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1. Nighttime Magnetic Perturbation Events Observed in Arctic Canada: 1. Survey and Statistical Analysis

   https://doi.org/10.1029/2019JA026794  

   Engebretson, M. J.; Pilipenko, V. A.; Ahmed, L. Y.; Posch, J. L.; Steinmetz, E. S.; Moldwin, M. B.; Connors, M. G.; Weygand, J. M.; Mann, I. R.; Boteler, D. H.; et al (September 2019, Journal of Geophysical Research: Space Physics)

   Abstract The rapid changes of magnetic fields associated with large, isolated magnetic perturbations with amplitudes |ΔB| of hundreds of nanotesla and 5‐ to 10‐min periods can induce bursts of geomagnetically induced currents that can harm technological systems. This paper presents statistical summaries of the characteristics of nightside magnetic perturbation events observed in Eastern Arctic Canada from 2014 through 2017 using data from stations that are part of four magnetometer arrays: MACCS, AUTUMNX, CANMOS, and CARISMA, covering a range of magnetic latitudes from 68 to 78°. Most but not all of the magnetic perturbation events were associated with substorms: roughly two thirds occurred between 5 and 30 min after onset. The association of intense nighttime magnetic perturbation events with magnetic storms was significantly reduced at latitudes above 73°, presumably above the nominal auroral oval. A superposed epoch study of 21 strong events at Cape Dorset showed that the largest |dB/dt| values appeared within an ~275‐km radius that was associated with a region of shear between upward and downward field‐aligned currents. The statistical distributions of impulse amplitudes of both |ΔB| and |dB/dt| fit well the log‐normal distribution at all stations. The |ΔB| distributions are similar over the magnetic latitude range studied, but the kurtosis and skewness of the |dB/dt| distributions show a slight increase with latitude. Knowledge of the statistical characteristics of these events has enabled us to estimate the occurrence probability of extreme impulsive disturbances using the approximation of a log‐normal distribution. 

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2. Decidability of Non-Interactive Simulation of Joint Distributions

   Ghazi, Badih; Kamath, Pritish; Sudan, Madhu (October 2016, Annual Symposium on Foundations of Computer Science)

   We present decidability results for a sub-class of “non-interactive” simulation problems, a well-studied class of problems in information theory. A non-interactive simulation problem is specified by two distributions P(x, y) and Q(u, v): The goal is to determine if two players, Alice and Bob, that observe sequences Xn and Y n respectively where {(Xi, Yi)}n i=1 are drawn i.i.d. from P(x, y) can generate pairs U and V respectively (without communicating with each other) with a joint distribution that is arbitrarily close in total variation to Q(u, v). Even when P and Q are extremely simple: e.g., P is uniform on the triples {(0, 0), (0, 1), (1, 0)} and Q is a “doubly symmetric binary source”, i.e., U and V are uniform ±1 variables with correlation say 0.49, it is open if P can simulate Q. In this work, we show that whenever P is a distribution on a finite domain and Q is a 2 × 2 distribution, then the non-interactive simulation problem is decidable: specifically, given δ > 0 the algorithm runs in time bounded by some function of P and δ and either gives a non-interactive simulation protocol that is δ-close to Q or asserts that no protocol gets O(δ)-close to Q. The main challenge to such a result is determining explicit (computable) convergence bounds on the number n of samples that need to be drawn from P(x, y) to get δ-close to Q. We invoke contemporary results from the analysis of Boolean functions such as the invariance principle and a regularity lemma to obtain such explicit bounds. 

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3. Demonstration of Wide Bandgap AlGaN/GaN Negative‐Capacitance High‐Electron‐Mobility Transistors (NC‐HEMTs) Using Barium Titanate Ferroelectric Gates

   https://doi.org/10.1002/aelm.202000074  

   Chandrasekar, Hareesh; Razzak, Towhidur; Wang, Caiyu; Reyes, Zeltzin; Majumdar, Kausik; Rajan, Siddharth (July 2020, Advanced Electronic Materials)

   Abstract A negative‐capacitance high electron mobility transistor (NC‐HEMT) with low hysteresis in the subthreshold region is demonstrated in the wide bandgap AlGaN/GaN material system using sputtered BaTiO₃as a “weak” ferroelectric gate in conjunction with a conventional SiN_xdielectric. An enhancement in the capacitance for BaTiO₃/SiN_xgate stacks is observed in comparison to control structures with SiN_xgate dielectrics directly indicating the negative capacitance contribution of the ferroelectric BaTiO₃layer. A significant reduction in the minimum subthreshold slope for the NC‐HEMTs is obtained in contrast to standard metal‐insulator‐semiconductor HEMTs with SiN_xgate dielectrics—97.1 mV dec⁻¹versus 145.6 mV dec⁻¹—with almost no hysteresis in theI_D–V_Gtransfer curves. These results are promising for the integration of ferroelectric perovskite oxides with III‐Nitride devices toward NC‐field‐effect transistor switches with reduced power consumption. 

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4. Estimating random close packing in polydisperse and bidisperse hard spheres via an equilibrium model of crowding

   https://doi.org/10.1063/5.0137111  

   Anzivino, Carmine; Casiulis, Mathias; Zhang, Tom; Moussa, Amgad Salah; Martiniani, Stefano; Zaccone, Alessio (January 2023, The Journal of Chemical Physics)

   We show that an analogy between crowding in fluid and jammed phases of hard spheres captures the density dependence of the kissing number for a family of numerically generated jammed states. We extend this analogy to jams of mixtures of hard spheres in d = 3 dimensions and, thus, obtain an estimate of the random close packing volume fraction, ϕRCP, as a function of size polydispersity. We first consider mixtures of particle sizes with discrete distributions. For binary systems, we show agreement between our predictions and simulations using both our own results and results reported in previous studies, as well as agreement with recent experiments from the literature. We then apply our approach to systems with continuous polydispersity using three different particle size distributions, namely, the log-normal, Gamma, and truncated power-law distributions. In all cases, we observe agreement between our theoretical findings and numerical results up to rather large polydispersities for all particle size distributions when using as reference our own simulations and results from the literature. In particular, we find ϕRCP to increase monotonically with the relative standard deviation, sσ, of the distribution and to saturate at a value that always remains below 1. A perturbative expansion yields a closed-form expression for ϕRCP that quantitatively captures a distribution-independent regime for sσ < 0.5. Beyond that regime, we show that the gradual loss in agreement is tied to the growth of the skewness of size distributions. 

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5. Challenge and Solution of Characterizing Glass Transition Temperature for Conjugated Polymers by Differential Scanning Calorimetry

   https://doi.org/10.1002/polb.24889  

   Qian, Zhiyuan; Galuska, Luke; McNutt, William W.; Ocheje, Michael U.; He, Youjun; Cao, Zhiqiang; Zhang, Song; Xu, Jie; Hong, Kunlun; Goodman, Renée B.; et al (November 2019, Journal of Polymer Science Part B: Polymer Physics)

   ABSTRACT Thermomechanical properties of polymers highly depend on their glass transition temperature (T_g). Differential scanning calorimetry (DSC) is commonly used to measureT_gof polymers. However, many conjugated polymers (CPs), especially donor–acceptor CPs (D–A CPs), do not show a clear glass transition when measured by conventional DSC using simple heat and cool scan. In this work, we discuss the origin of the difficulty for measuringT_gin such type of polymers. The changes in specific heat capacity (Δc_p) atT_gwere accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δc_pfrom flexible polymer (0.28 J g⁻¹K⁻¹for polystyrene) to rigid CPs (10⁻³J g⁻¹K⁻¹for a naphthalene diimide‐based D–A CP). When a conjugation breaker unit (flexible unit) is added to the D–A CPs, we observed restoration of the Δc_patT_gby a factor of 10, confirming that backbone rigidity reduces the Δc_p. Additionally, an increase in the crystalline fraction of the CPs further reduces Δc_p. We conclude that the difficulties of determiningT_gfor CPs using DSC are mainly due to rigid backbone and semicrystalline nature. We also demonstrate that physical aging can be used on DSC to help locate and confirm the glass transition for D‐A CPs with weak transition signals. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019, 57, 1635–1644 

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