The metal-insulator transition in disordered solids: How theoretical prejudices influence its characterization. A critical review of analyses of experimental data

In a recent experiment, Siegrist et al. [Nature Materials 10, 202 (2011)] investigated the metal-insulator transition (MIT) of GeSb_2Te_4 on increasing annealing temperature. The authors conclude that this material exhibits a discontinuous MIT with a finite minimum metallic conductivity. The striking contrast to reports on other disordered substances motivates the present in-depth study of the influence of the MIT criterion used on the characterization of the MIT. First, we discuss in detail the inherent biases of the various available approaches to locating the MIT. Second, reanalyzing the GeSb_2Te_4 measurements, we show that this material resembles other disordered solids to a large extent: according to a widely-used approach, these data may also be interpreted in terms of a continuous MIT. Checking the justification of the respective fits, however, uncovers inconsistencies in the experimental data. Third, comparing with previous experimental studies of crystalline Si:As, Si:P, Si:B, Ge:Ga, CdSe:In, n-Cd_{0.95}Mn$_{0.05}Se, Cd_{0.95}Mn_{0.05}Te_{0.97}Se_{0.03}:In, disordered Gd, and nano-granular Pt-C, we show that such an inconclusive behavior occurs frequently: the analysis of the logarithmic temperature derivative of the conductivity highlights serious inconsistencies in the original interpretations in terms of a continuous MIT. In part, they are common to all these studies and seem to be generic, in part, they vary from experiment to experiment and may arise from measurement problems. Thus, the question for the character of the MIT of these materials has to be considered as yet open. The challenges now lie in improving the measurement precision and in developing a microscopic theory capable of explaining the seemingly generic features.Comment: Revtex-file + 23 figures -> 51 pages. Revisions: Some arguments completed; structure slightly modified: mathematical part of former Subsection II.E is now presented as Appendix B. This version was accepted for publ. by Critical Reviews in Solid State and Materials Sciences at July 18, 2017. It differs from this publication concerning citation style, abstract, and some very minor modification

When Celebrities Speak: A Nationwide Twitter Experiment Promoting Vaccination in Indonesia

Celebrity endorsements are often sought to influence public opinion. We ask whether celebrity endorsement per se has an effect beyond the fact that their statements are seen by many, and whether on net their statements actually lead people to change their beliefs. To do so, we conducted a nationwide Twitter experiment in Indonesia with 46 high-profile celebrities and organizations, with a total of 7.8 million followers, who agreed to let us randomly tweet or retweet content promoting immunization from their accounts. Our design exploits the structure of what information is passed on along a retweet chain on Twitter to parse reach versus endorsement effects. Endorsements matter: tweets that users can identify as being originated by a celebrity are far more likely to be liked or retweeted by users than similar tweets seen by the same users but without the celebrities' imprimatur. By contrast, explicitly citing sources in the tweets actually reduces diffusion. By randomizing which celebrities tweeted when, we find suggestive evidence that overall exposure to the campaign may influence beliefs about vaccination and knowledge of immunization-seeking behavior by one's network. Taken together, the findings suggest an important role for celebrity endorsement.Comment: 55 pages, 13 tables, 6 figure

Studies of Interstellar Pickup Ions in the Solar Wind

The work under this grant involves studies of the interaction of interstellar pickup ions with the solar wind, with the goal of a comprehensive model of the particle distributions and wave intensities to be expected throughout the heliosphere, as well as the interactions of those distributions with the solar wind termination shock. In the past year, we have completed a number of projects, including observations and modeling of the effects of a large scattering mean free path on the pickup He(+) seen at AMPTE, an analytical model of anisotropic pickup tons in a steady radial magnetic field, and a derivation of a reduced solar wind Mach number due to increased estimates on the inflowing hydrogen density allowing for a weak termination shock. In the next year, we plan to investigate in more detail the correspondence between our models of anisotropic pickup ions and the data on spectra, variations, and proton-He(+) correlation provided by AMPTE, Ulysses, and our instrument on SOHO. We will model the time-dependent pickup ion density resulting from finite periods of radial magnetic field. We will also incorporate the effects of a large mean free path into our analysis of the He(+) focusing cone, leading to more accurate parameter values for the interstellar helium gas. This progress report also includes a discussion of our Space Physics Educational Outreach activities in the past year and plans for the next year

TRIANGULATION OF THE INTERSTELLAR MAGNETIC FIELD

Determining the direction of the local interstellar magnetic field (LISMF) is important for understanding the heliosphere's global structure, the properties of the interstellar medium, and the propagation of cosmic rays in the local galactic medium. Measurements of interstellar neutral atoms by Ulysses for He and by SOHO/SWAN for H provided some of the first observational insights into the LISMF direction. Because secondary neutral H is partially deflected by the interstellar flow in the outer heliosheath and this deflection is influenced by the LISMF, the relative deflection of H versus He provides a plane—the so-called B–V plane in which the LISMF direction should lie. Interstellar Boundary Explorer (IBEX) subsequently discovered a ribbon, the center of which is conjectured to be the LISMF direction. The most recent He velocity measurements from IBEX and those from Ulysses yield a B–V plane with uncertainty limits that contain the centers of the IBEX ribbon at 0.7–2.7 keV. The possibility that Voyager 1 has moved into the outer heliosheath now suggests that Voyager 1's direct observations provide another independent determination of the LISMF. We show that LISMF direction measured by Voyager 1 is >40° off from the IBEX ribbon center and the B–V plane. Taking into account the temporal gradient of the field direction measured by Voyager 1, we extrapolate to a field direction that passes directly through the IBEX ribbon center (0.7–2.7 keV) and the B–V plane, allowing us to triangulate the LISMF direction and estimate the gradient scale size of the magnetic field