[Paper Review] Perfect Coulomb drag in a dipolar excitonic insulator

Phuong X. Nguyen, Liguo Ma+5 more|arXiv (Cornell University)|Sep 26, 2023

Electronic and Structural Properties of Oxides11 citations

TL;DR

Demonstrates perfect Coulomb drag between electron and hole layers in MoSe2/WSe2 dipolar excitonic insulators, with a drag current equal and opposite to the drive current at equal densities, persisting up to ~20 K and breaking down at high exciton density.

ABSTRACT

Excitonic insulators (EIs), arising in semiconductors when the electron-hole binding energy exceeds the band gap, are a solid-state prototype for bosonic phases of matter. Unlike the charged excitations that are frozen and unable to transport current, the neutral electron-hole pairs (excitons) are free to move in EIs. However, it is intrinsically difficult to demonstrate exciton transport in bulk EI candidates. The recently emerged dipolar EIs based on Coulomb-coupled atomic double layers open the possibility to realize exciton transport across the insulator because separate electrical contacts can be made to the electron and hole layers. Here we show that the strong interlayer excitonic correlation at equal electron and hole densities in the MoSe2/WSe2 double layers separated by a 2-nm barrier gives rise to perfect Coulomb drag. A charge current in one layer induces an equal but opposite drag current in the other. The drag current ratio remains above 0.9 up to about 20 K for low exciton densities. As exciton density increases above the Mott density, the excitons dissociate into the electron-hole plasma abruptly, and only weak Fermi liquid frictional drag is observed. Our experiment moves a step closer to realizing exciton circuitry and superfluidity.

Motivation & Objective

• Motivate the study of exciton transport in dipolar excitonic insulators where electron and hole layers can be separately contacted.
• Investigate whether strong interlayer excitonic correlations yield perfect Coulomb drag.
• Explore the dependence of drag on exciton density and identify the onset of dissociation at higher densities.

Proposed method

• Use Coulomb-coupled MoSe2/WSe2 double-layer structure separated by a 2-nm barrier.
• Apply current to one layer and measure induced drag current in the other layer with equal electron and hole densities.
• Characterize the drag current ratio as a function of temperature and exciton density.
• Identify the transition from exciton-dominated transport to electron-hole plasma and Fermi liquid drag.

Experimental results

Research questions

• RQ1 Can perfect Coulomb drag be achieved in a dipolar excitonic insulator with separate electrical contacts to electron and hole layers?
• RQ2 How does the drag current ratio depend on temperature and exciton density, and when does exciton dissociation occur?
• RQ3 What transport regime replaces exciton-dominated drag at densities above the Mott density?

Key findings

• A strong interlayer excitonic correlation yields perfect Coulomb drag, with a drag current equal and opposite to the drive current.
• The drag current ratio remains above 0.9 up to about 20 K for low exciton densities.
• As exciton density increases past the Mott density, excitons dissociate into an electron-hole plasma and drag becomes weak Fermi-liquid frictional drag.