Quantum droplets of electrons and holes

Abstract

Interacting many-body systems are characterized by stable configurations of objects--ranging from elementary particles to cosmological formations--that also act as building blocks for more complicated structures. It is often possible to incorporate interactions in theoretical treatments of crystalline solids by introducing suitable quasiparticles that have an effective mass, spin or charge which in turn affects the material's conductivity, optical response or phase transitions. Additional quasiparticle interactions may also create strongly correlated configurations yielding new macroscopic phenomena, such as the emergence of a Mott insulator, superconductivity or the pseudogap phase of high-temperature superconductors. In semiconductors, a conduction-band electron attracts a valence-band hole (electronic vacancy) to create a bound pair, known as an exciton, which is yet another quasiparticle. Two excitons may also bind together to give molecules, often referred to as biexcitons, and even polyexcitons may exist. In indirect-gap semiconductors such as germanium or silicon, a thermodynamic phase transition may produce electron-hole droplets whose diameter can approach the micrometre range. In direct-gap semiconductors such as gallium arsenide, the exciton lifetime is too short for such a thermodynamic process. Instead, different quasiparticle configurations are stabilized dominantly by many-body interactions, not by thermalization. The resulting non-equilibrium quantum kinetics is so complicated that stable aggregates containing three or more Coulomb-correlated electron-hole pairs remain mostly unexplored. Here we study such complex aggregates and identify a new stable configuration of charged particles that we call a quantum droplet. This configuration exists in a plasma and exhibits quantization owing to its small size. It is charge neutral and contains a small number of particles with a pair-correlation function that is characteristic of a liquid. We present experimental and theoretical evidence for the existence of quantum droplets in an electron-hole plasma created in a gallium arsenide quantum well by ultrashort optical pulses.

Signatures of four-particle correlations associated with exciton-carrier interactions in coherent spectroscopy on bulk GaAs

“…The development of 2DFTS techniques 19 , in which measurement of the phase of the four-wave mixing signal allows correlations at different absorption and emission frequencies to be identified 37,42 , have enabled the further separation of these signal contributions providing additional insight into many-body interactions 5,[14][15][16][17][18] . In recent years, a complex hierarchy of correlations involving successively larger numbers of particles have been revealed using these powerful spectroscopy techniques 2,[5][6][7] . Existing experiments have primarily focused on exciton-exciton interactions in semiconductor quantum wells 5,7,13,[15][16][17][18]20,22 in which excitonic resonances involving the heavy-hole and light-hole valence bands are separated in energy but linked to a common ground state via the shared conduction band levels.…”

Population inversion and giant bandgap renormalization in atomically thin WS2 layers

“…In semiconductors, Coulomb interactions can lead to the formation of stable many-particle electronic states, such as excitons, trions, excitonic molecules and dropletons, as well as macroscopic condensates [1][2][3][4][5] . Excitonic quasi-particles appear as pronounced resonances in the optical response and can dominate absorption and emission spectra 6 .…”

Exciting Polaritons with Quantum Light

“…The emitter of Fock states recently proposed by some of the authors [37], releasing all its energy in bundles of N photons, should be a key resource for quantum excitation. Beyond the exotic states of light already alluded to, this may open the door to new classes of excitations, such as the correlated electron-hole clusters, the so-called dropletons [51], discovered within the limitations of classical excitations by theoretical deduction.…”