Excitonic States in Low-Dimensional Materials

Excitonic states arise when electrons and holes in a semiconductor or semimetal bind through Coulomb attraction, forming quasiparticles known as excitons. In low-dimensional materials—ranging from atomically thin two-dimensional sheets to quasi-one-dimensional chains—reduced dielectric screening and enhanced quantum confinement amplify electron–hole interactions, often yielding large exciton binding energies and novel collective phenomena. Under suitable conditions these bound states can precipitate an excitonic insulator phase, marked by a spontaneous opening of an energy gap at the Fermi level and, in some cases, a Bose–Einstein condensation of excitons. Experimental signatures include anomalies in optical absorption, transport gaps that evolve with temperature or pressure, and discrete collective modes observable via ultrafast spectroscopy and Raman scattering. Beyond fundamental interest, excitonic phases in low-dimensional platforms hold promise for ultrafast optoelectronics, coherent light control and sensing technologies, owing to their sharp excitonic resonances and sensitivity to external fields.

Research from Nature Portfolio

Recent studies have uncovered a precursor exciton gas phase in monolayer 1T-ZrTe₂, revealing distinct band-folding signatures that precede a charge-ordered ground state. High-resolution angle-resolved photoemission spectroscopy and scanning tunnelling microscopy demonstrate a two-step emergence of excitonic correlations, providing direct evidence for tunable excitonic condensation in a truly two-dimensional material. Investigations of monolayer WTe₂ report equilibrium exciton formation at elevated temperatures, with transport and chemical-potential measurements displaying a V-shaped doping dependence sharper than predicted by independent-electron models. First-principles calculations attribute this behaviour to binding energies exceeding 100 meV in a topologically non-trivial host. In bulk Ta₂NiSe₅, Raman spectroscopy has now directly captured critical fluctuations of the excitonic order parameter, revealing a diverging quasi-elastic scattering intensity of B₂g symmetry that abruptly halts at a concurrent structural transition. These findings unambiguously establish the electronic origin of the low-temperature phase and clarify the interplay between lattice and excitonic orders.

Excitonic States in Low-Dimensional Materials publication trend

The graph below shows the total number of articles in excitonic states in low-dimensional materials across all publications each year (not limited to Nature Index journals).

Technical terms

Exciton: A bound state of an electron and a hole held together by Coulomb attraction.

Excitonic insulator: A phase in which a macroscopic population of excitons opens a gap at the Fermi level through spontaneous electron–hole pairing.

Bose–Einstein condensation (BEC): The collective occupation of the lowest quantum state by bosonic quasiparticles, such as excitons, at low temperature or high density.

Angle-resolved photoemission spectroscopy (ARPES): An experimental technique that maps electronic band structures by measuring the energy and momentum of electrons ejected by photons.

References

1. Signatures of the exciton gas phase and its condensation in monolayer 1T-ZrTe2. Nature Communications (2023).
2. Evidence for equilibrium exciton condensation in monolayer WTe2. Nature Physics (2021).
3. Phononic soft mode behavior and a strong electronic background across the structural phase transition in the excitonic insulator Ta2NiSe5. Physical Review Research (2020).
4. Spontaneous Gap Opening and Potential Excitonic States in an Ideal Dirac Semimetal Ta2Pd3Te5. Physical Review X (2024).
5. Ultrafast Electronic Band Gap Control in an Excitonic Insulator. Physical Review Letters (2017).
6. Imaging the coherent propagation of collective modes in the excitonic insulator Ta2NiSe5 at room temperature. Science Advances (2021).