The observation of interaction-limited transport 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. This 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. Apart from that, we probed the electron-electron interactions in graphene by means of thermopower measurements. Our findings show that at high temperatures the measured thermopower S departs from the semiclassical Mott relation as the sample mobility increases. Our measured S/T is consistently larger than that of Mott's formula in the degenerate limit, but does not reach the universal hydrodynamic TEP in the limit of low disorder. By analyzing temperature dependent resistivity and extrapolating different scattering rates, we conclude that the intrinsic optical phonon scattering can suppress thermopower at high temperatures due to the strong coupling of these high energy phonons to electrons.
For further information please contact Nikolai Zhitenev, 301-975-6039, nikolai.zhitenev [at] nist.gov (nikolai[dot]zhitenev[at]nist[dot]gov)