Manipulation and detection of electronic states of single unit cell correlated oxides

Abstract: 

Two dimensional (2D) systems can not only have physical properties distinct from those of 3D materials but also allow control/manipulation of their properties. For example, Mott insulating and superconducting states, unavailable in a single layer graphene, are realized in twisted bilayer graphene systems. While these novel 2D systems are mostly obtained through exfoliation of van der Waals materials, a more conventional approach is to achieve it through thin film growth. In this presentation, I wish to introduce our research efforts to measure and manipulate electronic properties of a few unit-cell (uc) thick thin films by using thin film growth and in-situ angle resolved photoemission (ARPES).

We developed a way to study atomically thin films by using an additional ‘conducting layer’.[1] This method allows us to obtain the electronic structure of TMO films down to the thickness of 1 uc. Using the method, we were able to show that a few uc thick film of SrRuO₃ (SRO), a prototypical metallic ferromagnet with spin-orbit coupling, can exhibit various ground states such as ferro- and anti-ferromagnetic states. Moreover, the ground state can be manipulated by varying the thickness and strain (see the figure).[2,3,4]

We extend our research to SrIrO₃ (SIO) and a cuprate superconductor (La,Sr)₂CuO₄ (LSCO). It is found that SIO 1uc films have the electronic structure similar to that of Sr₂IrO₄, relativistic Mott insulating state with (short) AF order, which is not surprising considering the similarity in their crystal structure. However, it is found that 1 uc SIO film possesses split bands where a single band is expected for Sr₂IrO₄. The splits bands can be reconciled with existence of loop current or nematic order. Additional polarization dependence suggests that the ground state of 1 us SIO is a spin nematic state.[5] Meanwhile, we were able to grow 0.5 uc LSCO (a single CuO₂ plane) and obtain the electronic structure. The electronic structure shows a finite gap whose momentum dependence is consistent with a d-wave gap, strongly suggesting that superconductivity is retained even in a single CuO₂ plane.[6]