OBSERVATION OF THE FIELD-DRIVEN VORTEX PHASE TRANSITION IN LuNi_2B_2C SINGLE CRYSTALS

摘要

The family of rare-earth metal nickel-borocarbides RINi_2B_2C (R=Er, Lu, Tm, Ho and so on) is an object of considerable attention due to the interplay between magnetic and superconducting properties. Unconventional behavior of vortex lattices in these compounds and, in particular, the phase transition from triangular to square vortex lattice driven by the magnetic field, has been discovered. Various experimental methods of studying vortex lattices were employed: the small-angle neutron scattering (SANS), the scanning tunneling microscopy (STM), and the decoration by small magnetic particles. One should note that whereas SANS and STM studies are effective at rather high magnetic fields (from a few kOe to T), the decoration technique is commonly used in the low field domain (about 100 Oe or less). In ErNi_2B_2C, the phase transition from triangular to square vortex lattices has been observed in increasing magnetic fields by both decoration and SANS. Decoration patterns also revealed the coexistence of the square and triangular lattices in the field region of about 600 Oe. In LuNi_2B_2C single crystals, the square vortex lattice at high magnetic fields (more than 2kOe) has been seen in STM. However, no experiments in low magnetic fields were carried out. The theory based on nonlocal corrections to the London model is available for the behavior of vortex lattices in high-k su- perconductors in general, and in borocarbides, in particular. The model predicts many features of vortex lattices in low magnetic fields as well as the phase transition from triangular to square lattice in increasing fields. In this work, the vortex lattices in LuNi_2B_2C were investigated by using the decoration method. The goal was to study the magnetic field dependence of the vortex lattice anisotropy at a wide region of external magnetic fields.