Dynamic properties of Josephson junctions are interesting due to the emission of high frequency radiation (up to THz range) from Josephson junctions, closely related to fluxon dynamics. A better understanding of this dynamics can help to improve the Josephson devices used for applications. Josephson junctions can also be of great use as T-shaped multiple Josephson junctions in Josephson electronic circuits. In general, T-junctions consist of two attached Josephson transmission lines: a main Josephson transmission line (MJTL) along the -axis, and an additional Josephson transmission line (AJTL) along the -axis. These junctions can use to create fluxons (solitons) in junctions without applied magnetic field, (called flux cloning phenomenon). This work is devoted to contributing to a clarification of the dynamic behaviour of solitons (fluxons) in 2D extended conventional T-shaped Josephson junctions (extended means an AJTL is larger than MJTL). A conventional T-junction is a MJTL along the x-axis which divides into two Josephson transmission lines along the x- and y-axes. In addition, we also attempt to elucidate further the concept of flux cloning in rotated T-junctions, which are 90 degrees anticlockwise rotation of conventional T-junction. In rotated Tjunction, a MJTL along the x-axis divide into two Josephson transmission lines along the y-axis. We find the first evidence of moving semifluxon and observe for the first time new phenomena of semifluxons and anti-semifluxons in both extended conventional and rotated T-junctions. We numerically study the electrodynamics behaviour of solitons in the standard Tshaped Josephson junction (conventional T-junction) in a magnetic field. Therefore, we describe theoretically how flux cloning circuits exist and give an opportunity for use as flux flow oscillators operating without applied magnetic field. The results that emerge give further support to the flux cloning mechanism.

In this research we looked at how fluxons propagate in an annular Josephson junction containing a microshort. We studied this from a theoretical stance and looked at how a single fluxon based on the sine-Grodon soliton equation propagates in this type of junction. It has been seen from a variety of studies that fluxons have many applications through the use of Josephson junctions. The aim of this thesis was to see whether a fluxon will show new properties whilst coming into contact with a microshort located in the junction. We also explored the different geometries a Josephson junction can have and whether that would show the fluxon to present new phenomena. We will also examine point particle systems. With this in mind we took a keen interest in how the interaction between two of these particles in a double well potential would present itself and whether a relationship would become apparent. Alongside the point particle system we modelled fluxons in a double well potential and comment on the similarities with the point particle system. With the aid of the computer programmes Mathematica and COMSOL Multiphysics we were able to compute these different theoretical models and present the work in a logical order with a progression from a single point particle in a double well potential to a fluxon in a heart-shaped Josephson junction. We have looked at current theories and ideas present in this area of condensed matter physics and have explained these in the subsequent thesis.

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.

<p>In this dissertation, we present work on developing superconducting circuits intended to advance the implementation of Asynchronous Ballistic Reversible Computation using Fluxon Logic. In the first Chapter we introduce the need for developing reversible computing, and discuss implementing asynchronous reversible computing using fluxons in superconducting circuits. In Chapter 2, we introduce basic superconductivity physics, including the Josephson effects, which is necessary to know for understanding the behavior of Josephson junction transmission lines. In Chapter 3, we introduce tools to physically understand the behavior of topologically protected solitons, 'fluxons', in Josephson junction transmission lines. Finally, in Chapter 4, we briefly discuss the history of fluxon-based computation devices and present current state of the art design of such reversible computation devices, including the fluxon Rotary gate that we have developed. Taken together, these represent advances in the direction of implementing asynchronous reversible computing in practice.</p>

In this thesis, the tunneling between individual atomic layers in structures of Bi2Sr2CaCu2O8+x based high-temperature superconductors are experimentally studied employing the intrinsic Josephson effect. A special attention is paid to the fabrication of small mesa structures using micro and nanofabrication techniques. In the first part of the thesis, the periodic Fraunhofer-like modulation of the critical current of the junctions as a function of in-plane magnetic field is investigated. A transition from a modulation with a half flux quantum to a flux quantum periodicity is demonstrated with increasing field and decreasing junction length. It is interpreted in terms of the transformation of the static fluxon lattice of stacked, strongly coupled intrinsic Josephson junctions and compared with theoretical predictions. A fluxon phase diagram is constructed.Numerical simulations have been carried out to complement the experimental data. In the second part of the thesis, different resonant phenomena are studied in the dynamic flux-flow state at high magnetic fields, including Eck-resonances and Fiske steps. Different resonant modes and their velocities, including superluminal modes, are identified. In the third part, different experiments attempting to detect radiation from small mesa structures using different setups based on hot-electron bolometer mixers and calorimeters are described. No distinct radiation with emission powers higher than about 500pW could be detected. Furthermore, the interaction with external GHz-radiation is studied. Resonances attributed to an induced flux-flow are observed, and the reflectivity of the sample can be tuned by switching mesas between the superconducting and quasiparticle state. In the last part, the resistive switching of mesas at high bias is studied. It is attributed to a persistent electrical doping of the crystal. Superconducting properties such as the critical current and temperature and the tunneling spectra are analyzed at different doping states of the same sample. The dynamics of the doping is studied, and attributed to two mechanisms; a charge-transfer effect and oxygen reordering