23 research outputs found
Condensation of Excitons in Cu2O at Ultracold Temperatures: Experiment and Theory
We present experiments on the luminescence of excitons confined in a
potential trap at milli-Kelvin bath temperatures under cw-excitation. They
reveal several distinct features like a kink in the dependence of the total
integrated luminescence intensity on excitation laser power and a bimodal
distribution of the spatially resolved luminescence. Furthermore, we discuss
the present state of the theoretical description of Bose-Einstein condensation
of excitons with respect to signatures of a condensate in the luminescence. The
comparison of the experimental data with theoretical results with respect to
the spatially resolved as well as the integrated luminescence intensity shows
the necessity of taking into account a Bose-Einstein condensed excitonic phase
in order to understand the behaviour of the trapped excitons.Comment: 41 pages, 23 figure
Electron-Phonon Interaction in Tetrahedral Semiconductors
Effects of electron-phonon interactions on the band structure can be
experimentally investigated in detail by measuring the temperature dependence
of energy gaps or critical points (van Hove singularities) of the optical
excitation spectra. These studies have been complemented in recent years by
observing the dependence of such spectra on isotopic mass whenever different
stable isotopes of a given atom are available at affordable prices. In crystals
composed of different atoms, the effect of the vibration of each separate atom
can thus be investigated by isotopic substitution. Because of the zero-point
vibrations, such effects are present even at zero temperature (T = 0). In this
paper we discuss state-of-the-art calculations of the dielectric function
spectra and compare them with experimental results, with emphasis on the
differences introduced by the electron-phonon interaction. The temperature
dependence of various optical parameters will be described by means of one or
two (in a few cases three) Einstein oscillators, except at the lowest
temperatures where the T4 law (contrary to the Varshini T2 result) will be
shown to apply. Increasing an isotopic mass increases the energy gaps, except
in the case of monovalent Cu (e.g., CuCl) and possibly Ag (e.g., AgGaS2). It
will be shown that the gaps of tetrahedral materials containing an element of
the first row of the periodic table (C,N,O) are strongly affected by the
electron-phonon interaction. It will be conjectured that this effect is related
to the superconductivity recently observed in heavily boron-doped carbon.Comment: 17 pages, 17 fifure
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)
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