Dual Moiré System Creates New Correlated Electron Phases
Correlated Electron Phases
Quantum Materials Breakthrough: Scientists Find a Perfectly Aligned Dual Moiré System to Study Correlated Electron Phases
An multinational team from the Technical University of Munich and the National Institute for Materials Science designed a new platform for researching tightly correlated electron phases and uncommon collective occurrences, making substantial quantum research progress. The upcoming publication by Amine Ben Mhenni, Elif Çetiner, and colleagues introduces a novel dual moiré system, enabling unprecedented control and understanding of particle interactions as composite bosons.
Understanding Electron Phase Correlations
In materials with high electron interactions, correlated electron phases cause collective behaviors that defy particle models. Extreme interactions can create quantum circuits, topological states, superconductivity, and magnetism. Develop stable and adjustable solid-state platforms to explore these phenomena has always been tough.
The current study examines dual moiré systems, complicated layered structures with two Coulomb-coupled moiré lattices. Moiré patterns form by superimposing two periodic lattices with a tiny twist or misfit. A larger periodicity can drastically modify the material's electrical properties. These systems are promising for studying composite bosons with electrical tunability and highly connected dipolar excitons. Early attempts to develop dual moiré systems were difficult due to the two moiré patterns' misalignment and incommensurability, making controlled testing difficult.
Innovation: Twisted hBN Perfect Alignment
The researchers' primary discovery is their twin moiré system's perfect translational and rotational alignment. This was done with a twisted hexagonal boron nitride (hBN) bilayer. This hBN spacer physically separates MoSe2 and WSe2 monolayers, maintaining strong intralayer Coulomb coupling and creating an electrostatic moiré potential that penetrates both semiconductor layers. This architecture stabilizes long-lived dipolar excitons.
Charge transfer between boron and nitrogen atoms in the hBN bilayer creates local electric dipoles, which generate the electrostatic moiré potential. The shifting local stacking registry throughout the moiré unit cell spatially modulates this electric polarisation, resulting in a large triangle electrostatic moiré potential when hBN monolayers are twisted by a small angle. Importantly, this twisted hBN bilayer interfaces with the MoSe2 and WSe2 monolayers, exposing them to the same strong triangular moiré potential with the same periodicity and perfect relative alignment. A device with a twist angle of 1.7 degrees had an 8.5 nm moiré periodicity.
Strong Intralayer Phase Correlation
Scientists detected intralayer interactions linking electron phases in this meticulously built gadget. Gate-dependent reflection contrast tests showed polaronic branches of neutral excitons and substantial cusps in the trion spectra at particular, equally spaced gate voltages. Quantum phases with this unusual behavior are driven by strong correlations at specific fills of electrostatic moiré superlattices.
One notable example was the electron Mott insulating state at integer fills of the MoSe2 layer when the WSe2 layer was charge-neutral. An abrupt redshift of both exciton species and a stronger MoSe2 trion oscillator indicated the creation of this Mott insulator. Additional research showed band insulators at integer fillings and electron and hole GWCs at fractional fillings.
Rydberg Trion Probes for Interlayer Correlations
The tight coupling between moiré layers created interlayer Rydberg trions as well as intralayer effects. Even though WSe2 is charge-neutral, a 2s exciton in the WSe2 layer exploits charge-dipole interactions to attach to an electron in the MoSe2 layer in these new quasiparticles, named 2sI-. The interlayer trion proved effective for probing related states.
Interestingly, Rydberg interlayer trions were trapped at (vMo = 1, νW = 0) during the Mott insulating state. The 2sI-resonance showed a cusp in the Mott insulator state, while the 2sW exciton usually dissipates upon electron injection in MoSe2 due to screening. This trapping behavior allows access to the system's tiny properties, unlike optical probes.
The researchers also measured the Mott insulator's melting temperature by monitoring the 2sI-trin cusp. The Cusp gradually disappeared between 50 and 70 K as the temperature rose, indicating that the Correlated Electron Phases melted into a population of mobile charges and that the 2sI-trin could not be trapped. The Mott state's correlation gap was estimated as 6 meV using these results.
Dipolar Excitons and Electrostatic Programmable Geometry This innovative platform can implement multiple lattice symmetries with repulsive or attractive interlayer interactions due to its electrostatic programmability. The moiré potential's origin from the same hBN source ensures the rotational and translational alignment of the charge lattices in the two layers, which is essential for studying odd phases with charges in both layers.
The system shape depends on the injected charge sign:
When charges of the same kind are injected into both layers, the potential minima rotate 180 degrees, creating a hexagonal superlattice unit cell with broken inversion symmetry.
Since the electron and hole potential minima line up, injecting opposite charges creates a triangular unit cell with threefold rotational symmetry.
By optically injecting charges, the scientists created a dipolar excitonic phase. The coexistence of MoSe2 trions (XMo-) and WSe2 trions (XW+) and the unexpected appearance of the 2sW exciton show that charges did not adequately screen the 2s excitons. When electrons in MoSe2 bind to holes in WSe2 to form composite bosons, dipolar excitons are created. In an area of the electrostatic phase diagram where MoSe2 electron and WSe2 hole doping are virtually equivalent, the dipolar phase was observed.
Outlook for exotic quantum phenomena
This work establishes a flexible and reproducible environment for exploring exotic and topological bosonic quantum many-body phases. Controlling charge interactions and building well-defined geometries allows researchers to study exciton crystals, superfluids, supersolids, and topological exciton structures.
Layer pseudospin degrees of freedom and topological flat bands by using two layers of the same TMD could be discovered using this architecture's diversity in TMD materials and spacer thickness. Adding quantum emitters to the hBN spacer could enable quantum computing. This remarkable discovery unleashes strongly correlated systems' full potential and provides a clean, reproducible paradigm for quantum many-body physics.
