Soft vortices in type-II superconductors: TmNi2B2C

The structural distortions, magnetism and superconductivity coexist in single crystals of TmNi2B2C. It is antiferromagnetic below TN≈1.5 K and superconducting below Tc≈10.6 K. We present here a neutron diffraction study which shows that the apex angle β never reaches the value of 90° belonging to the square lattice because of the onset of a phenomenon that is a precursor to antiferromagnetism. At some field such as 3 kOe upon cooling below 4 K, instead of increasing, the apex angle starts reducing. The angle tends to zero at the Neel temperature and exhibits a mean field exponent of 1/2. Similarly, upon warming above 4 K, the angle becomes soft at the superconducting transition temperature, Tc. The change in London penetration depth as a function of temperature has been deduced from the intensity of the neutron diffraction. This measurement also shows node-like dependence as in the d-wave gap at low temperatures and the s-wave at high temperatures. The nodes are caused by d-wave admixture in the s wave. Therefore, we understand that the gap in the TmNi2B2C has a complex, d+is, symmetry.