Superconductivity and anomalous magnetic phase in LuGe$_2$ single crystals

LuGe$_2$ single crystals (ZrSi$_2$-type orthorhombic structure, $C_{mcm}$) were synthesized by the high temperature metal flux method. LuGe$_2$ was found to be the type-II superconductivity below superconducting transition temperature, $T_c$ = 2.3 K. An anomalous magnetic and electric phase below $T^*$ = 4 K was found only along $b$-axis in electrical transport measurement, and confirmed additionally by magnetization and heat capacity measurement. From the heat capacity data, LuGe$_2$ was found to be the weak coupling BCS type superconductivity at $T_c$, whereas the anomalous phase above $T_c$ seems to have a close correlation with the superconducting state below $T_c$. Upper critical field, $H_{c2}$($T$), of the superconductivity exhibits significant anisotropy either along $b$-axis or along $a$ (and $c$)-axis. Recently, a possible second superconducting state in superconducting YbSb$_2$ single crystals, which has the same structure as LuGe$_2$, was reported. [1] Thus, we will discuss in detail on the anisotropic superconducting properties and the anomalous phase above $T_c$ in terms of the possible second superconductivity as in YbSb$_2$.\\[4pt] [1] Liang L. Zhao, Stefan Lausberg, H. Kim, M. A. Tanatar, Manuel Brando, R. Prozorov, and E. Morosan, Phys. Rev. B 85, 214526 (2012)