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Excitons are electron-hole (e-h) pair quasiparticles, which may form a Bose-Einstein condensate (BEC) and collapse into the phase coherent state at low temperature. However, because of ephemeral strength of pairing, a clear evidence for BEC in electron-hole system has not yet been observed. Here, we report electron-hole pair condensation in graphene (Gr)/MoS2 heterointerface at 10K without magnetic field. As a direct indication of e-h pair condensation, we demonstrate a vanished Hall drag voltage and the resultant divergence of drag resistance. While strong excitons are formed at Gr/MoS2 heterointerface without insulating layer, carrier recombination via interlayer tunneling of carriers is suppressed by the vertical p-Gr/n-MoS2 junction barrier, consequently yielding high BEC temperature of 10K, ~1000 times higher than that of two-dimensional electron gas in III-V quantum wells. The observed excitonic transport is mainly governed by the interfacial properties of the Gr/MoS2 heterostructure, rather than the intrinsic properties of each layer. Our approach with available large-area monolayer graphene and MoS2 provides a high feasibility for quantum dissipationless electronics towards integration.
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We consider the pairing of electrons and holes due to their Coulomb attraction in two parallel, independently gated graphene layers, separated by a barrier. At weak coupling, there exist the BCS-like pair-condensed state. Despite the fact that electrons and holes behave like massless Dirac fermions, the problem of BCS-like electron-hole pairing in graphene bilayer turns out to be rather similar to that in usual coupled semiconductor quantum wells. The distinctions are due to Berry phase of electronic wave functions and different screening properties. We estimate values of the gap in one-particle excitation spectrum for different interlayer distances and carrier concentrations. Influence of disorder is discussed. At large enough dielectric susceptibility of surrounding medium, the weak coupling regime holds even at arbitrarily small carrier concentrations. Localized electron-hole pairs are absent in graphene, thus the behavior of the system versus coupling strength is cardinally different from usual BCS-BEC crossover.
We study the two-dimensional spatially separated electron-hole system with density imbalance at absolute zero temperature. By means of the mean-field theory, we find that the Fulde-Ferrell state is fairly stabilized by the order parameter mixing effect.
The superconducting pairing of electrons in doped graphene due to in-plane and out-of-plane phonons is considered. It is shown that the structure of the order parameter in the valley space substantially affects conditions of the pairing. Electron-hole pairing in graphene bilayer in the strong coupling regime is also considered. Taking into account retardation of the screened Coulomb pairing potential shows a significant competition between the electron-hole direct attraction and their repulsion due to virtual plasmons and single-particle excitations.
Band structure determines the motion of electrons in a solid, giving rise to exotic phenomena when properly engineered. Drawing an analogy between electrons and photons, artificially designed optical lattices indicate the possibility of a similar band modulation effect in graphene systems. Yet due to the fermionic nature of electrons, modulated electronic systems promise far richer categories of behaviors than those found in optical lattices. Here, we uncovered a strong modulation of electronic states in bilayer graphene subject to periodic potentials. We observed for the first time the hybridization of electron and hole sub-bands, resulting in local band gaps at both primary and secondary charge neutrality points. Such hybridization leads to the formation of flat bands, enabling the study of correlated effects in graphene systems. This work may also offer a viable platform to form and continuously tune Majorana zero modes, which is important to the realization of topological quantum computation.
We consider ground state of electron-hole graphene bilayer composed of two independently doped graphene layers when a condensate of spatially separated electron-hole pairs is formed. In the weak coupling regime the pairing affects only conduction band of electron-doped layer and valence band of hole-doped layer, thus the ground state is similar to ordinary BCS condensate. At strong coupling, an ultrarelativistic character of electron dynamics reveals and the bands which are remote from Fermi surfaces (valence band of electron-doped layer and conduction band of hole-doped layer) are also affected by the pairing. The analysis of instability of unpaired state shows that s-wave pairing with band-diagonal condensate structure, described by two gaps, is preferable. A relative phase of the gaps is fixed, however at weak coupling this fixation diminishes allowing gapped and soliton-like excitations. The coupled self-consistent gap equations for these two gaps are solved at zero temperature in the constant-gap approximation and in the approximation of separable potential. It is shown that, if characteristic width of the pairing region is of the order of magnitude of chemical potential, then the value of the gap in the spectrum is not much different from the BCS estimation. However, if the pairing region is wider, then the gap value can be much larger and depends exponentially on its energy width.
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