[diss] Perustieteiden korkeakoulu / SCI

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Browsing [diss] Perustieteiden korkeakoulu / SCI by Subject "1/f noise"

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  • Dynamics and correlations in low-dimensional electrical systems
    (2024) Will, Marco
     School of Science | Doctoral dissertation (article-based)
    In this thesis, the interaction of suspended graphene and carbon nanotube mechanical resonators with superconducting microwave cavities is investigated. Due to the photon interaction between both systems, small fluctuations in the mechanical resonators translate into measurable signals in the cavity frequency domain. The experiments were carried out at low temperatures, down to 10 mK, to reduce the noise level of the measurement setup. Such clean and low noise systems are also a good platform to probe 1/f noise. In the first part, the optomechanical coupling of graphene/NbSe2/graphene and graphene/hBN mechanical resonators with a superconducting coplanar waveguide is demonstrated. High quality factors over 200000 were measured with the ring-down measurement technique in our hybrid system. Lower electrical losses of the mechanical resonator can be attributed to the additional layers of NbSe2 compared to pure graphene devices. At low temperatures, the internal quality factor is limited by two-level system dissipation, which itself relaxes over the electronic system. In the second part, we study thermal self-oscillations in a superconductor - insulator - graphene - insulator - superconductor junction formed by a suspended piece of graphene coupled to a superconducting microwave cavity. The graphene piece has a strongly non-linear temperature dependence of resistance. This leads to a switching of dissipation once the Joule heating induced by the cavity pump can't be compensated by the heat transport of the junction. A high and low quality factor state is formed, which enables a fast cycle of heating and cooling of the system. When probing the thermal sidebands with a weak probe signal, amplification of the signal by up to 20 dB with a very low noise temperature of 1.4 K was measured. In the third part, 1/f noise in suspended and hBN encapsulated graphene was studied. Because graphene is two dimensional, noise from external sources such as the substrate or surface adsorbates is particularly noticeable. We studied the influence of adsorbed neon atoms on suspended graphene. Diffusion and clustering result in a 1/fγ dependence, with γ= 1.4-1.2 for temperatures between T=4-10 K. In hBN encapsulated graphene, we investigated the critical current noise. In the superconducting state, by tuning the charge carrier density away from the Dirac point, enhanced 1/f noise is measured with a noise level SIC/IC2≈10-6. In the last part, suspended, backgated carbon nanotubes are fabricated. We measured high supercurrents of up to 53 nA. The suspended nanotube resonator was used to investigate the phases of 3He dependent on 3He coverage and temperature down to 200 μK.The nanotube resonance shifts differently depending on the distinct phases of the 3He and a 3He phase diagram could be concluded.
  • 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.