Search

The coherent properties of a dense ensemble of triplet cyclotron magnetoexcitons in a quantum Hall insulator near the filling factor have been studied using shear interferometry methods. It has been found that a condensate of magnetoexcitons formed at fairly low temperatures by means of nonresonant photoexcitation has a higher degree of coherence compared to the gas of uncondensed magnetoexcitons: the transverse length of coherence reaches about 10 μm. The condensate can include up to 20% of all triplet magnetoexcitons. [ABSTRACT FROM AUTHOR]

The second- and third-order coherence functions g(2)(0) and g(3)(0), the number distribution, and the phase distribution of a polariton condensate are calculated by a quantum Monte Carlo wavefunction method using a one-dimensional model. The second- and third-order coherence functions change from g(n)(0) = n! to unity as the pumping intensity increases, as is the case for an ideal photon laser. However, the number and phase distributions demonstrate that the polariton condensate is quite different from a coherent state; this is owing to polariton-polariton scattering. We investigate the effects of this scattering upon the statistical properties of the condensates. [ABSTRACT FROM AUTHOR]

Two semiconductor systems, quantum wells with spatially indirect dipolar excitons and exciton polaritons in semiconductor microcavities, exhibiting the Bose condensation of excitons, are discussed. [ABSTRACT FROM AUTHOR]

If bosonic particles are cooled down below the temperature of quantum degeneracy, they can spontaneously form a coherent state in which individual matter waves synchronize and combine. Spontaneous coherence of matter waves forms the basis of a number of fundamental phenomena in physics, including superconductivity, superfluidity and Bose-Einstein condensation. Spontaneous coherence is the key characteristic of condensation in momentum space. Excitons-bound pairs of electrons and holes-form a model system to explore the quantum physics of cold bosons in solids. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy owing to their long lifetimes. Here we report measurements of spontaneous coherence in a gas of indirect excitons. We found that spontaneous coherence of excitons emerges in the region of the macroscopically ordered exciton state and in the region of vortices of linear polarization. The coherence length in these regions is much larger than in a classical gas, indicating a coherent state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. A pattern of extended spontaneous coherence is correlated with a pattern of spontaneous polarization, revealing the properties of a multicomponent coherent state. We also observed phase singularities in the coherent exciton gas. All these phenomena emerge when the exciton gas is cooled below a few kelvin. [ABSTRACT FROM AUTHOR]

We review, aiming at an audience of final year undergraduates, the phenomena observed in, and properties of, microcavity exciton-polariton condensates. These are condensates of mixed light and matter, consisting of superpositions of photons in semiconductor microcavities and excitons in quantum wells. Because of the imperfect confinement of the photon component, exciton-polaritons have a finite lifetime, and have to be continuously re-populated. Therefore, exciton-polariton condensates lie somewhere between equilibrium Bose-Einstein condensates and lasers. We review in particular the evidence for condensation, the coherence properties studied experimentally, and the wide variety of spatial structures either observed or predicted to exist in exciton-polariton condensates, including quantised vortices and other coherent structures. We also discuss the question of superfluidity in a non-equilibrium system, reviewing both the experimental attempts to investigate superfluidity to date, and the theoretical suggestions of how it may be further elucidated. [ABSTRACT FROM AUTHOR]