Abstract
We predict enhanced electron-hole superfluidity in two coupled electron-hole armchair-edge terminated graphene nanoribbons separated by a thin insulating barrier. In contrast to graphene monolayers, the multiple subbands of the nanoribbons are parabolic at low energy with a gap between the conduction and valence bands, and with lifted valley degeneracy. These properties make screening of the electron-hole interaction much weaker than for coupled electron-hole monolayers, thus boosting the pairing strength and enhancing the superfluid properties. The pairing strength is further boosted by the quasi one-dimensional quantum confinement of the carriers, as well as by the large density of states near the bottom of each subband. The latter magnifies superfluid shape resonances caused by the quantum confinement. Several superfluid partial condensates are present for finite-width nanoribbons with multiple subbands. We find that superfluidity is predominately in the strongly-coupled BEC and BCS-BEC crossover regimes, with large superfluid gaps up to 100 meV and beyond. When the gaps exceed the subband spacing, there is significant mixing of the subbands, a rounding of the shape resonances, and a resulting reduction in the one-dimensional nature of the system.
Highlights
The electronic properties of graphene nanoribbons depend on the type of edge termination[17]
In addition to reducing the effect of screening, electron-hole pairing strengths will be further boosted in our proposed system by the enhanced density of states near the minimum of each subband (see Fig. 1(b)) that arises from the van Hove singularities of the quasi-one-dimensional nanoribbons, and by the quantum confinement of the carriers in the nanoribbons
The superfluid gaps in our coupled electron-hole nanoribbon systems are large in absolute value and comparable to the Fermi energy
Summary
The electronic properties of graphene nanoribbons depend on the type of edge termination[17]. We focus on armchair-edge terminated nanoribbons since (i) their subbands are parabolic around their minima (Fig. 1(a)), (ii) there is a sizeable semiconductor-like energy gap between the conduction and valence bands, and (iii) the valley degeneracy of monolayer graphene is lifted. These properties combine to greatly reduce the strength of screening of the electron-hole pairing interaction. In addition to reducing the effect of screening, electron-hole pairing strengths will be further boosted in our proposed system by the enhanced density of states near the minimum of each subband (see Fig. 1(b)) that arises from the van Hove singularities of the quasi-one-dimensional nanoribbons, and by the quantum confinement of the carriers in the nanoribbons. Superconductivity has been observed in quasi-one-dimensional systems including Sn and Al metallic nanowires and carbon nanotubes, with enhanced transition temperatures as compared with their bulk values[22]
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