Although the two superconducting phase transitions manifest in the same manner, just one aspect of which is the abrupt disappearance of DC electrical resistance shown in Figure 1.1, scientists speculated they are probably a result of unlike physical processes.This claim is nowadays well accepted in the scientific community, there being much evidence about the different nature of the superconducting phase of high-T c and low-T c materials (even in the absence of an exact microscopic model for the former).Indeed, a few key characteristics differentiate the two groups of superconductors, the most prominent being the already mentioned unconventional order parameter symmetry.It is a feature superficially similar to anisotropy in crystals and results in a number of interesting effects distinctive to high-T c superconductors.The aspects of the unconventional order parameter symmetry relevant to superconducting circuits, which is the main focus of this thesis, are presented in Chapter 2.Chapter 4 describes the improvements to the hybrid YBCO-Nb fabrication process and discusses the results of the implemented changes that, together with the knowledge gained about particular procedures, show a positive trend in process robustness.Also presented are electromagnetic simulation techniques modelling electrical parameters of the superconducting thin films as well as comparison with experimentally obtained values.Subsequently, the same Chapter briefly addresses the experimental difficulties with magnetic shielding, where measurements were Superconducting (Josephson) Networks
Although the two superconducting phase transitions manifest in the same manner, just one aspect of which is the abrupt disappearance of DC electrical resistance shown in Figure 1.1, scientists speculated they are probably a result of unlike physical processes.This claim is nowadays well accepted in the scientific community, there being much evidence about the different nature of the superconducting phase of high-T c and low-T c materials (even in the absence of an exact microscopic model for the former).Indeed, a few key characteristics differentiate the two groups of superconductors, the most prominent being the already mentioned unconventional order parameter symmetry.It is a feature superficially similar to anisotropy in crystals and results in a number of interesting effects distinctive to high-T c superconductors.The aspects of the unconventional order parameter symmetry relevant to superconducting circuits, which is the main focus of this thesis, are presented in Chapter 2.Chapter 4 describes the improvements to the hybrid YBCO-Nb fabrication process and discusses the results of the implemented changes that, together with the knowledge gained about particular procedures, show a positive trend in process robustness.Also presented are electromagnetic simulation techniques modelling electrical parameters of the superconducting thin films as well as comparison with experimentally obtained values.Subsequently, the same Chapter briefly addresses the experimental difficulties with magnetic shielding, where measurements were Superconducting (Josephson) Networks
The first chapter of this book provides a brief history of the important developments in superconductivity. After a general introduction, the superconductor-insulator transition is discussed in regards to open questions in the field and some of the questions tackled in this dissertation. Also, a brief introduction to ferromagnetism and phase transition in ferromagnetic systems is presented. In the second chapter, the results of simulations of three types of random inductor-capacitor (LC) networks on square lattices are presented [1,2]. The dynamical conductivity was calculated using an equation-of-motion method. The critical exponent was extracted at low frequencies. The results suggest that there are three different universality classes and that classical percolative 2D superconductor-insulator transitions (SITs) generically have sigma(omega)->infinity as omega->0. The third chapter presents results of simulations of a quantum rotor model describing a Josephson junction array (JJA) in a perpendicular magnetic field B on a square lattice [3]. The SIT is tuned by the ratio of charging energy to Josephson coupling, U/J. Abrupt drops in the magnetization values were observed in the bigger lattices at certain values of B and U/J caused by the formation of vortices. Increasing U/J at a fixed B field causes quantum vortex lattice melting. The magnetization drops to zero around U/J ~ 5 indicating SIT. In the fourth chapter, results from simulations of anisotropic Ising models are presented. These simulations were performed for a Hf2MnC2O2 monolayer under uniaxial strain [4]. The Curie temperature increases with the increasing strain, which means magnetic ordering survives up to higher temperatures under strain. In the fifth and final chapter, important results accrued over the whole dissertation are presented.
This thesis is about unconventional superconductors out of equilibrium. More precisely, it summarizes our theoretical efforts in addressing a few questions related to the real-time evolution in models displaying unconventional superconductivity. Apart from the purely theoretical interest, the motivation for these studies comes from the recent achievement in the field of ultrafast time-resolved spectroscopy on correlated materials, such as the high-temperature copper-oxide superconductors. The task of describing such systems in nonequilibrium is a very difficult one. In this thesis we attack the problem from two different points of view: • the anisotropic unconventional d-wave symmetry, • the strong electron-electron correlations. The thesis is organized as follows: The first part introduces the basic concepts which lay at the foundation of the work presented in the other two parts. In Chapter 1, the concept of unconventional superconductivity is briefly introduced. This mainly includes the high-temperature copper-oxide superconductors and their phenomenology: d-wave symmetry of the super- conducting gap and strong electron-electron interaction. Chapter 2 is devoted to a review of the recent experimental advances in the field of ultrafast spectroscopy of high temperature superconductors and correlated materials, with a particular focus on time- and angle- resolved photoemission spectroscopy. Finally, Chapter 3 is a short review of some works in the field of cold-atom Fermi gases. In the second part we address the physics of the d-wave superconductor after a sudden excitation. First, in Chapter 4 we introduce in some detail a model of mean-field superconductor with a d-wave symmetry. The principal feature of this model is the anisotropy of the gap and the nodal lines along which the gap vanishes. The results concerning the dynamics of the gap after a “quantum quench” are exposed in Chapter 5 where we compare our model to the s-wave superconductor. This comparison allows to identify the consequences of the unconventional symmetry. In Chapter 6 we discuss the spectral features of the transient nonequilibrium state of the d-wave superconductor. This is particularly important to move a first step towards photoemission experiments. In the third part we concentrate on the attractive Hubbard model as a prototype of strongly correlated superconductor. In Chapter 8 we discuss the superconducting state at equilibrium with the use of dynamical mean-field theory. In Chapter 9 we discuss the extension of this technique to systems out of equilibrium. In particular it is described the implementation in the superconducting phase. At the end of the thesis two appendices give some details of the calculations.
Good order is the foundation of all good things. E. Burke, Reflections on the Revolution in France , 1790 Josephson effect Superconductivity: A crash course The transition from the theoretical description of hypothetical building blocks of a quantum coherent device to something which can be actually fabricated and controlled is made much easier by the existence of superconductivity . This phenomenon, roughly speaking, allows a macroscopic quantum coherent flow of electrons in a sufficiently cold piece of an appropriate material by establishing a specific long-range order among them. Due to this one can, for example, use macroscopically different states of a superconductor as quantum states of a qubit, with the obvious advantage over “microscopically quantum” systems (like actual atoms) from the point of view of control, measurement and, last but not least, fabrication of structures with the desired parameters and on the desired scale. The phenomenon of superconductivity – the history of its discovery, experimental manifestations, theoretical explanation, open questions, relevance to other branches of physics, and technological applications – requires a thorough treatment, which can be found in any number of books. For our purposes Tinkham (2004) will provide more than sufficient background. We will not wander into the field of exotic/high-temperature superconductors for two simple reasons: the quantum coherent behaviour of the kind we need to realize qubits or other quantum coherent devices has not yet been properly and/or routinely achieved in these systems; and the fabrication of such devices is not yet reliable enough or even feasible.
The performance and coherence of superconducting quantum systems are fundamentally tied to the quality of the materials from which they are built. In superconducting transmon qubits, many loss mechanisms and sources of decoherence originate from material imperfections or unwanted oxide formation. Likewise, semiconductor–superconductor hybrid systems depend on high-quality interfaces to ensure strong coupling between electronic states in the two materials. In such hybrid devices, the accessible operating regime is often dictated by the intrinsic material properties of either the semiconductor or the superconductor. Advancing quantum device performance therefore requires exploring new material systems and optimizing their physical characteristics. In this thesis, we present the development of several superconducting material platforms for quantum technologies, including superconducting qubits, single-photon detectors, and semiconductor superconductor hybrid devices. We focus on the growth and characterization of superconducting films and heterostructures, and we establish clear connections between their material properties and the resulting device performance. First, we will present a novel semiconductor-superconductor hybrid material based on in-situ deposited epitaxial layers of Nb and Al on a shallow InAs quantum well. In this structure, the Al interlayer provides an epitaxial connection between an in-situ magnetron sputtered Nb or NbTi thin film and a shallow InAs two dimensional electron gas. This metal-to-metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al-based hybrid materials and indicates the formation of highly-transparent interfaces that are required in high-quality hybrid material platforms. Then, we discuss a different approach in the next chapter where we develop a hybrid material based on a molecular beam epitaxy grown InAsSb surface quantum well with an in-situ deposited Nb layer directly on top of the quantum well. Relative to conventional Al-InAs based systems, the InAsSb surface quantum well offers a lower effective mass and stronger spin-orbit interaction, while the Nb layer has a higher critical temperature and a larger critical magnetic field. The in-situ deposition of the Nb layer directly on top of the quantum well results in a high-quality interface that enables strong coupling to the InAsSb quantum well. Transport measurements on Josephson junctions reveal an induced superconducting gap of 1.3 meV and a critical current density of 3.75 × 10³A/cm². Furthermore, a planar asymmetric SQUID is realized, exhibiting gate-tunable superimposed oscillations originating from both the individual Josephson junction and the full SQUID loop. The large induced superconducting gap combined with strong spin-orbit interaction position this material as an attractive platform for experiments exploring gate-tunable superconductivity and topological superconducting devices. In the next chapter, we will focus on establishing a link between Nb film crystal properties and their performance in superconducting circuits. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over 10⁶ at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasi-particle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Then, we explore yet a different material platform by attempting to realize onchip NbTiN single photon detectors on LiNbO₃, an emerging photonic platform with high potential for scalable quantum information processing. We use DC reactive magnetron sputtering to grow high-quality NbTiN thin films and investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature (𝑇𝑐) of 12.3 K grown at a substrate temperature of 825 K. We demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LiNbO3 platform and investigate the obtained interface quality. Finally, we fabricate superconducting nanowire single photon detectors from the NbTiN film on LiNbO₃ and characterize the detector performance. Lastly, we report a gate-tunable Josephson diode effect in hybrid nanowire junctions consisting of a spin-orbit-coupled semiconductor core coated with epitaxial ferromagnetic insulator and superconductor shells. The wires display a hysteretic superconducting window as a function of axial magnetic field. In the superconducting regime, the devices exhibit nonreciprocal current transport, with the diode efficiency showing a strong dependence on back-gate voltage. The effect persists in a remanent magnetization state following a controlled demagnetization procedure, establishing zero-field operation. The combination of results presented in this thesis stress the importance of a material-focused approach in advancing the performance of quantum hardware. The novel hybrid material platforms developed in this thesis expand the operating regime of devices, which could enable future experiments involving gate-tunable superconductivity or topological properties. Similarly, we demonstrate that optimizing superconductor crystal structure benefits quantum devices, such as single-photon detectors and superconducting resonators.
The predominantly dx2−y2-pairing symmetry in most high-Tc superconductors provides the opportunity to fabricate Josephson junction circuits in which part of the junctions are biased by a phase difference of the superconducting order parameter of π. We present fabrication and measurements of an all high-Tc dc superconducting quantum interference device (dc SQUID) realized with thin-film technology, of which the Josephson junctions consist of one standard junction and one junction with a π-phase shift. The characteristics of the π-SQUID are compared with the properties of a standard high-Tc SQUID.