09. Konferenssit, seminaarit ja kokoomateokset / Conferences, Seminars and Compiled works
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Yliopiston järjestämien konferenssien kokoomateoksia / Conference proceedings of the university's events
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Browsing 09. Konferenssit, seminaarit ja kokoomateokset / Conferences, Seminars and Compiled works by Department "Department of Applied Physics"
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Item Controlling supercurrents using single-walled carbon nanotube weak links(IOP Publishing, 2009) Wu, F.; Danneau, R.; Queipo, P.; Kauppinen, Esko I.; Tsuneta, T.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe have investigated proximity-induced supercurrents in single-walled carbon nanotubes. Phase diffusion is found to be present in the maximum measured supercurrent of 4.8 nA, which results in a minimum of 100Ω zero bias resistance in superconducting state. We also observe that the supercurrent is very sensitive to the measurement bandwidth and large phase fluctuations can even destroy the supercurrents. Our results shed light on the methods of how to improve the performance of high frequency superconducting single-walled nanotube devices.Item Diffusive Josephson junctions made out of multiwalled carbon nanotubes(IOP Publishing, 2009) Tsuneta, T.; Lechner, L.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe have investigated electrical transport in diffusive multiwalled carbon nanotubes (MWNT) contacted using superconducting leads made of Ti/Al/Ti sandwich structure. We measure proximity-induced supercurrents up to Icm = 1.3 nA and find tunability by the gate voltage due to variation of the Thouless energy via the diffusion constant that is controlled by scattering in the MWNT. The modeling of these results by long, diffusive SNS junctions, supplemented with phase diffusion effects is discussed: the agreement between theory and experiments is tested especially on the basis of the temperature dependence of the Josephson coupling energy. In order to prove conclusively that the diffusive model works for MWNT proximity junctions, we propose an improved measurement scheme that is based on the kinetic inductance of superconducting junctions.Item Measurements of shot noise in single walled carbon nanotubes(Aalto University, 2007) Wu, F.; Queipo, P.; Tsuneta, T.; Wang, T. H.; Kauppinen, E.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe have measured shot noise in single walled carbon nanotubes (SWNT) at 4.2K over frequencies f = 600 – 850 MHz. Here we report results obtained on shot noise without DC bias by applying an AC modulation at ω0 and recording the noise variation at 2ω0: the Fano factor is obtained by extrapolating down to zero excitation amplitude. We also discuss the applicability of this method in samples which have strongly non‐linear IV characteristics like carbon nanotubes. The obtained results are compared with regular differential noise measurements where both DC and AC bias are employed.Item Mesoscopic Josephson junction as a noise detector(SPIE-Intl Soc Optical Eng, 2004) Lindell, Rene; Delahaye, Julien; Sillanpää, Mika; Paalanen, Mikko; Sonin, E.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceSmall Josephson junctions are known to be very susceptible to noise. We have utilized this property in developing methods to measure noise as well as environmental resonance modes in mesoscopic systems. We review recent results on tunnel junction systems and show also that higher order moments of shot noise can be addressed with the present method based on the noise-induced modification of incoherent tunneling of Cooper pairs.Item Multiwalled carbon nanotubes as single electron transistors(Aalto University, 2001) Ahlskog, Markus; Tarkiainen, Reeta; Roschier, Leif; Paalanen, Mikko; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceSingle electron transistors (SET) are fabricated from multiwalled carbon nanotubes (MWNT) by manipulation with an atomic force microscope. The devices consist of either a single MWNT with Au contacts at the ends or of two crossing tubes. In the latter device, the lower nanotube acted as the central island of a single electron transistor while the upper one functioned as a gate electrode. Coulomb blockade oscillations were observed on the nanotube at low temperatures. The voltage noise of the nanotube-SET was gain dependent as in conventional SETs. The charge sensitivity at 10 Hz was 6x10 exp -4 e/√Hz. Furthermore, in another device where the MWNT is suspended above the substrate between the electrodes, we measure an extremely high charge sensitivity of 6x 10 exp -6 e/√Hz at 45 Hz, comparable to the best of the conventional SETs.Item Setup for shot noise measurements in carbon nanotubes(Aalto University, 2006) Wu, F.; Roschier, L.; Tsuneta, T.; Paalanen, M.; Wang, T. H.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe have constructed a noise measurement setup for high impedance carbon nanotube samples. Our setup, working in the frequency range of 600 – 900 MHz, takes advantage of the fact that the shot noise power is reasonably large for high impedance sources so that relatively large, fixed non‐matching conditions can be tolerated.Item Vacancy defect distributions in bulk ZnO crystals(SPIE-Society of Photo Optical Instrumentation Engineers, 2007) Tuomisto, Filip; Look, David C.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe have used positron annihilation spectroscopy to study vacancy defects in ZnO single crystals grown by various methods from both commercial and academic sources. The combination of positron lifetime and Doppler broadening techniques with theoretical calculations provides the means to deduce both the identities and the concentrations of the vacancies. The annihilation characteristics of the Zn and O vacancies have been determined by studying electron-irradiated ZnO grown by the seeded vapor phase technique. The different ZnO samples were grown with the following techniques: the hydrothermal growth method, the seeded vapor phase technique, growth from melt (skull melting technique), and both conventional and contactless chemical vapor transport. We present a comparison of the vacancydefects and their concentrations in these materials.