Bose-Einstein Condensation of Excitons in Bilayer Electron Systems

An ordered state of electrons in solids in which excitons condense was proposed many years ago as a theoretical possibility but has, until recently, never been observed. We review recent studies of semiconductor bilayer systems that provide clear evidence for this phenomenon and explain why exciton condensation in the quantum Hall regime, where these experiments were performed, is as likely to occur in electron-electron bilayers as in electron-hole bilayers. In current quantum Hall exciton condensates, disorder induces mobile vortices that flow in response to a supercurrent and limit the extremely large bilayer counterflow conductivity.Comment: 19 pages including 4 figure

Macroscopic coherence of a single exciton state in a polydiacetylene organic quantum wire

We show that a single exciton state in an individual ordered conjugated polymer chain exhibits macroscopic quantum spatial coherence reaching tens of microns, limited by the chain length. The spatial coherence of the k=0 exciton state is demonstrated by selecting two spatially separated emitting regions of the chain and observing their interference.Comment: 12 pages with 2 figure

Exciton Condensation and Perfect Coulomb Drag

Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, bilayer two-dimensional electron systems can support an exciton condensate consisting of electrons in one layer tightly bound to holes in the other. One thus expects "perfect" drag; a transport current of electrons driven through one layer is accompanied by an equal one of holes in the other. (The electrical currents are therefore opposite in sign.) Here we demonstrate just this effect, taking care to ensure that the electron-hole pairs dominate the transport and that tunneling of charge between the layers is negligible.Comment: 12 pages, 4 figure

Giant Superfluorescent Bursts from a Semiconductor Magnetoplasma

Currently, considerable resurgent interest exists in the concept of superradiance (SR), i.e., accelerated relaxation of excited dipoles due to cooperative spontaneous emission, first proposed by Dicke in 1954. Recent authors have discussed SR in diverse contexts, including cavity quantum electrodynamics, quantum phase transitions, and plasmonics. At the heart of these various experiments lies the coherent coupling of constituent particles to each other via their radiation field that cooperatively governs the dynamics of the whole system. In the most exciting form of SR, called superfluorescence (SF), macroscopic coherence spontaneously builds up out of an initially incoherent ensemble of excited dipoles and then decays abruptly. Here, we demonstrate the emergence of this photon-mediated, cooperative, many-body state in a very unlikely system: an ultradense electron-hole plasma in a semiconductor. We observe intense, delayed pulses, or bursts, of coherent radiation from highly photo-excited semiconductor quantum wells with a concomitant sudden decrease in population from total inversion to zero. Unlike previously reported SF in atomic and molecular systems that occur on nanosecond time scales, these intense SF bursts have picosecond pulse-widths and are delayed in time by tens of picoseconds with respect to the excitation pulse. They appear only at sufficiently high excitation powers and magnetic fields and sufficiently low temperatures - where various interactions causing decoherence are suppressed. We present theoretical simulations based on the relaxation and recombination dynamics of ultrahigh-density electron-hole pairs in a quantizing magnetic field, which successfully capture the salient features of the experimental observations.Comment: 21 pages, 4 figure

Transition to a Bose-Einstein condensate of excitons at sub-Kelvin temperatures

Bose-Einstein condensation (BEC) is a quantum mechanical phenomenon directly linked to the quantum statistics of bosons. While cold atomic gases provide a new arena for exploring the nature of BEC, a long-term quest to confirm BEC of excitons, quasi-Bose particles formed as a bound state of an electron-hole pair, has been underway since its theoretical prediction in the 1960s. Ensembles of electrons and holes are complex quantum systems with strong Coulomb correlations; thus, it is non-trivial whether nature chooses a form of exciton BEC. Various systems have been examined in bulk and two-dimensional semiconductors and also exciton-photon hybrid systems. Among them, the 1s paraexciton state in a single crystal of Cu2O has been a prime candidate for realizing three-dimensional BEC. The large binding energy and long lifetime enable preparation of cold excitons in thermal equilibrium with the lattice and decoupled from the radiation field. However, collisional loss severely limits the conditions for reaching BEC. Such a system with a large inelastic cross section is excluded in atomic BEC experiments, where a small inelastic scattering rate and efficient elastic scattering are necessary for evaporative cooling. Here we demonstrate that it is nevertheless possible to achieve BEC by cooling paraexcitons to sub-Kelvin temperatures in a cold phonon bath. Emission spectra from paraexcitons in a three-dimensional trap show an anomalous distribution in a threshold-like manner at the critical number of BEC expected for ideal bosons. Bosonic stimulated scattering into the condensate and collisional loss compete and limit the condensate to a fraction of about 1%. This observation adds a new class of experimentally accessible BEC for exploring a rich variety of matter phases of electron-hole ensembles.Comment: 19 pages, 3 figures, Supplementary Information (12 pages, 4 figures) include

High-temperature superfluidity with indirect excitons in van der Waals heterostructures.

All known superfluid and superconducting states of condensed matter are enabled by composite bosons (atoms, molecules and Cooper pairs) made of an even number of fermions. Temperatures where such macroscopic quantum phenomena occur are limited by the lesser of the binding energy and the degeneracy temperature of the bosons. High-critical temperature cuprate superconductors set the present record of ~100 K. Here we propose a design for artificially structured materials to rival this record. The main elements of the structure are two monolayers of a transition metal dichalcogenide separated by an atomically thin spacer. Electrons and holes generated in the system would accumulate in the opposite monolayers and form bosonic bound states--the indirect excitons. The resultant degenerate Bose gas of indirect excitons would exhibit macroscopic occupation of a quantum state and vanishing viscosity at high temperatures