Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures

We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi2Sr2CaCu2O8+x and Bi1.75Pb0.25Sr2CaCu2O8+x single crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on lambdaJ(Jc) and occur at about 0.4 and 1.3 fluxons per lambdaJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.Comment: Conference paper LT2

Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures

Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x mesa structures leads to persistent doping of the mesas. Here we employ this effect for analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are able to controllably and reversibly change the doping state of the same single crystal from underdoped to overdoped state, without changing its chemical composition. It is observed that such physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping the critical temperature and the superconducting gap decrease, with underdoping the c-axis critical current rapidly decreases due to progressively more incoherent interlayer tunneling and the pseudogap rapidly increases, indicative for the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even in voltage contribution, attributed to a charge transfer effect, and (ii) odd in voltage contribution, attributed to reordering of oxygen impurities

Confinement and resonant phenomena in small Bi2Sr2CaCu2O8 mesa structures

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

Properties of small Bi2Sr2CaCu2O8 intrinsic Josephson junctions: confinement, flux-flow and resonant phenomena

In this thesis, intrinsic Josephson junctions, naturally formed in the strongly anisotropic high-temperature superconductor Bi2Sr2CaCu2O8 (Bi-2212), are studied experimentally. For this purpose, small mesa structures are fabricated on the surface of single crystals using micro- and nano-fabrication tools, focused ion beam is used to reduce the area of the mesa-structures down to ≈ 1 × 1 μm2. The properties of charge transport across copper-oxide layers inside the mesas are studied by intrinsic tunneling spectroscopy. Temperature, bias and magnetic field dependences of current-voltage characteristics are examined. In the main part of the thesis, the behavior of intrinsic Josephson junctions in magnetic fields B parallel to the copper-oxide planes is studied. Parallel magnetic fields penetrate the junctions in the form of Josephson vortices (fluxons). At high magnetic fields, fluxons are arranged in a regular lattice and are accelerated by a sufficient high transport current. As the fluxon lattice is moving through the mesa, it emits electromagnetic waves in the important THz frequency range. Properties of Bi-2212 mesas in this flux-flow regime are studied in this thesis. The following new observations were made during the course of this work: a crossover from thermal activation above Tc to quantum tunneling below Tc is seen in the interlayer transport-mechanism, the Fraunhofer pattern of Ic(B) is observed clearly in Bi-2212, superluminal electromagnetic cavity resonances and phonon-polaritons are observed in Bi-2212. It is argued that the employed technique for miniaturization of mesas and the obtained results can be useful for a better understanding of fundamental properties of high-temperature superconductors and for the realizations of coherent flux-flow oscillators and coherent phonon-polariton generators in the important THz frequency range.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Manuscript