Electron-Hole Liquids in Semiconductor Systems

Electron-hole liquids represent a collective phase of charge carriers in semiconductors, arising when the density of photogenerated or electrically injected electron–hole pairs exceeds a critical threshold known as the Mott density. Below this threshold, electrons and holes bind to form excitons—hydrogen-like quasi-particles—whereas above it Coulomb screening and many-body interactions drive a transition to a correlated liquid or plasma state. In this high-density regime, the interplay between carrier–carrier interactions, dynamical screening and band-structure effects gives rise to complex phase behaviour, including metal–insulator transitions, excitonic condensates and quantum critical phenomena. Studies span bulk crystals, quantum wells, two-dimensional materials and interface systems, employing ultrafast optical spectroscopy, transport measurements and theoretical modelling to map phase diagrams in temperature–density space. Understanding electron-hole liquids is essential for the development of excitonic devices, novel light sources and quantum technologies that exploit collective many-body states.

Research from Nature Portfolio

Recent studies have employed time-resolved terahertz spectroscopy to probe exciton formation dynamics at passivated silicon interfaces. Measurements reveal a surprisingly long-lived population of free carriers at cryogenic temperatures, persisting well below the Mott threshold and attributed to surface photovoltage effects that transiently localise holes. These findings illuminate pathways to engineer excitonic devices operating at low temperature with optical access to condensed phases.

Spectroscopic and transport investigations of few-layer black phosphorus have uncovered quantum critical behaviour at the exciton Mott transition. By tuning photoexcitation density and temperature, researchers have mapped a comprehensive phase diagram delineating sharp excitonic insulating states and metallic electron-hole plasmas. Near the transition boundary, resistivity scales linearly with temperature, signalling strange-metal behaviour and offering a platform for exploring strongly correlated excitonic physics and potential crossover to superconducting or superfluid regimes.

Electron-Hole Liquids in Semiconductor Systems publication trend

The graph below shows the total number of articles in electron-hole liquids in semiconductor systems across all publications each year (not limited to Nature Index journals).

Technical terms

Exciton: A bound state of an electron and a hole in a semiconductor, held together by Coulomb attraction and behaving as a neutral quasi-particle.

Mott transition: The critical changeover from a regime of bound excitons to an unbound electron-hole plasma or liquid when carrier density surpasses the Mott density, due to screening of the Coulomb interaction.

Electron-hole liquid: A high-density, correlated phase of electrons and holes in a semiconductor, characterised by collective many-body interactions and often metallic transport properties.

Dynamical screening: The reduction of effective Coulomb interactions between carriers by the presence of other moving charges, dependent on frequency and carrier density.

References

1. Exciton formation dynamics at the SiO2/Si interface. Communications Materials (2023).
2. Quantum criticality of excitonic Mott metal-insulator transitions in black phosphorus. Nature Communications (2022).
3. Giant Exciton Mott Density in Anatase TiO2. Physical Review Letters (2020).
4. Mott transition of excitons in GaAs-GaAlAs quantum wells. New Journal of Physics (2012).
5. Transient transition from free carrier metallic state to exciton insulating state in GaAs by ultrafast photoexcitation. New Journal of Physics (2018).