Quantum droplets of electrons and holes
Quantum droplets of electrons and holes
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DOI：
10.1038/nature12994
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发表时间：
2014-02-27
期刊：
影响因子：
64.8
通讯作者：
Koch, S. W.
中科院分区：
文献类型：
--
作者：
Almand-Hunter, A. E.;Li, H.;Koch, S. W.
相互作用的多体系统的特征是从基本粒子到宇宙结构（1 - 3）等物体的稳定构型，这些构型也作为更复杂结构的构建单元。在晶体固体的理论处理中，常常可以通过引入具有有效质量、自旋或电荷（4,5）的合适准粒子来纳入相互作用，这反过来又会影响材料的电导率、光学响应或相变（2,6,7）。额外的准粒子相互作用也可能产生强关联构型，从而产生新的宏观现象，比如莫特绝缘体（8）的出现、超导性或高温超导体的赝能隙相（9 - 11）。在半导体中，导带电子吸引价带空穴（电子空位）形成束缚对，称为激子（12,13），这是又一种准粒子。两个激子也可能结合在一起形成分子，通常称为双激子（14），甚至可能存在多激子（15,16）。在锗或硅等间接带隙半导体中，一个热力学相变可能产生电子 - 空穴液滴，其直径可接近微米范围（17,18）。在砷化镓等直接带隙半导体中，激子寿命太短，无法发生这样的热力学过程。相反，不同的准粒子构型主要由多体相互作用而非热化来稳定。由此产生的非平衡量子动力学非常复杂，以至于包含三个或更多库仑相关电子 - 空穴对的稳定聚集体大多尚未被探索。在这里，我们研究了这类复杂聚集体，并确定了一种新的带电粒子稳定构型，我们称之为量子液滴。这种构型存在于等离子体中，由于其尺寸小而呈现量子化。它是电中性的，包含少量粒子，其对关联函数具有液体的特征。我们通过实验和理论证明了在砷化镓量子阱中由超短光脉冲产生的电子 - 空穴等离子体中存在量子液滴。
Interacting many-body systems are characterized by stable configurations of objects-ranging from elementary particles to cosmological formations(1-3)-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(4,5) which in turn affects the material's conductivity, optical response or phase transitions(2,6,7). Additional quasiparticle interactions may also create strongly correlated configurations yielding new macroscopic phenomena, such as the emergence of a Mott insulator(8), superconductivity or the pseudogap phase of high-temperature superconductors(9-11). In semiconductors, a conduction-band electron attracts a valence-band hole (electronic vacancy) to create a bound pair, known as an exciton(12,13), which is yet another quasiparticle. Two excitons may also bind together to give molecules, often referred to as biexcitons(14), and even polyexcitons may exist(15,16). 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(17,18). 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.