Exciton Physics in Low-Dimensional Quantum Systems

Excitons, bound states of electrons and holes, lie at the heart of optical and transport phenomena in two­-dimensional materials and layered heterostructures. In systems such as transition metal dichalcogenide monolayers, graphene bilayers and van der Waals stacks, reduced dielectric screening and quantum confinement enhance exciton binding energies by an order of magnitude compared with bulk semiconductors. This boost enables the study of excitonic insulating phases, Bose–Einstein condensation and superfluid transport at elevated temperatures. Control over layer alignment, carrier density and interlayer spacing allows precise tuning of exciton–exciton and exciton–charge interactions, opening routes to novel quantum phases. Practical applications range from ultrafast modulators and excitonic transistors to devices exploiting dissipationless counterflow. The interplay of many-body correlations, moiré engineering and external fields in low-dimensional architectures continues to drive advances in fundamental physics and optoelectronic technology.

Research from Nature Portfolio

Studies of coupled two-dimensional heterobilayers have revealed abrupt changes in electron and hole chemical potentials at matched densities, signalling an excitonic insulator ground state persisting up to the Mott transition and thermal ionisation near liquid-nitrogen temperatures. Optical spectroscopy in MoSe₂/hBN/WSe₂ stacks quantifies attractive exciton–exciton and exciton–charge interactions across the phase diagram, highlighting strongly correlated Bose–Fermi mixtures. In a highly doped van der Waals semiconductor, infrared and angle-resolved photoemission measurements identify tightly bound, room-temperature-stable excitons with binding energies approaching 400 meV at carrier concentrations above the conventional Mott criterion, defying standard polariton models. Earlier foundational work has predicted equilibrium excitonic condensation in chemically tailored bilayer heterostructures, in which lattice-matched, type-III band alignment yields spontaneous exciton superfluidity without external gating, offering a pathway to dissipationless charge counterflow and Josephson-like transport.

Research from all publishers

New theoretical analysis of excitonic insulators under applied electric fields uncovers a many-body breakdown mechanism distinct from conventional Zener tunnelling. Depending on system size, the excitonic gap either survives initial tunnelling or collapses abruptly at a critical field, producing a characteristic on–off current–voltage signature for distinguishing excitonic and band insulators. Experimental work in twisted bilayer graphene sheets demonstrates orbitally controlled fractional quantum Hall states mediated by strong interlayer Coulombic coupling. At half filling, a Bose–Einstein condensate of interlayer excitons emerges only in orbitally excited levels, suggesting a role for topological excitations in stabilising superfluidity. In double transition metal dichalcogenide monolayers separated by hexagonal boron nitride, multiband interactions and self-consistent screening yield electron-hole superfluidity with transition temperatures exceeding 100 K, driven by large effective masses, wide bandgaps and multiple condensate channels, thus extending excitonic superfluid behaviour into the regime of practical cryogenic operation.

Exciton Physics in Low-Dimensional Quantum Systems publication trend

The graph below shows the total number of articles in exciton physics in low-dimensional quantum systems across all publications each year (not limited to Nature Index journals).

Technical terms

Exciton: A quasiparticle formed by the Coulomb attraction between an electron and a hole in a semiconductor or insulator.

Excitonic insulator: A correlated phase in which bound electron–hole pairs condense and open an energy gap, rendering the system insulating.

Bose–Einstein condensation: The macroscopic occupation of a single quantum state by bosonic particles or quasiparticles at low temperature or high density.

Van der Waals heterostructure: A stack of atomically thin layers held together by weak interlayer forces, enabling new electronic and optical properties.

Coulomb drag: The transfer of momentum between spatially separated charge carriers via interlayer Coulomb interactions, often used to probe many-body effects.

References

1. Thermodynamic behavior of correlated electron-hole fluids in van der Waals heterostructures. Nature Communications (2023).
2. Tightly bound and room-temperature-stable excitons in van der Waals degenerate-semiconductor Bi4O4SeCl2 with high charge-carrier density. Communications Materials (2023).
3. Heterobilayers of 2D materials as a platform for excitonic superfluidity. Nature Communications (2020).
4. Electrical Breakdown of Excitonic Insulators. Physical Review X (2024).
5. Orbitally Controlled Quantum Hall States in Decoupled Two‐Bilayer Graphene Sheets. Advanced Science (2023).
6. Transition Metal Dichalcogenides as Strategy for High Temperature Electron-Hole Superfluidity. Condensed Matter (2020).