Inhomogeneous phases and Coulomb drag in electron-hole graphene bilayers

30/10/2018 @ 1:00 PM – 2:00 PM Asia/Singapore Timezone
S16 Level 6 – Theory Common Conference Room
6 Science Drive 2
Singapore 117546
Inhomogeneous phases and Coulomb drag in electron-hole graphene bilayers @ S16 Level 6 – Theory Common Conference Room | Singapore | Singapore

Speaker: Mohammad Zarenia
Affiliation: Department of Physics & Astronomy, University of Missouri, Columbia (MO), USA.
Host: Assistant Professor Vitor Manuel Pereira
Location: Click HERE for directions

Abstract Details: 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 the 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).

About the Speaker: Dr Mohammad Zarenia obtained his PhD in 2013 in the Condensed Matter Theory group (heading by Professor Francois Peeters) at the University of Antwerp (Belgium). Since 2008, he has been involved in the study of electronic properties of 2D materials, particularly graphene. After his PhD, he received the prestigious postdoctoral Flanders Research Foundation fellowship at the University of Antwerp. In recent years he expanded his research interests further from the single-particle physics towards the many-body aspects of 2D materials. To pursue this, he recently moved to the group of Professor Giovanni Vignale at the University of Missouri (Columbia, USA).