascinating aspects of the superconducting behaviour of LuNi2B2C is the occurrence of field driven transitions in its vortex lattice geometry. The evolution of the flux line lattice symmetry in LuNi2B2C, as a function of external field H, is clearly evident through Bitter decoration and SANS. Under weak fields H applied parallel to the crystal axis, the decoration method images a hexagonal to square vortex lattice transition . As the magnetic field H climbs from to , triangular flux line domains enlarge and one of their nearest neighbour directions becomes parallel with the or orientations. Raising the magnetic field H distorts the hexagonal configuration and local regions of square geometry appear above . Further magnetic field increase up to reveals an expanding square proportion co-existing with a heavily distorted triangular phase. At fields H upwards of , SANS records  a square vortex lattice which slowly becomes completely amorphous by . SANS also shows another vortex lattice symmetry transition occurring at an external field of . The hexagonal lattice reorients from having a nearest neighbour direction along the axis at lower fields H to having one along the axis at higher fields H. Whereas the geometrical transitions taking place in LuNi2B2C for applied fields arise from nonlocal interactions, those for fields stem from energy gap anisotropy .
The LuNi2B2C sample examined in this SR experiment was a single crystal in diameter and in mass. The crystal grew from a mixture of Ni2B flux and arcmelted and annealed polycrystalline LuNi2B2C as the solution cooled from to over several days . The sample formed as a plate, with the crystalline axis perpendicular to the plate plane. Thermal conductivity measurements  performed on this sample find an upper critical field . Its residual resistivity is and its electron mean free path is . Resistivity data  from similarly grown crystals indicate that this sample should have a critical temperature .
The expected Kramer-Pesch effect for the sample studied in this experiment is that the vortex core radius should contract linearly with temperature T on cooling from (= Tc) down to [= T0, assuming ]. Below the quantum limit temperature , the core radius should stay constant at (=1/kF). The experimental setup employed to investigate this effect in LuNi2B2C is described in the following chapter.