Magnon-Mediated Indirect Exciton Condensation through Antiferromagnetic Insulators

Electrons and holes residing on the opposing sides of an insulating barrier and experiencing an attractive Coulomb interaction can spontaneously form a coherent state known as an indirect exciton condensate. We study a trilayer system where the barrier is an antiferromagnetic insulator. The electrons and holes here additionally interact via interfacial coupling to the antiferromagnetic magnons. We show that by employing magnetically uncompensated interfaces, we can design the magnon-mediated interaction to be attractive or repulsive by varying the thickness of the antiferromagnetic insulator by a single atomic layer. We derive an analytical expression for the critical temperature $T_c$ of the indirect exciton condensation. Within our model, anisotropy is found to be crucial for achieving a finite $T_c$, which increases with the strength of the exchange interaction in the antiferromagnetic bulk. For realistic material parameters, we estimate $T_c$ to be around 7 K, the same order of magnitude as the current experimentally achievable exciton condensation where the attraction is solely due to the Coulomb interaction. The magnon-mediated interaction is expected to cooperate with the Coulomb interaction for condensation of indirect excitons, thereby providing a means to significantly increase the exciton condensation temperature range.

Figure 1

(a) An antiferromagnetic insulator (AFI) sandwiched between two separate fermion reservoirs, denoted by $L$ and $R$. We let the spins on sublattice $A$ (illustrated in blue) be down, and the spins on sublattice $B$ (illustrated in red) be up. (b) The fermions in the two reservoirs can interact through emission and absorption of magnons. For the process in the figure we have that either a spin-up fermion in $L$ emits a $S_z=+\hslash$ magnon (red arrow) which is absorbed by a spin-down fermion in $R$, or a spin-down fermion in $R$ emits a $S_z=-\hslash$ magnon (blue dashed arrow) absorbed by a spin-up fermion in $L$.

Figure 2

Dependence of the normalized critical temperature on the strength of the normalized magnetic anisotropy.