Analysis of Possible Quantum Metastable States in Ballistic Graphene-based Josephson Junctions

Graphene is a relatively new material (2004) made of atomic layers of carbon arranged in a honeycomb lattice. Josephson junction devices are made from graphene by depositing two parallel superconducting leads on a graphene flake. These devices have hysteretic current-voltage characteristics with a supercurrent branch and Shapiro steps appear when irradiated with microwaves. These properties motivate us to investigate the presence of quantum metastable states similar to those found in conventional current-biased Josephson junctions. We present work investigating the nature of these metastable states for ballistic graphene Josephson junctions. We model the effective Washboard potential for these devices and estimate parameters, such as energy level spacing and critical currents, to deduce the design needed to observe metastable states. We propose devices consisting of a parallel on-chip capacitor and suspended graphene. The capacitor is needed to lower the energy level spacing down to the experimentally accessible range of 1-20 GHz. The suspended graphene helps reduce the noise that may otherwise come from two-level states in the insulating oxide layer. Moreover, back-gate voltage control of its critical current introduces another knob for quantum control. We will also report on current experimental progress in the area of fabrication of this proposed device.Comment: 4 pages, 5 figures, Accepted for publication in IEEE Transactions on Applied Superconductivity from ASC 2010. Additional figures, additional calculation

Experimental Investigations of Magnesium Diboride Josephson Junctions

In this thesis, I present the results of experiments on Josephson tunnel junctions that incorporate magnesium diboride (MgB2) as one of the superconducting electrodes. Through explorations of their current vs. voltage and conductance vs. voltage characteristics, I produced high-resolution distributions of the energy gap structure of MgB2. I also explored their resonant modes by performing the first superconducting-to-normal state switching experiments on such "hybrid" junctions. Magnesium diboride exhibits two superconducting energy gaps, sigma and pi. The Josephson junctions measured for this thesis used clean MgB2 thin films grown by hybrid physical-chemical vapor deposition on single-crystal silicon carbide substrates. For some films, only the c-axis is exposed, allowing direct tunneling to the pi gap. In others, the a-b plane is exposed, allowing us to explore the sigma gap as well. Lead or tin were evaporated to form the superconducting counterelectrodes. I performed high-resolution tunneling spectroscopy measurements of these junctions. Their results demonstrate that, for films with little scattering, it is not possible to model the density of states with only two superconducting energy gaps. Instead, at least four gaps are needed and a distribution is more appropriate, as anticipated by several theoretical results. The switching of Josephson junctions from the superconducting to normal states reveals additional interesting physics. There has been some recent theoretical interest in the switching behavior of "hybrid" junctions -- those consisting of one single-gap electrode and one multi-gap electrode, separated by a tunneling barrier. I observed several features of the superconducting-to-normal switching behavior that are not yet explained by theory. However, most of my results are consistent with theories developed for conventional single-gap/single-gap Josephson junctions, suggesting that switching in these junctions is dominated by a single tunneling mode. By measuring the switching behavior across temperatures, with and without microwave excitation, I was able to characterize the resistance, capacitance, critical current, and quality factor of these junctions. Each junction proved to be highly underdamped (quality factor Q >> 1). Because my results also exhibit several signatures of quantum-mechanical behavior, similar junctions may be suitable for quantum device applications.Ph.D., Physics -- Drexel University, 201

Valvular heart disease: what does cardiovascular MRI add?

Although ischemic heart disease remains the leading cause of cardiac-related morbidity and mortality in the industrialized countries, a growing number of mainly elderly patients will experience a problem of valvular heart disease (VHD), often requiring surgical intervention at some stage. Doppler-echocardiography is the most popular imaging modality used in the evaluation of this disease entity. It encompasses, however, some non-negligible constraints which may hamper the quality and thus the interpretation of the exam. Cardiac catheterization has been considered for a long time the reference technique in this field, however, this technique is invasive and considered far from optimal. Cardiovascular magnetic resonance imaging (MRI) is already considered an established diagnostic method for studying ventricular dimensions, function and mass. With improvement of MRI soft- and hardware, the assessment of cardiac valve function has also turned out to be fast, accurate and reproducible. This review focuses on the usefulness of MRI in the diagnosis and management of VHD, pointing out its added value in comparison with more conventional diagnostic means

ACC/AHA Guidelines for Coronary Angiography. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Coronary Angiography)

"The ACC/AHA Task Force on Practice Guidelines herein revises and updates the original “Guidelines for Coronary Angiography,” published in 1987 (1). The frequent and still-growing use of coronary angiography, its relatively high costs, its inherent risks and the ongoing evolution of its indications have given this revision urgency and priority. The expert committee appointed included private practitioners and academicians. Committee members were selected to represent both experts in coronary angiography and senior clinician consultants. Representatives from the family practice and internal medicine professions were also included on the committee. The English-language medical literature was searched for the 10 years preceding development of the guidelines. The searches yielded >1,600 references that the committee reviewed for relevance. Evidence relative to the use of coronary angiography was compiled and evaluated by the committee. Whereas randomized trials are often available for reference in the development of treatment guidelines, randomized trials regarding the use of diagnostic procedures such as coronary angiography are rarely available (2). For development of these guidelines, when coronary angiography was a necessary procedure in describing a clinical subset or in choosing a course of treatment and that therapy was shown to have an advantage for the patient, especially in the context of a randomized trial, then the indication for angiography was given greater consideration than indications cited in less-rigorous evaluations of data.