Special seminar - Fereshte Ghahari, Columbia University, "Transport and Thermoelectric Studies of Correlated Phenomena in Graphene"

Fri, Mar 14, 2014, 1:30 pm to 2:30 pm
Location: 
Jadwin A06
The observation of correlated electron physics in graphene is mostly limited by strong electron scattering that is caused by charge impurities. We fabricate devices in which electrically contacted and electrostatically gated graphene flakes are either suspended over a SiO2 substrate or deposited on a hexagonal boron nitride layer such that a drastic suppression of disorder is achieved. The mobility of our graphene flakes exceeds 100,000 cm2/Vs. This very high mobility allows us to observe previously inaccessible transport regimes. In particular, we succeeded to observe the fractional quantum Hall Effect for the first time, hereby supporting the existence of interaction induced correlated electron states in the presence of a magnetic field. We were able to measure the energy gap associated with the fractional ν=1/3 state. This gap is at least 3 times larger than that of the 2DEGs in the best quality GaAs heterojunctions in a similar field range. In addition, at low carrier density graphene becomes an insulator with an energy gap tunable by a magnetic field. The insulating behavior at the charge neutrality point is independent of the in plane magnetic field indicating that the ν=0 quantum hall state in graphene is not spin polarized. Apart from that, we probed the e-e correlations in graphene by means of thermopower measurements. Our results show that at high temperatures the measured thermopower deviates from the generally accepted Mott's formula and that this deviation increases for samples with higher mobility. We quantify this deviation in both the degenerate and the non-degenerate regime using Boltzmann transport theory. We consider different scattering mechanisms in the system including electron-electron scattering.