Researchers from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, and collaborating institutions, reporting in ACS Applied Materials & Interfaces, have demonstrated a novel method to generate spin-dependent electrical currents at room temperature by precisely controlling the symmetry at the interface between two atomically thin materials. Their work highlights the critical, often overlooked, role of interfacial symmetry breaking in harnessing quantum geometric effects for advanced electronics.

Quantum geometry, describing the fundamental shape of electron wavefunctions in materials, underpins exotic phenomena like the quantum Hall effect and holds promise for next-generation technologies. Two key components are the Berry curvature, related to electron "twisting," and the quantum metric, describing distances in quantum state space. Isolating the effects of these components typically relies on manipulating global symmetries like parity (P, mirror reflection) and time-reversal (T) in complex antiferromagnetic structures. However, the specific role of symmetry breaking right at the interface where two materials meet – where perturbations are strongest – remained poorly explored. This study targeted that interface.

The team focused on a heterostructure combining hexagonal boron nitride (hBN), an insulator with specific hexagonal symmetry, and chromium thiophosphate (CrPS₄), an antiferromagnetic semiconductor with different mirror symmetries. By meticulously stacking these layers with their atomic lattices aligned either parallel or perpendicularly, they engineered interfaces with distinct symmetries: one preserving a mirror plane (C_1v symmetry) and one with all mirror symmetries broken (C₁ symmetry). Crucially, all experiments were performed at room temperature. This ensured CrPS₄ was in a non-magnetic state, eliminating any contributions from time-reversal symmetry breaking due to magnetic order and isolating the effect of structural (parity) symmetry manipulation.

Using spatially resolved photocurrent mapping, the researchers first confirmed that the observed electrical current originated directly from the hBN/CrPS₄ interface and not from other effects like Schottky barriers at the metal contacts. They then probed the photogalvanic effect (PGE), where light illumination generates a steady electrical current without an applied voltage. Linearly polarized light tests revealed a shift current, one component of PGE, whose direction and strength depended predictably on the light's polarization angle. This behavior matched theoretical predictions for an interface with a single mirror plane (C_1v symmetry), confirming successful symmetry engineering.

The critical test involved circularly polarized light. Under normal incidence, the heterostructure engineered with C_1v symmetry (preserved mirror plane) generated a spin-dependent current, known as the circular photogalvanic effect (CPGE) or injection current, only perpendicular to the mirror plane. When the mirror symmetry was completely broken (C₁ symmetry interface), spin-dependent currents emerged in both in-plane directions. This directional selectivity directly linked to the presence or absence of mirror symmetry. Symmetry analysis revealed that breaking the mirror symmetry lifted constraints on the Berry curvature distribution, allowing the formation of a Berry curvature dipole (BCD) – a net imbalance of this quantum geometric property across the material's momentum space. This BCD is responsible for converting the helicity (handedness) of the circularly polarized light into a directional flow of spin-polarized electrons.

Second-harmonic generation (SHG) measurements provided further evidence. The nonlinear optical response at the hBN/CrPS₄ interface differed significantly from the simple sum of the individual layers' responses. This deviation confirmed strong interfacial coupling and electronic interactions, proving that the hBN layer actively modified the symmetry and electronic properties of the CrPS₄ at the interface, rather than just acting as a passive cover.

This work establishes interfacial mirror symmetry engineering as a powerful and practical strategy. By simply controlling the stacking angle of common van der Waals materials like hBN and CrPS₄, researchers can create and manipulate the Berry curvature dipole at room temperature. This enables the generation and control of spin-polarized electrical currents using only circularly polarized light, a crucial functionality for spintronics. The approach bypasses the need for complex low-temperature magnetic setups typically associated with quantum geometric effects. It opens new avenues for designing quantum optoelectronic devices operating under ambient conditions, leveraging the vast library of 2D materials with different symmetries to create tailored interfacial quantum phenomena.