Scanning tunneling microscopy and spectroscopy studies of the heavy-electron superconductor TlNi2Se2

We report on the structural and superconducting electronic properties of the heavy-electron superconductor TlNi2Se2. By using a variable-temperature scanning tunneling microscopy (VT-STM) the coexistence of (root 2 x root 2)R45 degrees and (2 x 1) surface reconstructions is observed. Similar to earlier observations on the "122" family of Fe-based superconductors, we find that their respective surface fraction strongly depends on the temperature during cleavage, the measurement temperature, and the sample's history. Cleaving at low temperature predominantly results in the (root 2 x root 2)R45 degrees-reconstructed surface. A detailed analysis of the (root 2 x root 2)R45 degrees-reconstructed domains identifies (2 x 1)-ordered dimers, tertramers, and higher order even multimers as domain walls. Higher cleaving temperatures and the warming of low-temperature-cleaved samples increases the relative weight of the (2 x 1) surface reconstruction. By slowly increasing the sample temperature T-s inside the VT-STM we find that the (root 2 x root 2)R45 degrees surface reconstructions transforms into the (2 x 1) structure at T-s = 123 K. We identify the polar nature of the TlNi2Se2(001) surface as the most probable driving mechanism of the two reconstructions, as both lead to a charge density rho = 0.5 e(-), thereby avoiding divergent electrostatic potentials and the resulting "polar catastrophe." Low-temperature scanning tunneling spectroscopy (STS) performed with normal metal and superconducting probe tips shows a superconducting gap which is best fit with an isotropic s wave. We could not detect any correlation between the local surface reconstruction, suggesting that the superconductivity is predominantly governed by TlNi2Se2 bulk properties. Correspondingly, temperature-and field-dependent data reveal that both the critical temperature and critical magnetic field are in good agreement with bulk values obtained earlier from transport measurements. In the superconducting state the formation of an Abrikosov lattice is observed without any zero bias anomaly at the vortex core.