Anomalous Coulomb Drag in Electron-Hole Bilayers due to the Formation of Excitons

Anomalous Drag in Electron-Hole Condensates with Granulated Order

We explain the strong interlayer drag resistance observed at low temperatures in bilayer electron-hole systems in terms of an interplay between local electron-hole-pair condensation and disorder-induced carrier density variations. Smooth disorder drives the condensate into a granulated phase in which interlayer coherence is established only in well-separated and disconnected regions, or grains, within which the densities of electrons and holes accidentally match. The drag resistance is then dominated by Andreev-like scattering of charge carriers between layers at the grains that transfers momentum between layers. We show that this scenario can account for the observed dependence of the drag resistivity on temperature and, on average, charge imbalance between layers.

Frictional Drag Effect between Massless and Massive Fermions in Single-Layer/Bilayer Graphene Heterostructures

The emergence of two-dimensional layered materials offers an excellent opportunity for the fabrication of double-layer electron systems that are in close proximity but electronically isolated. In this work, heterostructures consisting of one single-layer graphene (SLG) and one bilayer graphene (BLG) separated by an ultrathin hBN layer are successfully fabricated, enabling the experimental investigation of the interlayer frictional drag effect between massless and massive fermions. With varying carrier densities, a giant positive peak of drag response emerges at the double neutrality point, around which nonmonotonic temperature dependent behaviors of drag resistance are further observed. These observations can be attributed to the anticorrelations in the distributions of e-h puddles between layers. More importantly, as the system shifts toward the strong coupling regime, the carrier density dependence of drag resistance R_drag shows a crossover from 1/n³ to 1/n² for the density matched cases, which is a unique characteristic predicted for massless-massive fermion systems. Consequently, a generalized carrier dependent expression (1/(|n_S| + |n_B|)²) is obtained for the strong coupling regime, where n_S and n_B are the carrier densities of SLG and BLG, respectively. Our study provides insight into the electronic frictional effects between massless and massive fermions and thus will promote the investigations of interlayer interactions in hybrid structures hosting different types of carriers.

Evidence of high-temperature exciton condensation in two-dimensional atomic double layers

A Bose-Einstein condensate is the ground state of a dilute gas of bosons, such as atoms cooled to temperatures close to absolute zero¹. With much smaller mass, excitons (bound electron-hole pairs) are expected to condense at considerably higher temperatures^2-7. Two-dimensional van der Waals semiconductors with very strong exciton binding are ideal systems for the study of high-temperature exciton condensation. Here we study electrically generated interlayer excitons in MoSe₂-WSe₂ atomic double layers with a density of up to 10¹² excitons per square centimetre. The interlayer tunnelling current depends only on the exciton density, which is indicative of correlated electron-hole pair tunnelling⁸. Strong electroluminescence arises when a hole tunnels from WSe₂ to recombine with an electron in MoSe₂. We observe a critical threshold dependence of the electroluminescence intensity on exciton density, accompanied by super-Poissonian photon statistics near the threshold, and a large electroluminescence enhancement with a narrow peak at equal electron and hole densities. The phenomenon persists above 100 kelvin, which is consistent with the predicted critical condensation temperature^9-12. Our study provides evidence for interlayer exciton condensation in two-dimensional atomic double layers and opens up opportunities for exploring condensate-based optoelectronics and exciton-mediated high-temperature superconductivity¹³.

Multiband Mechanism for the Sign Reversal of Coulomb Drag Observed in Double Bilayer Graphene Heterostructures

Coupled 2D sheets of electrons and holes are predicted to support novel quantum phases. Two experiments of Coulomb drag in electron-hole (e-h) double bilayer graphene (DBLG) have reported an unexplained and puzzling sign reversal of the drag signal. However, we show that this effect is due to the multiband character of DBLG. Our multiband Fermi liquid theory produces excellent agreement and captures the key features of the experimental drag resistance for all temperatures. This demonstrates the importance of multiband effects in DBLG: they have a strong effect not only on superfluidity, but also on the drag.

Phase diagram of a symmetric electron-hole bilayer system: a variational Monte Carlo study

We study the phase diagram of a symmetric electron-hole bilayer system at absolute zero temperature and in zero magnetic field within the quantum Monte Carlo approach. In particular, we conduct variational Monte Carlo simulations for various phases, i.e. the paramagnetic fluid phase, the ferromagnetic fluid phase, the anti-ferromagnetic Wigner crystal phase, the ferromagnetic Wigner crystal phase and the excitonic phase, to estimate the ground-state energy at different values of in-layer density and inter-layer spacing. Slater-Jastrow style trial wave functions, with single-particle orbitals appropriate for different phases, are used to construct the phase diagram in the (r _s , d) plane by finding the relative stability of trial wave functions. At very small layer separations, we find that the fluid phases are stable, with the paramagnetic fluid phase being particularly stable at [Formula: see text] and the ferromagnetic fluid phase being particularly stable at [Formula: see text]. As the layer spacing increases, we first find that there is a phase transition from the ferromagnetic fluid phase to the ferromagnetic Wigner crystal phase when d reaches 0.4 a.u. at r _s   =  20, and before there is a return to the ferromagnetic fluid phase when d approaches 1 a.u. However, for r _s   <  20 and [Formula: see text] a.u., the excitonic phase is found to be stable. We do not find that the anti-ferromagnetic Wigner crystal is stable over the considered range of r _s and d. We also find that as r _s increases, the critical layer separations for Wigner crystallization increase.

Exciton fission in monolayer transition metal dichalcogenide semiconductors

When electron-hole pairs are excited in a semiconductor, it is a priori not clear if they form a plasma of unbound fermionic particles or a gas of composite bosons called excitons. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states. Using state-of-the-art many-body theory, we show that the thermodynamic fission–fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping. We propose the observation of these effects by studying exciton satellites in photoemission and tunneling spectroscopy, which present direct solid-state counterparts of high-energy collider experiments on the induced fission of composite particles.

Owing to their atomically thin nature, 2D transition metal dichalcogenides host room temperature, strongly bound excitons. Here, the authors show that the thermodynamical balance between fission and fusion of excitons can be tuned by the dielectric environment and charge carrier doping and observed by photoemission spectroscopy.

Diffusion Monte Carlo study of excitons and biexcitons in a mass-asymmetric electron-hole bilayer

We employed the diffusion Monte Carlo method, under fixed node approximation, to investigate the various ground state properties of a mass-asymmetric electron-hole bilayer system. Particularly, we calculated the ground state energy, the condensation fraction c, and the pair correlation function g(r) at density r_s = 5 a.u. for the inter-planer distance d ≤ 0.5 a.u. Based on the characteristics of condensate faction and pair correlation functions, we found the phase transition from the excitonic fluid phase to the biexcitonic fluid phase at d = 0.24 a.u. We also analytically calculated the critical layer separation d_crit for biexciton stability, the value for which was almost double the d = 0.24 a.u.