Strongly bound excitons in gapless two-dimensional structures

Abstract

Common wisdom asserts that bound excitons cannot form in high-dimensional (d>1) metallic structures because of their overwhelming screening and unavoidable resonance with nearby continuous bands. Strikingly, here we illustrate that this prevalent assumption is not quite true. A key ingredient that has been overlooked is that of viable decoherence that thwarts the formation of resonances. As an example of this general mechanism, we focus on an experimentally relevant material and predict bound excitons in twisted bilayer graphene, which is a two-dimensional gapless structure exhibiting metallic screening. The binding energies calculated by first-principles simulations are surprisingly large. The low-energy effective model reveals that these bound states are produced by a unique destructive coherence between two alike subband resonant excitons. In particular, this destructive coherent effect is not sensitive to the screening and dimensionality, and hence may persist as a general mechanism for creating bound excitons in various metallic structures, opening the door for excitonic applications based on metallic structures.Bound excitons, electron-hole (e-h) pairs, are of particular interest because of their neat physics picture and intrinsic long lifetime that makes broad applications, including photovoltaic and photocatalytics [1][2][3]. However, the formation of bound e-h pairs had been thought to be impossible in metallic (gapless) systems due to their overwhelming screening effects. Moreover, e-h pairs in gapless structures tend to hybridize with continuous transitions nearby, forming resonant states, whose intrinsic lifetime is substantially shorter. To date, the only exception was found in metallic carbon nanotubes (mCNTs), in which the depressed one-dimensional (1D) screening together with the unique optical symmetry gap lead to the formation of a bound e-h pair [4][5][6][7][8]. Meanwhile, these studies ignite many obvious but fundamental questions: besides 1D metals, can we observe bound excitons in structures with stronger dielectric screening, e.g., higher dimensional (d>2) gapless materials? In addition to the symmetry-related reason revealed in mCNTs, are there any other general mechanisms responsible for bound exciton formation in gapless systems?

Optical and Electronic Properties of Two-Dimensional Layered Materials

“…The VBM at K shows contributions only from d orbitals of WX 2 , and the CBM at K only from d orbitals of MoX 2 , consistent with the formation of a type-II band alignment [37,42,43]. The inclusion of band structure corrections with the GW method increases the gap of the individual monolayers by~0.5-1 eV but does not change the type-II alignment predicted within DFT [44], similar to what was found in previous studies of 2D material interfaces [45].…”

“…Quantitative calculations of band offsets are significantly more complex. GW corrections to the band gap of the single materials plus band alignment using DFT interface dipoles appears to be a viable route for large systems [44,45,48]. Studies in which entire interfaces are computed using GW are still challenging due to computational cost.…”

Electronic properties of graphene-based bilayer systems

“…The possibility of the formation of excitons in tBLG has been proposed theoretically in Ref. [447]. The exciton is formed by the hole, located near the VHS above the Fermi level and the electron located close to the bottom of the upper electron band.…”

“…According to calculations done in Ref. [447], the electron-hole interaction leads to the formation of the bound exciton with binding energy E b ∼ 0.5 eV. The existence of these excitonic states should result in increasing the lifetime of optical excitations.…”

Many‐body effects in optical response of graphene‐based structures

“…Additionally, the existence of a strongly bound exciton with a binding energy of $0.5 eV was predicted for twisted graphene bilayers. [93] However, the optical oscillator strength for this exciton was calculated to be weak and hence two-photon techniques or applying of a magnetic field are needed for its detection.…”