Morpurgo, Alberto

Date:   Friday, Nov. 9, 2018
Time:   11:00
Place:   ETH Zurich, Hönggerberg, HPZ E 35
Host:    Lukas Novotny

Electronic phase transition in charge neutral graphene multilayers: Does graphene ever become graphite upon increasing thickness?

Alberto Morpurgo
University of Geneva, Switzerland

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. Here I will discuss experiments on ultra-clean suspended multilayer graphene devices showing that –at least up to 8 layer Bernal-staked graphene multilayers (8LG)– this is not the case. The experiments show that 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 Eg =1.5 meV, and in 7LG Tc = 100 K and Eg = 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.