Enhanced valley splitting in monolayer WSe2 due to magnetic exchange field (1610.04878v1)

Abstract: Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition metal dichalcogenide (TMDC) with broken inversion symmetry possesses two degenerate yet inequivalent valleys, offering unique opportunities for valley control through helicity of light. Lifting the valley degeneracy by Zeeman splitting has been demonstrated recently, which may enable valley control by a magnetic field. However, the realized valley splitting is modest, (~ 0.2 meV/T). Here we show greatly enhanced valley spitting in monolayer WSe2, utilizing the interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A valley splitting of 2.5 meV is demonstrated at 1 T by magneto-reflectance measurements. Moreover, the splitting follows the magnetization of EuS, a haLLMark of the MEF. Utilizing MEF of a magnetic insulator can induce magnetic order, and valley and spin polarization in TMDCs, which may enable valleytronic and quantum computing applications.

• The paper demonstrates that using a ferromagnetic EuS substrate significantly enhances valley splitting in monolayer WSe2, achieving 2.5 meV at 1 T.
• It employs magneto-reflectance measurements and DFT calculations to reveal that the valley splitting scales with the EuS magnetization, confirming the role of the magnetic exchange field.
• The study suggests that optimizing interfaces and exploring tunable methods like strain or electric fields may further improve valley polarization for valleytronic and quantum computing applications.

Enhanced Valley Splitting in Monolayer WSe2 Due to Magnetic Exchange Field

The paper discusses the advancement of monolayer transition metal dichalcogenides (TMDC), focusing on monolayer WSe2, which demonstrates enhanced valley splitting solely due to the interfacial magnetic exchange field (MEF) induced by a ferromagnetic EuS substrate. The research highlights novel prospects for valleytronics and quantum computing applications by exploiting the valley degree of freedom.

Key Contributions

1. Valley Splitting Enhancement:
   • The paper revealed a significant improvement in valley splitting, measured to be 2.5 meV at 1 T, using a magneto-reflectance technique. This enhancement is attributed to the use of an EuS substrate as opposed to the minimal splitting (~0.2 meV/T) achieved via conventional Zeeman splitting.
2. Mechanisms and Measurements:
   • Experimental data established that the valley splitting scales with the EuS magnetization's field-dependence—demonstrating a correlation with the MEF rather than external magnetic fields usually relied upon. Measurements across different temperatures and magnetic fields provide strong evidence that the observed splitting originates from the EuS substrate.
3. Theoretical Calculations:
   • Density Functional Theory (DFT) calculations correlated with experimental findings suggest that the EuS substrate amplifies the effects of valley exchange splitting in the WSe2 monolayer, with predicted splitting greater than experimentally observed due to the model's simplification of the EuS surface.

Implications and Future Directions

This paper significantly impacts both theoretical advancements and practical applications of TMDCs:

• Practical Implications: The enhanced splitting reported demonstrates the potential of using magnetic exchange fields to control valley properties in TMDCs more efficiently than traditional methods. Such control is invaluable for applications like valleytronic devices, which rely on robust valley polarization.
• Theoretical Advancements: The findings support theoretical predictions regarding enhanced valley splitting when interfaced with magnetic insulators, enriching our understanding of valley dynamics in TMDCs.
• Future Research Directions:
  • This work implies scope for optimizing interfaces between TMDCs and magnetic substrates to achieve even greater control over valley splitting.
  • Exploring other ferromagnetic substrates, potentially operating above room temperature, could propel practical device applications toward commercial viability.
  • Investigation into external tunability methods, through mechanical strain or electric fields, might amplify these findings further, broadening potential applications.

Conclusion

The paper compellingly demonstrates how interfacing TMDCs, specifically monolayer WSe2, with ferromagnetic substrates like EuS, can lead to substantial valley splitting enhancements. Such advancements pave the way for sophisticated material manipulations, enabling new devices operating at novel operational paradigms. These developments underscore the importance of methodical experimental designs supplemented by robust theoretical calculations in advancing condensed matter physics and material sciences.