2D bilayer electron-hole superfluidity with unequal and anisotropic masses

TL;DR

This study examines how unequal and anisotropic effective masses affect electron-hole superfluidity in 2D bilayers, revealing robustness of superfluidity despite reduced pairing strength and shifts in the BEC-BCS crossover.

Contribution

It provides a detailed analysis of mass imbalance and anisotropy effects on superfluidity, extending understanding beyond the ideal equal-mass case in bilayer systems.

Findings

Mass imbalance and anisotropy suppress critical temperature.

Superfluidity remains stable across various densities and separations.

Fermi surface nesting is not essential for bilayer superfluidity.

Abstract

We investigate the stability of electron-hole superfluidity in two-dimensional bilayers with unequal and anisotropic effective masses. Using a zero-temperature, self-consistent Hartree-Fock approach, we study two experimentally relevant deviations from the ideal equal-mass isotropic case: (i) isotropic but unequal conduction and valence band masses ($m_c^* \neq m_v^*$), and (ii) equal average masses with orthogonal in-plane anisotropies $(m_{c,x}^*, m^*_{c,y}) = (m_1^*, m_2^*)$ and $(m^*_{v,x}, m^*_{v,y}) = (m_2^*, m_1^*)$. For both scenarios, we compute the order parameter and analyze the BEC-BCS crossover as a function of layer separation and mass ratio. We find that both mass imbalance and mass anisotropy reduce the pairing strength and suppress the inferred critical temperature $T_c$ by breaking perfect Fermi surface nesting, and shift the BEC-BCS crossover. Despite these effects, superfluidity remains robust across the full range of densities and interlayer separations considered, with no transition to an unpaired plasma state in the absence of screening. Our results provide a baseline for understanding the interplay of mass mismatch and anisotropy in current and emerging bilayer platforms, including van der Waals heterostructures and anisotropic two-dimensional semiconductors. Our work also establishes that Fermi surface nesting is not a key ingredient for the bilayer superfluidity, which is always the ground state for all electron-hole bilayers although the resultant $T_c$ depends on the parameter details and may very well be unmeasurably low for large interlayer separations.