Unexpected Two Dimensional Electron Gas at the LaAlO$_3$/SrTiO$_3$ (110) Interface

The observation of two dimensional electron gas (2DEG), superconductivity and magnetism at the interfaces of LaAlO$_3$/SrTiO$_3$ has further amplified the potential of complex oxides for novel electronics. These multifunctional properties are strongly believed to originate from the polarization discontinuity at the interface between the two oxides along (001) direction. In this scenario, the crystal orientation plays an important role and no conductivity would be expected for e.g., the interface between LaAlO$_3$ and (110)-oriented SrTiO$_3$, which should not have a polarization discontinuity. In this talk, we will show the observation of conductivity at the LaAlO$_3$/SrTiO$_3$ interface prepared on (110)-oriented SrTiO$_3$. The strong evidence for 2DEG at the interface of LaAlO$_3$/SrTiO$_3$ (110) is the observed thickness dependence of insulator-metal transition at 3-4 u.c of LaAlO$_3$. Directional dependent electrical transport is observed in resistance versus temperature measurements. We propose some models for explaining the observed phenomena.