Interface induced magnetic polar metals by design*

Polar metals with ferroelectric-like (FEL) displacements offer the promise of designing ferroelectrics with tunable band gaps [1,2]. One efficient way to induce FEL displacements in a centrosymmetric phase is by interface-induced coupling [3]. Here, using first-principles density functional theory (DFT) and DFT+U (static d-d Coulomb interaction) we investigate (BaTiO$_3$)$_6$/(SrRuO$_3$)$_n$/(BaTiO$_3$)$_7$ heterostrcutures (6/n/7), with n=1, 2 and 3. DFT calculations predict that a FEL phase coupled with in-plane rotation of RuO$_6$ octahedral ($a^0a^0c^+$) can be induced in centrosymmetric SrRuO$_3$ unit when BaTiO$_3$ is in a ferroelectric phase. Moreover, a spin crossover from low spin (2$\mu_B$, Bulk SrRuO$_3$) to high spin (4$\mu_B$, 6/3/7) is found driven by the FEL distortion, which is a rare phenomenon in case of magnetic polar metals. Experimental investigation has confirmed the induced polar distortion and the spin crossover. Finally, we discuss the design principles for spin crossover magnetic polar metals. [1] W. Cochran, Phys. Rev. Lett., 3, 412 (1959), [2] S. Ghosh et. al. Phys. Rev. Lett. 119, 177603 (2017), [3] H. Guo, PANS, 114, E5062-E5069 (2017).