Correlated Insulating States at Fractional Fillings of the WS2/WSe2 Moiré Lattice (2007.11155v2)

Abstract: Moir\'e superlattices of van der Waals materials, such as twisted graphene and transitional metal dichalcogenides, have recently emerged as a fascinating platform to study strongly correlated states in two dimensions, thanks to the strong electron interaction in the moir\'e minibands. In most systems, the correlated states appear when the moir\'e lattice is filled by integer number of electrons per moir\'e unit cell. Recently, correlated states at fractional fillings of 1/3 and 2/3 holes per moir\'e unit cell has been reported in the WS2/WSe2 heterobilayer, hinting the long range nature of the electron interaction. In this work, employing a scanning microwave impedance microscopy technique that is sensitive to local electrical properties, we observe a series of correlated insulating states at fractional fillings of the moir\'e minibands on both electron- and hole-doped sides in angle-aligned WS2/WSe2 hetero-bilayers, with certain states persisting at temperatures up to 120 K. Monte Carlo simulations reveal that these insulating states correspond to ordering of electrons in the moir\'e lattice with a periodicity much larger than the moir\'e unit cell, indicating a surprisingly strong and long-range interaction beyond the nearest neighbors. Our findings usher in unprecedented opportunities in the study of strongly correlated states in two dimensions.

• The paper reveals multiple correlated insulating states at fractional fillings using advanced microwave impedance microscopy and Coulomb gas Monte Carlo simulations.
• It demonstrates both electron- and hole-doped scenarios with robust transition temperatures up to 120 K at specific fillings such as n = ±1/3 and ±2/3.
• The research highlights the critical role of long-range Coulomb interactions in tuning 2D heterostructures for potential applications in exotic quantum phases and electronic devices.

Insights into Correlated Insulating States in WS$_2$/WSe$_2$ Moiré Lattices

This paper presents a comprehensive paper of correlated insulating states at fractional fillings in WS$_2$/WSe$_2$ moiré lattice systems. Utilizing angle-aligned heterobilayers of transition metal dichalcogenides (TMDs), this work provides nuanced insights into the electron-electron interactions in two-dimensional heterostructures, employing advanced scanning microwave impedance microscopy (MIM) to probe local electrical properties.

Observations and Findings

The researchers report the discovery of a succession of correlated insulating states across both electron- and hole-doped scenarios in the moiré superlattice. Particularly, insulating states observed at fillings such as $n = \pm1/6$, $\pm1/4$, $\pm1/3$, $\pm1/2$, $\pm2/3$, $\pm3/4$, $\pm3/2$, and further extremes such as $n = -8/9$, $-5/6$, $-7/9$, $+5/9$, and $+6/7$ point to the strong, long-range interaction underlying these systems. The Monte Carlo simulations based on a Coulomb gas model attribute these states to complex electron ordering patterns, such as triangular and stripe phases, emphasizing the substantial inter-site Coulomb interactions extending beyond the nearest neighbors. The high transition temperatures for some states, such as $T_c = 120 \text{K}$ for $n = \pm1/3$ and $\pm2/3$, underscore the robust interaction strength achieved in these systems.

Technical Approach

MIM serves as the principal tool in this paper, allowing precise mapping of resistivity variations as a function of gate voltage to identify insulating states. MIM measures the complex impedance between a metallic tip and sample and separates it into imaginary (MIM-Im) and real (MIM-Re) components. These components vary with the local resistivity of the sample, providing insights into the transitions between different states.

Implications and Future Directions

The results indicate a pathway towards realizing strong long-range electron interactions in TMD-based moiré superstructures. Given their gate tunability, such heterostructures open new opportunities for studying exotic correlated states like the fractional quantum Hall effect, superconductivity, and quantum spin liquids. The significant long-range electron interactions and commensurate charge ordering patterns reported in this paper contribute to the emerging understanding of moiré systems and their potential applications in fundamental research and future electronic devices.

Continuous advancements in stacking and characterizing heterostructures will likely enhance the ability to control and manipulate these correlated states, setting the stage for groundbreaking developments in two-dimensional material science. Further experiments are warranted to unravel the detailed mechanisms that underpin these high-temperature phase transitions and robust interaction phenomena, potentially involving new computational models or hybrid scanning techniques that could magnify spatial and temporal resolution. In sum, this research demonstrates the rich physics accessible through careful engineering and examination of moiré superlattices, underscoring the critical role of electron-electron interactions in determining the electronic properties of 2D materials.