Inhomogeneous phases and Coulomb drag in electron-hole graphene bilayers

Date: 30 April 2019

Venue: Campus Groenenborger, Building U, Room 244 - Groeneborgerlaan 171 - 2020 Antwerpen

Time: 4:00 PM - 5:00 PM

Organization / co-organization: cmt

Short description: CMT Lecture presented by Dr Mohammad Zarenia, Univ. of Missouri, USA

Closely coupled two-dimensional electron-hole sheets are attracting great interest as they should generate novel quantum phases driven by the strong Coulomb attractions between the sheets. In the first part of my talk I demonstrate that coupled electron-hole bilayer graphene as well as coupled few-layer graphene sheets with carrier densities in a range accessible to experiments, can access the regime of strong pairing necessary for superfluidity [2]. For the coupled bilayer graphene system, we find two new inhomogeneous ground states, a one-dimensional Charge Density Wave (1D-CDW) phase, i.e. density modulations in one planar direction, and a coupled electron-hole Wigner crystal (c-WC) in association with the superfluid phase [3]. A very interesting aspect of the system is that the elementary crystal structure of bilayer graphene plays no role in generating these new quantum phases, which are completely determined by the electrons and holes simply interacting through the Coulomb interaction.

To account for the strong inter-layer correlation energy accurately, I introduce a new approach which is based on a random phase approximation at high densities and an interpolation between the weakly- and strongly-interacting regimes. The approach gives excellent agreement with available Quantum Monte Carlo calculations for single layer two-dimensional-electron-gas systems [3]. Coulomb drag of carriers in one sheet by carriers moving in the other is a powerful tool to study Fermi liquid properties and identify formation of these phases. Two independent Coulomb drag experiments on electron-hole sheets in graphene double bilayers have reported an unexplained and puzzling sign reversal of the Coulomb drag signal. In the next part of my talk, I show that this unusual effect can be explained by the multiband character of bilayer graphene and the temperature dependence of effective mass at low densities caused by the electron-electron interactions [4]. The theory produces excellent agreement with the observed structure in the Coulomb drag resistance, capturing the key features of the recent experiments over the full reported range of temperatures.

[1] M. Zarenia, A. Perali, D. Neilson, and F.M. Peeters, Scientific Reports 4, 7319 (2014).
[2] M. Zarenia, D. Neilson, and F.M. Peeters, Scientific Reports 7, 11510 (2017).
[3] M. Zarenia, D. Neilson, B. Partoens, and F. M. Peeters, Phys. Rev. B 95, 115438 (2017).
[4] M. Zarenia, A. R. Hamilton, F. M. Peeters, and D. Neilson, Phys. Rev. Lett. 121, 036601 (2018).

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