[qmeso-seminar] семинар сектора 12 сентября

[Mikhail A. Skvortsov]

                     СЕМИНАР СЕКТОРА КВАНТОВОЙ МЕЗОСКОПИКИ

                        пятница 12 сентября, 15:00, ИТФ


                      Artem Sboychakov (ITAE RAS, Moscow)

           AA-stacked bilayer graphene in an applied electric field: 
       Tunable antiferromagnetism and coexisting exciton order parameter


We study electronic properties of AA-stacked bilayer graphene. In the
single-particle approximation such a system has one electron band and
one hole band crossing the Fermi level. If the bilayer is undoped,
the Fermi surfaces of these bands coincide. Such a band structure 
is unstable with respect to a set of spontaneous symmetry violations.
Specifically, strong on-site Coulomb repulsion stabilizes
antiferromagnetic order. At small doping and low temperatures, 
the homogeneous phase is unstable and experiences phase separation 
into an undoped antiferromagnetic insulator and a metal. The metallic
phase can be either antiferromagnetic (commensurate or
incommensurate) or paramagnetic depending on the system parameters.
We derive the phase diagram of the system on the doping-temperature
plane and find that under certain conditions, the transition from 
the paramagnetic to the antiferromagnetic phase may demonstrate
reentrance. The application of the transverse voltage induces the
exciton order parameter coexisting with antiferromagnetism. The value
of this second order parameter is proportional to the biased voltage
and the value of the nearest- neighbor inter-plane Coulomb repulsion.