Novel Anisotropy of Upper Critical Fields in Fe${}_{1+y}$Te${}_{0.6}$Se${}_{0.4}$

Abstract

Studying the upper critical field (${\mu }_0$$H$${}_{c2}$) and its anisotropy of superconductors is of great importance because it can provide an unusual insight into the pair-breaking mechanism. Since Fe${}_{1+y}$Te${}_{1-x}$Se${}_x$ exhibits the high ${\mu }_0$$H$${}_{c2}$ and small anisotropic superconductivity, it has attracted considerable attention. However, some issues related to ${\mu }_0$$H$${}_{c2}$ are still unknown, including the effect of excess Fe content on ${\mu }_0$$H$${}_{c2}$ behavior and the origin of the crossover of the ${\mu }_0{H}_{c2}^c$ -- $T$ and ${\mu }_0{H}_{c2}^{ab}$ -- $T$ curves. In this work, the value of ${\mu }_0$$H$${}_{c2}$ of Fe${}_{1+y}$Te${}_{0.6}$Se${}_{0.4}$ single crystals with controlled amounts of excess Fe was obtained by resistivity measurements over a wide range of temperatures down to $\sim$ 1.5 K, and magnetic fields up to $\sim$ 60 T. The crossover of the ${\mu }_0{H}_{c2}^c$ -- $T$ and ${\mu }_0{H}_{c2}^{ab}$ -- $T$ curves was found to be independent of the excess Fe content. The angle dependence of ${\mu }_0{H}_{c2}$ was also checked. The ${\mu }_0{H}_{c2}(\theta )$ symmetry at higher temperature near $T_c$ could be fitted by anisotropic G-L model, and novel fourfold symmetry of ${\mu }_0{H}_{c2}$ at lower temperature was found. Based on our spin-locking pairing model, the crossover behavior originates from the anisotropic spin-paramagnetic effect, and the novel fourfold symmetry of ${\mu }_0{H}_{c2}$ could be understood by our extended anisotropic G-L model.

14 pages; 10 figures