Generation and Electric Control of Spin-Coupled Valley Current in WSe2 (1403.2696v1)

Abstract: The valley degree of freedom in layered transition-metal dichalcogenides (MX2) provides the opportunity to extend functionalities of novel spintronics and valleytronics devices. Due to spin splitting induced by spin-orbital coupling (SOC), the non-equilibrium charge carrier imbalance between two degenerate and inequivalent valleys to realize valley/spin polarization has been successfully demonstrated theoretically and supported by optical experiments. However, the generation of a valley/spin current by the valley polarization in MX2 remains elusive and a great challenge. Here, within an electric-double-layer transistor based on WSe2, we demonstrated a spin-coupled valley photocurrent whose direction and magnitude depend on the degree of circular polarization of the incident radiation and can be further greatly modulated with an external electric field. Such room temperature generation and electric control of valley/spin photocurrent provides a new property of electrons in MX2 systems, thereby enabling new degrees of control for quantum-confined spintronics devices.

• The paper demonstrates generation and electric control of a spin-coupled valley photocurrent in WSe₂ using circularly polarized light and EDLTs.
• The study employs CPGE measurements to reveal how helicity and an external electric field modulate the photocurrent at room temperature.
• The results provide a framework for advancing semiconductor spintronics and valleytronics by leveraging spin-orbital coupling in 2D materials.

Overview of Spin-Coupled Valley Current in WSe₂

The research delineated in this paper presents a paper on the generation and electric control of spin-coupled valley current in WSe₂, particularly through the application of circularly polarized light and the utilization of electric-double-layer transistors (EDLTs). The investigation is grounded in the phenomenon correlated with the non-equilibrium charge carrier imbalance between two degenerate and inequivalent valleys made possible by spin-orbital coupling (SOC). This paper succeeds in demonstrating a modulated valley/spin photocurrent at room temperature, an important step towards advancing semiconductor spintronic applications.

Key Findings

A focal point of this paper is observing the Circular Photogalvanic Effect (CPGE) in a two-dimensional electronic system (2DES) with broken spin degeneracy. This permits the generation of a non-uniform distribution of photo-excited carriers in k-space, producing spin and valley currents. The experimental results leverage electric-double-layer transistors (EDLTs) made from WSe₂ to produce and control a spin-coupled valley photocurrent. Remarkably, the research shows that the photocurrent's direction and magnitude can be modulated by both the helicity of the incident radiation and an external electric field.

Through extensive experimentation, consisting of CPGE current measurements induced by varying degrees of circularly polarized light, the team effectively demonstrated that the generated photocurrent is not merely a result of thermal effects but is truly dependent on the intricate k-space distribution. Particularly significant is the expression of the CPGE current that displays dependency on crucial factors such as the incident angle and helicity, aligning with prior assumptions based on Rashba 2DES behaviors.

Theoretical Implications

On a theoretical front, the paper provides an analytical framework to examine the light absorption of electrons at distinct valley points, elucidating how the electric field alters the CPGE's magnitude. The valley-dependent phenomena also display coupling with spin, which is inherently linked with WSe₂'s unique electronic structures and lack of inversion symmetry.

The paper explores the spin physics in transition metal dichalcogenides, specifically focusing on the breakdown of inversion symmetry and its pivotal role in engendering spin-polarized photocurrents. Utilizing ab initio calculations, the paper effectively elucidates the role of crystal symmetry and orbital contributions to the CPGE, offering insight into potential valley and spintronic applications.

Practical Implications and Future Research Directions

Practically, the electric modulation and control over the spin photocurrent heralds promising advancements in the domain of semiconductor spintronics and valleytronics. The findings could well pave the way to more efficient and tunable quantum-confined devices, potentially relevant for information processing systems that harness electron spin and valley degrees of freedom.

Future research may explore the extension of these results to other 2D electron systems with similar valley structures and symmetries, broadening the scope of technologically viable spintronic devices. Moreover, given that the fundamental mechanisms of achieving and modulating spin-photocurrents should extend beyond WSe₂, subsequent studies could focus on material systems like MoS₂ and WS₂, further catalyzing developments in this field.

Conclusion

This research elucidates a crucial aspect in the paper of valley and spintronics, namely demonstrating the effective manipulation of spin-coupled valley currents via an external electric field in WSe₂ EDLTs. A foundational framework is laid, allowing a deeper understanding of electric field modulation and control over valley/spin photocurrents, with implications enhancing the versatility of transition metal dichalcogenides in future technological applications.