Quantum droplets of electrons and holes

摘要

激子、等离子体激元和声子是一些比较有名的准粒子（行为在某些方面像正常粒子的奇特体）。虽然新类型的准粒子并不总是经常出现，但现在就有这么一个。它是一个全新的多体粒子，被命名为"dropleton" 。Mackillo Kira 及同事识别出了这一新的量子体，它是当四个或更多的电子和空穴（电子空位）在砷化镓等direct-gap半导体中通过库仑吸引力形成一个微小的"关联泡"(correlation bubble)时所生成的一种量子微滴。本期封面所示为量子微滴的"对相关函数"(pair-correlation function) g®。该"相关函数''的"中央峰''显示电子和空穴可能是在同一位置;同时该函数的波纹显示，否则的话它们会形成间隔规则的壳。%Interacting many-body systems are characterized by stable configurations of objects-ranging from elementary particles to cosmologi-cal 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 qua-siparticles 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 exist15,16. 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 ther-malization. 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.