arXiv subjects

Monique Combescot

Publications and source records attributed to Monique Combescot.

40 records · Page 3Linked to original sources

arXiv2007

Bose-Einstein condensation in semiconductors: the key role of dark excitons

The non elementary-boson nature of excitons controls Bose-Einstein condensation in semiconductors. Composite excitons interact predominantly through Pauli exclusion; this produces dramatic couplings between bright and dark states. In microcavities, where bright excitons and photons form polaritons, they force the condensate to be linearly polarized--as observed. In bulk, they also force linear polarization, but of dark states, due to interband Coulomb scatterings. To evidence this dark condensate, we thus need indirect processes, like the shift it induces on the (bright) exciton line.

cond-mat.mes-hall↗

arXiv2006

The carrier ''antibinding'' in quantum dots: a charge separation effect

We show that the carrier ''antibinding'' observed recently in semiconductor quantum dots, i.e., the fact that the ground state energy of two electron-hole pairs goes above twice the ground-state energy of one pair, can entirely be assigned to a charge separation effect, whatever its origin. In the absence of external electric field, this charge separation comes from different ''spreading-out'' of the electron and hole wavefunctions linked to the finite height of the barriers. When the dot size shrinks, the two-pair energy always stays below when the barriers are infinite. On the opposite, because barriers are less efficient for small dots, the energy of two-pairs in a dot with finite barriers, ends by behaving like the one in bulk, i.e., by going above twice the one-pair energy when the pairs get too close. For a full understanding of this ''antibinding'' effect, we have also reconsidered the case of one pair plus one carrier. We find that, while the carriers just have to spread out of the dot differently for the ''antibinding'' of two-pairs to appear, this ''antibinding'' for one pair plus one carrier only appears if this carrier is the one which spreads out the less. In addition a remarkable sum rule exists between the "binding energies" of two pairs and of one pair plus one carrier.

cond-mat.mes-hall↗

arXiv2004

Dressed atom versus exciton polariton: From Rabi oscillations to the Fermi Golden rule

We rederive the dressed atom and the exciton polariton within the {\it same} framework to make clear that their difference only comes from the number of electrons available for photoexcitations. Using it, we analytically show how the time dependence of the photon number transforms from an oscillating behavior (at the stimulated or vacuum Rabi frequency) to an exponential decay (identical for atom and semiconductor) when the excited state lifetime decreases. Although the matter ground state is in both cases coupled by monochromatic photons {\it not to a continuum but to a discrete state}, this decay yet follows a kind of Fermi Golden rule. The energy conservation it contains, is however conceptually different.

cond-mat.str-el↗

arXiv2001

New criteria for bosonic behavior of excitons

Dimensional arguments lead to say that $N$ excitons in a sample of volume $V$ behave as bosons for $λN a_x^3/V \ll 1$, $a_x$ being the exciton radius and $λ$ a dimensionless factor. The Mott criterion, which is based on the disappearance of all exciton bound states because of screening, gives $λ\simeq 1$. Since excitons feel each other through both Coulomb interaction and Pauli exclusion between their electrons and holes, criteria based on the underlying fermionic character of the exciton should be even more relevant. Two significant quantities are the boson departure $1- [B,B^{\dagger}]$ and boson number $B^{\dagger} B$, where $B^{\dagger}$ is the exciton creation operator. Their expectation values in the $N$-exciton state are close to their boson values for $λ\sim 100$ and $λ\sim 50$, respectively. By contrast, the expectation value of $B^N (B^{\dagger})^N$ in the vacuum state is close to $N!$ for $25 N^2 a_x^3/V \ll 1$ only. This surprising $N^2$ dependence comes from the intrinsic $N$-body character of Pauli exclusion. Consequences of these new criteria on the Bose condensation of excitons are discussed.

cond-mat.mtrl-sci↗