Magnetic Control of Valley Pseudospin in Monolayer WSe2 (1407.2645v1)

Abstract: Local energy extrema of the bands in momentum space, or valleys, can endow electrons in solids with pseudo-spin in addition to real spin. In transition metal dichalcogenides this valley pseudo-spin, like real spin, is associated with a magnetic moment which underlies the valley-dependent circular dichroism that allows optical generation of valley polarization, intervalley quantum coherence, and the valley Hall effect. However, magnetic manipulation of valley pseudospin via this magnetic moment, analogous to what is possible with real spin, has not been shown before. Here we report observation of the valley Zeeman splitting and magnetic tuning of polarization and coherence of the excitonic valley pseudospin, by performing polarization-resolved magneto-photoluminescence on monolayer WSe2. Our measurements reveal both the atomic orbital and lattice contributions to the valley orbital magnetic moment; demonstrate the deviation of the band edges in the valleys from an exact massive Dirac fermion model; and reveal a striking difference between the magnetic responses of neutral and charged valley excitons which is explained by renormalization of the excitonic spectrum due to strong exchange interactions.

• The paper reveals a linear valley Zeeman splitting of excitonic pseudospin with a slope of −0.11 ± 0.01 meV/T, confirming sizable valley magnetic moments.
• The paper shows that polarized photoluminescence data indicate magnetic field-induced symmetry breaking, leading to asymmetric valley pseudospin relaxation.
• The paper highlights deviations from the massive Dirac fermion model, demonstrating distinct behaviors of neutral and charged excitons under strong exchange interactions.

Overview of Magnetic Control of Valley Pseudospin in Monolayer WSe₂

This paper investigates the magnetic manipulation of the valley pseudospin in monolayer tungsten diselenide (WSe₂), contributing fundamentally to the understanding of valleytronics. Centered around the concepts of valley pseudospin and its manipulation, the authors explore the Zeeman effect's implications on valley states through experimental magnetophotoluminescence (MPL) spectra analyses in an external magnetic field. This paper provides pivotal insight into the interplay between valley pseudospins and magnetic fields.

Core Findings

1. Valley Zeeman Splitting: The research demonstrates the observed valley Zeeman splitting of the excitonic valley pseudospin in WSe₂ monolayers. The experimental results emphasize that the valley Zeeman splitting is notably linear, with the field-induced splitting being characterized by a distinct slope of −0.11 ± 0.01 meV/T. This finding confirms the existence of sizable valley magnetic moments, further linked to Berry curvature phenomena.
2. Polarization Dependence: Through the analysis of polarized photoluminescence, it becomes evident that magnetic fields break valley degeneracy, enabling asymmetric valley pseudospin relaxation processes. The evident symmetry in the "X" and "V" patterns observed suggests a controllable polarization dependent on the applied magnetic field's orientation.
3. Deviation from the Dirac Model: The research spots significant deviations from the massive Dirac fermion model when explaining band-edge behaviors, substantiated through calculated effective masses and valley magnetic moments. It highlights comprehensive differences in the behavior of neutral and charged excitons under the influence of strong exchange interactions and magnetic fields.
4. Valley Coherence: The suppression of valley coherence is meticulously reported, as the application of a magnetic field induces a “Λ” pattern, underscoring the magnetic field's effect on quantum coherence. Such decoherence is traced back to exciton formation processes more than to recombination stages.

Implications and Future Directions

The paper offers considerable contributions to the theoretical and practical landscape of valleytronics, with implications spanning to optoelectronic applications in quantum devices where valley pseudospin manipulation is pivotal. Specifically, the ability to modulate excitonic valley pseudospin and achieve optical and electronic control highlights potential applications in developing high-speed, low-power electronic devices that utilize this additional degree of electron freedom.

Future research could pursue:

• Advanced Material Synthesis: Further investigations into other monolayer materials that might exhibit superior or complementary properties for valley control could enrich the diversity of usable materials in valleytronics applications.
• Quantum Device Implementation: The exploration of device architecture exploiting valley pseudospins, such as valley transistors and valley-based logic circuits, can propel technological applications.
• Comprehensive Theoretical Models: Expanding the theoretical framework to include more complex interactions and refined models beyond the tight-binding approximation could clarify phenomena currently unexplained by simple Dirac models.

In conclusion, this paper accentuates the nuanced relationship between magnetic fields and valley pseudospins, setting a foundational foray into valley-based technologies, which promise to pave new directions in advancing quantum material sciences.