There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This book presents the conceptual framework underlying the atomistic theory of matter, emphasizing those aspects that relate to current flow. This includes some of the most advanced concepts of non-equilibrium quantum statistical mechanics. No prior acquaintance with quantum mechanics is assumed. Chapter 1 provides a description of quantum transport in elementary terms accessible to a beginner. The book then works its way from hydrogen to nanostructures, with extensive coverage of current flow. The final chapter summarizes the equations for quantum transport with illustrative examples showing how conductors evolve from the atomic to the ohmic regime as they get larger. Many numerical examples are used to provide concrete illustrations and the corresponding Matlab codes can be downloaded from the web. Videostreamed lectures, keyed to specific sections of the book, are also available through the web. This book is primarily aimed at senior and graduate students.

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Quantum Transport: Atom to Transistor

Advanced gravitational wave interferometers, currently under realization, will soon permit the detection of gravitational waves from astronomical sources. To open the era of precision gravitational wave astronomy, a further substantial improvement in sensitivity is required. The future space-based Laser Interferometer Space Antenna and the third-generation ground-based observatory Einstein Telescope (ET) promise to achieve the required sensitivity improvements in frequency ranges. The vastly improved sensitivity of the third generation of gravitational wave observatories could permit detailed measurements of the sources' physical parameters and could complement, in a multi-messenger approach, the observation of signals emitted by cosmological sources obtained through other kinds of telescopes. This paper describes the progress of the ET project which is currently in its design study phase.

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The Einstein Telescope: a third-generation gravitational wave observatory

The Advanced LIGO gravitational wave detectors are next generation instruments which will replace the existing initial LIGO detectors. They are currently being constructed and installed. Advanced LIGO strain sensitivity is designed to be about a factor 10 better than initial LIGO over a broad band and usable to 10 Hz, in contrast to 40 Hz for initial LIGO. This is expected to allow for detections and significant astrophysics in most categories of gravitational waves. To achieve this sensitivity, all hardware subsystems are being replaced with improvements. Designs and expected performance are presented for the seismic isolation, suspensions, optics and laser subsystems. Possible enhancements to Advanced LIGO, either to resolve problems that may arise and/or to allow for improved performance, are now being researched. Some of these enhancements are discussed along with some potential technology being considered for detectors beyond Advanced LIGO.

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Advanced LIGO: the next generation of gravitational wave detectors

This review provides a theoretical basis for understanding the current-phase relation (CPhiR) for the stationary (dc) Josephson effect in various types of superconducting junctions The authors summarize recent theoretical developments with an emphasis on the fundamental physical mechanisms of the deviations of the CPhiR from the standard sinusoidal form A new experimental tool for measuring the CPhiR is described and its practical applications are discussed The method allows one to measure the electrical currents in Josephson junctions with a small coupling energy as compared to the thermal energy A number of examples illustrate the importance of the CPhiR measurements for both fundamental physics and applications

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The current-phase relation in Josephson junctions

In applications for high-Tc superconducting DC-SQUIDs such as biomagnetism, nondestructive evaluation and the relaxation of magnetic nanoparticles, it is important to maintain reliable sensor performance over an extended time period. We designed and produced DC-SQUID gradiometers based on YBCO thin films which are inductively coupled to a flux transformer to achieve a higher sensitivity. The gradiometers are protected against ambient atmosphere and humidity by SiO2 and amorphous YBCO layers. 
The noise properties of the sensor in flip-chip configuration, especially in unshielded environments, are shown. We present a comparison of TBCCO thin films on buffered sapphire or LaAlO3 substrates for the flux transformer in shielded and unshielded environments. We reach a low white field gradient noise of 72 fT/(cm sqrt(Hz)) with the TBCCO on LaAlO3 flux transformer. The electric properties of the gradiometers (critical current IC, normal state resistance RN and the transfer function VPhi) were measured over a period of one year and do not show significant signs of degradation.

Long-time stable HTSC DC-SQUID gradiometers with silicon dioxide passivation for measurements with superconducting flux transformers

In order to improve transport and noise properties of YBa2Cu3O7–δ thin films, we studied a technique to produce gold nano-particles in these films. We report on the formation of these particles from thin gold layers during pulsed laser deposition of YBa2Cu3O7–δ. Depending on initial gold layer thickness we analyzed their size distribution and structural properties. Gold particles preferentially grow at the YBa2Cu3O7–δ surface and the SrTiO3/YBa2Cu3O7–δ interface and show an epitaxial alignment parallel to the 〈001〉YBCO direction. We discuss the influence of these particles on the crystal structure of YBa2Cu3O7–δ in regard to their application as artificial pinning centers. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of gold nano‐particles during pulsed laser deposition of YBa2Cu3O7–δ thin films

Focused Ion Beams and Some Selected Applications

We investigate the iron-based superconductor Ba(Fe 1−x
 Co x
 ) 2As 2 (Ba-122) regarding its superconducting properties and possible applications. Therefore, Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and a
 b-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I– V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I– V characteristics and I
 c
 R
 n– T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I
 c
 R
 n product of the interface-engineered barrier junction was 12 μV and the planar junction with Au barrier showed 18 μV, we could increase the I
 c
 R
 n to 90 μV for planar TiO x
 barrier junctions.

Hybrid Josephson Junctions with Iron-based and Conventional Superconductor Electrodes

We have investigated the behaviour of differential conductance of YBa2Cu3O7−δ-Au point contacts depending on applied bias voltage. The change of differential conductance of YBa2Cu3O7−δ-Au contact from tunneling to metallic type and vice versa, are present. These effects we explain in terms of oxygen replacement in the unit cell of high Tc superconductor in close proximity to the contact.

Paul Seidel

Papers

Long-time stable HTSC DC-SQUID gradiometers with silicon dioxide passivation for measurements with superconducting flux transformers

Formation of gold nano‐particles during pulsed laser deposition of YBa2Cu3O7–δ thin films

Focused Ion Beams and Some Selected Applications

Hybrid Josephson Junctions with Iron-based and Conventional Superconductor Electrodes

Modification of YBa2Cu3O7−δ-Au point contact interface properties by applied electric voltage