of fermion exchanges on the polarization of exciton condensates

Fabrice P. Laussy, Ivan Andreevich Shelykh, Alexey Vitalievich Kavokin +1 more

Ming‐Tsung Lee

We present a canonical ensemble approach for weakly interacting composite boson systems, which is confined with a finite number of particles. Composite bosons, made of fermion pairs, correlate to each other through the Coulomb interaction associated with the fermion exchange, and are treated perturbatively. We study Bose–Einstein condensation of composite bosons according to the specific heat capacity, the composite-boson distribution function, and the condensate fraction up to the first-order perturbations. As an example, we investigate two-dimensional confined excitons. The study shows that the fermion exchange tends to assist the exciton condensation by raising the temperature. This stabilization of the exciton condensation owing to the fermion exchange is characterized by a small trough in the specific heat capacity profile. As the temperature becomes higher than the transition temperature, the effective Coulomb interaction reduces the thermal fluctuation of excitons, leading to a negative correction to the specific heat capacity in the normal phase. Weakly interacting excitons stay in a more stable thermodynamic state in comparison with ideal exciton gases.

Gerhart Lüders

Christo I. Ivanov, H. Barentzen, Marvin D. Girardeau

Monique Combescot, R. Combescot, Mathieu Alloing +1 more

We propose optical means to reveal the presence of a dark exciton condensate that does not yield any photoluminescence at all. We show that i) the dark exciton density can be obtained from the blueshift of the excitonic absorption line induced by dark excitons; ii) the polarization of the dark condensate can be obtained from the blueshift dependence on the probe photon polarization as well as from the Faraday effect. All these effects result from carrier exchanges between dark and bright states.

Yun-Pil Shim

In this thesis we study the spin or pseudospin singlet pair condensation of two different kinds of polarized fermion systems.Using generalized BCS mean-field theories we study how pairing adapts to unequal spin or pseudospin populations.After briefly reviewing the basic physics of superconductivity in Chapter 2, in Chapter 3 the mean-field theory for electron-hole bilayer systems is derived to describe the condensation of excitons which is analogous to the Cooper pair condensation in superconductors.Self-consistent solution of the exciton system gap equation shows that the excitation energy spectrum is qualitatively the same as in superconductors.In Chapter 4 the role of the spin degree of freedom in the bilayer system is investigated by generalizing the two-component mean-field theory developed in Chapter vi 3 to four-component cases.The main consequence is that population polarization leads to ferromagnetism.The interplay between exciton condensation and spontaneous spin-order is the most important consequence of the presence of both spin and pseudospin degrees of freedom in excitonic condensates.In a sense that we explain in this Chapter, both normal and condensed fluids are present in the ferromagnetic excitonic state.Using the Rashba spin-orbit interaction model derived in the appendix, we show that an external electric field can alter the characteristics of the ferromagnetic condensate phase.The spin splitting by the spin-orbit interaction and its different spin state structures lead to qualitatively different magnetic properties for electron and hole layers.In Chapter 5 we turn our attention to a second class of polarized fermion systems that is of great current interest.A fully quantum mechanical treatment of a rotating fermion atom cloud is developed and implicit equations determining the critical temperatures for all center-of-mass Landau level pairings are obtained.In Chapter 6 the condition for the realization of higher center-of-mass Landau level pairing, which corresponds to FFLO state in spin split superconductors, is determined by calculating the critical temperatures for all possible pairing channels.It is shown that FFLO states can be realized in the strong interaction and low rotation frequency regimes in parameter space, where the pairing energy can survive the high polarization.