Institute for Solid State Physics finds that spin-polarized photocurrent occurs at the interface of different two-dimensional crystal stacks

Two-dimensional crystals consisting of several atomic layers obtained by exfoliating three-dimensional layered materials can be freely stacked to create interfaces, regardless of the type of material to which they are attached and have a characteristic structure and physical properties and functionality not found in the original two-dimensional crystals.

A research group led by Associate Professor Toshiya Ideue of the Institute for Solid State Physics at the University of Tokyo and Professor Yoshihiro Iwasa of the Graduate School of Engineering at the University of Tokyo (Team Leader, RIKEN Center for Emergent Matter Science), in collaboration with groups at Nanjing University, Princeton University, and the University of California, Berkeley, discovered that when an interface fabricated by layering two different types of two-dimensional crystals—tungsten diselenide (WSe₂) and silicon phosphide (SiP)—was irradiated with circularly polarized light, a spin-polarized photocurrent flowed in a specific direction at the interface. This finding was published in Nature Nanotechnology.

WSe₂ and SiP have rotational symmetry and multiple mirror-image symmetries, however the rotational symmetry disappears and only one mirror-image symmetry exists at the interface formed by overlapping them. When the photocurrent flowing through this interface was investigated while changing the polarization of the irradiating light, a photocurrent component dependent on circular polarization was observed in the direction perpendicular to the mirror image plane.

However, no such response was observed in the direction parallel to the mirror plane.

Because the irradiation of WSe₂ with circularly polarized light is known to produce spin-polarized carriers, this result suggests that the spin-polarized carriers are rectified in the direction perpendicular to the mirror plane and observed as a circularly polarized light-dependent photocurrent.

Furthermore, measurements using a device with magnetic electrodes revealed that the photocurrent is actually spin polarized.

In addition, by investigating the irradiation-wavelength dependence of the photocurrent component that depends on circularly polarized light, it was found that the observed photocurrent can be explained by a quantum-mechanical mechanism that reflects the geometric properties of electrons.

In the future, the optimization of the combination of two-dimensional materials is expected to produce large spin-polarized photocurrents, and advanced optical spintronics functions are expected to be realized by combining them with more complex device structures.

In addition, this result indicates the possibility of controlling various quantum-mechanical degrees of freedom, including spin, by controlling the symmetry of the two-dimensional crystal interface. The development of various quantum functionalities based on symmetry control is expected to accelerate.