Optical detection of Mott and generalized Wigner crystal states in WSe2/WS2 moiré superlattices (1910.09047v1)

Abstract: Moir\'e superlattices are emerging as a new route for engineering strongly correlated electronic states in two-dimensional van der Waals heterostructures, as recently demonstrated in the correlated insulating and superconducting states in magic-angle twisted bilayer graphene and ABC trilayer graphene/boron nitride moir\'e superlattices. Transition metal dichalcogenide (TMDC) moir\'e heterostructures provide another exciting model system to explore correlated quantum phenomena, with the addition of strong light-matter interactions and large spin-orbital coupling. Here we report the optical detection of strongly correlated phases in semiconducting WSe2/WS2 moir\'e superlattices. Our sensitive optical detection technique reveals a Mott insulator state at one hole per superlattice site ({\nu} = 1), and surprising insulating phases at fractional filling factors {\nu} = 1/3 and 2/3, which we assign to generalized Wigner crystallization on an underlying lattice. Furthermore, the unique spin-valley optical selection rules of TMDC heterostructures allow us to optically create and investigate low-energy spin excited states in the Mott insulator. We reveal an especially slow spin relaxation lifetime of many microseconds in the Mott insulating state, orders-of-magnitude longer than that of charge excitations. Our studies highlight novel correlated physics that can emerge in moir\'e superlattices beyond graphene.

• The paper identifies a Mott insulating state with a ~10 meV gap at one hole per moiré unit cell and reports fractional fillings indicating generalized Wigner crystallization.
• It utilizes an optically detected resistance and capacitance (ODRC) technique that leverages strong light-matter interactions to probe correlated electronic states without direct electrical measurements.
• The study shows that Mott insulating states persist up to 45K while generalized Wigner crystal states appear up to 10K, underscoring potential applications in quantum devices.

Optical Detection of Correlated States in Moiré Superlattices

The paper presented investigates the emergence of correlated electronic states in transition metal dichalcogenide (TMDC) heterostructures, particularly focusing on $\text{WSe}_2/\text{WS}_2$ moiré superlattices. Moiré superlattices have garnered attention as a versatile platform for exploring correlated phenomena due to their ability to alter electronic band structures significantly. This investigation is critical for understanding the physics underlying such systems, especially in the context of optical detection techniques.

The paper reports the optical detection of Mott insulating states and generalized Wigner crystal states within $\text{WSe}_2/\text{WS}_2$ moiré superlattices. The experimental approach capitalizes on the TMDCs' strong light-matter interactions and employs an Optically Detected Resistance and Capacitance (ODRC) technique to circumvent challenges associated with high contact resistance. The ODRC technique provides a novel method to measure the quantum capacitance and resistance without direct electrical transport measurements, which are often hindered by the semiconducting nature of TMDCs.

Key Findings

1. Mott Insulator and Generalized Wigner States:
   • The paper identifies a Mott insulating state at one hole per moiré unit cell ($\nu = 1$), with an unusually high Mott-Hubbard gap of ~10 meV, which is notable compared to graphene-based systems.
   • In addition to the Mott insulator, the paper reports insulating states at fractional fillings ($\nu = 1/3$ and $\nu = 2/3$), suggesting generalized Wigner crystallization. These findings necessitate an extension of the Hubbard model to include longer-range interactions, highlighting the complexity of electron correlations in these materials.
2. Optical Detection of Spin Excitations:
   • The paper utilizes the unique spin-valley selection rules inherent in TMDC materials to generate and probe spin excitations. The spin relaxation lifetime in the Mott insulating state exceeds several microseconds, which is significantly longer than typical charge excitation lifetimes.
3. Temperature Dependence and Gap Estimation:
   • Mott insulator states remain detectable up to 45 K, while generalized Wigner crystal states persist until 10 K.
   • The examination of the temperature-dependent resistance provides estimates of the Mott gap and the inscrutable but smaller gaps associated with the generalized Wigner crystal states.

Methodology

The paper elaborates on constructing near-zero twist angle $\text{WSe}_2/\text{WS}_2$ heterostructures and implementing an ODRC measurement technique. The measurements involve configuring a heterostructure with locally gated regions where carrier doping is modulated using a top gate, and optical responses are captured via changes in reflectivity. The analysis of these optical signals in terms of quantum capacitance and resistance offers insights into electronic correlations.

Implications and Future Directions

The implications of this paper are manifold. The ability to optically detect and manipulate strongly correlated phases, such as Mott insulators and Wigner crystals, in TMDC moiré heterostructures opens avenues for new quantum materials and devices. These findings could inform future theoretical models to predict exotic quantum phases in moiré superlattices. The paper suggests further exploration into spin dynamics, which might uncover rich physics such as quantum spin liquids within these materials.

The future of research in this area appears to be promising, particularly in terms of scaling these techniques to various TMDC materials and exploring other heterostructures with different twist angles or lattice mismatches. Additionally, integrating these insights into practical applications could involve developing next-generation optoelectronic devices that exploit the tunable electronic correlations of moiré superlattices.