Confinement and resonant phenomena in small Bi2Sr2CaCu2O8 mesa structures

Sammanfattning: In this thesis, intrinsic Josephson junctions, naturally formed in the stronglyanisotropic high-temperature superconductor Bi2Sr2CaCu2O8, are studied experimentally.Small mesa structures are fabricated on the surface of singlecrystals using micro- and nano-fabrication tools, focused ion beam is used toreduce the mesa-area to ≈ 1 × 1 μm2.Intrinsic tunneling spectroscopy is performed to study the properties ofcharge transport between adjacent copper-oxide layers. For this purpose, currentvoltage characteristics along the crystallographic c-axis are measured and temperature and bias-dependence of the c-axis quasiparticle resistance is studied. A change in the interlayer transport-mechanism is observed at the criticaltemperature Tc: from thermal activation like above Tc to multiparticle transport below Tc.In magnetic fields B parallel to the copper-oxide planes, Josephson-vortices (fluxons) penetrate the intrinsic Josephson junctions in the form of a vortexlattice.Information about the static lattice structure can be obtained fromthe magnetic field dependence of the critical current Ic. In this thesis, Ic(B) is measured for mesas with different sizes in magnetic fields up to 4 T and at atemperature of 1.6 K. Modulations similar to the Fraunhofer diffraction patternare found. For long mesas, the period of oscillations is equal to half a fluxquantum per junction, but it changes to one flux quantum per junction if the size of the mesa is reduced, or if the magnetic field is increased. This indicatesa transition from a triangular to a rectangular fluxon-lattice by geometrical confinement in small mesas.Flux-flow characteristics and resonance phenomena in mesa structures of Bi2Sr2CaCu2O8+x and Bi2−yPbySr2CaCu2O8+x are finally studied. From measurementsof the flux-flow voltage in magnetic fields up to 17 T, the maximumvelocity of fluxons is calculated to cmax = 4.3 × 105 m/s. Phonon resonances are observed in the current-voltage characteristics. Their voltage positions are shown to be temperature-independent. The analysis of phonon-resonances in magnetic fields reveals a shift in the phonon frequencies with increasing fields. Evidence for an interaction between flux-flow and phonon resonances is found. In addition, Fiske steps (cavity-resonances) are observed in the current-voltagecharacteristics.It is argued that the employed technique for miniaturization of mesas andthe obtained results can be useful for better understanding of fundamentalproperties of high-temperature superconductors and for realization of a coherentflux-flow oscillator in the important THz frequency range.