Spinor Exciton Condensation in Coupled Quantum Wells

寻找新的量子现象和物质的新量子态是目前实验和理论研究的一个高度活跃的领域。目标有两个：了解物质的基本性质和我们可以将其推向的极端，并将其用于实际应用。这个项目的目的是探索半导体中一种新的量子态的性质，并帮助实验实现，这种量子态被称为：旋量激子凝聚体。最令人兴奋的宏观量子态之一，有时被称为“物质的第五态”，是玻色-爱因斯坦凝聚体（BEC）。凝聚体的一个重要性质是量子相干性--一种特殊类型的秩序，它也强调了激光、超导体和无阻力流动的超流体的独特性质。1995年，在冷却到纳开尔文温度的铷原子气体中首次实现了BEC。半导体电子-空穴系统中的BEC的想法，由BCS理论的公式引发，可以追溯到BEC和超导研究的早期。已经讨论过，BCS-BEC态可以通过电子的外部激发在半导体中产生，导致形成电子-空穴束缚态--激子--氢的固态类似物。由于激子的轻有效质量，预期激子BEC在开尔文量级的温度下发生，即比原子碱性气体的温度高几个数量级，为实际器件应用带来希望。然而，BEC只有在激子复合速率足够慢时才有机会形成，即比热化和冷凝物形成速率慢，并且如果在激子的寿命内可以实现足够大的激子密度。这已被证明是实现激子BEC的主要技术障碍，并且在理论提出后的几乎半个世纪，这种状态的实验证据仍然不令人信服。然而，由于近年来在样品生长和有效的激子捕获方面的巨大技术进步，可以预期，继微腔极化激元BEC在过去三年中经历真实的蓬勃发展的例子之后，激子BEC应该在实验范围内。在这种背景下，在过去的十年中，耦合量子威尔斯已经成为一个有前途的系统，以实现激子的玻色凝聚，与许多实验研究，旨在证明这种效果。由于两个威尔斯阱中的电子和空穴波函数几乎没有重叠，因此这种类型的结构中的激子具有长得多的寿命。此外，通过施加机械应力，可以产生有效的激子陷阱，因此密度足够BEC。在应力下的耦合量子威尔斯的物理是相当复杂的：应变诱导耦合，自旋轨道和压电效应导致各种激子自旋态的混合。因此，在这些结构中，我们应该期待旋量亮-暗凝聚。最终目标是实现和研究激子BEC。然而，为了实现这一目标，它将有助于如果基本的基本问题，激子在耦合量子威尔斯应变下理解：（一）什么是性质（自旋结构）的基态？(ii)具有旋量结构和偶极相互作用的激子的散射性质是什么？(iii)多体的特性，以及BEC和超流性的特征是如何受到这种结构和散射的影响的？通过有效地结合我们的理论分析和我们的学术合作者的实验研究，我们的项目将解决这些问题。量子凝聚已经在高精度测量、原子钟和精度空前的惯性传感器中得到应用。随着最近在固态中实现这些独特的状态，肯定会有更多。

The search for new quantum phenomena and new quantum states of matter is currently a highly active area of experimental and theoretical investigations. The aims are twofold: to understand fundamental properties of matter and the extremes to which we can push it, and to use them for practical applications. The aim of this project is to explore the properties, and to help in experimental realisation, of a novel quantum state in semiconductors, called: spinor exciton condensate.One of the most exciting macroscopic quantum states, sometimes called ``the fifth state of matter'', is a Bose-Einstein condensate (BEC). An important property of condensates is quantum coherence -- a special type of order which also underlines the unique properties of laser light, superconductors and superfluids flowing without resistance. The first realisation of BEC took place in 1995 in a gas of rubidium atoms cooled to nano-Kelvin temperatures. The idea of BEC in semiconductor electron-hole systems, triggered by the formulation of the BCS theory, dates back to the early days of research on BEC and superconductivity. It has been discussed, that the BCS-BEC state can be created in semiconductors by external excitations of electrons, leading to the formation of electron-hole bound states -- the excitons -- solid state analog of hydrogen. Due to the light effective mass of excitons, excitonic BEC is expected to take place at temperatures of the order of kelvins, i.e. orders of magnitude higher than that for atomic alkali gases, bringing hope for practical device applications. However, BEC has the chance to form only if the excitonic recombination rate is sufficiently slow, i.e. slower than the thermalisation and condensate formation rates, and if sufficiently large densities of excitons can be achieved within their lifetime. This has proven to be the major technical obstacle in the realisation of excitonic BEC, and almost half a century after the theoretical proposal the experimental evidence of this state remains unconvincing. However, due to the large technological progress in the sample growth and effective exciton trapping in recent years it is expected that, following the example of microcavity polariton BEC undergoing a real blossoming in the last three years, the exciton BEC should be within experimental reach. In this context, over the last decade coupled quantum wells have emerged as a promising system to achieve Bose condensation of excitons, with numerous experimental studies aimed at the demonstration of this effect. Since the electron and hole wavefunctions in the two wells have little overlap, excitons in this type of structure have much longer lifetimes. Further, by applying mechanical stress one can create effective exciton traps, and thus densities sufficient for BEC.In coupled quantum wells under stress the physics is quite complex: strain induced coupling, spin-orbit and piezoelectric effects lead to mixing of various exciton spin states. Thus, we should expect a spinor bright-dark condensate in these structures. The ultimate goal is to realise and study exciton BEC. However, in order to achieve this goal it would help if basic fundamental questions concerning excitons in coupled quantum wells under strain were understood: (i) what is the nature (spin structure) of the ground state? (ii) what are the scattering properties of excitons with the spinor structure and dipolar interactions? (iii) how are the many-body features, and signatures of BEC and superfluidity, affected by this structure and scattering? By effectively combining our theoretical analysis and experimental investigations of our academic collaborators, our project will address these questions. Quantum condensates have already found applications in high precision measurement, atomic clocks and inertial sensors of unprecedented accuracy. With the recent realisation of these unique states in the solid state, there is definitely more to come.