Carrier Anisotropy and Impurity Scattering in Germanium at mK Temperatures: Modeling and Comparison to Experiment

International audienceImproving upon the present background rejection capabilities of the cryogenic Ge detectors for direct dark matter search involves an in-depth comprehension of the charge collection process in these devices. Experimental data point to the combined effects of lattice and impurity scattering on the anisotropy of electron transport in germanium at mK temperatures. A Monte Carlo simulation code has been implemented to incorporate these features in a consistent model of charge collection. In a novel approach to carrier scattering by charged impurities, the scattering field of the impurities is treated statistically as a random contribution to the collection field, described by the Holtsmark distribution function with a single adjustable parameter, the mean density of the charged centers. Simulation of charge collection along these lines in devices different by their impurity content shows excellent agreement to experiment. Especially noteworthy is the fact that the strength of impurity scattering is reversed from the known concentration of dopant impurities in the crystals, as the crystal with the higher dopant concentration shows lower scattering at low field than the one with the lower concentration. This raises as an issue for further improvement of these devices, the question of the nature of the scattering centers in high-purity Ge crystals at cryogenic temperatures, associated presumably with deep level impurities or crystal defects

Flexibility, Complexity, and Controllability in Large Scale Systems

System structure is a key determinant of system behavior. There is a particularly strong link between a system’s structure and its flexibility – it’s capacity to respond to changes. Often, adding flexibility entails adding complexity. In this paper, we propose measures for a system’s complexity that are complementary to existing flexibility measures. Furthermore, flexibility often comes at the cost of some measure of control over the system’s behavior. We therefore propose a metric for system controllability that is complementary to our flexibility metric

Analogies Between Complex Systems and Phases of Matter

The behavior of a complex system in a changing environment is strongly affected by the system's architecture. We present an analogy between the major phases of matter (solid, liquid, gas) and three major generic architectures of complex systems: tree structures, layered structures and grid networks. This analogy is realized using a graph-based formalism, with nodes and edges in a given configuration. Solid materials are akin to tree structures, especially when we consider that most solids actually have cracks. Solids with cracks between their components can be modeled by nodes (representing each component) and their interconnection, leading to a tree structured hierarchy. Gases made up of molecules can be modeled by nodes (the molecules) with local interconnections representing nearby molecules in space, thus forming a grid network. Liquids can form layers as in a mixture of oil and water. We represent this by connections that are densely horizontal within layers as well as sparsely vertical between layers. A key issue for complex systems is the ease by which they may be changed, which we call the system’s flexibility. Our definition of flexibility indicates that tree structures, like solids, are relatively inflexible and that grid networks, like gases, are extremely flexible, possibly leading to loss of control and chaotic behavior. Like liquids, layered systems are intermediate in flexibility and controllability. Solids, even with cracks, are relatively difficult to modify, whereas gases change internal form so quickly that they can only be constrained; not controlled. Liquids are intermediate in their ability to change form internally. Just as heating solids can lead to liquids, and heating liquids can result in gases, we shall present transformations in the interconnection structure of systems, analogous to heating, that change tree structures into layered ones and layered structures into networks

Gist and Verbatim in Narrative Memory

A major concern regarding the study of narratives regards how they are indexed and retrieved. This is a question which touches on the structure of human memory in general. Indeed, if narratives capture the substance of human thought, then data that we have already collected regarding human memory is of central importance to the computational study of narrative. Fuzzy Trace Theory assumes that memory for narrative is simultaneously stored at multiple levels of abstraction and, whenever possible, decision-makers interpret a stimulus qualitatively and therefore operate on a simple - typically categorical - "gist" representation. Here, we present a computational model of Fuzzy Trace Theory applied to explain the impact of changes in a narrative upon risky-choice framing effects. Overall, our theory predicts the outcome of 20 experimental effects using only three basic assumptions: 1) preference for lowest level of gist, that is, categorical processing; 2) decision options that fall within the same categorical description are then interpreted using finer-grained (ordinal or verbatim) distinctions; and 3) once the options are mentally represented, decision preferences are generated on the basis of simple positive vs. negative valences stored in long-term memory (e.g., positive value for human lives). A fourth assumption - that negatively-valenced decision options are preferentially converted to positive decision options - is used when categories are not otherwise comparable

Effects of a Weak Magnetic Field on Carrier Transport in a Cryogenic Germanium Detector for Dark Matter Search

International audienceA magnetic field of a few gauss produces sizeable effects on carrier trans-port and charge collection in a germanium dark matter detector operated at millikelvin temperatures. The magnitude of the effects is explained by the large values of the velocities imparted to the carriers, even under the low electric field conditions typical for charge collection in these devices (a few 106 cm/s at ~ 1 V/cm). Using a suitable experimental setup, effects of the magnetic field on electron and hole transport were investigated separately. A dependence of these effects on the orientation of the field relative to the detector axis is demonstrated, arising in part from magnetic flux conser-vation through the superconducting (Al) annular electrodes on these devices

Identifying Nuances in Fake News vs. Satire: Using Semantic and Linguistic Cues

The blurry line between nefarious fake news and protected-speech satire has been a notorious struggle for social media platforms. Further to the efforts of reducing exposure to misinformation on social media, purveyors of fake news have begun to masquerade as satire sites to avoid being demoted. In this work, we address the challenge of automatically classifying fake news versus satire. Previous work have studied whether fake news and satire can be distinguished based on language differences. Contrary to fake news, satire stories are usually humorous and carry some political or social message. We hypothesize that these nuances could be identified using semantic and linguistic cues. Consequently, we train a machine learning method using semantic representation, with a state-of-the-art contextual language model, and with linguistic features based on textual coherence metrics. Empirical evaluation attests to the merits of our approach compared to the language-based baseline and sheds light on the nuances between fake news and satire. As avenues for future work, we consider studying additional linguistic features related to the humor aspect, and enriching the data with current news events, to help identify a political or social message.Comment: Accepted to the 2nd Workshop on NLP for Internet Freedom (NLP4IF): Censorship, Disinformation, and Propaganda. Co-located with EMNLP-IJCNLP 201

Thermally-Stimulated Current Investigation of Dopant-Related D- and A+ Trap Centers in Germanium for Cryogenic Detector Applications

International audienceThermally-stimulated current measurements provide a sensitive tool to char-acterize carrier traps in germanium detectors for dark matter search. Using this technique at cryogenic temperatures, very shallow traps have been detected with binding energies of a fraction of a meV, associated with the dopant species in the D-(A+) charge states. A positive identification of these traps is achieved through an analysis of the field dependence of the carrier emission rates, which demonstrates a potential well for the trapped carriers in the form of a polarization well in r-4, consistent with Lax's model for carrier trapping by a neutral center. The density of these traps is assessed, and implications for the space-charge cancellation procedure in cryogenic Ge detectors are discussed