Supercurrent Spectroscopy of Andreev States

We measure the excitation spectrum of a superconducting atomic contact. In addition to the usual continuum above the superconducting gap, the single particle excitation spectrum contains discrete, spin-degenerate Andreev levels inside the gap. Quasiparticle excitations are induced by a broadband on-chip microwave source and detected by measuring changes in the supercurrent flowing through the atomic contact. Since microwave photons excite quasiparticles in pairs, two types of transitions are observed: Andreev transitions, which consists of putting two quasiparticles in an Andreev level, and transitions to odd states with a single quasiparticle in an Andreev level and the other one in the continuum. In contrast to absorption spectroscopy, supercurrent spectroscopy allows detection of long-lived odd states.Comment: typos correcte

Comparison of musculoskeletal networks of the primate forelimb

Anatomical network analysis is a framework for quantitatively characterizing the topological organization of anatomical structures, thus providing a way to compare structural integration and modularity among species. Here we apply this approach to study the macroevolution of the forelimb in primates, a structure whose proportions and functions vary widely within this group. We analyzed musculoskeletal network models in 22 genera, including members of all major extant primate groups and three outgroup taxa, after an extensive literature survey and dissections. The modules of the proximal limb are largely similar among taxa, but those of the distal limb show substantial variation. Some network parameters are similar within phylogenetic groups (e.g., non-primates, strepsirrhines, New World monkeys, and hominoids). Reorganization of the modules in the hominoid hand compared to other primates may relate to functional changes such as coordination of individual digit movements, increased pronation/supination, and knuckle-walking. Surprisingly, humans are one of the few taxa we studied in which the thumb musculoskeletal structures do not form an independent anatomical module. This difference may be caused by the loss in humans of some intrinsic muscles associated with the digits or the acquisition of additional muscles that integrate the thumb more closely with surrounding structures

First anatomical network analysis of fore- and hindlimb musculoskeletal modularity in bonobos, common chimpanzees, and humans

Studies of morphological integration and modularity, and of anatomical complexity in human evolution typically focus on skeletal tissues. Here we provide the first network analysis of the musculoskeletal anatomy of both the fore- and hindlimbs of the two species of chimpanzee and humans. Contra long-accepted ideas, network analysis reveals that the hindlimb displays a pattern opposite to that of the forelimb: Pan big toe is typically seen as more independently mobile, but humans are actually the ones that have a separate module exclusively related to its movements. Different fore- vs hindlimb patterns are also seen for anatomical network complexity (i.e., complexity in the arrangement of bones and muscles). For instance, the human hindlimb is as complex as that of chimpanzees but the human forelimb is less complex than in Pan. Importantly, in contrast to the analysis of morphological integration using morphometric approaches, network analyses do not support the prediction that forelimb and hindlimb are more dissimilar in species with functionally divergent limbs such as bipedal humans

Exciting Andreev pairs in a superconducting atomic contact

The Josephson effect describes the flow of supercurrent in a weak link, such as a tunnel junction, nanowire, or molecule, between two superconductors. It is the basis for a variety of circuits and devices, with applications ranging from medicine to quantum information. Currently, experiments using Josephson circuits that behave like artificial atoms are revolutionizing the way we probe and exploit the laws of quantum physics. Microscopically, the supercurrent is carried by Andreev pair states, which are localized at the weak link. These states come in doublets and have energies inside the superconducting gap. Existing Josephson circuits are based on properties of just the ground state of each doublet and so far the excited states have not been directly detected. Here we establish their existence through spectroscopic measurements of superconducting atomic contacts. The spectra, which depend on the atomic configuration and on the phase difference between the superconductors, are in complete agreement with theory. Andreev doublets could be exploited to encode information in novel types of superconducting qubits.Comment: Submitted to Natur

Phase controlled superconducting proximity effect probed by tunneling spectroscopy

Using a dual-mode STM-AFM microscope operating below 50mK we measured the Local Density of States (LDoS) along small normal wires connected at both ends to superconductors with different phases. We observe that a uniform minigap can develop in the whole normal wire and in the superconductors near the interfaces. The minigap depends periodically on the phase difference. The quasiclassical theory of superconductivity applied to a simplified 1D model geometry accounts well for the data.Comment: Accepted for publication in Physical Review Letter

Electron transport through a metal-molecule-metal junction

Molecules of bisthiolterthiophene have been adsorbed on the two facing gold electrodes of a mechanically controllable break junction in order to form metal-molecule(s)-metal junctions. Current-voltage (I-V) characteristics have been recorded at room temperature. Zero bias conductances were measured in the 10-100 nS range and different kinds of non-linear I-V curves with step-like features were reproducibly obtained. Switching between different kinds of I-V curves could be induced by varying the distance between the two metallic electrodes. The experimental results are discussed within the framework of tunneling transport models explicitly taking into account the discrete nature of the electronic spectrum of the molecule.Comment: 12 pages, 12 figures to appear in Phys. Rev. B 59(19) 199

Theory of microwave spectroscopy of Andreev bound states with a Josephson junction

We present a microscopic theory for the current through a tunnel Josephson junction coupled to a non-linear environment, which consists of an Andreev two-level system coupled to a harmonic oscillator. It models a recent experiment [Bretheau, Girit, Pothier, Esteve, and Urbina, Nature (London) 499, 312 (2013)] on photon spectroscopy of Andreev bound states in a superconducting atomic-size contact. We find the eigenenergies and eigenstates of the environment and derive the current through the junction due to inelastic Cooper pair tunneling. The current-voltage characteristic reveals the transitions between the Andreev bound states, the excitation of the harmonic mode that hybridizes with the Andreev bound states, as well as multi-photon processes. The calculated spectra are in fair agreement with the experimental data.Comment: 8 pages, 6 figure

Collective action problems in the contracting of public services: Evidence from the UK’s Ministry of Justice

In this paper, we examine collective action problems in the UK government in the process of contracting public services to the private sector. In particular, we examine the Ministry of Justice (MoJ) and its evolution in contract monitoring as part of a larger effort of the government to join up departments in contract management. By analyzing MoJ’s management of the electronic tagging contract with G4S and Serco, we show that a lack of coordination within the department and with other departments was a major reason for the overbilling by the two companies. Recent efforts to join up contract management efforts throughout government show promise in rectifying these contracting issues

Management Roles in Political and Senior Civil Servant Positions: A Multiple-Study Approach

A recent trend in public administration studies has been to assess how public management is performed by politicians and top civil servants. Surprisingly, little is known about how these different posts develop their management roles. Following seminal management studies, we focus on core management roles to compare the ways in which politicians and top civil servants develop their management responsibilities within public organizations. Empirical evidence is provided by triangulating three separate studies developed in Spain. Results from the triangulation show that there is unmistakable evidence of differentiations between politicians and top civil servants when performing their managerial roles

Evidence for long-lived quasiparticles trapped in superconducting point contacts

We have observed that the supercurrent across phase-biased, highly transmitting atomic size contacts is strongly reduced within a broad phase interval around {\pi}. We attribute this effect to quasiparticle trapping in one of the discrete sub-gap Andreev bound states formed at the contact. Trapping occurs essentially when the Andreev energy is smaller than half the superconducting gap {\Delta}, a situation in which the lifetime of trapped quasiparticles is found to exceed 100 \mus. The origin of this sharp energy threshold is presently not understood.Comment: Article (5 pages) AND Supplemental material (14 pages). To be published in Physical Review Letter