Electron-hole bilayers: Excitonic phases and collective modes

当两个不同的半导体接触时，它们的导带和价带通常在界面上不连续。这些能带的轮廓可能类似于一个能捕获电子和空穴的势垒。这些被捕获的载流子平行于界面高度移动，但在第三维度被限制在几十纳米范围内。这种能带工程原理催生了硅MOSFET和基于砷化镓的高电子迁移率晶体管(HEMT)，它们给电子学带来了革命性的变化。与此同时，这些设备开辟了令人着迷的二维电子系统世界。最近的技术进步使器件的制造成为可能，在这种器件中，一片二维电子与一片二维空穴保持10纳米的均匀距离。电流可以独立地通过每一张纸。在这些双层器件中，电子和空穴之间的吸引(层间)相互作用强于同一层中电子或空穴之间的排斥层内相互作用。这是一种新的半导体制度，几十年来一直在设想，但直到最近才实现。这些设备处于当今技术可能的前沿-它们需要高度发达的分子束外延(MBE)、微米和亚微米级的光刻处理以及在毫秒级温度下的测量专业知识。通过摇动一层中的粒子并测量另一层中的粒子倾向于移动多少来响应，可以直接测量电子和空穴层之间的相互作用强度。这种吸引的相互作用可以导致电子和空穴的束缚态。从实验上看，这可能表现为一个层与另一个层中的运动同步移动的趋势增加。因为电子和空穴被限制在各自的层中，所以它们不能相互坍塌和湮灭。在这样的系统中，最低的能态可能是由类似于氢原子的束缚对(间接激子)形成的，或者它可能包括一个更复杂的态，其中电子和空穴的密度在某些波矢上经历自发调制。间接激子具有整数自旋角动量，因为它的组成电子和空穴都是自旋为半整数的费米子。这个束缚对的行为就像一个玻色子--所有自旋为整数的粒子都必须这样。玻色气体的基态可以是凝聚态，其中大量粒子被锁定为零动量状态。这种被称为玻色-爱因斯坦凝聚的非凡现象已经在稀薄的原子云中观察到，温度很低。量子力学清楚地预测，较轻的玻色子(如间接激子)可以在更高的温度下转变为凝聚态，这很容易使用液体氦而不是激光冷却。几十年来，人们一直期待着一个非常丰富的电子-空穴双层相图。我们提出的研究将对双层系统中的散射过程和集合态提供基本的见解，并导致在受控固体系统中实现具有超流性质的玻色凝聚体的现实可能性。

When two dissimilar semiconductors are in contact their conduction and valence bands are generally not continuous across the interface. The profile of the bands may resemble a potential well that can trap electrons and holes. These trapped carriers are highly mobile parallel to the interface but are confined to a few tens of nanometers in the third dimension. This band engineering principle gave birth to Silicon MOSFETs and Gallium Arsenide based High electron mobility transistors (HEMT) that revolutionised electronics. At the same time these devices opened up the fascinating world of 2-dimensional electronic systems. Very recent technological advances have enabled the fabrication of devices in which a sheet of 2-dimensional electrons is maintained at a uniform distance of 10 nanometers from a sheet of 2-dimensional holes. Electrical current can be passed through each sheet independently. In these bilayer devices, the attractive (interlayer) interaction between the electrons and holes is stronger than the repulsive intralayer interactions between electrons or holes in the same layer. This is a new regime in semiconductors that has been envisioned for a few decades but only recently realised. These devices are at the very frontier of what is technologically possible today - they require a confluence of highly developed Molecular Beam Epitaxy (MBE), photolithographic processing at micron and submicron level as well as expertise in measurements at millikelvin temperatures.The interaction strength between the electron and hole layers can be directly measured by shaking the particles in one layer and measuring how much the particles in the other layer tends to move in response. The attractive interaction, can lead to bound states of an electron and a hole. Experimentally this may appear as an increased tendency of one layer to move in phase with motions in the other layer. Because the electrons and holes are confined to their respective layers they cannot collapse and annihilate each other. The lowest energy state in such systems may be formed of bound pairs (indirect excitons) analogous to the Hydrogen atom or it may involve a more complex state where the densities of the electrons and holes undergo spontaneous modulations at certain wavevectors. The indirect exciton has an integer spin angular momentum, because its constituent electron and hole are both fermions with half-integer spins. This bound pair behaves like a boson - as all particles with integer spin must. The ground state of a bose gas can be a condensate where a large number of particles are locked into a zero momentum state. This remarkable phenomena known as Bose-Einstein Condensation has been observed in dilute clouds of atoms at few microkelvin temperatures. Quantum mechanics clearly predicts that lighter bosons (like indirect excitons) can undergo a transition to a condensate state at much higher temperatures, easily achievable using liquid Helium rather than laser cooling. A remarkably rich phase diagram of the electron-hole bilayer has been anticipated for decades. Our proposedstudy will give fundamental insights to scattering processes and collective states in bilayer systems as wellas lead to realistic possibilities of achieving a Bose condensate with superfluid like properties in a controlled solid state system.