Electron-hole bilayers: Excitonic phases and collective modes

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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.