60 research outputs found
Composite boson signature in the interference pattern of atomic dimer condensates
We predict the existence of high frequency modes in the interference pattern
of two condensates made of fermionic-atom dimers. These modes, which result
from fermion exchanges between condensates, constitute a striking signature of
the dimer composite nature. From the 2-coboson spatial correlation function,
that we derive analytically, and the Shiva diagrams that visualize many-body
effects specific to composite bosons, we identify the physical origin of these
high frequency modes and determine the conditions to see them experimentally by
using bound fermionic-atom pairs trapped on optical lattice sites. The dimer
granularity which appears in these modes comes from Pauli blocking that
prevents two dimers to be located at the same lattice site.Comment: 10+7 pp, 3 figures. v2: version accepted for publication in New J.
Phy
Way to observe the implausible "trion-polariton"
Using the composite boson (coboson) many-body formalism, we determine under
which conditions "trion-polariton" can exist. Dipolar attraction can bind an
exciton and an electron into a trion having an energy well separated from the
exciton energy. Yet, the existence of long-lived "trion-polariton" is a priori
implausible not only because the photon-trion coupling, which scales as the
inverse of the sample volume, is vanishingly small, but mostly because this
coupling is intrinsically "weak". Here, we show that a moderately dense Fermi
sea renders its observation possible: on the pro side, the Fermi sea overcomes
the weak coupling by pinning the photon to its momentum through Pauli blocking,
it also overcomes the dramatically poor photon-trion coupling by providing a
volume-linear trion subspace to which the photon is coherently coupled. On the
con side, the Fermi sea broadens the photon-trion resonance due to the
fermionic nature of trions and electrons, it also weakens the trion binding by
blocking electronic states relevant for trion formation. As a result, the
proper way to observe this novel polariton is to use doped semiconductor having
long-lived electronic states, highly-bound trion and Fermi energy as large as a
fraction of the trion binding energy.Comment: 6 pages, 3 figure
Cross-over from trion-hole to exciton-polaron in n-doped semiconductor quantum wells
We present a theoretical study of photo-absorption in n-doped two-dimensional
(2D) and quasi-2D semiconductors that takes into account the interaction of the
photocreated exciton with Fermi-sea (FS) electrons through (i) Pauli blocking,
(ii) Coulomb screening, and (iii) excitation of FS electron-hole pairs---that
we here restrict to one. The system we tackle is thus made of one exciton plus
zero or one FS electron-hole pair. At low doping, the system ground state is
predominantly made of a "trion-hole"---a trion (two opposite-spin electrons
plus a valence hole) weakly bound to a FS hole---with a small exciton
component. As the trion is poorly coupled to photon, the intensity of the
lowest absorption peak is weak; it increases with doping, thanks to the growing
exciton component, due to a larger coupling between 2-particle and 4-particle
states. Under a further doping increase, the trion-hole complex is less bound
because of Pauli blocking by FS electrons, and its energy increases. The lower
peak then becomes predominantly due to an exciton dressed by FS electron-hole
pairs, that is, an exciton-polaron. As a result, the absorption spectra of
-doped semiconductor quantum wells show two prominent peaks, the nature of
the lowest peak turning from trion-hole to exciton-polaron under a doping
increase. Our work also nails down the physical mechanism behind the increase
with doping of the energy separation between the trion-hole peak and the
exciton-polaron peak, even before the anti-crossing, as experimentally
observed.Comment: 20 pages, 10 figure
Exciton ground-state energy with full hole warping structure
Most semiconductors, in particular III-V compounds, have a complex valence
band structure near the band edge, due to degeneracy at the zone center. One
peculiar feature is the warping of the electronic dispersion relations, which
are not isotropic even in the vicinity of the band edge. When the exciton, all
important for the semiconductor optical properties, is considered, this problem
is usually handled by using some kind of angular averaging procedure, that
would restore the isotropy of the hole effective dispersion relations. In the
present paper, we consider the problem of the exciton ground-state energy for
semiconductors with zinc-blende crystal structure, and we solve it exactly by a
numerical treatment, taking fully into account the warping of the valence band.
In the resulting four-dimensional problem, we first show exactly that the
exciton ground state is fourfold degenerate. We then explore the ground-state
energy across the full range of allowed Luttinger parameters. We find that the
correction due to warping may in principle be quite large. However, for the
semiconductors with available data for the band structure we have considered,
the correction turns out to be in the range.Comment: 10 pages, 1 figur
Correlated Pair Approach to Composite Boson Scattering Lengths
We derive the scattering length of composite bosons (cobosons) within the
framework of the composite boson many-body formalism that uses correlated-pair
states as a basis, instead of free fermion states. The integral equation
constructed from this physically relevant basis makes transparent the role of
fermion exchange in the coboson-coboson effective scattering. Three potentials
used for Cooper pairs, fermionic-atom dimers, and semiconductor excitons are
considered. While the s-wave scattering length for the BCS-like potential is
just equal to its Born value, the other two are substantially smaller. For
fermionic-atom dimers and semiconductor excitons, our results, calculated
within a restricted correlated-pair basis, are in good agreement with those
obtained from procedures numerically more demanding. We also propose model
coboson-coboson scatterings that are separable and thus easily workable, and
that produce scattering lengths which match quantitatively well with the
numerically-obtained values for all fermion mass ratios. These separable model
scatterings can facilitate future works on many-body effects in coboson gases.Comment: 10 pages, 6 figure
- …