Nonreciprocal perfect Coulomb drag in electron-hole bilayers: coherent exciton superflow as a diode

Abstract

Distinguishing an exciton condensate from an excitonic gas or insulator remains a fundamental challenge, as both phases feature bound electron-hole pairs but differ only by the emergence of macroscopic phase coherence. Here, we theoretically propose that a spin-orbit-coupled bilayer system can host a finite-momentum exciton condensate exhibiting a nonreciprocal perfect Coulomb drag -- the coherent-exciton diode effect. This effect arises from the simultaneous breaking of inversion and time-reversal symmetries in the exciton condensate, resulting in direction-dependent critical counterflow currents. The resulting nonreciprocal perfect Coulomb drag provides a clear and unambiguous transport signature of phase-coherent exciton condensation, offering a powerful and experimentally accessible approach to identify, probe, and control exciton superfluidity in solid-state platforms.

Figures (4)

• Figure 1: (a) An illustration of Coulomb drag experiment. A current $I_{\rm drive}$ in top layer will spontaneously induce $I_{drag}$ in bottom layer. (b) A schematic plot for drag ratio $\zeta=I_{\rm drag}/I_{\rm drive}$: at small $I_{\rm drive}$, $|\zeta|$ is close to 1, corresponding to the perfect Coulomb drag; as $I_{\rm drive}$ exceeds its critical value $I_{c,\pm}$ in either direction, the condensate is broken down, corresponding to the transition to the frictional drag regime between the two Fermi liquids on each layer Narozhny2016dragreview.
• Figure 2: (a) Electron dispersions and Fermi energies in both layers. Gating, spin–orbit coupling, and Zeeman fields create an electron Fermi sea in the top layer and a hole Fermi sea in the bottom layer. The gray dashed line marks the Fermi energy. (b) Schematic of electron and hole Fermi contours. The distortion and shift of the electron contour indicate finite-momentum $\bf q$ electron–hole pairing .
• Figure 3: (a) Exciton condensate momentum $q_0$ as a function of $B/k_BT_c$ at $0.90T_c$. At $B/k_BT_c$=3.0, we calculate the Ginzburg-Landau coefficients of the condensate as a function of exciton momentum $(q_x,q_y)$, based on which we calculate free energy and extract $q_0$. (b, c, d) Normalized values of $\alpha(\textbf{q}), \beta(\textbf{q})$ and $F(\textbf{q})$ at $T/T_c=0.90, B/k_BT_c =3.0$, where red dot represents exciton momentum $\textbf{q}_0$ that minimizes $F(\textbf{q})$. $E_u=k_BT_c$ and $l_u=\hbar/\sqrt{m_ek_BT_c}$ are energy and length units, respectively.
• Figure 4: (a) Supercurrent density $J_x$ as a function of exciton momentum $q_x$ at $T/T_c=0.90, B/k_BT_c =3.0$. (b) At $T/T_c=0.90$, diode efficiency $\eta$ as a function of $B/k_BT_c$.