[diss] Perustieteiden korkeakoulu / SCI

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  • Transport Experiments on Suspended Graphene Devices
    (2018) Laitinen, Antti
     School of Science | Doctoral dissertation (article-based)
    In this thesis, sophisticated conductance and noise measurements were employed for studying electron transport through suspended graphene devices in order to understand the fundamental properties of graphene. The experiments were conducted at low temperatures down to T = 10 mK, and at high magnetic fields up to B = 9 T on suspended graphene devices. In these devices, graphene is connected only to the metallic contacts leaving the graphene flake intact of outside disturbances, and close to ideal theoretical behavior. The work was divided into two segments: quantum transport studies in the zero magnetic field using rectangular bi- and monolayer graphene devices, and magnetotransport measurements at high magnetic fields on Corbino ring devices.  In the case of the rectangular graphene devices, a model for contact doping in monolayer graphene by the metal leads was developed first. This facilitated understanding of the transport through the whole device and served as a basis for understanding the origin of the observed Fabry-Pérot resonances. The resonances were used to demonstrate the phase-coherent transport and long mean free path in the devices. Two sets of noise measurements were performed on these devices. First, low frequency 1/f noise measurements on suspended bilayer graphene (BLG) devices revealed extremely low flicker noise levels that was contributed to the substrate-free form of the devices and the effective screening of fluctuations in BLG. The low intrinsic noise level was exploited in a gas sensing application, where adsorbed gases were detected through the extra noise caused by molecules that had landed on the device. In the later set of noise measurements at higher frequencies, f = 600 - 900 MHz, noise thermometry was employed for characterization of the electron-phonon coupling in bi- and monolayer graphene.  Finally, suspended graphene Corbino devices were developed for studying integer and fractional quantum Hall effect (IQHE and FQHE). The observed FQHE was explained with the established theory of composite fermions. Based on the measurements, it was concluded that the composite fermions in graphene are Dirac particles with cyclotron mass around one electron rest mass. At very high fields and low charge carrier densities, evidence of Wigner crystallization was obtained. Additionally, the breakdown of quantum Hall effect was studied at the filling factor ν = 0 in the middle of the lowest Landau level. Zener tunneling between Landau sublevels was found to facilitate the breakdown at fields below 7 T, while a more standard behavior due to bootstrapped electron heating was observed at higher fields.