Electron-Electron Interaction Phenomena in Ultraclean Graphene

May 2nd, 2018 ALBERTO MORPURGO University of Geneva

More than 10 years after its experimental discovery, graphene and graphene based- structure continue to reveal new interesting phenomena that are either entirely unexpected or that can be only observed now, thanks to technical progress in device fabrication. This is the case for many physical phenomena driven by interactions between electrons. In my talk, I will discuss two topics: 1) finite-temperature electronic phase transitions in Bernal-stacked multilayer graphene that exhibit an impressively systematic behavior and an even-odd effect upon increasing the number of layers, and 2) electron-hole scattering as the limiting mechanism of transport below 100 K in charge neutral bilayer and trilayer graphene.

1) Upon increasing the number of layers, it is expected that the electronic properties of Bernal stacked multilayers should approach those of graphite, which is a semi-metal. I will show that -at least up to 8 layer Bernal-staked graphene multilayers (8LG)- this is not the case and increasing thickness makes the behavior deviate more from that of graphite. In particular -starting from bilayers- we observe phase transitions occurring in all Bernal stacked multilayers irrespective of whether they are even or odd. The phase transition is such that, at low temperature, all even layers become insulating and all odd layers remains conducting, exhibiting conduction due to an individual Dirac band. We can measure precisely the transition temperature and the gap for all thicknesses (so far) up to 7LG. We find that in bilayers Tc = 12 K and  =1.5 meV, and in 7LG Tc = 100 K and  = 12 meV. The behavior of all multilayers -even and odd- is perfectly described by a second order phase transition, in which the order parameter is a staggered potential whose sign alternates from one layer to the next.

2) Above the critical temperature of the transition discussed above, we have studied transport in graphene bilayers and trilayers at charge neutrality (that is, for kT > EF). In this regime, both electrons and holes are present due to thermal activation. We find that in ultraclean devices transport is entirely determined by electron-hole collisions and, if the carrier density is increased so that EF > kT, transport becomes ballistic. When both electrons and holes are present, transport is diffusive and the behavior of the conductivity is reproduced quantitatively, with no free parameters, by considering the effect of inelastic electron-hole scattering occurings with a rate Γ=C kT/ℏ (with C ~ 1), in agreement with expectations based on quantum criticality.

Seminar, May 2, 2018, 12:00. ICFO Seminar Room

Hosted by Adrian Bachtold