Direct observation of ballistic Andreev reflection
An overview is presented of experiments on ballistic electrical transport in inhomogeneous super
conducting systems which are controlled by the process of Andreev reflection. The initial experiments based
on the coexistence of a normal phase and a superconducting phase in the intermediate state led to the concept
itself. It was followed by a focus on geometrically inhomogeneous systems like point contacts, which provided
a very clear manifestation of the energy and direction dependence of the Andreev reflection process. The
point contacts have recently evolved towards the atomic scale due to the use of mechanical breakjunctions,
revealing a very detailed dependence of Andreev reflection on the macroscopic phase of the superconducting
state. In presentday research, the superconducting in homogeneity is constructed by clean room technology
and combines superconducting materials, for example, with lowdimensional materials and topological insu
lators. Alternatively, the superconductor is combined with nanoobjects, such as graphene, carbon nano
tubes, or semiconducting nanowires. Each of these “inhomogeneous systems” provides a very interesting
range of properties, all rooted in some manifestation of Andreev reflection.
Ferromagnetic-insulator (FI) based Josephson junctions are promising candidates for a coherent superconducting quantum bit as well as a classical superconducting logic circuit. Recently the appearance of an intriguing atomic-scale 0-pi transition has been theoretically predicted. In order to uncover the mechanism of this phenomena, we numerically calculate the spectrum of Andreev bound states in a FI barrier by diagonalizing the Bogoliubov-de Gennes equation. We show that Andreev spectrum drastically depends on the parity of the FI-layer number L and accordingly the pi (0) state is always more stable than the 0 (pi) state if L is odd (even).
We argue that electron-electron interactions fundamentally restrict the penetration length of Cooper pairs into a diffusive normal metal from a superconductor. At low temperatures, this Cooper pair dephasing length remains finite and does not diverge. We evaluate the subgap conductance of hybrids in the presence of electron-electron interactions and demonstrate that this length Lϕ can be directly extracted from conductance measurements in such structures.
The Andreev current and the subgap conductance in a superconductor/ insulator/ ferromagnet (SIF) structure in the presence of a small spin-splitting field show novel interesting features (A. Ozaeta et al., Phys. Rev. B 86, 060509(R), 2012). For example, the Andreev current at zero temperature can be enhanced by a spin-splitting field h, smaller than the superconducting gap, as has been recently reported by the authors. Also at finite temperatures the Andreev current has a peak for values of the spin-splitting field close to the superconducting gap. Finally, the differential subgap conductance at low temperatures shows a peak at the bias voltage eV = h. In this paper we investigate the Andreev current and the subgap conductance in SFF structures with arbitrary direction of magnetization of the F layers. We show that all aforementioned features occur now at the value of the "effective field", which is the field acting on the Cooper pairs in the multi-domain ferromagnetic region, averaged over the decay length of the superconducting condensate into a ferromagnet. We also briefly discuss the heat transport and electron cooling in the considered structures.
The dynamics of a two-component Davydov-Scott (DS) soliton with a small mismatch of the initial location or velocity of the high-frequency (HF) component was investigated within the framework of the Zakharov-type system of two coupled equations for the HF and low-frequency (LF) fields. In this system, the HF field is described by the linear Schrödinger equation with the potential generated by the LF component varying in time and space. The LF component in this system is described by the Korteweg-de Vries equation with a term of quadratic influence of the HF field on the LF field. The frequency of the DS soliton`s component oscillation was found analytically using the balance equation. The perturbed DS soliton was shown to be stable. The analytical results were confirmed by numerical simulations.
Radiation conditions are described for various space regions, radiation-induced effects in spacecraft materials and equipment components are considered and information on theoretical, computational, and experimental methods for studying radiation effects are presented. The peculiarities of radiation effects on nanostructures and some problems related to modeling and radiation testing of such structures are considered.
This volume presents new results in the study and optimization of information transmission models in telecommunication networks using different approaches, mainly based on theiries of queueing systems and queueing networks .
The paper provides a number of proposed draft operational guidelines for technology measurement and includes a number of tentative technology definitions to be used for statistical purposes, principles for identification and classification of potentially growing technology areas, suggestions on the survey strategies and indicators. These are the key components of an internationally harmonized framework for collecting and interpreting technology data that would need to be further developed through a broader consultation process. A summary of definitions of technology already available in OECD manuals and the stocktaking results are provided in the Annex section.