Signatures of moiré-trapped valley excitons in MoSe$_2$/WSe$_2$ heterobilayers (1809.04562v1)

Abstract: The creation of moir\'e patterns in crystalline solids is a powerful approach to manipulate their electronic properties, which are fundamentally influenced by periodic potential landscapes. In 2D materials, a moir\'e pattern with a superlattice potential can form by vertically stacking two layered materials with a twist and/or finite lattice constant difference. This unique approach has led to emergent electronic phenomena, including the fractal quantum Hall effect, tunable Mott insulators, and unconventional superconductivity. Furthermore, theory predicts intriguing effects on optical excitations by a moir\'e potential in 2D valley semiconductors, but these signatures have yet to be experimentally detected. Here, we report experimental evidence of interlayer valley excitons trapped in a moir\'e potential in MoSe$_2$/WSe$_2$ heterobilayers. At low temperatures, we observe photoluminescence near the free interlayer exciton energy but with over 100 times narrower linewidths. The emitter g-factors are homogeneous across the same sample and only take two values, -15.9 and 6.7, in samples with twisting angles near 60{\deg} and 0\deg, respectively. The g-factors match those of the free interlayer exciton, which is determined by one of two possible valley pairing configurations. At a twist angle near 20\deg, the emitters become two orders of magnitude dimmer, but remarkably, they possess the same g-factor as the heterobilayer near 60\deg. This is consistent with the Umklapp recombination of interlayer excitons near the commensurate 21.8{\deg} twist angle. The emitters exhibit strong circular polarization, which implies the preservation of three-fold rotation symmetry by the trapping potential. Together with the power and excitation energy dependence, all evidence points to their origin as interlayer excitons trapped in a smooth moir\'e potential with inherited valley-contrasting physics.

• The paper demonstrates that moiré patterns trap interlayer valley excitons in MoSe₂/WSe₂ heterobilayers, evidenced by narrow linewidths (~100 μeV).
• It employs photoluminescence and magneto-PL spectroscopy to analyze twist-angle dependence and determine distinct Landé g-factors of excitonic states.
• The findings advance 2D valleytronics by highlighting how moiré-induced potentials control exciton behavior, paving the way for future quantum applications.

Moiré-Trapped Valley Excitons in MoSe₂/WSe₂ Heterobilayers

The paper, "Signatures of moiré-trapped valley excitons in MoSe₂/WSe₂ heterobilayers," presents significant advancements in the experimental observation and understanding of interlayer excitons in twisted transition metal dichalcogenide (TMD) heterobilayers. By employing moiré patterns, the research provides evidence of excitonic trapping and its implications in these two-dimensional (2D) semiconductor structures.

Key Findings and Methodology

The primary focus of the paper is on MoSe₂/WSe₂ heterobilayers, in which interlayer excitons are optically probed and found to exhibit moiré-trapped characteristics. These excitons, resulting from Coulomb interactions between electrons and holes located in different monolayers, are significantly influenced by the lattice mismatch and twist angle between the layers, leading to a moiré superlattice potential.

Experimental Techniques

1. Sample Fabrication: Monolayers of MoSe₂ and WSe₂ were exfoliated and stacked using a dry-transfer technique. Devices were fabricated with specific twist angles to explore the excitonic behavior across different angular configurations.
2. Photoluminescence Spectroscopy: Low-temperature photoluminescence (PL) measurements reveal distinct narrow emission lines from the trapped excitons, showcasing significantly reduced linewidths (~100 μeV) compared to previous studies, indicating the presence of confinement potentials.
3. Magneto-PL Spectroscopy: The Zeeman effects were investigated to identify the Landé g-factor of the trapped excitons, providing insights into the valley configurations. Analytical comparisons between the PL spectra under varying magnetic fields elucidate the valley physics underlying their behavior.

Numerical Results and Analysis

1. Linewidth and Intensity: At specific twist angles, particularly near 60° and 0°, the excitonic emissions showcased homogeneously narrow linewidths, aligning with the free interlayer exciton g-factors of -15.9 and 6.7, respectively. This demonstrates the distinct impact of twist angles on excitonic properties.
2. Valley Polarization: Strong circular polarization was observed, implying the preservation of valley contrasting physics by the moiré potential. The PL's polarization dependencies substantiate the valley contrasting nature of these excitonic states.
3. Twist Angle Dependency: A decreased emission intensity in heterobilayers twisted near 20°, yet with consistent g-factors with 60° heterobilayers, aligns with Umklapp recombination effects, further corroborating theoretical predictions.

Implications and Future Directions

The discovery of moiré-trapped valley excitons with well-defined optical and magnetic properties advises multiple implications for future research and applications:

1. 2D Moiré Optics: The manipulation of moiré potentials via twist angles provides a novel mechanism to control optical properties in 2D materials, paving the way for advances in valleytronics and optoelectronics.
2. Quantum Information: The potential for encoding information in valley degrees of freedom could facilitate developments in quantum information applications. This may also lead to new pathways for studying fundamental excitonic processes in solid-state systems.
3. Exploration of Theoretical Proposals: The paper affirms and encourages further exploration into areas such as spin-orbit coupling and topological exciton states in 2D heterostructures.

Continued research is warranted to address fabrication challenges and enhance the uniformity of moiré patterns, which may unlock additional functionalities and applications of these heterobilayer systems. Future work employing advanced microscopy techniques or exploring different TMD combinations could provide further insights into the inherently rich physics of moiré-induced trapping of valley excitons.